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The gharial (Gavialis gangeticus Gmelin) is a fish-eating specialist crocodylian, endemic to south Asia, and critically endangered in its few remaining wild localities. A secondary gharial population resides in riverine-reservoir habitat adjacent to the Nepal border, within the Katerniaghat Wildlife Sanctuary (KWS), and nests along a 10 km riverbank of the Girwa River. A natural channel shift in the mainstream Karnali River (upstream in Nepal) has reduced seasonal flow in the Girwa stretch where gharials nest, coincident with a gradual loss of nest sites, which in turn was related to an overall shift to woody vegetation at these sites. To understand how these changes in riparian vegetation on riverbanks were related to gharial nesting, we sampled vegetation at these sites from 2017 to 2019, and derived an Enhanced Vegetation Index (EVI) from LANDSAT 8 satellite data to quantify riverside vegetation from 1988 through 2019. We found that sampled sites transitioned to woody cover, the number of nesting sites declined, and the number of nests were reduced by > 40%. At these sites, after the channel shift, woody vegetation replaced open sites that predominated prior to the channel shift. Our findings indicate that the lack of open riverbanks and the increase in woody vegetation at potential nesting sites threatens the reproductive success of the KWS gharial population. This population persists today in a regulated river ecosystem, and nests in an altered riparian habitat which appears to be increasingly unsuitable for the continued successful recruitment of breeding adults. This second-ranking, critically endangered remnant population may have incurred an "extinction debt" by living in a reservoir that will lead to its eventual extirpation.

The gharial (Gavialis gangeticus) is a critically endangered, long‐snouted crocodilian, endemic to the Indian sub‐continent. Today, the species' distribution and numbers have reduced by more than 95% in all the large rivers where it was formerly abundant. Living upstream in a reservoir dammed in 1976, the Katerniaghat gharial population has continued to nest along the Girwa River, subject to seasonal flooding in recent decades. In 2010, a natural flood upstream in Nepal resulted in a permanent reduction in the mainstream river flow. As a consequence of reduced flow, the formerly open sand banks and mid‐river islands have converted gradually to riverbanks with woody vegetation. Coincident with the increased vegetation growth, gharial nesting sites and nest numbers declined by more than 40% by 2018. In an attempt to reverse the observed decline in nesting, we intervened with vegetation removal (VR) in 2019 and sand addition (SA) in 2020, to augment available nesting opportunities at previous and potential nesting locations. The number of nests increased with SA (n = 36 in 2020) but decreased with VR (n = 19 in 2019), relative to the prior year without intervention (n = 25 in 2018). Furthermore, hatching success increased significantly to 93% with SA, compared to 63% in VR. Creating an artificial sand bank required approximately one‐third work hours and cost much less than removing vegetation. Substrate temperatures in and around nests approximated the viable incubation range (29–33.5°C) when vegetation is absent, but were lower in sites covered with woody vegetation and/or dense, high grass. Our study indicates that gharial will respond favourably to newly created sand banks that provide open, sandy riverside nesting areas, in contrast to cut‐over sites with dense vegetation removed. Finally, we note that this strategy of augmenting nesting sites is only an interim attempt to solve the ‘nesting site’ dilemma for the river‐adapted gharial. Landscape‐level solutions related to resumption of seasonal flooding, and particularly natural flow regimes that are dynamic, rather than steady, will likely be needed to avoid local extirpation of gharial in river‐reservoir habitats.

The sex of American Alligator (Alligator mississippiensis) hatchlings is deter- mined by the egg temperature during the middle third of the 9-12 week incubation period. As a consequence, predictable sex ratios are possible for clutches incubated in constant temperatures in the laboratory, but naturally occurring sex ratios of American Alligator hatchlings from wild nests exposed to fluctuating temperatures are not well documented. Over a 5-year period (1995-1999), we determined the sex of American Alligator hatchlings from wild nests left in the field until after sex was irreversibly determined. A total of 6226 hatchlings from 232 naturally incubated wild nests showed a strong female bias (71.9% females, yearly range = 62.3-89.4% females). Most nests (64.2%) produced hatchlings of both sexes. Of the remaining clutches that produced exclusively one sex (83 nests), 78 nests produced all females, and 5 nests produced only male hatchlings. For the 2 years in which nest-cavity temperatures were known, higher temperatures led to production of significantly more male hatchlings (P < 0.001 for both 1997 and 1999). Knowledge of natural sex ratios of hatchlings can aid in the management and harvest of this commercially valuable species, and in understanding sex-ratio bias in American Alligator populations.

