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Introduction: Reproduction in Crocodilians
Author(s): Carlos I. Piña Mark E. Merchant and Luciano M. Verdade
Source: South American Journal of Herpetology, 10(1):1-3.
Published By: Brazilian Society of Herpetology
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Introduction: Reproduction in Crocodilians
Carlos I. Piña1,2,3,*, Mark E. Merchant4, Luciano M. Verdade5
1 Centro de Investigaciones Científicas y Transferencia de Tecnología a la Producción, Consejo Nacional de Investigaciones Científicas y Técnicas.
Matteri y España s/n, CP E3105BWA, Diamante, ER, A rgentina.
2 Proyecto Yacaré, Laboratorio de Zoología Aplicada: Anexo Vertebrados (Facultad de Humanidades y Ciencias–Universidad Nacional del Litoral/
Ministerio de Aguas, Ser vicios Públicos y Medio Ambiente), A. del Valle 8700, CP3000, Santa Fe, Argentina.
3 Facultad de Ciencias y Tecnología, Universidad Autónoma de Entre Ríos, Diamante, ER, Argentina.
4 Department of Chemistry, McNeese State University, Box 90455, Lake Charles, LA 70609, USA.
5 Laboratório de Ecologia Isotópica, Centro de Energia Nuclear na A gricultura, Universidade de São Paulo. Avenida Centenário, 303, Caixa Postal 96,
CEP 13400–970, Piracicaba, SP, Brazil.
* Corresponding author. Email:
etal., 2013), and multiple paternity appears to be com-
mon, although reproduction tends to be limited to a few
dominant males (Zucoloto et al., 2006, 2009; Amavet
etal., 2008; McVay etal., 2008; Lance etal., 2009b; Budd
etal., 2015). So far, all extant crocodilian species studied
exhibit temperature-dependent sex determination (Piña
etal., 2003, 2007b; Deeming, 2004). We have also learned
that weather conditions appear to influence the repro-
ductive efforts of females (Lance etal., 2009a; Simoncini
etal., 2011), and that a great portion of the reproductive
energy is usually lost due to nest predation or hatchling
mortality during the first year of life (Larriera and Piña,
2000; Platt etal., 2008; Somaweera etal., 2013; Mazzotti
etal., 2014; Parachú-Marcó etal., 2015a).
This issue of the South American Journal of Herpetol-
ogy is dedicated to crocodilian reproduction and is derived
from a series of papers presented at the 23rd Working
Meeting of the Crocodile Specialist Group, held in Lake
Charles, Louisiana, USA, in May 2014. These papers in-
clude studies on diverse crocodilian species and topics, all
of them related to reproduction.
Two studies (Parachú Marcó etal., 2015b; Iungman
etal., 2015) are related to stress hormones (or their syn-
thetic homologues) and their possible effects on develop-
ment and sexual determination. Stress hormones do not
appear to affect sexual determination (at least at the stud-
ied dosage and time of application), but the synthetic ho-
mologues increased the body size of hatchlings. Such re-
sults can be relevant in ranching and farming operations.
Two other papers provide novel information on the
reproduction of crocodiles on oceanic islands, where they
present a high mortality rate of hatchlings (Balaguera
Reina etal., 2015). Budd etal. (2015) report that mate fi-
delity and high relatedness values could be a consequence
of the smaller number of adult crocodiles in those popula-
tions or an indication of a small local number of dominant
Moore and Kelly (2015) describe the histological
structure and functionality of the American alligator phal-
lus. This study reveals that the crocodilian penis contains
rigid structures in order to facilitate cloacal intromission
Crocodilians were used for medicinal and religious
purposes by the Aztecs and the Mayas in the Americas
and the Egyptians in Africa. Romans printed figures of
crocodiles on their coins, and images of crocodiles appear
in stone on the temples of Angkor Wat in Cambodia. More
recent cultures have overhunted crocodiles for their skin.
As a result, by the 1970s all crocodilian species had been
included in AppendixI of the Convention on Internation-
al Trade in Endangered Species, with consequent banning
of their international trade.
