Efecto-del-clima-sobre-la-proporcion-de-sexos-del-cocodrilo-americano-crocodylus-acutus-y-cocodrilo-de-pantano-c -moreletii-y-posibles-implicaciones-ante-el-cambio-climatico

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UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO 
 
 
 DOCTORADO EN CIENCIAS 
BIOMÉDICAS 
 
 
 Instituto de Ecología 
 
 
 
EFECTO DEL CLIMA SOBRE LA PROPORCIÓN DE 
SEXOS DEL COCODRILO AMERICANO (Crocodylus 
acutus) Y COCODRILO DE PANTANO (C. moreletii), 
Y POSIBLES IMPLICACIONES ANTE EL CAMBIO 
CLIMÁTICO 
 
 
 
 T E S I S 
 
 QUE PARA OBTENER EL GRADO ACADÉMICO DE 
 
 DOCTOR EN CIENCIAS 
 
 P R E S E N T A 
 
 
 ARMANDO H. ESCOBEDO GALVÁN 
 
 
 
DIRECTOR DE TESIS: DR. ENRIQUE MARTÍNEZ MEYER 
COMITÉ TUTORAL: DRA. NORMA A. MORENO MENDOZA 
 DR. GERARDO CEBALLOS GONZÁLEZ 
 
 
MÉXICO, D.F. NOVIEMBRE, 2012 
 
 
UNAM – Dirección General de Bibliotecas 
Tesis Digitales 
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respectivo titular de los Derechos de Autor. 
 
 
 
Agradecimientos 
 
 
 Agradezco al Programa de Doctorado en Ciencias Biomédicas por la oportunidad y el 
apoyo recibido para realizar mis estudios de doctorado en la Universidad Nacional 
Autónoma de México. También quiero extender mis agradecimientos al Consejo Nacional 
de Ciencia y Tecnología (CONACYT), por la beca otorgada para llevar acabo estudios de 
doctorado. 
 
 Quiero agradecer el apoyo y tiempo dedicado en estos cinco años por parte del Dr. 
Enrique Martínez Meyer, y el comité tutoral: Dra. Norma A. Moreno Mendoza y Dr. 
Gerardo Ceballos, para mi formación personal y académica. 
 
 Esta tesis fue posible gracias al apoyo de: La Dirección General de Asuntos del 
Personal Académico de la Universidad Nacional Autónoma de México, bajo el proyecto 
DGAPA-PAPIIT IN221208, la Organización para Estudios Tropicales por la beca post-curso 
(INECOL LSU OTS 07-19), la fundación IdeaWild proporcionó parte del equipo (Proyecto 
1775) y la beca Scott Neotropical Fund del Cleveland Metropark Zoo and Cleveland 
Zoological Society durante el último año de tesis. 
 
 
 
 
 
 
 
 i
Agradecimientos personales 
 
 Quiero agradecer a mis padres: María del Socorro Galván Carreón y Luis Escobedo 
Cantú, por su apoyo, cariño y la motivación para salir adelante. Mis hermanos, Luis y 
Evelyn, gracias por ser mis compañeros y amigos de vida. A Miryam Venegas y Héctor 
Anaya (q.e.p.d.) por su cariño y apoyo en mi desarrollo personal y profesional. 
 
 A mi comité tutoral: Dr. Enrique Martínez Meyer, Dra. Norma A. Moreno Mendoza y Dr. 
Gerardo Ceballos, por su apoyo y entusiasmo en cada parte de la investigación. 
 
 Agradezco al comité de candidatura por los comentarios y observaciones en el 
proyecto: Dr. Horacio Merchant Larios, Dr. Víctor Magaña Rivera, Dra. Robyn Hudson, Dr. 
Fausto Méndez de la Cruz y Dra. Norma A. Moreno Mendoza. 
 
 A Sylvia de la Parra y Camila de la Parra por su apoyo incondicional y forma parte de 
este proyecto. 
 
 A mis compañeros de laboratorio: Constantino González Salazar, Edith Calixto, Mariana 
Munguía, Carolina Ureta, Karina Ramos, Yajaira García, Julián Velasco, Bárbara Ayala, 
Marta Suárez, Saúl López Alcaide, Miguel Rivas Soto y Baruch Arroyo Peña, por todas las 
aventuras durante mi tiempo en el laboratorio. 
 
 A mis colegas y compañeros de aventura en el mundo de los cocodrilos: M. en C. Sergio 
Padilla Paz, M. en C. Mauricio González Jáuregui, Biol. Ernesto Perera Trejo, M. en C. 
 ii
Javier Gómez Duarte, M. en C. Claudia Monzón, Dr. Pierre Charruau, Dr. Fabio G. Cupul 
Magaña, M. en C. Marco A. López Luna, M. en C. José F. González Maya, Armando Rubio 
Delgado, M. en C. Hernán Mandujano, M. en C. Alejandro Reyes, Dr. Juan Gaviño, Dr. 
Gustavo Casas Andreu y Biol. Gabriel Barrios Quiroz. 
 
 Agradezco los comentarios a cada capítulo que forman parte de esta tesis: Dr. Forest 
Isbell, Dra. Katherine Renton, Dr. Steven Platt, Dr. Charles Deeming, Dra. Roxana Torres 
y Dr. Sherman Silber. 
 
 Al personal del programa de doctorado en ciencias biomédicas y del Instituto de 
Ecología: Patricia Martínez Reyes, Angélica Téllez Garmendia y Zenaida Martínez 
Estrella. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 iii
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
A mis padres por su cariño y apoyo 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 iv
 
Contenido 
 
 
Resumen…………………………………………………………………………………………....…1 
 
Abstract…………………………………………………………………………………………….....2 
 
Introducción…………………………………………………………..……………….…………….3 
 
Capítulo 1: 
Variation in sex ratio of Morelet’s crocodile in northern wetlands of 
Campeche, Mexico ……………………………………………………………….…….……....7 
 
Capítulo 2: 
Temperature fluctuation within and between nests of Morelet’s crocodile: 
implications to embryo development and sex determination..…………..18 
 
Capítulo 3: 
Survival and extinction on sex-determining mechanisms of Cretaceous 
tetrapods……………………………………………………………………………………………….27 
 
Capítulo 4: 
Will all species with temperature-dependent sex determination respond 
the same way to climate change?............................................32 
 
Conclusiones..……………………………………………………………………………………...35 
 
Bibliografía..……………………………………………………………………………….........39 
 
Anexos……………………………………………………………………………………………………51 y 58
 v
Resumen 
 
 
En años recientes, la atención de los científicos ha incrementado en relación a las 
consecuencias ecológicas y evolutivas por efectos climáticos sobre especies con 
determinación sexual por temperatura (TSD). Para esto, los cocodrilos son un modelo 
interesante para entender respuestas adaptativas de la plasticidad fenotípica ante las 
condiciones ambientales. Para evaluar cómo las poblaciones de cocodrilos (Crocodylus 
acutus y C. moreletii) podrían ser afectadas por las condiciones ambientales, en este 
estudio se consideró los efectos del clima sobre la proporción de sexos y la temperatura 
de los nidos en Crocodylus moreletii, y las implicaciones de esta relación ante el actual 
cambio clima y la conservación de los cocodrilos. Los resultados mostraron que la 
proporción de sexos de C. moreletii presentó una variación anual, la cual no se relacionó 
con las variaciones climáticas. La temperatura de los nidos de C. moreletii varió en 
respuesta a las condiciones externas (fluctuación de temperatura y lluvia). Las 
modificaciones en los patrones de temperatura debido al cambio climático actual no 
necesariamente podría resultar en un sesgo de la proporción de sexos, debido a que la 
selección de sitios para anidación y el comportamiento de la hembra en el caso de los 
cocodrilos son claves para la fluctuación térmica del nido, lo cual podría llevar a un 
equilibrio en la proporción de sexos. 
 
