Crocodiles have remained virtually unchanged for the last 80 million years, demonstrating the success of their evolutionary adaptations, including their reproductive strategy. One such adaptation is their use of temperature-dependent sex determination -- a mechanism that uses incubation temperatures to determine the sex of the developing embryos. Unfortunately, a changing climate threatens the survival of some crocodiles, as it is altering the ratio of males and females in populations.
Basic Crocodile Biology
Fourteen crocodile species inhabit south Asia, Australia, Africa and the Americas. The species comprising the group have broadly similar biology, but they do exhibit some variations in size: dwarf crocodiles (Osteolaemus tetraspis) rarely exceed 5 feet in length, while estuarine crocodiles (Crocodylus porosus) sometimes exceed 20 feet. Crocodiles are all predators, but their prey varies with the size of the individual -- smaller species consume insects, fish and amphibians while the largest crocodiles hunt animals as large as wildebeests, zebras and other crocodiles. In general, crocodiles are extraordinarily social non-avian reptiles; crocodiles form loose groups, communicate visually and acoustically and invest a significant amount of resources to provide parental care for their young.
Crocodile Nesting and Eggs
Shortly after breeding, female crocodiles begin searching for appropriate nest sites. They typically dig shallow pits or make mounds of vegetation; sometimes they locate their nests in shady places. Large females lay up to 90 eggs at a time, though small females deposit only a fraction of that. After depositing the eggs, the mother returns to the water, but remains close to protect the eggs through the entire incubation period, which lasts about three months.
Temperature Dependent Sex Determination
While genetic information determines the gender of most mammals, snakes, birds and many other animals, environmental conditions determine the gender of many other species -- including crocodilians. Though scientists are still investigating the specifics of the various species, crocodilians -- including crocodiles, alligators, caimans and gharials -- typically produce inherently female embryos, which can become male when certain environmental conditions are met. Typically, when a sufficiently high temperature is reached, the embryo begins producing androgenic hormones, which cause male sex organs to develop. In some species a second temperature threshold exists; when temperatures reach this point, they again spur the production of females, rather than males. Often, the egg deposition location -- sunny or shady -- determines the temperatures of the nest, and therefore the sex of the embryos.
Consequences of Climate Change
In addition to deforestation, habitat destruction and outright persecution, crocodiles must cope with another daunting challenge: climate change. As global temperatures climb, it is altering the sex ratios of the species. In the case of some Costa Rican American crocodile (Crocodylus acutus) populations, the high temperatures are already affecting the gender ratio. Typically five females hatch for every male; however, recent clutches are demonstrating a 1:1 sex ratio. Laura Porras, a biologist studying the Costa Rican crocodiles, estimates that given the variety of pressures now facing these crocodiles, they will probably vanish entirely by 2040.
- Animal Diversity Web: Crocodylus Porosus
- National Geographic: Nile Crocodile (Crocodylus Niloticus)
- Ecological Genetics and Physiology: Temperature-Dependent Sex Determination in Crocodilians
- Journal of Zoology: Incubation Temperatures, Sex Ratios and Sex Determination in a Population of Nile Crocodiles (Crocodylus Niloticus)
- Global Post: Missing in Costa Rica: Female Crocodiles
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