Currently, most populations have recovered and are
being exploited under regulated sustainable programs. It
is known that populations can be subjected to manage-
ment if the number of reproductive females is not reduced
(Abercrombie and Verdade, 1995), both in ranching pro-
grams based on the collection of eggs or hatchlings from
wild populations of Alligator mississippiensis (Daudin,
1802) in the USA (Elsey etal., 2001; Elsey and Woodward,
2010), Caiman latirostris (Daudin, 1802) and Caiman ya-
care (Daudin, 1802) in Argentina (Larriera et al., 2008),
Crocodylus porosus Schneider, 1801 in Australia (Webb
et al., 2010), and Crocodylus niloticus Laurenti, 1768 in
many African countries (Fergusson, 2010), and hunting
programs of Alligator missippiensis in the USA (Elsey and
Woodward, 2010), Caiman crocodilus (Linneaus, 1758) in
Venezuela (Velasco and Ayarzagüena, 2010), Caiman ya-
care in Bolivia (Campos etal., 2010), and Crocodylus niloti-
cus in many African countries (Fergusson, 2010). Farming
operations (i.e., closed production cycle) are also carried
out with crocodilians in many countries (e.g., Verdade,
2001a). In these three types of management systems
(ranching, hunting, and farming) it is necessary to main-
tain sustainable levels of reproduction.
Since the 1970s, our understanding of reproduction
in crocodilians has increased considerably. We now know
that both males and females take a relatively long time to
reach sexual maturity (Verdade etal., 2003, Larriera etal.,
2006; Platt et al., 2008; Lance et al., 2015)—although
they have significant sexual dimorphism (Verdade 2001b;
Verdade, 2003; Piña etal., 2007a)—females guard nests
and hatchlings (Charruau and Hénaut, 2012; Somaweera
South American Journal of Herpetology, 10(1), 2015, 1–3
© 2015 Brazilian Society of Herpetology
Submitted: 14 April 2015
Accepted: 21 April 2015
Handling Editor: Taran Grant
doi: 10.2994/SAJH-D-15-00006.1
10(1), 2015, 1
14 April 2015
21 April 2015
Taran Grant
latirostris in Santa Fe, Argentina. Journal of Experimental Zoolog y Part
A: Ecological Genetics and Physiology 309:637–642. doi:10.1002/jez .496
Balaguera-Reina S.A., Venegas-Anaya M., Sanjur O.I., Lessios
H.A., Densmore L.D. III. 2015. Reproductive ecology and
hatchlings’ growth rates of American Crocodile (Crocodylus acutus)
on Coiba Island, Panama. South American Journal of Herpetology
10:10–22. doi:10.2994/SAJH-D-14-00024.1
Budd K.M., Spotila J.R., Mauger L.A. 2015. Preliminary mating
analysis of American Crocodiles, Crocodylus acutus, in Las Baulas,
Santa Rosa, and Palo Verde National Parks, Guanacaste, Costa
Rica. South American Journal of Herpetology 10:4–9. doi:10.2994/
Campos Z., Llobet A., Piña C.I. Magnusson W.E. 2010. Yacare
Caiman Caiman yacare. Pp. 23–28, in Manolis S.C., Stevenson C.
(Eds.), Crocodiles. Status Survey and Conservation Action Plan.
Third Edition, Crocodile Specialist Group: Darwin.
Charruau P., Hénaut Y. 2012. Nest attendance and hatchling care
in wild American crocodiles (Crocodylus acutus) in Quintana Roo,
Mexico. Animal Biology 62:29–51. doi:10.1163/157075511X597629
Daudin F.M. 1802. Histoire Naturelle, Générale et Particulière des
Reptiles; ouvrage faisant suit à l’Histoire naturell générale et
particulière, composée par Leclerc de Buffon; et rédigee par C.S.
Sonnini, membre de plusieurs sociétés savantes, Vol.2. F. Dufart,
Deeming D.C. 2004. Prevalence of TSD in crocodilians. Pp. 33–41,
in Valenzuela N., Lance V.A. (Eds.), Temperature Dependent Sex
Determination in Vertebrates. Smithsonian Books, Washington.
Elsey R.M., McNease L., Joanen T. 2001. Louisiana’s alligator
ranching programme: a review and analysis of releases of captive-
raised juveniles. Pp.426–441, in Grigg G.C., Seebacher F., Franklin
C.E. (Eds.), Crocodilian Biology and Evolution. Surrey Beatty &
Sons, Chipping Norton.
Elsey R.M., Woodward A.R. 2010. American alligator Alligator
mississippiensis. Pp. 1–4, in Manolis S.C., Stevenson C. (Eds.),
Crocodiles. Status Survey and Conservation Action Plan. Third
Edition, Crocodile Specialist Group: Darwin.
