Success of male tits is set from birth
25 November 2009, by Sara Coelho
The breeding success of female great tits depends on where they raise their young - good habitat means healthy chicks. But for male tits, the game is rigged from the start. No matter where they live later on in life, their chances are forever tied to what happens in the first autumn of their life.
Some animals are more successful than their fellows and manage to live longer, raise more young and pass on their genes to more descendents. Professor Ben Sheldon and Dr Teddy Wilkin, evolutionary biologists at the Edward Grey Institute in the University of Oxford, were interested to find out what is behind these differences.
They analysed 47 years of data on the life history of generations of great tits of Wytham Woods, near Oxford, collecting information about clutch size, lifespan or number of grandchildren (called recruits). Then they looked for connections between breeding success parameters and the environments where the parents grew up or where they decided to breed.
They found that female great tits lay eggs earlier and raise healthier chicks if they breed in a good environment - an area of woodland with many oak trees, which support many insects, good nesting places and not many other great tits around to compete for resources.
'For females, it doesn't matter where they were born,' says Sheldon. 'Once they survive the first few months, a poor early environment has no carry on effect into later life.'
Great tits nesting in the Wytham Woods
But for males, raising a family in a breeding paradise doesn't make a lot of difference. Their lifelong breeding success depends mostly on their home environment. Males that grew up in plenty will have more and healthier offspring than fellow great tits raised under a tight belt.
Sheldon thinks that the difference in fate is controlled by the female great tits who make most of the breeding decisions - when and where to breed and with whom. 'Females have an in built flexibility and they can adjust their breeding effort every year, according to circumstances.'
Males, on the other hand, have little control over breeding decisions. What makes them attractive to females, a good territory and, probably, bright plumage, is decided early in life. 'For males events up to the first autumn are important determinants, after that they are relatively fixed,' says Sheldon, who published his findings last week in Current Biology.
If males are to be successful, they need a good start and the right amount of nutrients from birth to allow them to grow strong and attractive. That first autumn is their best chance to become competitive enough to acquire the best territories, attract the best females and raise more young.
This trend for different controls of local environment in male and female life histories does not happen only in great tits. 'There is some evidence that the red deer on the Isle of Rum follow the same trend,' says Sheldon.
'It's interesting to see the same thing happening in two such different types of animal,' he adds. 'Perhaps this is more common than previously thought.'
Teddy A. Wilkin, Ben C. Sheldon. Sex Differences in the Persistence of Natal Environmental Effects on Life Histories. Current Biology, Available online 19 November 2009. doi:10.1016/j.cub.2009.09.065
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