Southeastern Naturalist
191
R.M. Elsey and J.W. Lang
22001144 SOUTHEASTERN NATURALIST 1V3o(2l.) :1139,1 N–1o9. 92
Sex Ratios of Wild American Alligator Hatchlings in
Southwest Louisiana
Ruth M. Elsey1,* and Jeffrey W. Lang2
Abstract - The sex of American Alligator (Alligator mississippiensis) hatchlings is determined
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.
Introduction
Multiple crocodilian species, including Alligator mississippiensis Daudin
(American Alligator; hereafter also Alligator) exhibit temperature-dependent sex
determination (hereafter TSD; Ferguson and Joanen 1982, 1983; Lang and Andrews
1994). Refined laboratory studies with accurate temperature controls have examined
this phenomenon in detail (see Western 1999 and references therein). The influence
of temperature on the sex of developing Alligator hatchlings is complicated. Under
constant temperature incubation in the laboratory, only a narrow range of intermediate
temperatures produce males, with females being produced over a much broader
range of both high and low temperatures (Lang and Andrews 1994). Specifically,
laboratory incubation at a constant temperature of ≤31 °C produces only females, 32
°C results in mixed sex ratios, and 33 °C only produces males (Lang and Andrews
1994). Mixed sex ratios occur at 34 °C, and very high temperatures (35 °C) produce
exclusively females; however, mortality is high at this temperature (only 11%
embryonic survival at 35 °C; Lang and Andrews 1994) and thus the production of
high-temperature females may rarely be seen in nests incubated in the wild. Therefore,
the TSD pattern in American Alligators in a controlled laboratory setting is a
bimodal distribution of exclusively females at low and at high temperatures, males
1Louisiana Department of Wildlife and Fisheries, 5476 Grand Chenier Highway, Grand
Chenier, LA 70643. 2Department of Fisheries, Wildlife and Conservation Biology, University
of Minnesota, Saint Paul, MN 55108. *Correspnding author - relsey@wlf.la.gov.
Manuscript Editor: Thomas Rainwater
Southeastern Naturalist
R.M. Elsey and J.W. Lang
2014 Vol. 13, No. 2
192
at a narrow range of intermediate temperatures, and mixed sex ratios on either side
of the male-producing temperatures. At constant temperatures, the incubation period
(i.e., the time between oviposition and hatching) ranges from 63 days at 33 °C to 84
days at 29 °C (Lang and Andrews 1994). In American Alligators, the thermosensitive
period when sex is irreversibly determined occurs during the middle third of incubation,
or at approximately days 30–45 (Lang and Andrews 1994).
Less is known about sex ratios of Alligator hatchlings in the wild, where nests,
eggs, and embryos are exposed to fluctuating temperatures (e.g., range of 23.3–32.8
°C; Joanen 1969). It is important for wildlife biologists to have knowledge of sex
ratios of Alligators from all size/age classes for use in management of the population
and establishment of harvest quotas (which may select for one sex rather than
the other). In Louisiana, harvest programs exist for both eggs and sub-adult and
adult Alligators.
Given the data from constant incubation thermal regimes in the laboratory,
female-biased sex ratios might be predicted among hatchlings because only a
narrow range of nest temperatures result in males. However, because natural nest
temperatures vary throughout the day and season it is difficult to accurately predict
whether naturally produced Alligator hatchling sex ratios will be biased toward
males or females (Rhodes and Lang 1995, 1996). In fact, reviews of crocodilian sex
ratios found that many populations of a variety of species examined are male-biased
(Lance et al. 2000, Thorbjarnarson 1997). For example, in an extensive 6-year survey
of over 3000 juvenile American Alligators at 11 sites in Louisiana, Lance et al.
(2000) reported a significant male-biased (58%) sex ratio that varied by year and
by site.
The inconsistency between lab-based predictions and field measurements suggest
more work is required to determine sex ratios of Alligator clutches incubated
naturally in wild nests. Information obtained from additional studies could be of
great importance as projected global temperature changes affect wetlands and
wetlands management (Withey and van Kooten 2011) and therefore Alligators.