 
 
 
 1
 
Abstract 
 
 
In recent years, scientific attention has increased around ecological and evolutionary 
consequences of climate effects on temperature-dependent sex determination species 
(TSD), which may threaten their population viability in the face of changing 
environmental conditions. Therefore, crocodiles could be interesting model group to 
understand the adaptive phenotypic plasticity response to environmental conditions. To 
address how wild populations of crocodilians (Crocodylus acutus and C. moreletii) could 
potentially be affected by environmental conditions, we considered climate effects on 
population sex-ratio, nests temperature and implications to future environmentalchanges. We found balanced and skewed populations sex-ratios towards the core and the 
edges of crocodiles range distribution. Nests temperature varied in response to external 
conditions (temperature fluctuation, and rainfall). The alterations of temperature 
fluctuation patterns due to current climate change do not necessarily result on a skewed 
sex ratio of crocodile populations because nesting site selection and nesting behavior 
becomes a key issue regarding nest thermal daily fluctuations, which could balance 
offspring sex-ratio. 
 
 
 
 
 
 2
 
Introducción1 
 
Los cocodrilos han sobrevivido millones de años con pocos cambios en su 
morfología y anatomía. Una de las características más interesantes, que comparten con 
sus antepasados los dinosaurios, es la determinación sexual por temperatura (TSD por 
sus siglas en ingles). Esto quiere decir que durante la incubación de sus huevos, la 
temperatura determina el sexo de los descendientes (Deeming, 2004). Se ha observado, 
principalmente bajo condiciones de laboratorio, que para las especies de cocodrilos 
distribuidos a nivel mundial su patrón de determinación sexual se caracteriza por 
producir 100% hembras a temperaturas bajas y altas (< 32° C y > 33.5° C); mientras que 
los machos se producen a temperaturas intermedias (Deeming, 2004). Esta característica 
de los cocodrilos, de diferenciar el sexo de los embriones durante el desarrollo según la 
temperatura, también se presenta en diversas especies de tortugas terrestres y 
dulceacuícolas, así como en algunas lagartijas (Valenzuela, 2004). Tal condición provoca 
que los cocodrilos sean susceptibles a los cambios ambientales ocurridos donde habitan, 
por lo que las tendencias climáticas pronosticadas por efecto del cambio climático 
(aumento de temperatura, disminución de la precipitación, mayor frecuencia de eventos 
extremos y el fenómeno El Niño más intenso; IPCC, 2007), pueden constituir factores 
clave para la prevalencia de los cocodrilos en el futuro. Hace unos cuantos años se 
demostró que muchos de los fósiles de dinosaurios correspondían en su mayoría al mismo 
sexo, existiendo la posibilidad de que estas poblaciones estuvieran sesgadas hacia uno 
 
1Escobedo-Galván, A. (2010) Cocodrilos y cambio climático. Ambientico 205: 7-8; Escobedo-
Galván A.H. (2011) Investigando el efecto del cambio climático en cocodrilos. Boletín Aluna 3(2): 
43. 
 3
de los sexos debido a las condiciones climáticas, lo que, posiblemente aumentó su riesgo 
de extinción (Miller et al., 2004). 
En el caso particular de los cocodrilos, es poco lo que se conoce sobre el efecto 
del cambio climático en las proporciones de sexos y en cómo influye para que sus 
poblaciones sean viables en el futuro. No obstante, se ha observado que las proporciones 
de sexos en las poblaciones de cocodrilos varían considerablemente dependiendo de los 
cambios ambientales durante la época de anidación. Por ende, cambios climáticos 
direccionales podrían propiciar sesgos crónicos en las proporciones sexuales que, a su 
vez, podrían llevar a la extinción de poblaciones en un plazo relativamente corto. En las 
tuataras (género Sphenodon), reptil endémico con determinación sexual por 
temperatura de Nueva Zelanda, se ha observado que la especie puede mantenerse viable 
a futuro si su población está compuesta por al menos 75% de machos. Sin embargo, si el 
porcentaje de machos aumenta hasta un 85% la población puede encaminarse a la 
extinción en pocas generaciones. Este riesgo es mayor debido a que se pronostica, con 
base en las tendencias del calentamiento global, que la especie solo producirá machos 
alrededor del año 2085 (Mitchell et al., 2010). Para el caso de los cocodrilos, el aumento 
en la temperatura podría sesgar la proporción de sexos para algunas poblaciones. 
Además, el incremento de temperatura puede afectar el desarrollo embrionario ya que 
su intervalo óptimo de temperatura oscila entre 28º y 34º C (Deeming, 2004). De 
sobrepasarse el intervalo óptimo podría promoverse la aparición de malformaciones, 
afectando la sobrevivencia de las crías e incrementando la muerte embrionaria y, por 
consecuencia, el éxito de eclosión (Webb and Cooper-Preston, 1989). Un bajo 
reclutamiento, es decir, el número de crías que sobreviven el primer año, afectaría la 
viabilidad de algunas poblaciones de cocodrilos. 
 4
Lo anterior permitió plantear este trabajo de tesis para responder a una de las 
preguntas más frecuentes al respecto, aunque los cocodrilos han sobrevivido durante 
millones de años a númerosos cambios climáticos, ¿por qué el actual evento de 
calentamiento global sí les afectaría? Para responder a esta pregunta se utilizaron a dos 
especies, el cocodrilo americano (Crocodylus acutus) y el cocodrilo de pantano (C. 
moreletii) como grupos modelo con el objetivo de determinar los efectos del clima sobre 
la proporción y determinación de sexos y las implicaciones de esta relación ante el 
actual cambio de clima. 
En el primer capítulo, se determinó si la proporción de sexos de una población de 
Crocodylus moreletii es el reflejo de las condiciones ambientales durante la época de 
anidación en años anteriores. Modelos teóricos sugieren que los factores que afectan la 
mortalidad entre hembras y machos durante los primeros años de vida no generan 
variaciones suficientemente importantes para sesgar la proporción de sexos que nació 
(Kallimanis, 2010). Por ello, mediante la estimación de la edad y el sexo de C. moreletii 
de diferentes edades (juvenes, sub-adultos y adultos) se estimaron las variaciones 
anuales en la proporción de sexos. En el segundo capítulo, se evaluó las variaciones 
térmicas en nidos naturales de C. moreletii y el efecto de la variabilidad climática en su 
fluctuación, al mismo tiempo que se discute cómo el gradiente térmico a diferentes 
profundidades del nido podría balancear las proporciones de sexos. En el tercer capítulo 
se evaluó la tasa de sobrevivencia de diferentes grupos de vertebrados (cocodrilos, 
lagartijas, tortugas, anfibios y mamíferos) con diferentes mecanismos de determinación 
sexual, con el objetivo de conocer si las especies con determinación sexual por 
temperatura podrían presentar una menor resiliencia a los cambios climáticos, tomando 
como ejemplo las variaciones ambientales ocurridas hace 65.5 millones de años. Por 
 5
último, en el cuarto capítulo se discuten las estrategias fenológicas, fisiológicas y 
conductuales que las especies con determinación sexual por temperatura presentan ante 
las condiciones climáticas y las implicaciones ecológicas y evolutivas del cambio 
climático actual. 
 
