The Fats Of Life On Balancing The Sexes
Mice given more fat tend to bear males

By Robert Cooke STAFF WRITER

April 29, 2003

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Strange results from experiments in mice show it may be possible to change the balance of the sexes - males vs. females - by interfering with mama's diet, scientists report.

In work done at the University of Missouri, animal science experts found that female mice given a high- fat diet bore many more male pups, and far fewer females, than normal. The opposite was seen from mothers fed a low-fat, high-carbohydrate diet: They got more girls than boys.

"The fact that we got such a dramatic difference was a surprise," said animal physiologist Michael Roberts, "particularly as the litter numbers were about the same, and the weights of the offspring weren't any different."

The researchers are assessing evidence that some animals, such as opossums, change the balance of male to female births in response to changes in nutrition. "There have been suspected shifts in ratios in mammals, but these [studies] were not well controlled and were done in the wild," Roberts said.

To put the work on firmer ground, Roberts and five colleagues at the university ran their experiments with captive animals living under rigidly controlled conditions. Two groups of female mice were treated exactly the same, except for a huge difference in the amount of fat they consumed.

During the experiment, each of the dozens of female mice produced four litters, for a total of 1,048 offspring. In all, the mothers eating a fat-laden diet bore 67 percent males, while mothers on the ultra-lean diet had only 39 percent males. That was a "striking" shift from the normal 50-50 sex ratio, the researchers said.

The difference in dietary fat was extreme. One group got 60 percent of their calories from fat - actually, lard - while the other group got only 10 percent from fat. Their diets were otherwise identical in terms of proteins, vitamins and minerals. All animals were allowed to eat as much as they wanted.

The experiment was done "because of the suspicion that it might be possible to shift sex ratios," Roberts said.

He cautioned that the results from mice might not be applicable to humans. Still, there is sketchy evidence that harsh conditions - such as famine during wartime - can influence human sex ratios.

"After the war in Europe there were some changes in sex ratio, but maybe because female fetuses seem less susceptible to these sorts of stresses than males are," Roberts said. For example, "there were more girls [born] in Holland after World War II. This was probably because females have a survival advantage in the womb" when mothers are under stress.

In contrast, the differences seen in the mice were not the result of stress.

A full report on the research appeared recently in the Proceedings of the National Academy of Sciences. The experimenters were from the Animal Sciences Center at the Columbia campus.

"It's a far more significant result than from other studies," said zoologist Steven Austad at the University of Idaho. "Most of the time you get, say, 55 percent males or females," considerably different from what Roberts' team found.

Physiologist George Seidel agreed: "It's an exciting result because it is a clear demonstration of this phenomenon" - the skewing of sex ratios because of environmental conditions.

According to Seidel, an animal reproductive physiologist at Colorado State University, the changing of sex ratios in response to environmental conditions is a known phenomenon, but it's still poorly understood.

"In nature there are a number of situations where sex ratios are skewed so that when times are good there are more males, and when times are bad there are more females." Theoretically this occurs, he explained, because "in nature most males do not succeed in siring offspring, while all females that are reasonably healthy do reproduce."

So "when a female is pregnant, if the objective is for her to promote her own genes, it's silly to waste them on a male that isn't going to be big, healthy and competitive," capable of spreading her genes farther.

Instead, Seidel said, "in order to get her own genes into the population when times are bad, she's much better off having females, because they are more likely to reproduce." Yet, "when times are really good, she may gamble on having a big, fat, competitive male who is going to sire multiple offspring by making many females pregnant."

He has worked with Roberts studying cattle reproduction, and there is some evidence that male fetuses are more likely than females to be lost in bad times. In fact, Seidel said, Roberts recently discovered that female fetuses seem to send a stronger chemical signal to their mother's hormone system, a signal that may help them avoid being aborted.