Regional climate changes could also affect many Louisiana Alligator “ranchers”,
who are licensed to collect eggs from wild Alligator nests to provide stock for commercial
Alligator farms (Elsey et al. 2001), as climatic factors affect the timing and
degree of Alligator nesting, age of sexual maturity, and growth rates (Joanen and
McNease 1987) as well as the sex ratio of hatchlings. Louisiana’s Alligator industry
can be a $60 million industry in peak market years (Louisiana Department of Wildlife
and Fisheries 2009), and thus, data relative to Alligator ecology is of significant
economic interest to managers and landowners. This study was initiated to evaluate
naturally occurring sex ratios in nests of American Alligators in Louisiana. We
also attempted to correlate nest-incubation temperatures during the thermosensitive
period with resulting hatchling sex ratios.
Methods
We conducted this study on portions of Rockefeller Wildlife Refuge, a 30,700-ha
coastal marsh in Cameron and Vermilion parishes, in southwestern Louisiana. The
Southeastern Naturalist
193
R.M. Elsey and J.W. Lang
2014 Vol. 13, No. 2
refuge boundaries and predominant vegetation were described by Joanen (1969);
current marsh types and management units were described recently by Selman and
Baccigalopi (2012). Nests were located by helicopter and marked with 3.05-m PVC
pipes. We plotted nest sites on an aerial map for later use by ground crews. The
great majority of nests were later accessed by airboat, with the exception of a few
nests that were adjacent to roads and were approached by vehicle and on foot. During
June 1995, we placed temperature recorders (Optic StowAway, Onset Computer
Corp., Pocasset, MA) in 10 Alligator nests soon after oviposition. We also collected
16 other clutches late in incubation (after the thermosensitive period) to examine
the hatchling sex ratio, for a total of 26 clutches examined in 1995. We expanded
the study from 1996–1999, and placed temperature recorders in additional nests
(2, 37, 42, and 40 nests with recorders in each successive year) and collected more
nests in August of each year, after the thermosensitive period.
Nests were opened to reveal the egg cavity. If temperature recorders were placed
in nests, 2 eggs were removed and sacrificed to determine the stage of incubation
of the developing embryos by examining the size of the embryo (Ferguson 1985).
This step was not done for nests that did not receive data loggers as embryos were
not needed for aging, and instead the entire clutch was collected in August. We
placed the temperature recorder near the middle of the clutch of eggs, with care
being taken to minimize disturbance to nest integrity and not inadvertently rotate
eggs. We re-covered the cavity with the original nest media, predominantly Spartina
patens. We programmed the temperature recorders, which had a sensitivity of
±0.2 °C, to take a reading every 15 minutes.
In August, after the thermosensitive period was completed, we revisited the
nests containing temperature recorders (the only visit after the initial visit, to limit
disturbance). We marked the eggs for upright orientation, packed them carefully
in layers of nest media, and transported them to the lab at Rockefeller for artificial
incubation (approximately 30.5–31.5 °C) until hatching (Joanen et al. 1987), which
generally occurred one to three weeks later. We collected full clutches from numerous
other nests not containing temperature recorders in the same manner during
this time and transported them to the field laboratory for incubation under the same
conditions as clutches that contained recorders.
During incubation and after hatching, each clutch was held in a separate container
with nest media from its corresponding nest used for insulation and cushioning.
Hatchlings were maintained by clutch until sex was verified. We determined sex for
each hatchling by visual examination of the genitalia within the cloaca as described
by Allsteadt and Lang (1995), using a lighted magnifying glass for clarity. J.W.
Lang made all observations to minimize error, as determining the sex of hatchling
crocodilians is challenging (Joanen and McNease 1978, Otano et al. 2010, Ziegler
and Olbort 2007). In some cases, embryos that died late in incubation were saved
for possible sex determination, if tissue degradation did not preclude it.
For nests that contained data loggers, we used a simple linear regression to
determine if there was a relationship between mean daily temperatures in the nest
cavity during the thermosensitive period for sex determination (days 30–45) and
Southeastern Naturalist
R.M. Elsey and J.W. Lang
2014 Vol. 13, No. 2
194
the percentage of hatchlings within the clutch that were male. Unfortunately, a
computer-system change led to loss of electronic temperature-data–logger files;
however, nest-cavity–temperature data from 1997 and 1999 could be gleaned from
quarterly reports provided by the second author as part of a research contract. We
also evaluated ambient temperature data for the month of July (the thermosensitive
period for sex determination at the study site occurs primarily in July) for each of
the 5 years, to determine if there was a correlation between ambient minimum temperatures
and overall hatchling sex ratio. Ambient temperature was recorded at the
Rockefeller refuge headquarters weather observation station.