 6
Capítulo 1 
 
 
Variation in sex ratio of Morelet’s crocodile in 
northern wetlands of Campeche, Mexico2 
 
Sex ratio is an essential demographic parameter for population dynamic in 
crocodilian species. Understanding the consequences of sex ratio variation on different 
life stages is required to implement effective conservation and management strategies. 
There are many studies that report sex ratios in crocodilian, but the information is 
usually presented as an overall population sex ratio, or in some cases, just considered a 
particular life-stage such as the hatchlings or adults, thereby the effect of intrinsic and 
extrinsic factors on sex ratio could varies among life-stage. For instance, crocodilians 
exhibit temperature-dependent sex determination, wherein sex is determined by 
thermal conditions experienced in the thermo-sensitive period during gonadal 
development (Ferguson and Joanen, 1982). As result, sex ratios among crocodilian 
hatchlings can vary year-to-year depending on nesting sites and local climatic conditions 
(Campos, 1993; Rhodes and Lang, 1996; Lance et al., 2000; Simoncini et al., 2008). Some 
problems in measuring sex ratio in earlier life-stage may cause strong sampling bias as 
sex-specificmortality during offspring development can change sex ratio at hatch, and 
the inability to accurately determine the sex of hatchling and juvenile crocodiles 
(Thorbjarnarson, 1997). On the other hand, measuring sex ratio of mature crocodile 
 
2With contributions from Marta Suárez Coya and Sergio E. Padilla Paz 
 7
population may bias sampling by behavioral differences in habitat preferences 
(Rosenblatt and Heithaus, 2011). Adult females, for example, tend to be move between 
different habitats, while adult males tend to be sedentary, therefore, more easily 
encountered than females (e.g., Subalusky et al., 2009). Thus, for reduce biased 
sampling, studies of population sex ratio in crocodilians should be done on different 
habitat and multiyear period, mainly for endangered species or those with low 
population densities in the wild. 
Some theoretical models concerning adaptive significance of temperature-
dependent sex determination in crocodilians suggest a strongly skewed female-biased in 
natural populations (Phelps, 1992; Woodward and Murray, 1993), which agrees with the 
early observations in crocodilian population (Ferguson and Joanen, 1983; Hutton, 1987; 
Webb et al., 1987a). Subsequent studies show unclear evidence of female-biased sex 
ratios in crocodilians, suggesting that should be differences, both male- and female-
biased, at the intra-specific level (Thorbjarnarson, 1997; Lance et al., 2000). For 
Morelet’s crocodile Crocodylus moreletii some studies have been conducted to 
determine many aspects of its life history, but there has been little analysis in regard to 
population sex-ratio. Herein, we evaluate the variation in sex ratio of Morelet’s 
crocodile over five years at three survey-areas in the northwest coastal wetlands of 
Yucatan Peninsula; we also discuss theoretical assumptions and ecological implications 
in the population dynamics of crocodilians. If population sex-ratios varied among age 
classes are a primarily reflex of the offspring sex ratio, which suggest that the 
mechanisms causing sex differential (differential mortality, sexually dimorphic patterns 
of habitat use and accurately sex determination) are not strong enough to modify 
hatchling sex ratio, then we expect that population sex-ratios may be explained by local 
 8
climatic variability. If crocodile size-classes varied among survey-areas, thus we expect 
that sex-ratios are based primarily on ontogenetic changes in habitat use. In this study, 
we hope to demonstrate the effect of local climatic variability and/or ontogenetic 
changes in habitat use in population sex-ratios of Morelet’s crocodile, as first step to 
understanding its population dynamics. 
 
MATERIALS AND METHODS 
The study was conducted in Los Petenes Biosphere Reserve in Campeche, Mexico. 
This reserve is characterized by the presence of hillocks, known locally as “petenes” 
(CONANP, 2006). Annual mean temperature varied from 26.4° C to 27.8° C, while 
precipitation varied from 725.5 mm to 1049.7 mm. The dry season is from November to 
April, while the rainy season is from May to October (CONANP, 2006). We undertook 
searches for crocodiles at three locations within the reserve: Isla Arena (20º33’47.23” N, 
90º25’3.90” W); Hampolol (19º56’31.84’’ N, 90º22’40.75” W); and Petenes (20º12’35.11” 
N, 90º29’6.65” W). The distances between localities were from 32.0 km to 82.6 km. In 
2007, we monitored from April to August (24 night/survey); in 2008, we monitored from 
July to October (6 night/survey); in 2009, we monitored in April (2 night/survey); in 
2010, we monitored in June (2 night/survey); and in 2011, we monitored in March (1 
night/survey in Isla Arena). We followed specific search routes at each site, according to 
site-survey of Padilla et al. (2011) and Gonzalez-Jauregui et al. (2012) in Los Petenes 
Biosphere Reserve. 
Crocodile searches were conducted at night between 21:00h and 05:00h using a 
spotlight, known as the detection visual nocturne method (Bayliss, 1987). Whenever we 
spotted a crocodile it was captured, either by hand or with a 2.5 m aluminum tube with 
 9
steel loop at the end, depending on the size and proximity of the crocodile. After 
capture, we measured total length (TL) and snout-vent length (SVL), ventrally of each 
crocodile using a tape measure with accuracy ± 0.01mm. With exception of recent 
hatchlings, crocodiles were sexed by manual probing of the interior of the cloaca 
(Ziegler and Olbort, 2007). Previously to be released at its capture site, each crocodile 
were permanently marked by notching the dorsal edge of a unique series of caudal 
scutes (Rainwater et al., 2007). 
Total length was used to create age-size classes according to the classifications 
of Platt and Thorbjarnarson (2000a) for Morelet’s crocodile: hatchlings (TL < 30 cm), 
yearlings (TL = 30.1 to 50 cm), juveniles (TL = 50.1 to 100 cm), subadults (TL = 100.1 to 
150 cm), or adults (TL > 150 cm). While snout-vent length was used to calculate 
crocodile age using the von Bertalanffy model to determine the year when each 
captured crocodile was hatched. In contrast with invasive technique (sacrifice) to known 
the age-growth of the organism, the von Bertalanffy model is a non-invasive technique in 
function of the life history of the organism. The von Bertalanffy models have been 
successfully used in a number of studies of crocodilians (Webb et al., 1983; Rebêlo et 
al., 1997; Cupul-Magaña et al., 2004; Charruau, 2011), and also have been suggested as 
an accurate method to estimated age from capture-recapture data from wild 
populations (Eaton and Link, 2011). 
Sex-ratios were tested against a null hypothesis of a 1:1 sex ratio using Chi-
square test. Differences in the frequency of males or females among size-classes, sites 
and year-survey were tested by Chi-square. We also compare the frequency of size class 
allocations among sites using Chi-square test. We used the age structure data to 
estimate the hatchling sex-ratios over the past years assumed that there was no 
 10
difference in mortality between males and females, and that current sex ratios reflected 
hatchling sex ratios. We compared the frequency of females and males among years with 
a Chi-square test. We also evaluated whether the number of males and females was 
related to climatic variables of maximum and minimum temperature, and total rainfall 
from 1993 to 2008 collected by the weather station maintained by the Comisión Nacional 
del Agua in Isla Arena town. All statistical tests were done with Statgraphics 5.1plus, and 
we considered P < 0.05 to be significant. 
 
RESULTS 
We captured a total of 119 crocodiles, of which 109 were captured and 10 of 
them were re-captured at least once. The total length ranged from 27.3 cm to 212.5 cm. 
Sixty-two were males and 37 were females (20 hatchlings were not sexed). The overall 
sex ratio for C. moreletii was not significantly different from 1:1 (X2 = 3.22, df = 1, P = 
0.072). Even though we captured a higher number of male juveniles and adults, Chi-
square contingency-table analysis showed non significant variation in sex ratio among 
size classes (X2 = 4.08, df = 3, P = 0.25; Fig. 1.1). Also, sex ratio did not vary among 
localities (X2 = 1.56, df = 2, P = 0.45; Fig. 1.2), while the sex ratio was not different 
from 1:1 among year-survey (Table 1). 
 
 
 
 
 11
0
0.2
0.4
0.6
0.8
1
Yearling Juvenil Subadult Adult
Size class
Se
x 
ra
ti
o 
(f
em
al
e/
m
al
e)
 
Figure 1.1. Sex ratio among size classes in Los Petenes Biosphere Reserve, 
Mexico. 
0
0.2
0.4
0.6
0.8
1
Petenes Isla Arena Hampolol
Sites
S
ex
 ra
tio
 (f
em
al
e/
m
al
e)
 
Figure 1.2. Sex ratio among site-survey in Los Petenes Biosphere Reserve, 
Mexico. 
 
The frequency of size classes varied significantly among the three localities for 
both males (X2 = 66.69,df = 6, P < 0.01) and females (X2 = 128.40, df = 6, P < 0.01). Isla 
Arena had a greater percent of males in yearling and juvenile classes (Fig. 1.3a) and 
females of subadults and adults (Fig. 1.3b) than in the other localities. 
 