Fergusson R.A. 2010. Nile crocodile Crocodylus niloticus. Pp. 84–89,
in Manolis S.C., Stevenson C. (Eds.), Crocodiles. Status Survey
and Conservation Action Plan. Third Edition, Crocodile Specialist
Group: Darwin.
Iungman J.L., Somoza G.M., Piña C.I. 2015. Are stress-
related hormones involved in the temperature-dependent sex
determination of the Broad-Snouted Caiman? South Amarican
Journal of Herpetology 10:41–49. doi:10.2994/SAJH-D-14-00027.1
Lance V.A., Rostal, D.C., Elsey, R.M., Trosclair P.L. III. 2009a.
Ultrasonography of reproductive structures and hormonal correlates
of follicular development in female American alligators, Alligator
mississippiensis, in southwest Louisiana. General and Comparative
Endocrinology 162:251–256. doi:10.1016/j.ygcen.2009.03.021
Lance S.L., Tuberville T.D., Dueck L., Holz-Schietinger C.,
Trosclair P.L. III, Elsey R.M., Glenn T.C. 2009b. Multiyear
multiple paternity and mate fidelity in the American alligator,
Alligator mississippiensis. Molecular Ecology 18:4508–4520.
Lance V.A., Elsey R.M., Trosclair P.L. III. 2015. Sexual maturity in
male alligator in southwest Louisiana. South American Journal of
Herpetolog y 10:58–63. doi:10.2994/SAJH-D-15-00005.1.
Larriera A., Piña, C.I. 2000. Caiman latirostris (broad-snouted
caiman) nest predation: does low rainfall facilitate predator access?
Herpetological Natural History 7:73–77.
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de conservación y manejo del género Caiman en Argentina. Pp.143–
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Larriera A., Siroski P., Piña C.I., Imhof A. 2006. Sexual maturity
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wild. Herpetological Review 37:26–28.
and that the sulcus spermaticus (i.e., the open groove
that runs along the ventral aspect of the phallic shaft) is
a spatially heterogeneous reproductive structure contain-
ing a complex architecture of multiple tissue types, and
its morphology changes from its proximal origin between
the crurae to its distal exit from the phallus tip.
Studies of reproduction in the wild are usually diffi-
cult to conduct, but new technologies can now help us un-
derstand many aspects of crocodilian reproduction that
have been obscure until recently. Portelinha etal. (2015)
used radio transmitters to follow reintroduced females
and compared their behavior to wild females. These au-
thors also used echographic images and hormone profiles
to understand egg development. Their results show that
wild and reintroduced females seem to have similar be-
havior, with 35–50% of females reproducing per year on
average and ovulation occurring one or two months be-
fore nesting. Lance etal. (2015) describe the onset of sex-
ual maturation of male American alligators, with males
presenting a clear cycle of testosterone levels, peaking in
breeding season, but varying ontogenetically.
Crocodilians comprise a group of amazing species
that have faced profound environmental changes over
the course of their long evolutionary history, from the ef-
fects of asteroids to the devastating impact of humans.
Their reproductive biology has shown an incredibly ro-
bust adaptive capacity. Crocodilians are still used as nat-
ural resources in many regions of the world, and, thus,
understanding their reproductive biology is paramount
for maintaining their sustainable use. We hope this the-
matic issue of the South American Journal of Herpetology
on crocodilian reproductive biology can help improve our
knowledge on such an intriguing subject.
We would like to thank all the people involved in
the production of this thematic issue on the reproduction
of crocodilians. In special, we would like to express our
appreciation for the anonymous reviewers and the chief
editor of the journal for their enthusiasm and support.
Last but not least, we would like to thank the Crocodile
Specialist Group for their support during the whole edit-
ing process of this thematic issue of the South American
Journal of Herpetology.
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Introduction: Reproduction in Crocodilians
Carlos I. Piña, Mark E. Merchant, Luciano M. Verdade
South American Journal of Herpetology, 10(1), 2015, 1–3
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Introduction: Reproduction in Crocodilians
Carlos I. Piña, Mark E. Merchant, Luciano M. Verdade 3
South American Journal of Herpetology, 10(1), 2015, 1–3
... It is a federally threatened species that may be affected by ecosystem restoration projects, particularly those related to the greater Everglades and Florida Bay (Mazzotti et al., 2007a;Mazzotti et al., 2009). As documented across other crocodylian species (Thorbjarnarson, 1996;Cañas & Anderson, 2002;Piña, Merchant & Verdade, 2015), environmental changes within American crocodile nests can significantly affect the health and survival of the developing young (Lutz & Dunbar-Cooper, 1984;Mazzotti et al., 1986;Mazzotti, Kushlan & Dunbar-Cooper, 1988;Mazzotti, 1989;Charruau, 2012;Charruau, Hénaut & Álvarez Legorreta, 2013;Murray et al., 2016). Crocodylus acutus exhibits temperature-dependent sex determination (Murray et al., 2016;Charruau, Cantón & De la Cruz, 2017), and developing embryos are vulnerable to both flooding and desiccation (Mazzotti, Kushlan & Dunbar-Cooper, 1988;Kushlan & Mazzotti, 1989;Charruau, Thorbjarnarson & Hénaut, 2010). ...