Results
During the 5-year study, we evaluated a total of 6226 hatchlings and observed
a strong overall female bias for hatchlings produced each year (Table 1). The percentage
of females ranged from 62.3% in 1998 to 89.4% in 1995. Seventy-eight
of 232 nests (33.6%) produced only female hatchlings, while there were only 5 of
232 nests (2.2%) that produced only males; all 5 of these nests were from the first
3 years of the study.
Data-logger temperatures from 1997 revealed that cool nest temperatures (29–
31 °C) during the temperature-sensitive period produced 100% females, intermediate
temperatures produced varying sex ratios (including 100% males produced in
one clutch at 32.8 °C), 2 nests at higher temperatures (33–34 °C) produced 74 and
78% males, and the warmest nest (35 °C) produced predominately (79.2%) females
(Fig. 1). There was a significant Pearson correlation coefficient (R) of 0.772 such
that increasing nest cavity temperatures led to development of more male hatchlings
(P < 0.001).
In 1999, one nest with a relatively cool initial temperature of 29 °C produced
all female hatchlings. In contrast, a relatively warm nest with an initial temperature
of 32.8 °C produced 66% males. The nest-cavity temperatures during the
thermosensitive period again correlated with the percent males produced, with
warm nests producing a majority of males (R = 0.566, P < 0.001), and the coolest
nests producing only females. Despite a severe drought in spring/summer 1996 in
southwestern Louisiana in which warm, dry conditions perhaps should have fa-
Table 1. Sex ratio of hatchling American Alligators, Alligator mississippiensis from nests on Rockefeller
Refuge 1995–1999.
Nests Nests
Study Number Number Total hatchlings Total producing producing
year of males of females or embryos nests 100% females 100% males
1995 81 (10.6%) 683 (89.4%) 764 26 9 1
1996 254 (34.1%) 491 (65.9%) 745 32 13 3
1997 372 (25.7%) 1074 (74.3%) 1446 47 21 1
1998 510 (37.7%) 844 (62.3%) 1354 62 20 0
1999 531 (27.7%) 1386 (72.3%) 1917 65 15 0
Total 1748 (28.1%) 4478 (71.9%) 6226 232 78 5
Southeastern Naturalist
195
R.M. Elsey and J.W. Lang
2014 Vol. 13, No. 2
vored the production of males (assuming high ambient temperatures cause higher
temperatures in nest cavities), only 34.1% of the hatchlings were male from the
nests studied (n = 32). This was the second highest percentage of male hatchlings
produced during the 5-year study.
The Pearson correlation coefficient for the relationship between the minimum
ambient July temperature for the 5-year study period and the percentage of male
hatchlings produced each year was 0.76, P = 0.14, indicating a non-significant
but positive association between warmer minimum July temperatures and more
male hatchlings.
Discussion
In all 5 years of this study, a strong female-bias (71.9%) was noted among hatchlings
produced from 232 clutches examined. This result is a marked difference to
the male-biased juvenile sex ratio (58%) we have documented in tag-and-release
efforts in southwestern Louisiana for some 3000 American Alligators from 1991–
1995 (Lance et. al. 2000). Thus, there may be differential survival rates that tend
to favor male juveniles; the faster growth rates of males (Elsey et. al. 1992, Joanen
et. al. 1987) may give them a competitive advantage over juveni le females.
Figure 1. Nest temperatures (°C) from data loggers during the thermosensitive period (days
30–45) for sex determination for American Alligators (Alligator mississippiensis) from the
nest cavity of wild alligator nests on Rockefeller Refuge in 1997, and the resulting sex ratio
expressed as percent male hatchlings for each clutch. Increasing nest-cavity temperatures
led to the development of more male hatchlings (R = 0.772, P < 0.001).
Southeastern Naturalist
R.M. Elsey and J.W. Lang
2014 Vol. 13, No. 2
196
The few nests with higher nest-cavity temperatures led to the production of more
male hatchlings. In the field, we rarely recorded the very high temperatures (35 °C)
that lead to production of females in controlled laboratory experiments. Ambient
air temperatures were also qualitatively associated with sex ratios of hatchlings
produced, with higher temperatures leading to a higher proportion of males within
a clutch. We anticipated ambient temperature would not be as rigorous a predictor
of sex ratios as actual nest-cavity temperatures, but suspect that knowledge of these
trends may assist in assessment of sex-ratio data in regions where nest-cavity–temperature
data may not be available to researchers.