 
 12
Table 1.1. Comparison of the sex ratio of Morelet’s crocodile captured in Los 
Petenes (2007-2011) 
 
Year Females Males Sex ratio (F:M) Chi-square (X2) P 
2007 18 31 1:1.7 1.77 0.18 
2008 12 14 1:1.2 0.08 0.78 
2009 5 7 1:1.4 0.17 0.68 
2010 0 6 - - - 
2011 2 4 1:20 0.34 0.55 
 
 
 
Figure 1.3. Size-class percentage of males (a) and females (b) in three sites at Los 
Petenes Biosphere Reserve (Hampolol, black; Petenes, white; Isla Arena, gray). 
 
 13
According to the von Bertalanffy model, the crocodiles captured were between 1 
and 10 years old. The largest crocodile, predicted to be 16 years old, was a male of 2.12 
m TL. The sex ratio, taken as percent of males, fluctuated greatly over the estimated 
hatch years (Fig 1.3). The number of males or females from each of the hatch years was 
not related with the climatic variables of maximum temperature (males: R2 = 0.01, F1,17 
= 0.19, P = 0.77; females: R2 = 0.01, F1,17 = 0.20, P = 0.78), minimum temperature 
(males: R2 = 0.007, F1,17 = 0.12, P = 0.30; females: R2 = 0.000, F1,17 = 0.007, P = 0.46), or 
rainfall (males: R2 = 0.045, F1,17 = 0.79, P = 0.70; females: R2= 0.028. F1,17 = 0.49, P = 
0.74). 
0
10
20
30
40
50
60
70
80
90
100
93
(1:0)
94
(1:1)
95
(1:2)
96
(4:0)
97
(1:0)
98 99
(3:1)
00
(4:5)
01
(2:3)
02
(3:0)
03
(2:5)
04
(5:1)
05
(8:1)
06
(10:8)
07
(6:7)
08
(5:1)
Year
Se
x 
ra
ti
o 
(m
al
es
)
 
Figure 1.3. Sex ratio, taken as percent of males, of Crocodylus moreletii by estimated 
hatch year in Los Petenes Biosphere Reserve. 
 
DISCUSSION 
The sex ratios of C. moreletii varied among size-classes. These results could 
suggest that differences in our estimated sex ratios are related to ontogenetic shifts in 
 14
habitat use (Subalusky et al., 2009). Crocodilians exhibit different feeding habits during 
ontogenetic development (Erickson et al., 2003), which means that crocodiles of 
different size-classes may move between habitats to find specific prey (Rosenblatt and 
Heithaus, 2011). 
The greater abundance of yearlings and juveniles crocodiles in Isla Arena could 
be related to low water level which reduces flow to a few streams and crocodiles are 
concentrated in ponds where water depth is < 1 m, thereby affecting the encounter rate 
of subadults and adults. In contrast, the homogeneous conditions of water level 
observed during spotlight surveys at Petenes may favor an encounter rate representative 
of all size-classes. The higher percentage of yearlings and juveniles at Hampolol, 
coupled with the low number of adult crocodiles, suggests that Hampolol may be used as 
a nesting site and nursery area (Padilla et al., 2010). Previous studies showed that 
variations in aquatic vegetation, nest sites, and basking sites have differing effects in 
the encounter rate among size-classes (Coutinho and Campos, 1996; Da Silveira et al., 
2008; Downs et al., 2008). 
The overall sex ratio reported in this study was not different from 1:1 (1.6:1 
male/female), as earlier repots (1:1.3 male/female, Cedeño-Vázquez et al., 2006; 1:1 
male/female, Cedeño-Vázquez and Pérez-Rivera, 2010; 0.8:1 male/female, Merediz-
Alonso, 1999), which suggest that some Morelet’s crocodile populations can maintain a 
balanced sex-ratio even though annual variation of offspring sex. Theoretical models on 
interplay between sex ratio and survivorship in crocodilians suggest that population sex-
ratios are markedly female biased, providing an advantage for population dynamics 
(Phelps, 1992; Woodward and Murray, 1993). This could not explain our results; though, 
the theoretical assumptions can not be valid for all crocodilians. Therefore, 
 15
demographic models for assessing the optimal sex ratio in Morelet’s crocodile are 
necessary to understand not only the evolutionary advantage of temperature-dependent 
sex determination, but to know how variation of sex ratio can affect the long term 
dynamics of crocodilian populations. 
We found annual variation in sex ratio of Morelet’s crocodile from estimated-age 
data, which accords with studies suggesting that crocodilian sex ratio varies among years 
and sites (Lance et al., 2000). For instance, Rhodes and Lang (1996) observed that sex 
ratio of the American alligator Alligator mississippiensis in natural conditions is 
associated with local climatic variability from year to year, in which wet conditions from 
high rainfall result in low incubation temperatures producing a female-biased sex ratio, 
and dry conditions produced a higher proportions of males. On the other hand, some TSD 
species produce unisexual nest as a response to local climate variations (Shine, 1999). 
Simoncini et al. (2008) observed in Broad-snouted caiman Caiman latirostris that natural 
nests producing only females varied between 14% and 45% during four nesting seasons. 
Nevertheless, in the present study no relationship between sex ratio and climate 
conditions was found; which suggests other ecological or behavioral factors specific to 
TSD species might be implicated. Nesting behavior and nest site selection of Morelet’s 
crocodile could have effects on sex ratios. Some species may alter the breeding and 
nesting season according to climate conditions to sustain balanced sex ratios (Schwanz 
and Janzen, 2008). The Chinese alligator Alligator sinensis showed changes in the 
breeding and nesting seasons in recent years in response to temperature variations 
(Zhang et al., 2009). The nesting season of Morelet’s crocodile at Los Petenes Biosphere 
Reserve is earlier than other nesting areas. (Platt et al., 2008; Escobedo-Galván et al., 
2009). The changes in nesting season not only affect offspring traits (sex ratio and 
 16
performance) but also environmental conditions for hatchling development (Kushlan and 
Jacobsen, 1990; Charruau et al., 2010). The sex ratio in the population structure of 
Morelet’s crocodile may also be due to a lower survival rate in the early years of life, 
considering nest thermal fluctuations. Temperature-independent differential mortality 
between post-hatching males and females has been suggested as a bias in crocodilian sex 
ratio (Thorbjarnarson, 1997). Further study will be needed to better understand the 
relationship between climatic variability and sex ratios, while assessing the climate 
effect on differential survival rate between post-hatching males and females. 
 
 
 
 
 
 
 
 
 17
Capítulo 2 
 
 
Temperature fluctuation within and between nests of 
Morelet’s crocodile: implications to embryo 
development and sex determination 
 