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Several data loggers deployed to monitor temperature and humidity of Crocodylus acutus (American crocodile) nests in South Florida could not be located after hatching. One badly damaged data logger was retrieved, providing insight into the possible fate of the others. Using a taphonomic approach, we identified numerous indentations, consistent with crocodylian bite marks, and inconsistent with potential mammalian or squamate bites. It seems most likely that the data logger was damaged by the nesting C. acutus rather than during attempted nest predation. Estimated bite forces for reproductive age, female C. acutus exceed the predicted material properties of the data logger’s housing, suggesting that the bites were exploratory in nature. We suggest that data loggers be removed prior to hatching or permit remote data storage.
... Se consideran depredadores topes en la cadena trófica, por lo que los individuos adultos no encuentran predadores naturales, a excepción del hombre.Son ovíparos y generalmente entierran sus huevos en diferentes tipos de sustratos: arena, tierra, turba, entre otros(HICKMAN et al. 2013). Presentan cuidado parental, caracterizado porque la hembra custodia el nido durante los meses de incubación y acude al llamado de los neonatos ayudándoles en el momento de la eclosión(PIÑA et al. 2015). Esta ayuda implica esfuerzo en la apertura del nido, así como recoger a los neonatos en su boca para transportarles hasta el agua, hacia lugares seguros y protegidos. ...
Entre los crocodilianos del Nuevo Mundo, Crocodylus rhombifer presenta la menor distribución natural y es además uno de los más amenazados. Esta especie endémica coexiste en Cuba con el cocodrilo americano (C. acutus), el segundo miembro del género que habita en la isla. Ambas especies se hibridan, tanto en poblaciones naturales como en cautiverio. El presente estudio se centra en la caracterización genética de poblaciones de Crocodylus que habitan en Cuba, mediante el empleo de marcadores nucleares y mitocondriales. Un total de 329 individuos provenientes de las ciénagas de Zapata y Birama se incluyeron en el estudio. El grado de hibridación interespecífico estimado a partir de nueve loci microsatélites resultó ser elevado y subestimado, tanto en el medio natural (49,1%), como en cautiverio (16,1%). El análisis del polimorfismo del ADNmt aportó evidencias de hibridación bidireccional entre C. rhombifer y C. acutus en su medio natural. Por otro lado, reveló una acentuada variación intraespecífica para C. acutus en contraste con una falta de diferenciación genética a este nivel entre las entidades morfológicas C. rhombifer y C. acutus que habitan en Cuba.
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Sexual identification of crocodilians is important in population studies and provides useful information for conservation and management plans and monitoring populations over time. It is possible to distinguish between male and female Caiman latirostris by cloacal palpation or eversion of the penis in individuals larger than 75 cm total length, but smaller animals possess a barely differentiable cliteropenis. In hatchlings, sex determination methods involve surgical examination, necropsy, or analysis of cranial dimorphism, which cannot be applied in the field. We classified hatchlings of C. latirostris by observing the color and shape of their genitals. The penis is a milky white organ with a rounded shape at the tip and a purple hue at the end, whereas the clitoris is shorter, whitish, and has a pointed end. The procedure was tested on hatchlings from three nests at the Proyecto Yacar study area (Santa Fe province); half of the eggs of each nest were incubated at a constant temperature of 31C (producing females) and the other half at 33C (producing males). To observe the sexual organs by cloacal inspection, we used a modified instrument whose function during palpation is like that of a finger applied in large animals to evert the penis or clitoris. In the first days after hatching we correctly scored the sex of 80% of the individuals. The number of correct identifications was slightly lower for males than for females. This technique might be a useful tool for field studies, as it allows the sex of small caimans to be estimated in situ.