In a controlled laboratory setting at constant incubation temperatures, the pattern
of percent males produced vs. incubation temperature is unimodal, with high
proportions of male hatchlings at a single range of intermediate to high temperatures,
and female production is bimodal, with high proportions of female hatchlings
at high and low temperatures above and below the male-producing mid-range (Lang
and Andrews 1994). In the present study, we observed the same phenomenon in
wild Alligator nests in 1997, with the coolest nests producing all females, the intermediate
temperatures producing more males, and the warmest nest (35 °C during
the thermosensitive period) producing 79% females. These results are likely influenced
by natural variability in field-nest temperatures leading to the production of
some hatchings of each sex in most years.
In a related study, results similar to ours were documented in coastal South
Carolina (Rhodes and Lang 1995, 1996) where sex ratios of naturally incubated
Alligator clutches (778 hatchlings from 25 nests) also exhibited a strong female
bias (79.2%) in 1994. Likewise, in 1995 Rhodes and Lang (1995, 1996)
monitored an additional 20 nests and the sex ratio of the resulting 648 hatchlings
was also female biased (58.2%). The South Carolina study was continued
for four more years, and in one year, 100% of hatchlings produced from all
clutches evaluated were female (Lance et al. 2000). We are not aware of data on
subsequent juvenile sex ratios in coastal South Carolina to evaluate sex-specific
differential survival.
Growth rates in Alligators vary by sex; males tend to grow faster and attain
larger maximum total lengths than females (Chabreck and Joanen 1979, Elsey et
al. 1992, Wilkinson and Rhodes 1997, Woodward et al. 1995). Because survival
in Alligators is size-dependent (Nichols et al. 1976, Rootes 1989), the sex of the
hatchling (and resulting growth rate and size) will affect its survival rate. Thus,
knowledge of the sex ratio of a population is essential to management of the species;
particularly if harvest programs preferentially target one sex.
Detailed studies exist on the relationship between nest temperature, sex ratios,
hatching success, and survivorship in other crocodilians, including Paleosuchus
trigonatus Schneider (Dwarf Caiman; Magnusson et al. 1990) and Caiman latirostris
Daudin (Broad-Nosed Caiman; Piña et al. 2003). In the closely related Caiman
crocodilus yacare Daudin (Yacare Caiman), nests constructed of varying types of
vegetation may have differing incubation temperatures (Campos 1993). Similarly,
we also have preliminary data suggesting different types of vegetation can affect
Southeastern Naturalist
197
R.M. Elsey and J.W. Lang
2014 Vol. 13, No. 2
nest-cavity temperatures, which would affect Alligator egg survival as well as
hatchling sex ratio. This finding is interesting from an evolutionary standpoint,
as perhaps female nest-site selection could influence hatchling sex ratio.
Many commercial Alligator egg ranchers in several southeastern states (Louisiana,
Texas, and Florida) collect large numbers of wild Alligator eggs to incubate
and then raise the hatchlings in captivity to produce juveniles for the international
leather goods market. Being that the incubation period of Alligator eggs is brief,
many of the eggs are often collected after the thermosensitive period for sex determination.
Commercial Alligator ranchers/producers may prefer to obtain hatchlings
of one sex or the other; males often grow faster, which could minimize production
costs, whereas females may be preferred to ensure that mandatory releases to the
wild will include females for future nesting on egg-collection sites (a juvenile headstart
release program is a required part of the egg-collection permit in Louisiana
[Elsey et al. 2001]). Thus, Alligator farmers may choose to coordinate egg collecting
efforts in varying regions based on which eggs they prefer to incubate early
(so they can influence the sex of the hatchlings by incubation temperature) and in
which regions they might collect eggs after the sex has been influenced by ambient
nest temperatures, such as areas of lower nest densities.