Detailed observations on groups of captive saltwater crocodile (Crocodylus porosus) hatchlings revealed sporadic periods of intense agonistic interactions, with 16 highly distinctive behaviours, in the morning (06:00–08:00 h) and evening (17:00–20:00 h) in shallow water. Ontogenetic changes in agonistic behaviour were quantified by examining 18 different groups of hatchlings, six groups each at 1 week, 13 weeks and 40 weeks after hatching. Agonistic interactions between hatchlings at 1 week of age (mean 7.3 ± 0.65/night) were not well-defined and varied in intensity (low, medium, high), number of individuals that were aggressive, and the outcome, while most interactions involved contact (94.5%). There were also clutch specific differences in the frequency of agonistic interactions. At 13 and 40 weeks, a more hierarchal dominance relationship appeared to be established which primarily involved aggression–submission interactions. Agonistic interactions were more frequent (13 weeks 9.7 ± 0.61/night; 40 weeks 22.2 ± 0.61/night) and intense (medium, high), but shorter in duration, in which the subordinate individual fled in response to an approach by a dominant animal that often gave chase but did not make contact. While the full repertoire of behaviour was displayed by hatchlings at 1 week of age, a smaller subset based on dominance status was displayed among 13- and 40-week-old hatchlings. Agonistic behaviour occurs in C. porosus shortly after hatching and is important in establishing and maintaining dominance hierarchies that are characterised by aggression–submission interactions. This type of interaction appears typical for C. porosus both in the wild and in captivity, and may be important in preventing serious injury in a species equipped with formidable armoury. Dispersal by hatchling C. porosus at around 13 weeks of age appears to be driven by a growing intolerance of conspecifics, while territoriality is apparent at an early age. Consequently, agonistic behaviour and social status may be major contributors to the observed differences in growth rates and survival in captivity.

Recent theoretical papers on temperature-dependent sex determination in reptiles are based on the assumption that crocodilian populations, particularly adults, are markedly female biased. While there is evidence that some crocodile populations may conform to this expectation, there is no compelling evidence that American alligator populations are female biased anywhere in the species' range. Previous data had indicated that some populations of juvenile and/or adult alligators were significantly male biased. However, these studies were criticized for sampling errors. Adult males typically occupy a different habitat from that of females, and males frequent areas where they are more likely to be caught. In contrast, in juveniles, both sexes occupy the same habitat. We determined the sex ratios of c. 3000 juvenile alligators collected from 11 sites over 6 years in south Louisiana. Our results indicate a significant sex bias (58% male), but variation was evident among samples. Sex ratios varied by year and site, and the interaction of these factors was significant. At one site in one year, there was a female bias (71%), but in no year was there an overall female bias across sites. From this study and earlier reports, we conclude that there is no evidence at present of female-biased sex ratios in the juvenile and/or adult alligator populations, and consequently, that certain models of temperature-dependent sex determination in reptiles require re-evaluation. Furthermore, our data suggest a pattern of differential mortality of females vs males during the first years of life, a pattern consistent with a key prediction of several differential fitness models for the adaptive significance of temperature-dependent sex determination in reptiles.

Unlike birds and mammals, in many reptiles the temperature experienced by a developing embryo determines its gonadal sex. To understand how temperature-dependent sex determination (TSD) evolves, we must first determine the nature of genetic variation for sex ratio. Here, we analyze among-family variation for sex ratio in three TSD species: the American alligator (Alligator mississipiensis), the common snapping turtle (Chelydra serpentina) and the painted turtle (Chrysemys picta). Significant family effects and significant temperature effects were detected in all three species. In addition, family-by-temperature interactions were evident in the alligator and the snapping turtle, but not in the painted turtle. Overall, the among-family variation detected in this study indicates potential for sex-ratio evolution in at least three reptiles with TSD. Consequently, climate change scenarios that are posited on the presumption that sex-ratio evolution in TSD reptiles is genetically constrained may require reevaluation.