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Resumo Ao longo deste capítulo, discutimos as limitações e virtudes da aplicação de modelos matemáticos ao manejo de populações de crocodilianos. Utilizando-se de uma matriz de Leslie (Leslie, 1945 e 1948), com dados de literatura basicamente sobre o Caiman crocodilus, exemplificamos, através da elaboração de um modelo hipotético, a utilização prática de modelos em situações reais. A dificuldade de obtenção no campo de informações seguras sobre a demografia da população em questão, aliada ao limitado número de variáveis possivelmente equacionadas por um modelo, diminuem sua eficiência como instrumento de tomada de decisões. Por outro lado, sua relativa simplicidade e seu caráter didático tornam o processo de elaboração de um modelo possivelmente mais rendoso a alguém que trabalhe com manejo de fauna, que sua utilização em si.
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Although there is much information available about reproduction in Caiman latirostris, knowledge related to steroid hormone levels and follicle development for wild adult females is still lacking. In this study we monitored and assessed the development of follicles and eggs and correlated these results with plasma steroid hormone levels in 32 adult females captured in Santa Fe, Argentina. Fieldwork was carried out over two reproductive seasons (October-January) between 2010 and 2012. Using an ultrasound device to take images of the reproductive structures of adult females, we observed individuals with vitellogenic follicles (n = 5), eggs (n = 4), atretic follicles (n = 11), and inactive reproductive structures (n = 12). We found no reproductive females smaller than 65 cm snout-vent length. High levels of estradiol were found during the ovulation period (November) only in reproductive females. Reproductive females showed no differences in progesterone levels during the study period (November-January) compared to non-reproductive females; however, reproductive females showed higher progesterone levels during nesting (December). We found no differences in progesterone levels between reproductive females and females with atretic follicles at the end of the nesting period (January). Ultrasound imaging was found to be an efficient technique to study reproductive structures at the beginning of reproductive cycle of the broad-snouted caiman. Isolated analyses of hormonal levels are not sufficient to determine the reproductive condition of C. latirostris females.
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Studying the mating system of wild populations of American crocodiles, Crocodylus acutus, has important conservation implications. We conducted a preliminary analysis of the mating system of C. acutus in Las Baulas (2007 and 2008), Santa Rosa (2007) and Palo Verde (2008 and 2009) National Parks in Guanacaste, Costa Rica. We captured hatchlings during crocodile surveys and analyzed them with nine polymorphic microsatellite loci to determine relatedness values. High relatedness values indicated that full and half siblings were sampled in a single locality and season. We found full siblings between the years that hatchlings were collected in Las Baulas and Palo Verde National Parks, which suggested mate fidelity. The mate fidelity and high relatedness values could be a consequence of the smaller number of adult crocodiles found within these areas or indicative of a small number of dominant males in the populations. Our results support the need to conduct future studies describing the mating system and nesting success within populations of C. acutus. Understanding of these population factors is crucial to the continued success and maintenance of viable populations of C. acutus.
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Very little is known about the attainment of puberty in reptiles. In the American alligator (Alligator mississippiensis) males are assumed to be sexually mature at about 1.8 m in total length, but it is not clear at what size they produce testosterone, spermatozoa and mate successfully. We re-examined this question by studying plasma testosterone levels in blood samples from a large sample of alligators (∼1,500) collected every month of the year and ranging in size from approximately 61 cm (2 ft) to 360 cm (11.5 ft). Testosterone values ranged from 0.05-115.41 ng/mL. All size classes of alligators exhibited a seasonal cycle in testosterone levels, but the concentrations were size-dependent: the larger the alligator the higher the testosterone. In all size-classes testosterone reached a peak in the breeding season (March-May). Mean testosterone in the largest size-class during breeding was 75 ng/mL whereas in the smallest size-class peak testosterone was less than 3 ng/mL. The smallest size-class (59-89 cm) showed an additional rise in testosterone in late summer. The attainment of sexual maturity in alligators appears to be closely associated with growth and is a gradual process lasting several years. Sexually immature alligators show a seasonal pattern of testosterone secretion similar to that of adults, but the values are significantly lower.