In several reptiles, sex is determined by heteromorphic sex chromosomes 
recombination, but some species like crocodilians, most turtles, tuataras and some 
lizards exhibit temperature-dependent sex determination (TSD), wherein sex is 
determined by thermal conditions experienced in the lability period during gonadal 
development (Bull, 1980; Pieau, 1996). As a consequence, incubation temperature 
influences phenotypic characteristics of hatchlings during embryo development, 
affecting directly their performance, survival and reproduction (Du and Ji, 2003; 
Willingham, 2005; Booth, 2006; Charruau, 2012). For this reason, TSD species are an 
interesting model group to understand the adaptive phenotypic plasticity response to 
contemporary changes in environmental conditions (Janzen, 1994). 
In recent years, scientific attention has increased around the ecological and 
evolutionary consequences of climate change for TSD species, including consideration of 
how climate change may threaten their population viability (Hulin et al., 2009; Mitchell 
and Janzen, 2010; Parrotand Logan, 2010; Wedekind and Stelkens, 2010). Climate 
change may lead to imbalanced sex ratios for TSD species, threatening their population 
 18
viability (Janzen, 1994, Wapstra et al., 2009), and eventually drive local populations to 
extirpation (Mitchell et al., 2010). 
Some studies have demonstrated that TSD species can shift the timing of nesting, 
nest depth, or nest-site, as responses to changes in environmental conditions (Doody et 
al., 2006; McGaugh et al., 2010; Weishampel et al., 2010); but these changes occur only 
prior to egg laying, since conditions during incubation are beyond the female’s control 
(Telemeco et al., 2009). Therefore, from this point on, offspring phenotype and sex 
differentiation depends on nest temperature fluctuations. Our basic knowledge of TSD 
suggests that a change of temperature above or below the pivotal temperature 
(temperature which produces a balanced sex ratio at constant incubation temperature) 
during the sensitive period, would drastically alter the offspring sex ratio. However, 
from their general framework important aspects have been largely unknown, for 
example the potential effects of daily fluctuations of the differential temperatures 
within the nest (Du et al., 2009; Paitz et al., 2010). 
Recently, under controlled conditions in different TSD models, it has been shown 
that offspring sex-determination can be reversed under increasing thermal fluctuation 
(McGaugh and Janzen, 2011; Neuwald and Valenzuela, 2011; Warner and Shine, 2011). 
These observations challenge simple ecological and evolutionary models that attempt to 
predict the consequences of environmental changes on TSD species. Data from field 
studies about thermal variation and climatic effects on nest temperature in TSD species 
are needed to develop a general framework for predicting how environmental changes 
impact the TSD process. Understanding the responses of TSD species to changing 
environmental conditions will be critical for developing long-term conservation 
strategies for these species. Here, I analyzed the thermal variation of individual 
 19
crocodile nests at three locations and the effect of climate nesting-site on nest 
temperature, and finally discuss ecological implications to TSD species. 
 
METHODS 
Field data collection 
The study was conducted at the Centro de Estudios Tecnológicos del Mar N° 2, 
and a local breeding farm, both close to Campeche City. Both sities kept crocodiles in 
outdoor enclosures of different sizes. The study carried out through three consecutive 
breeding season of Crocodylus moreletii (May to August, 2007-2009). Sixteen nests were 
monitored, to determine daily thermal fluctuations under natural conditions in relation 
to external temperature conditions for multiple nest depths (between 15 and 40 cm), at 
each nest I set dataloggers (UA-002-08, Onset Comp. Corp., Bourne, MA, USA) at the 
bottom, middle and top of the egg cavity. Dataloggers were programmed to record nest 
temperature every 30 minutes until hatching. In this study, nests were monitored at 
least 60 days, corresponding to 80% of the incubation time reported for Morelet’s 
crocodile (i.e., 75 days, Platt et al., 2008). This recording period included the 
temperature-sensitive period (TSP), which is the period during which the sex of the 
embryo is irreversibly fixed. Based on available information from the laboratory 
conditions, TSP occurs between days 30 and 45 for some cocodrilians (Lang and Andrews, 
1994; Piña et al., 2007). Air temperature was also recorded in study sites with a 
datalogger (UA-002-08, Onset Comp. Corp., Bourne, MA, USA) set in a shadow hanging 
from a tree at 1.5 m high within 2 or 3 m distance from the nest. Finally, temperature 
and rainfall data were drawn from the Comisión Nacional del Agua (CONAGUA data 
base). The Morelet’s crocodiles exhibit the TSD II pattern, in which females are 
 20
produced at low and high temperatures (< 31.5°C, and > 33.5°C), and males between 
31.5°C and 33.5°C (Lang and Andrews, 1994). 
 
Data analysis 
Nest temperature data were averaged daily and nest to facilitate analysis and 
interpretation. Thermal fluctuations were considered as daily difference between 
maximum and minimum. After testing for normality and homogeneity of variances, a 
parametric analysis of varianza (ANOVA) was used to examine thermal differences among 
depths and temperature-sensitive period (TSP), pre-TSP and post-TSP. A two-way ANOVA 
was also used to test differences during the temperature-sensitive period (TSP), pre-TSP 
and post-TSP among depths within the nest. A cross-correlation was used to evaluate the 
effect of external temperature on nest temperature. A linear regression analysis was 
used to examine the relation between nest temperatures in the three parts, and rainfall 
on nest thermal fluctuations. 
 
RESULTS 
The mean of clutch size in eight nests was 27.71 ± 10.22 eggs (ranged from 14 to 
46 eggs). Under natural incubation conditions thermal fluctuations varied among the 
three locations within individual nests as a function of depth. The daily thermal 
fluctuation was higher at the top of the nest cavity than the other two sections (ANOVA 
test, F2,30 = 4.71, P = 0.016; Fig. 2.1), although the magnitude of fluctuations differed 
among nests (top: 0.52º C to 6.21º C; middle: 0.43º C to 3.67º C; bottom: 0.23º C to 
2.19º C). Thermal fluctuation during the TSP was monitored in eleven nests. Thermal 
 21
daily range increased after TSP; however, non differences was find among TSP, pre-TSP 
and post-TSP (ANOVA test, F2,30 = 0.39, P = 0.67; Fig. 2.1). 
 
Figure 2.1. Thermal daily range 
among nest depth and temperature-
sensitive period in Morelet’s crocodile 
nests. 
In general, temperature exhibited daily cycles that were independent of nest 
depth, showing significant temporal hourly shift among nest regions (ANOVA two-way, 
F2,47 = 5.4, P < 0.01). Daily thermal fluctuations were smallest at the bottom region of 
the nests; but temperature was unexpectedly warmest at the bottom region of the nests 
during some hours of the day (Fig. 2.2). 
 
Figure 2.2. Nest temperature along 
time of the day at three sections 
within Morelet’s crocodile nests. 
Nest temperature followed the external air temperature pattern, but was 
delayed (2 days of delay: r = 0.76, r2 = 0.57, F1,52 = 69.86, P < 0.01; Fig. 2.3). As 
expected, temperature at the top of the nest was closest to external temperature 
 22
fluctuations (r2 = 0.93, p < 0.01), followed by temperature at the middle (r2 = 0.93, p < 
0.01) and finally at the bottom (r2 = 0.96, P < 0.01). Rainfall increased thermal daily 
fluctuation significantly (r = 0.67, r2 = 0.45, F1,32 = 26.3, P < 0.01). 
 
Figure 2.3. Cross-correlation 
between nest temperature and 
external temperature. The gray bar 
shows the highest correlation. 
 