Differences in nesting habitats and seasonal weather patterns in various regions
may influence hatchling sex ratios from nests incubated in the wild. For
example, results from our study showed production of 89.4% females in 1995,
which differs markedly from hatchling sex ratios in South Carolina the same
year, in which more males (42% of 648 hatchlings) were produced (Rhodes and
Lang 1995, 1996). Similar to the range in sex ratios we observed among years
in Louisiana, a hot, dry summer likely contributed to the 42% males produced in
South Carolina in 1995 as compared to the cooler, wetter summer in 1994 when
only 20% males were produced at the same site (Lang 1995). Recent speculation
about global climate change and possible effects on crocodilian reproduction and
sex ratios is of interest (Rao et al. 2013). Indeed, some authors have speculated
that global warming may affect reptiles exhibiting temperature-dependent sex
determination, as altered incubation temperatures in nests could potentially result
in skewed sex ratios or hatchlings of only one sex (Charruau 2012 and references
therein), or that past climate-change influences on sex ratios may have contributed
to selective extinction of some Archosaurs (Ferguson and Joanen 1982).
However, normal expected cyclical weather patterns and sex-specific differential
survival may ensure adequate production of both sexes of crocodilians, which
have survived successfully for millions of years.
Acknowledgments
We thank all Louisiana Department of Wildlife and Fisheries employees who assisted
with Alligator egg collection and incubation, including Phillip “Scooter” Trosclair, Jeb
Linscombe, George Melancon, and Leisa Nunez. We thank Lisa Morris of the Department
of Experimental Statistics at Louisiana State University for assistance with analyses.
Southeastern Naturalist
R.M. Elsey and J.W. Lang
2014 Vol. 13, No. 2
198
Literature Cited
Allsteadt, J., and J.W. Lang. 1995. Sexual dimorphism in the genital morphology of young
American Alligators, Alligator mississippiensis. Herpetologica 51:314–325.
Campos, Z. 1993. Effect of habitat on survival of eggs and sex ratio of hatchlings of Caiman
crocodilus yacare in the Pantanal, Brazil. Journal of Herpetology 27:127–132.
Chabreck, R.H., and T. Joanen. 1979. Growth rates of American Alligators in Louisiana.
Herpetologica 35:51–57.
Charruau, P. 2012. Microclimate of American Crocodile nests in Banco Chinchorro biosphere
reserve, Mexico: Effect on incubation length, embryos survival, and hatchlings
sex. Journal of Thermal Biology 37:6–14.
Elsey, R.M., T. Joanen, L. McNease, and N. Kinler. 1992. Growth rates and body condition
factors of Alligator mississippiensis in coastal Louisiana wetlands: A comparison of wild
and farm-released juveniles. Comparative Biochemistry and Physiology 103A:667–672.
Elsey, R.M., L. McNease, and T. Joanen. 2001. Louisiana’s alligator ranching program: A
review and analysis of releases of captive-raised juveniles. Pp. 426–441, In G. Grigg, F.
Seebacher, and C.E. Franklin (Eds.). Crocodilian Biology and Evolution. Surrey Beatty
and Sons Pty. Limited, Chipping Norton, NSW, Australia. 446 pp.
Ferguson, M.W.J., and T. Joanen. 1982. Temperature of egg incubation determines sex in
Alligator mississippiensis. Nature 298:850–853.
Ferguson, M.W.J., and T. Joanen. 1983. Temperature-dependent sex determination in Alligator
mississippiensis. Proceedings of the Zoological Society of London 200:143–177.
Ferguson, M.W.J. 1985. The reproductive biology and embryology of crocodilians. Pp.
329–491, In A.C. Gans, F. Billet, and P.F.A. Maderson (Eds). Biology of the Reptilia.
14. Development. John Wiley and Sons, New York, NY. 778 pp.
Joanen, T. 1969. Nesting ecology of alligators in Louisiana. Proceedings of the Annual
Conference of Southeastern Association of Game and Fish Commissioners 23: 141–151.
Joanen, T., and L. McNease. 1978. The cloaca sexing method for immature alligators. Proceedings
of the Southeastern Association of Fish and Wildlife Agencies 32:179–181.
Joanen, T., and L. McNease. 1987. Alligator farming research in Louisiana, USA. Pp.
329–340, In G.J.W. Webb, S.C. Manolis, and P.J. Whitehead (Eds). Wildlife Management:
Crocodiles and Alligators. Surrey Beatty and Sons Pty. Limited, Chipping Norton,
Australia. 552 pp.
Joanen, T., L. McNease, and M.W.J. Ferguson. 1987. The effects of egg incubation temperature
on post-hatching growth of American Alligators. Pp. 533–537, In G.J.W. Webb, S.C.
