At that time, ecologist Bob Trivers and mathematician Dan Willard said that large healthy mammals produce more male offspring when living in good conditions, such as areas where there is an ample food supply. Conversely, female mammals living in less desirable conditions would tend to have female offspring.
According to Cameron, the hypothesis demonstrated the idea that having more male offspring leads to greater evolutionary success for mammal parents, if living conditions support larger populations. Should conditions be less desirable, having female offspring would be a better investment for mammal parents.
"Male zebras can father more than a hundred offspring in a lifetime, whereas female zebras are constrained to minimal reproductive rates--about one a year," Cameron said. "Sons, therefore, offer higher breeding rates to zebra parents, while female offspring are a lower-risk investment.
"Therefore, if the animal's living conditions aren't suitable, giving birth to a female would better ensure the animal's genetic success in the long term. Daughters of less healthy mothers would out-reproduce sons in poorer conditions because males that are unsuccessful in having mates have few offspring while most females breed throughout their lifetime."
The problem with this idea is that, until now, only about 30-percent of studies have supported the hypothesis, which has been debated because there have been so many studies that have attempted to test the idea. The results have been contradictory--some find support for the hypothesis, some do not, Cameron said.
But in a journal article published toda
Contact: Bob Conrad
University of Nevada College of Agriculture, Biotechnology, and Natural Resources