 
DISCUSSION 
Previous laboratory studies have found that thermal fluctuations can shift sex 
ratio balance, and have encouraged field investigations of the influence of daily 
temperature fluctuations on TSD species (Neuwald and Valenzuela 2011; Warner and 
Shine 2011). Our results provide evidence that nest temperatures not only fluctuate 
considerably over time, but also that the extent of these fluctuations depends on nest 
depth. At the top of the egg cavity thermal fluctuation was highest and responded faster 
to changes in external temperature fluctuations, while at the bottom of nest 
temperature remained relatively constant and was often within the temperature range 
that produces male-biased sex ratios. Thus, unlike laboratory experiments, in a single 
natural nest it is possible to observe different thermal conditions, which may 
compensate sex ratio (McGaugh and Janzen, 2011; Paitz et al., 2010; Warner and Shine, 
2011). This study do not showed that thermal fluctuation varied from pre-TSP to post-
 23
TSP. Thesecould suggest that nesting females choose nest-site and nest depth to 
maintain relatively stable incubation temperatures despite wide ambient fluctuations; 
nevertheless, this idea must be studied in more detail in crocodilians. 
Nest temperature responds to environmental variations. Delayed effects of 
external temperature on thermal variation within nests may dampen high external 
variations that occur over a short time, reducing the risk of lethal temperatures that 
may threaten embryonic development and offspring performance. Since nest 
temperature varies daily in response to external temperature fluctuations, maternal 
nesting site selection and nesting behavior becomes a key issue regarding nest thermal 
daily fluctuations (e.g., Doody et al., 2006; Freedberg et al., 2011; McGaugh et al., 
2010; Weber et al., 2012). Rainfall is the major source of nest temperature decreases, 
affecting reproductive success and embryo survival of TSD species (e.g., Charruau, 2012; 
Charruau et al., 2010; Houghton et al., 2007). Yet, this study showed that rainfall 
increases thermal daily range, and thus changes in rainfall could lead to a shift in 
offspring sex-ratio at sites where environmental conditions, temperature and rainfall 
patterns are typically homogeneous during nesting season. Despite the limited sample 
size of this study, our results are an additional step toward a better understanding of 
the complex relationships between environmental conditions and temperature 
fluctuations in the nest cavity, with potential implications for sex determination. 
On the other hand, we still know surprisingly little about the embryo’s capacity 
to respond to thermal heterogeneity within the egg. An experimental study showed that 
temperature varied at different parts of egg and that embryos moved within the egg 
toward heat sources, like a behavioral response to thermal variation within the nest (Du 
et al., 2011). In crocodilians, the embryo attaches to the shell membrane within 24 
 24
hours of laying (Webb et al., 1987b), which could limit embryo thermoregulation within 
the egg. The embryo position and egg temperature may affect not only sexual 
determination but also enhance hatchling fitness (Du et al., 2011). Moreover, 
crocodilians exhibit a shorter thermo-sensitive period that some TSD species (Shine et 
al., 2007), which reduces the effect of thermal variations on gonadal development. In 
this study, the nests showed a cyclic daily thermal conversion that could allow a 
homogeneous heat distribution within the nest. Thus, the effect of temperature 
variations at the egg surface could influence a suitable embryonic development during 
the incubation period. Further research is needed to test this hypothesis. 
In the context to current climate change, projections indicate a rise of mean 
temperature, but offer little information regarding changes in thermal fluctuations. This 
limits our ability to predict the potential effects of climate change on TSD species under 
natural conditions. Silber et al. (2011) showed that TSD species exhibited a higher 
survival rate than genotypic sex determination (GSD) species to climatic fluctuations 65 
Ma ago. This suggests that previous climate changes have not necessarily result in a 
skewed sex ratio for TSD species (Miller et al., 2004; Warner and Shine 2008), possibly 
because the amplitude of temperature fluctuations enhanced offspring sex ratio 
balance. 
Investigations regarding the ecology and evolution of TSD in the context of 
environmental change continue to mount (Escobedo-Galván et al., 2011; Mitchell and 
Janzen, 2010; Warner and Shine, 2008; Wedekind and Stelkens, 2010). Climate 
variations, both spatial (nest sites over latitudes or altitudes) and temporal (weather 
conditions across nesting season) can affect incubation temperature and the associated 
offspring traits (Doody et al., 2006; Ewert et al., 2005; López-Luna, 2010; Pen et al., 
 25
2010; Weber et al., 2012). However, additional information is needed to get a broader 
view about the effects of environmental variations and offspring responsiveness. Future 
work should address questions regarding how offspring sex determination will respond to 
temperature fluctuations in field studies, and how environmental change could affect 
offspring fitness. 
 26
Capítulo 3 
 
 
Survival and extinction of sex-determining 
mechanisms in Cretaceous tetrapods3 
 
Understanding how species managed to survive climatic changes in the past can help 
us to foresee the fate of some of them in the future. However, recent research efforts 
considering the potential aftermath of current climate change have dismissed the 
relevance of plasticity (genotypic and phenotypic) of ancestral species to climatic 
alterations. For instance, analyzing the survival/extinctions patterns in previous 
climatic-change events may be useful for understanding how these phenomena impact 
biodiversity and thus anticipate it to some extent. 
We know that the impacts of mass extinctions on biodiversity were not random 
(Purvis et al., 2000). Although similar events have been involved, biodiversity loss was 
different for each mass extinction event (Buffetaut, 2006; Feulner, 2011). One of the 
most significant of biodiversity extinction events, because of the disappearance of all 
non-avian dinosaurs, was that associated with the Cretaceous/Palaeogene boundary, 
approximately 65 million years ago. One way of assessing the impact of climate change 
at this time is to speculate on the ecological implications of vertebrate sex- determining 
mechanisms (SDM) of Cretaceous tetrapods. 
 
3Escobedo-Galván, A.H. and C. González-Salazar. (2012) Survival and extinction of sex-
determining mechanisms in Cretaceous tetrapods. Cretaceous Research 36: 116-118. 
 27
Differences in SDMs indirectly show variations in a diverse array of traits, many of 
which would plausibly affect probability of survival. Recently, Silber et al. (2011) tested 
the hypothesis that genetic sex determination (GSD) may have an adaptive advantage 
over temperature-dependent sex determination (TSD). They showed, for the first time, 
the responsiveness of TSD species to climatic fluctuations at the Cretaceous/Palaeogene 
boundary. Their results suggested that TSD species are able to adapt to current climate 
change, which is a controversial topic (Hulin et al., 2009; Mitchell and Janzen, 2010; 
Escobedo-Galván et al., 2011). We suggest that evaluation of survival rates between 
specific sex-determining mechanisms (XX-XY, ZZ-ZW, and TSD) allow a better 
understanding how species have adapted to environmental changes. Here, we examine 
tetrapod survival patterns at a Cretaceous/Palaeogene boundary locality in relation to 
their sex-determination mechanisms with the aim of understanding the vulnerability of 
vertebrates to climatic changes. 
 
METHODS 
The sex-determining mechanisms of 62 survivor, non-dinosaur, tetrapods from 
the Hell Creek and Tullock formations of Garfield and McCone counties, Montana, USA, 
have been inferred by Silber et al. (2011). They used a parsimony reconstruction 
concerning the ancestral family-level condition of sex determination of extant taxa 
employed by Organ and Janes (2008). For the present study, of those 62 species (8 
amphibians, 27 reptiles, and 27 mammals), five were not considered for this analysis 
because of a lack of clear sex mechanism. Of the 57 species with an inferred sex-
determining mechanism 16 exhibited TSD (turtles and crocodilians), 13 exhibited ZZ-ZW 
(squamates and amphibians), and 28 mammals exhibited XX-XY. The chi-square test was 
 28
used to compare survival/extinction frequencies among sex-determination mechanisms 
(TSD, ZW and XY), and an additional Fisher’s exact test to determine differences only 
between TSD and ZW species.RESULTS AND DISCUSSION 
Of the 16 taxa with TSD, 13% went extinct. Of 28 taxa with XY, 79% went extinct, 
and of 13 taxa with ZW, 23% went extinct. Statistical analysis showed a significantly 
higher survival rate of TSD species to environmental variation at Cretaceous/Palaeogene 
boundary (Chi-square test: X2 = 21.81, P < 0.01; Fig. 1). However, if mammal species 
(XX-XY) are excluded from the analysis, TSD and ZW species showed no statistical 
differences (Fisher’s exact test: P = 0.39). 
 
Figure 3.1. Number of Cretaceous tetrapods in each sex-determining mechanism that 
survived (white) and became extinct (grey) at Cretaceous/Palaeogene boundary. TSD 
and ZW species showed a similar pattern. 
 
