The food of survival

Monday 8 March 2004

Biologists have discovered that female birds can bias the sex of their chicks. Whether a bird is more likely to lay a male or female egg depends on which sex will have the greatest chance of doing well.

The researchers, based at the University of St Andrews, have made the finding through adjusting the food intake of zebra finches. They found that well-fed females were more likely to produce daughters, while less well nourished birds were more likely to have sons. This is exactly as predicted by the fact that female offspring need to be better nourished than males if they are to survive and grow well.

The research, conducted by Dr Alison Rutstein, Dr Jeff Graves and Professor Peter Slater of the University’s School of Biology, clearly demonstrates that the mother-to-be can bias the sex of her offspring so that she produces more of those with the best chances.

They said: “In most animals sex ratio is close to 50:50 and extremely resistant to change. In mammals, including humans, the sex of the baby is determined by whether the sex chromosome in the sperm is male or female. But in birds it is the female’s egg rather than the male’s sperm that determines what sex the chick will be. Thus the female has the potential to determine the sex of her young by whether she ovulates male or female eggs.

“In some ways female zebra finches seem to be able to exert control over whether to produce a male or female egg depending on which of the two is most likely to be successful. Our research tells us that they do it, and we understand why. The big question is: how do they do it?”

In many animals females need to be well-nourished and in good condition if they are to breed, as eggs are costly to produce. Bigger eggs tend to lead to bigger young that are more likely to survive.

The idea that females in good condition should produce a greater number of sons was put forward over 30 years ago, based on a study of mammalian breeding systems, in which the male is often the one that needs to be bigger and stronger if he is to fight off the opposition and breed successfully. Such ‘sex ratio adjustment’ is well documented in certain insects, such as bees and wasps, but is less well understood in birds and mammals.

Birds are an excellent model to use in the study of sex ratio adjustment because, using molecular techniques, scientists can establish the sex of each egg soon after it has been laid. Furthermore, all the resources given to the developing embryo are present in the egg at laying. Thus the size and the content of the egg are measures of the amount of resources that the female has allocated to that egg, which affects its subsequent survival chances.

The team explained: “We manipulated the diet quality of zebra finches to look at the effects of body condition on female investment. We found that females were able to exert a strong degree of control over the production of male and female eggs. When females were fed on a low quality diet, they laid eggs that were considerably lighter than those laid when they were fed on a high quality diet, and they also laid far more male eggs on a low quality diet.

“This is the converse situation to that described 30 years ago for mammals, but it makes sense for zebra finches. Previous research has shown that under poor nutritional conditions, female zebra finches grow more slowly and survive less well compared to males. Therefore, females are producing more of the sex with the highest survival chances under those conditions.”

The St Andrews study adds to a growing body of evidence that suggests that female birds can adjust the number of sons and daughters that they produce in relation to environmental conditions.




PROFESSOR PETER SLATER – 01334 463500 (TODAY AND MONDAY) pjbs@st- DR JEFF GRAVES – 01334 463518 (MONDAY ONLY) [email protected]

The research paper ‘Diet quality and resource allocation in the zebra finch’ by Dr A N Rutstein, Professor PJB Slater and Dr JA Graves is published by the Royal Society’s Biology Letters.



Issued by Beattie Media On behalf of the University of St Andrews Contact Gayle Cook on 01334 467227, mobile 07900 050 103, or email [email protected] Ref: birds and daughters pr 080304 View the latest University news at

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