American Alligators’ Sex Ratios Could Go Haywire

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American alligators wade in the alligator lagoon at Everglades Alligator Farm in Homestead, Florida.
Photo: Rhona Wise/AFP (Getty Images)

Many of the effects of climate change are obvious, like bizarre weather patterns, rising sea levels, and species loss as a result of deforestation and habitat degradation. Some, though, are a little bit weird.

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One of those is that the sex ratios of certain reptile species, which might shift to be either entirely male or entirely female as the planet heats up. Populations of American alligators, in particular, may eventually become majority female, which isn’t the utopia it might sound like (alas) and could result in population declines and subsequent butterfly effects throughout the ecosystems they inhabit.

“Many reptiles lack sex chromosomes entirely (unlike humans, where your sex is determined by whether you receive an X or Y chromosome from your dad), and rely completely on temperature to determine an individual’s sex,” said Samantha Bock, a PhD candidate at the University of Georgia’s Savannah River Ecology Lab. “This is a process known as temperature-dependent sex determination.”

Temperature-dependent sex determination occurs in many, but not all, reptiles, including a large proportion of turtles and all crocodilians. Depending on the species, temperature shifts sex ratios in different directions. In turtles, females are produced at high temperatures, and males at low temperatures, while in tuatara (lizard-like reptiles native to New Zealand), males are produced at high temperatures and females at low temperatures. Crocodilians are particularly groovy, with females being produced at extreme temperatures (both low and high) and males at the sweet spot in the middle.

American alligators (Alligator mississippiensis) are one such crocodilian. Nest temperatures between 90.5 and 92.3 degrees Fahrenheit (32.5 and 33.5 degrees Celsius) will produce mostly males, whereas temperatures above or below these produce mostly females.

Bock said that different levels of warming may affect American alligator sex ratios in the future. A study she led, published in Proceedings of the Royal Society B: Biological Sciences early last year, shows what exactly we could expect. Bock and her team measured the temperatures of 86 nests at two sites in the northern and southern regions of the American alligator’s geographical range, and examined the influence of daily maximum air temperatures on nest temperature.

The sex ratios of species with temperature-dependent sex determination can fluctuate after even small changes in nest temperatures: A change of 1.8 degrees Fahrenheit (1 degree Celsius) can shift the sex ratio of a nest from 100% males to 100% females. Bock’s study found that a small nest temperature increase of between 2 and 2.5 degrees Fahrenheit (1.1 and 1.4 degrees Celsius) may shift the sex ratio to produce mostly males. However, larger increases in temperature like those predicted by the year 2100 even if the world begins to draw down emissions by midcentury are likely to produce highly female-biased sex ratios, with almost no males hatching at all.

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This isn’t great for the long-term outlook of the species since both males and females are required to make the babies that will continue the next generation. If the sex ratios shift far enough towards the male or the female end, there might not be enough of the opposite sex to prevent the species from going extinct. Additionally, developing reptiles may not be able to survive in the nest if temperatures become too warm due to heat shock, meaning that offspring dying before their egg hatches will become the main reason for population decrease.

Other studies support these forecasts for different reptiles in the same boat as alligators. A paper in the journal of Physiological and Biochemical Zoology found that individual variability of painted turtles (Chrysemys picta) in the timing of nesting will not be enough to offset the effects of climate change on sex ratios in the study population. An assessment of the Great Barrier Reef described how climate change could shift the sex ratios of various reptile populations including marine turtles, sea snakes, and estuarine crocodiles in addition to the threats posed by sea level rise and other climate impacts.

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In ecological systems, every organism serves a role in the greater food web. Even small changes to the status quo of single species, such as biased sex ratios and the resulting reduced population sizes, can send ripple effects throughout the whole community.

“The associated consequences of declining populations of alligators and other reptiles can have broader effects for the ecosystems these species inhabit”, said Bock. “Alligators play important roles in ecosystems as ecosystem engineers, as they provide habitats for other species through nest building and creating alligator holes and dens. They are also apex predators that keep their prey populations in check. These important ecosystem roles also apply to many other reptile species. Declines in populations of these species with shifting sex ratios associated with climate change can therefore have widespread effects on the natural system as a whole.”

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But while climate change could wreak havoc on ecosystems, it may not be the end of reptiles as we know them. Bock said they may need a little help from conservationists, however, particularly as other pressures such as human encroachment on their habitat make life difficult for them and other species around the world.

“One way that we might be able to ensure reptiles are able to withstand shifted sex ratios in the wild is by implementing artificial incubation programs where we collect eggs from nests, incubate them at the temperature that produces the rare sex, and release hatchlings back to the wild,” said Bock. “In addition, habitat protection will become even more important in ensuring reptiles can recover from/persist in the face of shifted sex ratios by allowing ample opportunities for females to nest in a variety of locations and by facilitating population connectivity to preserve genetic diversity.”

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Based in London, Jess is a big fan of carb-based foods and small dogs. You can find her writing on Twitter at @thomsonjessic.

DISCUSSION

dnapl
Dense non aqueous phase liquid

Googling around a bit it seems like the American alligator goes back awhile. It’s hard to ferret out from skimming whether or not that species specifically extends as far back as the more recent hothouse periods.  This would be the Cretaceous (about 90 million years ago) and the Paleocene-Eocene Thermal Maximum (about 50 odd million years ago). Maybe the American alligator evolved from another alligator species due to some sort of environmental perturbation like relatively rapid temperature change. Or maybe not. The alligator in general reportedly goes back to the triassic period (somewhere around 200 million years ago).

Nest temperatures between 90.5 and 92.3 degrees Fahrenheit (32.5 and 33.5 degrees Celsius) will produce mostly males, whereas temperatures above or below these produce mostly females.

Anyway, maybe thermodynamically there’s a Goldilocks temperature range for males. Conditions outside that range requires more females for better propagation. And then again, maybe not.