Birds' Cooperative Breeding Sheds Light on Altruism
Friday, August 17, 2007
Why do we help out our relatives when one of them needs a buck or a meal, and who gains the most from such acts of generosity?
It's a tough question to answer in human populations, where self-awareness and cultural expectation cloud the biological forces that underlie behavior. But there are hints of answers in other species.
Two studies of birds published this week report surprising insights into the evolution of altruistic behavior.
One study suggests that sometimes the greatest beneficiaries are neither those giving or receiving alms but those whose main job is the care and feeding of the neediest members of the population.
The second study provides evidence that living in unpredictable conditions is a big motivator for mutual assistance, that a kind of "we're all in it together here in the Jamestown Colony" consciousness drives selfless behavior.
It is believed that about 10 percent of bird species show "cooperative breeding" behavior, in which one or more mated pairs produce chicks that are then fed not only by the parents but by other birds sharing the territory.
The helpers are usually non-breeding males from the female's broods of the previous year -- in other words, the brothers of the hatchlings they are helping to feed.
In an Australian bird species called the superb fairy-wren, some breeding pairs have helpers and some do not. The baby birds of pairs with helpers get 20 percent more food than young fairy-wrens fed only by their parents, but curiously that gives them no long-term survival advantage.
"This has been a big mystery. The nestlings get a huge amount more food, but it doesn't translate into any tangible benefit," said Rebecca M. Kilner, a zoologist at the University of Cambridge in England. She helped lead a study that appears today in the journal Science.
To find an answer, Kilner and her colleagues looked at what happened before the baby birds hatched. They compared the eggs laid by females that had helpers with those laid by solo-breeding females.
They found that when helpers are at hand, female fairy-wrens produce eggs with 12 percent less fat, 13 percent less protein, and less carbohydrate, than eggs produced by females that do not have helpers. The hatchlings of those "lite" eggs are smaller than normal chicks, but their initial scrawniness is quickly overcome by the extra food brought by the non-breeding helpers.
The one who benefits is the mother.
Cooperatively breeding females have a 1-in-5 chance of dying over the next year, compared with a 1-in-3 chance for females without helpers. This is presumably because they are slightly healthier and stronger, having expended less energy to produce their eggs and feed their young. Their longer life span, in turn, gives them a chance to leave more offspring behind, the ultimate measure of evolutionary success.
"The mothers are stealing child care from their current young and spending it on their future young," Kilner said.
Scott Forbes, a biologist at the University of Winnipeg and author of the book "A Natural History of Families," says the study reveals a surprising and devious strategy for turning the help of relatives to the parents' advantage.
"We had overlooked one of the key elements in reproduction, the cost of manufacturing eggs. In the last decade or so, we've begun to appreciate that this is much more expensive to mothers than we had previously thought," he said.
In the second study, published yesterday in the online edition of Current Biology, Dustin R. Rubenstein of the University of California at Berkeley analyzed 45 species of African starling. He looked at the relationship between breeding behavior -- about 40 percent of the species use helpers -- and the environment.
Cooperative breeders were far more likely to live in savanna habitats -- grasslands with islands of trees or bushes -- than in forests or deserts. In the savanna, rainfall is highly seasonal and the amount of rain varies substantially from year to year. Rain brings insects, which are the starlings' chief food.
Rubenstein theorizes that in lean times, cooperative breeding increases the chance that baby birds will get enough food to survive. Although the helpers do not have their own offspring, they share some genes with the hatchlings, so natural selection would favor this behavior. In flush times, cooperative breeding allows mates to produce multiple clutches of eggs and ultimately more offspring overall.
"Having helpers may actually be able to help you in both the good years and the bad," he said.
So if it's such a good idea, why don't all birds do it?
The answer is that many variables push species toward or away from cooperative breeding.
Where there is a shortage of territory for young pairs to move into, cooperative breeding is favored.
But if mortality is very high, it may be worth it for birds to try to breed as soon as possible -- if they take a "postgraduate" year as a helper, they may not live long enough to become parents themselves.