by Joseph DeSisto
If you are careless in wandering along the swamps of the southeastern United States, you may hear this sound emanating from the brush:
[Recording by Adam Britton, used with permission.]
That is the hiss of an angry American alligator — if you hear it on land, you may have stumbled upon an alligator nest. If so, do not delay in your retreat. A mother alligator’s warning is no bluff.
If you could stay, however, you might be surprised at the tenderness with which alligators treat their offspring. When a female is ready to lay, she hauls herself on shore and finds a shaded, protected area not too far from the water. She lays her eggs in a pile of mud and leaf litter, then heaps more litter on top of them, so that the end result is a leaf-and-mud pile 2-3 feet tall and 5-7 feet wide (McIlhenny 1935).
A thick layer of insulating leaves also keeps the eggs at a more-or-less constant temperature. On a daily basis, even though the environment might go through wild changes in temperature, the inside of the nest stays within 3° F (Chabreck 1973). Alligator eggs usually take around 2 months to develop, and stable temperatures are critical.
In human beings, the presence or absence of a Y chromosome decides whether one develops into a male or female. In other words, human sex determination is chromosome-dependent. Alligators instead, like many reptiles, show temperature-dependent sex determination. Between days 20 and 35 of incubation, if eggs are kept between 86° and 93°F, a roughly even mixture of females and males will be the result (Ferguson and Joanen 1983). If, however, the batch stays above 93°, only males will emerge, and if below 86°, only females.
Alligator babies aren’t the only things that grow in alligator nests. The heap of dead leaves, twigs, and mud provides a haven for bacteria and other microorganisms. As bacteria digest the rotting vegetation, they produce heat — enough to keep the eggs 3-4° warmer than the habitat outside the nest (Chabreck 1973). In fact, bacteria keep alligator nests so consistently warm that the nests are also home to unique, heat-loving fungi (Tansey 1973).
Eggs, alligator or otherwise, look simple but are surprisingly complex. The “solid” shell is mostly made of calcium, but it’s far from a perfect seal — the whole surface is peppered with thousands of tiny holes, allowing the egg to take in oxygen and water, while “exhaling” carbon dioxide (Kern and Ferguson 1997). The thickness of the shell must be precise — too thin and the egg is easily crushed or infected by disease, but too thick and breathing, drinking, and hatching become difficult.
On alligator farms, where alligators are bred and raised for their skins, roughly 30-60% of eggs hatch successfully. Meanwhile more than 90% of alligator eggs hatch successfully in the wild (Kern and Ferguson 1997), as long as the nest isn’t flooded or raided by predators first. Experiments have shown that captive alligator eggs are less porous than their wild counterparts, and of the captive eggs, the least porous are doomed to die before hatching (Wink et al. 1990). Captive alligator eggs also have much thicker shells than wild eggs. So where is the difference coming from?
The bacteria in wild alligator nests, aside from producing heat, also produce acids. These acids aren’t strong or abundant enough to harm the developing reptiles, but over the 2 months it takes for them to develop, acids gradually erode the hard, calcium shell around each egg (Ferguson 1981). By the time the alligator is ready to hatch, its shell is significantly thinner than when the egg was first laid — just thin enough for the hatchling to easily break through.
[Recording by Adam Britton, used with permission.]
As they leave their eggs, baby alligators sound an alarm to their mother, who industriously digs them out of the nest where they spent the first two months of their lives. Although these months might seem uneventful, they are in fact full of challenges, which alligator eggs, however simple and unassuming, have ways to overcome. Those hatchlings that survive face yet another gauntlet of obstacles, including predators and ruthless competition from their siblings. It’s tough being a baby alligator, and maybe even tougher being an egg, but the toughest few have a chance to become some of the most awe-inspiring top predators in North America.
Dr. Adam Britton, a crocodile researcher at the Charles Darwin University in Northern Territory, Australia, has graciously allowed me to use the audio files in this article. More files, along with a wealth of information about crocodilian biology and conservation, can be found at his website, crocodilian.com.
Chabreck R.H. 1973. Temperature variation in nests of the American alligator. Herpetologica 29(1): 48-51.
Ferguson M.W.J. 1981. Increased porosity of the incubating alligator eggshell caused by extrinsic microbial degradation. Experientia 37(3): 252-255.
Ferguson M.W.J. and T. Joanen. 1983. Temperature-dependent sex determination in Alligator mississippiensis. Journal of Zoology 200(2): 143-177.
Kern M.D. and M.W.J. Ferguson. 1997. Gas permeability of American alligator eggs and its anatomical basis. Physiological Zoology 70(5): 530-546.
McIlhenny E.A. 1935. The Alligator’s Life History. Christopher Publishing House, Boston. 117 pp.
Tansey M.R. 1973. Isolation of thermophilic fungi from alligator nesting material. Mycologia 65(3): 594-601.
Wink C.S., R.M. Elsey, and M. Bouvier. 1990. Porosity of eggshells from wild and captive, pen-reared alligators (Alligator mississippiensis). Journal of Morphology 203(1): 35-39.