Male Combat Favors Female Biased Populations in Species

A group of herpetologists from Indiana University’s Biology Department, Steven Freedberg and Michael Wade, decided to combine several theories of environmental sex determination, and female gender selection to provide a more inclusive theory on reptilian sex determination. There was no actual experiment done to test their hypothesis, but the evidence and proven background theories provide a realistic commentary within their proposal.

Male vs. Male combat is present in many species throughout the animal world. However it is more rare in cold-blooded animals. Both the snapping turtles and in many crocodilians this male combat has shown determine male territories and establish a social hierarchy throughout the population. In snapping turtles males often have a significant amount of scars and missing claws when compared with females, which are indicative of a male cost to reproduction. As many of you know, the males at the top of the hierarchy achieve more access to mates and therefore have a higher reproductive success rate. This means that a male’s adult morphology plays a much larger role in how successful they are in passing their genes on to the next generation.

On the other side of the spectrum are the females. Crocodilian females typically live and breed in a male’s territory that he has established through fighting. A number of females can live within one male’s territory and all mate with the male who supplies the environment for them.

One theory that plays a role in sex determination in crocodilians is ESD or Environmental sex determination. This theory is relatively self-explanatory. It simply means that sex is determined through environmental cues such as temperature. This theory yields the hypothesis that an equilibrium sex ration is not 1:1, but rather favors the gender that is produced more frequently in poorer environments

Another theory that actually calculates which sex should be produced in these particular environments is the Trivers-Willard Hypotheses, which states that mothers produce females in poorer environments (particularly in species with male combativeness). In reptilians, temperature does play a role in gender identification, however there is no temperature pattern that coincides with gender determination across all species of reptilians.

A last theory that contributes to this overall picture is Shuster-Wade’s sexual dimorphism theory. In species where there is male combativeness there seems to be two outstanding male attributes. The first being that males have a selective pressure on size represented in their disparity in size when compared with females of the same species and secondly in their forced insemination patterns.
Freedberg and Wade are conveying a summary of these theories that pertains to species of Herps that display male combativeness. Their theory states that females are more common within these species because it is more fitness effective. Males have a larger selective pressure (competitive combat) in their adult stages to be successful breeders than females do. Therefore mothers will produce more females in poorer environments (which are more common in nature) and more males in richer environments to achieve their own reproductive success. These species achieve this gender determination by through environmental cues specific to their species. (ESD)