Manolis, and P.J. Whitehead (Eds.). Wildlife Management: Crocodiles and Alligators.
Surrey Beatty and Sons Pty. Limited, Chipping Norton, Australia. 552 pp.
Lance, V.A., R.M. Elsey, and J. Lang. 2000. Sex ratios of American Alligators (Crocodylidae):
male or female biased? Journal of Zoology (London) 252:71 –78.
Lang, J.W. 1995. Nest temperatures, egg mortality, and sex ratios of hatchling alligators.
Progress report to Louisiana Department of Wildlife and Fisheries, Baton Rouge, LA. 1
July 1995–30 September 1995. 4 pp.
Lang, J.W., and H.V. Andrews. 1994. Temperature-dependent sex determination in crocodilians.
Journal of Experimental Zoology 270:28–44.
Louisiana Department of Wildlife and Fisheries. 2009. 2008–2009 annual report. Baton
Rouge, LA. 98 pp.
Magnusson, W.E., A.P. Lima, J. Hero, T. Sanaiotti, and M. Yamakoshi. 1990. Paleosuchus
trigonatus nests: Sources of heat and embryo sex ratios. Journal of Herpetology
24:397–400.
Southeastern Naturalist
199
R.M. Elsey and J.W. Lang
2014 Vol. 13, No. 2
Nichols, J.D., L. Viehman, R.H. Chabreck, and B. Fenderson. 1976. Simulation of a commercially
harvested alligator population in Louisiana. Louisiana Agricultural Experiment
Station Bulletin No. 691. Baton Rouge, LA. 59 pp.
Otano, N.B.N., A. Imhof, P.B. Bolcatto, and A. Larriera. 2010. Sex differences in the genitalia
of hatchling Caiman latirostris. Herpetological Review 41:32–35.
Piña, C.I., A. Larriera, and M.R. Cabrera. 2003. Effect of incubation temperature on incubation
period, sex ratio, hatching success, and survivorship in Caiman latirostris (Crocodylia,
Alligatoridae). Journal of Herpetology 37:199–202.
Rao, M., S. Htun, S.G. Platt, R. Tizard, C. Poole, T. Myint, and J.E.M. Watson. 2013. Biodiversity
conservation in a changing climate: A review of threats and implications for
conservation planning in Myanmar. Ambio 42:789–804.
Rhodes, W.E., and J.W. Lang. 1995. Sex ratios of naturally incubated alligator hatchlings:
Field techniques and initial results. Proceedings of the Annual Conference of Southeastern
Association of Fish and Wildlife Agencies 49:640–646.
Rhodes, W.E., and J.W. Lang. 1996. Alligator nest temperatures and hatchling sex ratios in
coastal South Carolina. Proceedings of the Annual Conference of Southeastern Association
of Fish and Wildlife Agencies 50:521–531.
Rootes, W.L. 1989. Behavior of the American Alligator in a Louisiana freshwater marsh.
Ph.D. Dissertation. Louisiana State University, Baton Rouge, LA. 108 pp.
Selman, W., and B. Baccigalopi. 2012. Effectively sampling Louisiana Diamondback Terrapin
(Malaclemys terrapin) populations, with description of a new capture technique.
Herpetological Review 43:583–588.
Thorbjarnarson, J. 1997. Are crocodilian sex rations female biased? The data are equivocal.
Copeia 1997:451–455.
Western, P.S. 1999. Gene expression during temperature-dependent sex determination in
the American Alligator (Alligator mississippiensis). Ph.D. Dissertation. La Trobe University,
Bundoora, Victoria, Australia. 229 pp.
Wilkinson, P.M., and W.E. Rhodes. 1997. Growth rates of American Alligators in coastal
South Carolina. Journal of Wildlife Management 61:397–402.
Withey, P., and G.C. van Kooten. 2011. The effect of climate change on optimal wetlands
and waterfowl management in Western Canada. Ecological Economics 70:798–805.
Woodward, A.R., J.H. White, and S.B. Linda. 1995. Maximum size of the Alligator (Alligator
mississippiensis). Journal of Herpetology 29:507–513.
Ziegler, T., and S. Olbort. 2007. Genital structures and sex identification in crocodiles.
Crocodile Specialist Group website. Available online at http://www.iucncsg.org/pages/
Publications.html. Accessed 17 March 2010.