These results suggest some ecological and evolutionary implications. Firstly, TSD 
(turtles and crocodilians) and ZW (squamates and amphibians) species show similar 
 29
responses (e.g., behaviour and phenology) to environmental fluctuations even though 
they do not exhibit phylogenetic relatedness (Losos, 2008). Secondly, it has been 
suggested that some TSD species adapted to environmental fluctuations by shifting from 
TSD to GSD (ZZ-ZW in turtles principally; Valenzuela and Adams, 2011), which could 
stabilize sex ratios as these species respond to climatic extremes; however, this analysis 
shows that shifting between sex-determining mechanism does not seem to affect 
survival rate. It is possible that a TSD system may help rather than reduce survival if the 
global temperature change is in the direction of producing more females than males, as 
in tortoises and iguanas in the Galapagos today, but the evolution to a ZZ-ZW system 
may also favour survival by balancing the sex ratio and not skewing it towards males. 
This is the first evidence of the benefits of sex-determination changes. On the other 
hand, the TSD mechanism may be advantageous, potentially increasing survival rate in 
some species that retain it, as in crocodilians and some turtles (Organ and Janes, 2008). 
Undoubtedly, answers remain unclear and caveats deserve attention, the first of 
these being the accurate inference of a sex-determining system in extinct species. Some 
lineages are fairly conservative (e.g., mammals, XY, and crocodilians, TSD), whereas 
others, such as turtles and lizards, exhibit intra-family variation of sex-determining 
mechanisms (Organ and Janes, 2008) and sometimes variation within the same species 
(Pen et al., 2010). This variation may induce errors when inferring sex-determining 
systems of extinct species, not to mention the effect of phylogenetic relationships. To 
minimize this situation, additional analyses were carried out considering that TSD turtles 
and lizards exhibited ZW mechanism. We observed that survival rates between TSD and 
ZW showed no significant differences (Fisher’s exact test: P = 0.62). This result suggests 
 30
considerable ecological plasticity of TSD and ZW species during environmental 
modifications at the Cretaceous/Palaeogene boundary. 
The second caveat that deserves attention is that survival rate of Cretaceous 
tetrapods cannot only be attributable to sex-determining mechanism. For example, GSD 
and TSD species differ in a diverse array of traits (e.g., the requirements of ectotherms 
versus endotherms, Pough, 1980; habitat variation, Benson et al., 2011), many of which 
could plausibly affect their probability of extinction. It seems obvious at first that the 
survival of turtles and crocodilians during the K/Pg extinction argues against sex-ratio 
skewing having an impact. However, their differing habitats could have given these TSD 
species special advantages. Nevertheless, this idea must be subjected to further 
research. 
Finally, it is important to bear in mind that the factors involved in the extinction 
across species are not necessarily the same (Feulner, 2009; Purvis et al., 2000). 
Therefore, the resilience of TSD species to environmental variations may vary from one 
event to another, and could also require consideration of the landscape conditions and 
climatic regimes of each geological event (Buffetaut, 2006; Silber et al., 2011). This 
additional information would further improve our understanding regarding the plasticity 
of TSD and GSD species to mass extinction processes and future events. 
 
 31
Capítulo 4 
 
 
Will all species with temperature-dependent sex 
determination respond the same way to climate 
change?4 
 
Janzen (1994) published a fascinating paper on the effect of climate change on 
sex ratios in the freshwater painted turtle Chrysemys picta with temperature-dependent 
sex determination (TSD), motivating research into the ecology and evolution of TSD in 
the context of environmental change. Although the evolutionary origin of sex 
determination and the transition from environmental to genotypic sex determination 
remain an enigma and topics of discussion (Wedekind and Stelkens, 2010), provocative 
speculations have been made regarding possible responses of TSD species to climate 
change. Recently, Kallimanis (2010) proposed that: (1) ‘under stable climatic conditions, 
populations of some TSD species at the edge of their range are regulated by reduced 
growth rate’, and (2) ‘under climate change, these populations face new climatic 
conditions that trigger fast population growth’. Kallimanis (2010) stated that these 
arguments are based on the assumptions that: (1) TSD species have balanced sex ratios 
in the core but biased at the edges of their ranges; and (2) that under climate change, 
leading-edge populations (those that colonize new suitable areas; conversely, trailing-
 
4Escobedo-Galván, A.H., C. González-Salazar, S. López-Alcaide, V.B. Arroyo-Peña and E. 
Martínez-Meyer. (2011) Will all species with temperature-dependent sex determination respond 
the same way to climate change? A reply to Kallimanis (2010). Oikos 120: 795-799. 
 32
edge populations are those that go extinct when areas become unsuitable) will benefit 
by increasing population size and consequently allowing colonization of newly suitable 
sites. 
We appreciate these ideas because they open the possibility to constructive 
discussions on this fascinating topic; however, we think that Kallimanis’ model is an 
oversimplification of ecological and evolutionary processes that may produce more 
complex and site-specific responses of TSD species to climate change. We do not totally 
agree with several of Kallimanis’ postulates and underlying assumptions in this regard, 
for the following reasons. 
 
RESPONSES OF TSD SPECIES TO CHANING ENVIRONMENTAL CONDITIONS 
TSD is found in fish and reptiles (Valenzuela and Lance, 2004). Kallimanis (2010) 
includes some birds within TSD species based on Goth and Booth (2005); however, TSD 
has not been demonstrated in birds. These authors showed that incubation temperature 
influences off spring sex ratios through differential mortality and not because of a TSD 
mechanism, as supported by Eiby et al. (2008). 
In TSD species, phenology, behavior, and physiology during reproduction vary 
depending on environmental conditions. For example, the nesting season in many 
species (like crocodiles) changes over latitude, starting earlier at low latitudes 
(Thorbjarnarson, 1989). Furthermore, some species of crocodilians, marine and 
freshwater turtles have seen changes in the breeding and nesting seasons in recent years 
in response to sustained temperature increases in different parts of the world 
(Weishampel et al., 2004; Hawkes et al., 2007; Mazaris et al., 2008; Pike, 2008; Schwanz 
and Janzen, 2008; Tucker et al., 2008; Zhang et al., 2009). 
 33
Changes in reproductive behavior across ranges of several TSD species have been 
recorded at both macro- (e.g. different habitat, latitude) and micro- (e.g. nest-site in 
the same habitat) scales in response to variation in environmentalconditions (Juliana et 
al., 2004; Doody et al., 2006; Schofield et al., 2009; McGaugh et al., 2010). Nesting 
behavior and nest-site choice have effects on sex ratios and consequently on 
reproductive fitness. For instance, Doody et al. (2006) observed that water dragon 
Physignathus lesueurii selects different nesting sites across its range depending on 
thermal conditions, and Telemeco et al. (2009) observed that threelined skinks Bassiana 
duperreyi shift nest depth and laying season in response to environmental temperatures. 
In the mugger crocodile Crocodylus palustris, nests built early in the breeding season are 
more exposed to open sky than late nests, which are located mainly in shaded sites 
(Lang et al., 1989). Similarly, snapping turtle’s Chelydra serpentina nests at low 
latitudes are located preferably in shady sites, while those at high latitudes are set in 
exposed sites (Ewert et al., 2005). Tabet and Rodríguez (1998) observed that clutch 
deposition in crocodiles may be delayed under cold conditions. This evidence 
collectively suggests that behavioral plasticity plays a central role for sex determination 
and may buffer potential negative impacts of climatic changes on population structure. 
In terms of physiological adjustments to environmental changes, species can shift 
their pivotal temperature range (the constant temperature to produce 1:1 sex 
proportion) and extend the thermal range for embryo development. Ewert et al. (2005) 
observed geographic variation in temperature thresholds for sex determination in 
snapping turtles, with thresholds increasing with latitude. Interestingly, shifts from TSD 
to genotypic sex determination (GSD) also occur (Janzen and Krenz, 2004; Janzen and 
Phillips, 2006). Huey and Janzen (2008) argued that these changes have arisen 
 34
independently several times among amniotes, as suggested by the presence of conserved 
genes that play a role in sex determination in TSD species (Anand et al., 2008). 
Moreover, Warner and Shine (2010) observed that, in the jacky dragon Amphibolorus 
muricatus, daily fluctuations in nest temperature can have an effect on offspring sex, 
and that interactions of thermal incubation conditions could yield balanced sex ratios for 
TSD species. 
 
