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Review backs theory on sex selection in mammals

August 28, 2009

Having more male offspring leads to greater evolutionary success for mammal parents, if living conditions support larger populations.

Can we manage factors that may influence whether mammals such as horses produce a male or female offspring?

The issue is one of the most debated hypotheses in evolutionary biology.

A study by a New Zealand-educated scientist at the University of Nevada has helped to disprove critics of the theory, which was developed in 1973.

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.

Dr Elissa Cameron, a mammal ecologist in the Department of Natural Resources and Environmental Science at the university, has produced evidence supporting the theory.

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 says.

"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 per cent 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 says.

But in a journal article recently published in the "Proceedings of The Royal Society of London", Cameron asserts there is good reason to support the hypothesis.

It could all boil down to the amount of glucose, or blood sugar, in a female mammal's body around the time of conception, Cameron says.

She conducted an analysis of 1000 studies that examined the Trivers-Willard hypothesis and sex ratios in mammals.

Her study found that female mammals that were in better body condition during the early stages of conception were more likely have male offspring.

Body fat and diet can affect levels of glucose circulating in a mammal's body, and Cameron suggests that the levels of glucose around the time of conception could be influencing the sex of the animal's offspring.

"A high-fat diet can result in higher levels of glucose, thereby supporting the hypothesis that glucose may be contributing to the sex of the mammal's offspring," she says.

This finding is key to the Trivers-Willard debate, and if supported in future studies, Cameron's theory could have dramatic influence on wildlife control and animal production.

"If you can get dairy cows to have more female calves, it would have huge implications for the dairy industry," she says.

Cameron says she would like to test her hypothesis by reducing the amount of glucose in selected populations of wild mammals. An injected steroid can block glucose circulation, and potentially increase the number of female offspring, while in the long term the same result may be accomplished through diet.

Such a study would be able to show whether mammal populations in different regions could be increased or decreased, based on the needs of those areas, she says.

In eastern Nevada, for example, declining populations of mule deer could be better researched by taking into account Cameron's contribution to the decades-old scientific controversy.

Though her analysis has rekindled debate of the Trivers-Willard hypothesis, and its implications for mammal ecology, Cameron said that she hopes her broader accomplishment would be to generate further inquiry.

"Right or wrong, the journal article will spark research to confirm or deny my theory about the role of glucose in determining gender in reproduction," she says. "The idea is that it will hopefully encourage more research."

Cameron, who obtained her bachelor of science, masters degree, and a doctorate in ecology from New Zealand universities, has also conducted research in the field in New Zealand, centering on the wild Kaimanawa Horse population of the central North Island.

In a six-year long study funded by the Department of Conservation, Cameron and Wayne Linklater found that it was the change in condition of the mares around the time of conception, as opposed to her actual condition, that better predicted the sex of foals.

The pair, students in the Ecology Group of the Institute of Natural Resources at Massey University at the time, conducted the research between August 1994 and 1999.

Cameron had set out to test the expectation that more males should be born to better-conditioned mares. The study was the topic for her PhD thesis.



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