The leading explanation of temperature-dependent sex determination (TSD) in reptiles postulates that (1) ovarian differentiation is directed by estrogen and that (2) estrogen is synthesized in the developing gonad following induction of aromatase expression. However, the source of steroid substrate for aromatization has not yet been identified. In addition, sex ratios vary as a function of clutch, but such biases are as yet unexplained. To address these issues, we measured estradiol, testosterone, and androstenedione in yolks of the American alligator (Alligator mississippiensis) before, during, and after the period of gonadal differentiation in this TSD species. Eggs were collected from a wild population in Louisiana and were incubated at male- and female-determining constant temperatures in the lab, as well as at intermediate temperatures that produced both sexes. Steroids were assayed in yolk extracts after celite column chromatography. All three steroids were found to be in the range of nanograms/gram of yolk at stage 16. Androstenedione was the predominant steroid, 2- to 3-fold higher in concentration than estradiol and 15- to 20-fold higher than testosterone. The levels of these steroids declined (5- to 30-fold) between stages 16 and 25, most markedly between stages 21 and 23, regardless of incubation temperature. The chronology of this sharp decline in steroid levels in our study coincides with the timing of gonadal differentiation in this species, between stages 21 to 23 based on previous reports. Estradiol levels in yolks differed by 3-fold in some clutches relative to others, whereas, no clutch differences were apparent for either androstenedione or testosterone. These data demonstrate that alligator yolk contains high concentrations of two steroid substrates utilized for estrogen synthesis, as well as significant quantities of estradiol itself. We hypothesize that estradiol levels in yolk provide a steroid background, variable among and within clutches, on which gonadal development is initiated and proceeds. As a consequence, we suggest that yolk provides an epigenetic maternal contribution that modulates the effect of incubation temperature on hatchling sex.

Sexual differentiation of the gonads in Alligator mississippiensis and many other oviparous reptiles is controlled by egg incubation temperature. Estrogens are thought to play a role in this process, and it has been hypothesized that estrogen production is thermosensitive in species with temperature-dependent sex determination (TSD). Using the tritiated water assay, we measured the activity of the critical estrogen-synthesising enzyme, P450 aromatase, in the gonad-adrenal-mesonephric kidney complex (GAM) of alligator embryos incubated at male- and female-producing temperatures. Aromatase activity increased in the GAM of developing embryos incubated at 30° C (100% female-producing) and 34.5° C (predominantly female-producing), while it remained very low throughout development in embryos incubated at the intermediate temperature of 33° C (100% male-producing). However, it is unclear whether enhanced aromatase activity represents the initial signal for ovary differentiation or whether it lies downstream in the female developmental pathway. For embryos incubated at 30° C (female-producing), there was no detectable increase in aromatase activity until developmental stage 24, which is after the temperature-sensitive period for sex determination. This suggests that aromatase may be a downstream component of the ovary-determining cascade. In female alligator hatchlings, most of the aromatase activity was localised in the ovary, activity being low in the adrenal-mesonephros. Aromatase assays carried out at 30° C and at 33° C indicated that, at viable incubation temperatures, aromatase activity is not thermosensitive. This suggests that temperature directly or indirectly influences enzyme synthesis. Radioimmunoassay of estradiol synthesised by the GAM during the aromatase assay confirmed increased enzyme activity during female development but not during male development. Increased aromatase activity and estrogen synthesis during female development were correlated with the timing of ovary differentiation, particularly proliferation of the gonadal cortex. These findings implicate aromatase in temperature-dependent gonadal sex differentiation in alligator embryos, higher enzyme activity being associated with ovary development.