GEOGRAPHIC VARIATION IN TSD SPECIES SEX RATIO 
Kallimanis (2010) proposed that ‘the sex ratio varies geographically, with 
balanced sex ratio in the core of the species range and skewed sex ratios at the edge of 
the species range’. We contend these assertions for the following reasons: some of the 
information to support this idea is based on species that exhibit GSD and not TSD, 
according to his cited references (Bishop and Echternacht, 2003; Valenzuela, 2004: 216). 
On the other hand, recently, Witt et al. (2010) compiled information on offspring sex 
ratios in the sea turtle Caretta caretta across a latitudinal gradient, showing that sex 
ratios varied independently of latitude. It should be noted, though, that sea turtle 
species follow TSD pattern Ia, wherein males are produced at low temperatures and 
females at high temperatures (Valenzuela, 2004). To compare and illustrate, we 
compiled information on sex ratios in representative populations across most of the 
range of the American crocodile Crocodylus acutus, which follows TSD pattern II (Fig. 
4.1), in which females are produced at low and high temperatures, and males at 
intermediate temperatures (Deeming, 2004). 
 35
 
Figure 4.1. Top: proportion of males (black) and females (white) of the American 
crocodile Crocodylus acutus across range (grey). 1 = M.Venegas-Anaya (unpubl. data), 2 
= A. E. Seijas (unpubl. data), 3–5 = Porras-Murillo (2004), 6 = Barrantes (2008), 7–9 = A. 
E. Seijas (unpubl. data), 10 = Espinal et al. (2010), 11 = García-Grajales et al. (2007), 12 
= Platt and Thorbjarnarson (2000b), 13 = Cedeño-Vázquez et al. (2006), 14 = Charruau et 
al. (2005), 15 = Thorbjarnarson (1989), 16 = Huerta (2005), 17 = González-Cortés (2007), 
18 = Huerta (2005), 19 = Brandt et al. (1995) and 20 = Mazzotti (1983). Bottom: 
relationship between sex ratio (female-biased) and latitude (r2 = 0.0003, F1,18 = 0.00, P = 
0.97). 
 
 
 36
Based on Kallimanis (2010), we should expect biased sex-ratios towards the edge 
of the species’ range; instead, we observed no clear patterns, as both relatively 
balanced and skewed sex ratios are found towards the core and the edges of the species’ 
range (Fig. 1). Furthermore, nearby populations present very different sex ratios 
depending upon local conditions; e.g. populations 12, 13, 15 and 17 (Fig. 1). 
 
EFFECTS OF CLIMATE CHANGE ON SEX RATIO 
Kallimanis’ (2010) prediction that current climatic change will benefit TSD 
species by triggering fast population growth at the range margins may also be another 
oversimplification. Rather, if populations are already sex-biased and temperature rises, 
the specific outcome will depend on the TSD pattern of the species. Where populations 
are already under stress conditions, temperature increases may take them outside of 
their thermal tolerance limits for embryo development (Hawkes et al., 2007), eventually 
driving local populations to extirpation (Mitchell et al., 2010). On the other hand, 
warming may increase viability in sex-skewed populations when elevated temperatures 
approximate the thermal range that produces even sex proportion. 
Pivotal temperatures are quite narrow in range in some species (Hulin et al., 
2009). Hence, it is frequent to find male- or female-biased populations; however, sex 
ratio varies year-to-year depending on local climatic conditions across species’ ranges 
(Janzen, 1994; Lance et al., 2000; Wapstra et al., 2009). In species with narrow pivotal 
temperature ranges, sex ratios will shift on short time scales, whereas species with 
broader pivotal temperature ranges will show more balanced sex ratios in spite of 
environmental variation (Hulin et al., 2009). 
 37
Hence, is it likely that TSD species will benefit broadly and consistently under 
global warming, building pools of potential dispersers to colonize new areas? Not 
necessarily: Kallimanis’ model considered only the effect of temperature on TSD species 
at the macro-scale, but microclimate exerts the most important influence on sex ratios 
and hatching success (e.g. daily temperature fluctuation), and the relationship between 
temperature at the macro- and micro-scales is not always linear because it is strongly 
affected by other factors, such as precipitation and humidity (Houghton et al., 2007); 
thus these variables should be incorporated into a model to produce more reliable 
predictions (Warner and Shine, 2010). Finally, range expansion is not only a matter of 
higher population growth rate at range margins, dispersal plays a central role to colonize 
new habitats (Pearson, 2006), and many TSD species are poor dispersers. 
In general, Kallimanis’ (2010) model corresponds to a specific instance among 
many possible responses of TSD species to climate change. The author considered only 
one of the three TSD patterns, and presented an overly simple model of climate change 
implications. We conclude that general models will not be broadly applicable; 
projections of TSD species’ responses to climate change will need to be more specific to 
groups with similar ecologies and modes of TSD, not to mention specific landscapes and 
climatic regimes. 
 
 38
Conclusiones 
 
 
En los últimos años los científicos han sugerido que el cambio climático podría 
conducir a un desequilibrio en la proporción de sexos y por consiguiente poner en peligro 
la viabilidad poblacional de las especies con determinación sexual por temperatura (TSD, 
por sus siglas en inglés). En este trabajo pusimos a prueba varios de los supuestos y los 
argumentos con los que se ha abordado el tema; al mismo tiempo que se discutieron las 
posibles consecuenciasecológicas y evolutivas para las especies con TSD, en lo que 
respecta al cambio climático. 
Los resultados de este trabajo permiten tener un panorama más amplio sobre las 
consecuencias ecológicas y evolutivas de la relación entre clima y proporción de sexos. 
Para el caso particular de los cocodrilos, este es el primer esfuerzo que evalúa en forma 
integral la relación entre el clima y la proporción de sexos, y las implicaciones ante el 
cambio climático. 
Con los diferentes capítulos de este trabajo demostramos que el efecto de la 
variabilidad climática sobre la proporción de sexos es más complejo de lo que trabajos 
anteriores han mostrado (Escobedo-Galván et al., 2011). La aplicación de modelos 
simples (capítulo I: relación macro-clima y proporción de sexos) no resulta ser el método 
adecuado -usando a los cocodrilos como especies modelo- para evaluar los efectos 
potenciales de las variaciones ambientales en especies con determinación sexual por 
temperatura. Algunos factores externos, como el uso de hábitat y la tasa de mortalidad 
entre sexos, están influyendo en la estructura sexual de la población. Sin embargo, estos 
 39
factores no han sido tomados en cuenta al momento de evaluar las variaciones en la 
proporción de sexos en poblaciones bajo condiciones naturales. 
Por otro lado, las interpretaciones sobre los efectos del clima en la proporción de 
sexos que se han venido planteando en los últimos años, podrían no ser adecuadas para 
algunas de las especies con determinación sexual por temperatura. Esto debido a que la 
fluctuación de la temperatura varía entre las capas de huevos dentro del nido y, por 
ende, la proporción de sexos en cada capa podría ser diferente, por ello no debe 
tomarse en cuenta solo la temperatura de una parte del nido para estimar la proporción 
de sexos. 
 
Esquema de los efectos de 
las variaciones climáticas a 
diferentes escalas sobre la 
fluctuación térmica y 
determinación del sexo en 
cocodrilos. 
 Además, la estimación del sexo en condiciones naturales para algunas especies se 
hace a partir de la información que se ha generado en el laboratorio, lo cual no permite 
conocer realmente lo que está pasando en condiciones naturales y cómo la 
determinación del sexo responde a las fluctuaciones climáticas en condiciones in situ. El 
 40
sexo de los embriones en ambientes naturales va estar determinado por las fluctuaciones 
térmicas en el nido y no por la temperatura promedio de incubación; al mismo tiempo 
que la variación térmica en el nido va a estar influenciada por las condiciones climáticas 
en los sitios de anidación y las variaciones macro-climáticas (Capítulo 2). 
En relación al cambio climático, las especies con determinación sexual por 
temperatura son consideradas vulnerables y con menor capacidad de respuesta ante los 
futuros cambios ambientales. Sin embargo, las especies con determinación sexual por 
temperatura, comparadas con otras especies de vertebrados con mecanismos genéticos 
de determinación sexual, han demostrado una mayor capacidad de respuesta 
(fenológicas, fisiológicas y conductuales) ante los cambios ambientales a los que se han 
enfrentado (Escobedo-Galván and González-Salazar, 2012). Por consiguiente, las 
especies con determinación sexual por temperatura tienen la capacidad de compensar 
los efectos de las condiciones climáticas a las que se encuentran sometidas las 
poblaciones para mantener la viabilidad de las especies en el futuro. 
 41
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