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We assessed the reproductive ecology of the American crocodile (Crocodylus acutus) on Coiba Island, Panama from January–December 2013. We examined nest site characteristics from January–April and hatchling survivorship from April–December. Ten nests were examined at three nesting localities where 30% of the nests were found under forest canopies and 70% were exposed to sunlight (distance to nearest tree = 280 ± 110 cm). Half of the nests were built closer to the sea and the other half closer to bodies of freshwater (700 ± 360 cm). The nest dimensions were 17.5 ± 7.8 cm from the top of the clutch to the surface, 2.9 ± 9.9 cm from the bottom of the clutch to the surface, and 35.9 ± 3.6 cm wide at the top of the nest cavity. The average soil conditions in the nests consistently had high concentrations of potassium (69.3 mL/L) and manganese (9.2 mg/L), moderate concentrations of phosphorus (6.6 mg/L) and iron (3.7 mg/L), and low concentrations of zinc (0.5 mg/L) and copper (0.0 mL/L). Cation exchange capacity showed consistently high concentrations of calcium (2.2 cmol/kg), moderate of magnesium (1.1 cmol/kg), and low in aluminum (0.1 cmol/L). Volumetric water content was about 25.0 ± 2.6% at the bottom and 22.8 ± .3% in the middle of the clutches. Hatching success was 88.9%, of which 68.3% hatched by themselves or with the mother’s aid and 20.6% hatched with our aid. Mean size of the mother was 219 ± 6.2 cm total length (TL) and 115.9 ± 3.0 cm snout–vent length (SVL). The incubation period was estimated to be 85–88 days. TL and SVL growth rate of those individuals were 0.03–0.16 cm/day and 0.00–0.09 cm/day, respectively. Population size was estimated to be 218.6 hatchlings in 22.4 km2; the hatchling population declined 65.7% after the first 2 months (May and June) and 95.9% by July, leaving only 0.5% remaining by December. This is the first study to assess nest-site characteristics and estimate hatchling survival in a Pacific population of American crocodiles.
In some reptiles, gonadal outcome is regulated by temperature during a critical period of the embryonic development. Gonadal steroid hormones are seen as effectors of the gonadal differentiation process. Recently, stress and glucocorticoids (GCs), stress-related hormones in vertebrates, have been considered as potential modulators of the sex determination process in some vertebrates that present temperature-dependent sex determination (TSD). In reptiles, corticosterone is the main GC produced, and its administration to eggs causes a bias in sex ratio in some lizards. In this context, we aim at assessing whether dexamethasone (Dex), a potent synthetic glucocorticoid, can modify the sex ratio in Caiman latirostris, a species with strong TSD. As a first step, we incubated embryos at masculinizing temperatures (33°C; 100% males). Different doses of Dex were topically applied to the eggshell at stage 20, prior to gonadal differentiation. We assessed embryonic development at stages 22 and 25 and evaluated some physiological and morphological hatchling traits. Embryonic mortality was not affected by dexamethasone manipulation. No effects of Dex on sex ratio were found and all animals analyzed histologically possessed testes. However, older embryos and hatchlings from Dex treated eggs were heavier, larger, and hatched earlier than control individuals. Our results do not account for Dex involvement in the process of ovarian differentiation, at least under a strong masculinizing temperature. Nevertheless, they suggest that Dex might accelerate embryo development by enhancing intermediate metabolism and/or by stimulating growth hormone secretion.
Male intromittent organs serve two primary reproductive functions: the physical entry into the female body during copulation and the effective delivery of gametes resulting in internal fertilization. Here we present a histological examination of the adult male American alligator phallus semen delivery apparatus, the sulcus spermaticus. While the highly collagenous basal crurae and more distal shaft of the alligator penis contain the rigid structures that facilitate cloacal intromission, the sulcus is more functionally intricate. Here we show the sulcus spermaticus (an open groove that runs along the ventral aspect of the phallic shaft) to be a spatially heterogeneous reproductive structure containing a complex architecture of multiple tissue types. Sulcus morphology markedly changes from its proximal origin between the crurae to its distal exit from the phallus tip. At the proximal origin of the sulcus, the ductus deferens vent semen into an expanded lumen lined by a convoluted secretory epithelium. Along the length of the phallic shaft, an arrangement of longitudinally and radially oriented smooth muscle bundles may act via rhythmic contractions to produce peristaltic sperm conveyance through the sulcus. An extensive vascular network of blood and lymph vessels putatively engorges the sulcus tissues during reproductive activity, increasing tension on an internal network of connective tissues and leading to localized inflation and increased tissue rigidity. We hypothesize that this engorgement works to seal the sulcus groove and allow the structure to convey semen through a functionally closed tube. Further, numerous epithelial secretory cells contribute seminal fluids to the ejaculate and may aid in as yet uncharacterized aspects of sulcus functioning. Together, these observations establish that the sulcus spermaticus is far more than a simple furrow in the phallus shaft for sperm conduction: it contains elements that form a complex functional gamete delivery system.