Mama's Little Helpers Ease Her Burden

tags: evolution, helping behavior, cooperative breeding, suberb fairy-wren, Malurus cyaneus, birds

Here's a question for all of you: whose offspring would do better; those raised only by their parents, or those raised by their parents in addition to an extended family group?

Cooperative breeding is a breeding strategy where some individuals postpone their own reproductive efforts in order to help others in the family group to raise their offspring. Typically, these helpers are genetically related to the offspring that they are helping to raise.

A research team led by Andrew F. Russell of the University of Sheffield in England recently asked that very question in their research of superb fairy-wrens, a species where some breeding pairs have helpers while others do not. They found that both groups of chicks did equally well, even though those that lived in cooperative situations were fed 19 percent more food than those fed only by their parents (figure 1).

"This has been a big mystery. The nestlings get a huge amount more food, but it doesn't translate into any tangible benefit," said co-author Rebecca M. Kilner, a zoologist at the University of Cambridge in England.

Mystified by this observation, the team then looked at the quality of the eggs produced by the mother. They found that when helpers are present, female fairy-wrens produce eggs that are 5.3 percent smaller than those of parents without help. The eggs are also poorer in quality; containing 12 percent less fat, 13 percent less protein, and less carbohydrate, than eggs produced by females that do not have helpers, "suggesting that mothers invest less energy in their eggs when breeding in the presence of helpers," the scientists wrote (figure 2).

The hatchlings from those smaller eggs are correspondingly smaller than chicks hatched from larger eggs, but their small size is quickly overcome by the extra food brought by the non-breeding helpers. So it isn't the hatchlings who benefit at all, but rather, it's their mother who benefits.

In superb fairy-wrens, 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 (figure 4). This longer life span, in turn, gives them and their "helping genes" the opportunity to leave more offspring behind, which is the ultimate measure of evolutionary success. These offspring also possess the genes for helping behavior, thereby ensuring it will spread throughout the population.

The most likely reason that cooperative breeding has evolved in these birds is because the females that are breeding in groups reduced their own investment in reproduction, thereby saving their resources for future breeding attempts. As a result, they live longer and produce more chicks, and thus increase the "helping genes" that are contributed to the next generation.

"The mothers are stealing child care from their current young and spending it on their future young," explained Kilner.

So, to answer the initial question: whose offspring would do better; those raised only by their parents, or those raised by their parents in addition to an extended family group? Neither group of chicks does better, but the mothers that were helped by their offspring lived longer and more productive lives.

Of course, I have a question regarding the chicks; did the chicks that were raised by helpers fledge earlier? Given the fact that chicks in the nest are vulnerable to predation, getting them out of the nest early is a huge advantage when it comes to increasing their odds for long-term survival.

This research was published in Science.

Sources

"Reduced Egg Investment Can Conceal Helper Effects in Cooperatively Breeding Birds" by A. F. Russell, N. E. Langmore, A. Cockburn, L. B. Astheimer, R. M. Kilner. Science. August 2007: 317 (5840) 941-944. DOI: 10.1126/science.1146037 [abstract or PDF] (data images)

MSNBC (quotes, bird image)