Incubation temperature determines sex in crocodilians and influences other features of hatchlings. We investigated the effects of incubation temperature, clutch, and sex on the dimensions, mass, and energy reserves of hatchling Alligator mississippiensis. Eggs from three clutches were incubated to pipping at constant temperatures of 29°, 31°, 32°, and 33° C; females were produced at 29° and 31° C, mixed sex ratios at 32° C, and males at 33° C Incubation temperature affects hatchling size and yolk utilization in a consistent, but complex, pattern. Maximum yolk-free hatchling mass occurred at 32° C, and minimum mass at 31° C. Similarly, dimensions were maximized at 32° C and minimized at 29° C Residual yolk mass was inversely related to yolk-free hatchling mass. Higher yolk mass occurred at 31° and 33° C; at 29° and 32° C, values were lower. However, fat mass remained constant in all clutches. Strong clutch effects were present in nearly all traits. At 32° C, males were heavier and had less yolk than female...

Half of the 22 extant crocodilians show evidence of temperature-dependent sex determination (TSD). We examine evidence for TSD in 11 species by reviewing reports on five and presenting new data for six. The female-male pattern (FM; females at low temperature, males at high temperature) attributed to Alligator mississippiensis and Caiman crocodilus are here revised to be female-male-female (FMF; males at intermediate temperature, females at low and high temperatures). A similar pattern characterizes Crocodylus palustris, C. moreletii, C. siamensis, and Gavialis gangeticus based on new data; published accounts establish a FMF pattern in Crocodylus porosus, C. johnstoni, and C. niloticus. TSD apparently occurs in Paleosuchus trigonatus and Alligator sinensis, but patterns are not yet documented. In the well-studied species, the incubation temperatures for FM transitions are congruent, but MF transition temperatures differ among species. In A. mississippiensis, 100% males are produced over a range of constant incubation temperatures, whereas in C. johnstoni, only low proportions of males are produced at any constant temperature. The thermosensitive period (TSP) for A. mississippiensis occurs during stages 21 to 24 (days 30–45 at intermediate temperatures) and coincides with gonadal differentiation. A similar scenario is suggested in other species. The TSP in A. mississippiensis (and possibly other crocodilians) encompasses the third quarter of development and occurs later than in turtles and a lizard. In A. mississippiensis as in turtles, the duration (cumulative effect) and/or the magnitude (potency effect) of incubation temperatures during the TSP predictably alter sex ratios. TSP chronologies and features which are shared among TSD reptiles suggest common, underlying mechanisms; A. mississippiensis is an appropriate model for further study. In crocodilians, clutch effects are a significant source of variation in TSD response. Hatchling sex ratios previously reported for A. mississippiensis are reconsidered in light of our new data. © 1994 Wiley-Liss, Inc.

We compare and contrast the signalling systems and social behaviors of Alligator mississippiensis, Crocodylus aculus, and Crocodylus mloticus . Our qualitative analysis focuses primarily on the behaviors of adults during three phases of reproduction: I. Defense of Territory and Courtship, II. Nesting and Incubation, and III. Hatching and Post Hatching. Signals and signal elements are very similar among the three species. For example, all have vocal, non-vocal acoustic, and visual signals, some transmitted through air or water and others through both media. In addition, each species' repertoire is composed of discrete, graded and complex signals. A few signals are unique to each species. However, their signalling systems differ in the temporal organization of the behaviors, and in the relative frequency in which certain functional groups of signals occuror in which signals occur in a particular sensory mode. Apparently, the signalling systems of C. acutus and C. niloticus are more similar to each other than either is to the signalling system of A. mississippiensis . The signalling systems of the crocodile species appear to be adapted to open water habitats in which visual signals are advantageous and to high density breeding groups and post-copulatory intersexual contact. In contrast, the Alligatorsignalling system appears adapted to a marsh habitat in which vocal signals are likely favored and to low density breeding groups.

Juvenile American alligators in outdoor pens moved out of and into the water at sunrise and sunset, respectively. When the natural light cycle was extended with artificial illumination, these movements gradually shifted into phase with the altered light cycles; therefore, the amphibious behavior was modulated by a circadian rhythm cued by light. Movement between land and water was characterized by a decrease in body temperature, which suggests that it is was not simply a proximate heat-seeking response. After the movements had been in phase with the altered light cycles for a time, they spontaneously shifted back into phase with the natural light cycl. A changing response to light is viewed as an adaptation to seasonal changes in heat availability.