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Conflict over parental care

My wife and I have been watching the Planet Earth series. Week after week, mother polar bears and mother snow leopards care for their young, while fathers are either absent or dangerous. But then we got to Emperor penguins. What's the difference? This week's paper Parental conflict in birds: comparative analyses of offspring development, ecology and mating opportunities tries to answer this question.

Every baby bird -- except some turkeys -- has two parents. How much care does each parent provide for their chicks, and why? If animal behavior were ordained by a god, as a guide to human behavior, then we might expect all wild species to exhibit the same exemplary behavior. Or maybe those species that have more opportunities to interact with and influence humans -- ducks, say -- would exhibit divinely inspired behavior, while those remote from human settlements -- Emperor penguins, for example -- are left to the whims of natural selection?

The authors of this week's paper didn't waste time testing such nature-as-morality-lesson hypotheses, which are left as an exercise for creationists. Instead, they explored how parental care behaviors have evolved in response to various factors. These factors include how chicks of different species depend on parental care, and also whether a bird that leaves its mate alone to care for their chicks has additional opportunities to reproduce.

They compared parental care in 193 species of birds, using "phylogenetic comparative analysis." This method attempts to subtract out correlations that are simply the result of sharing a recent common ancestor. For example, suppose that none of the bird species that can carry a coconut is migratory. Does that indicate some intrinsic trade-off between migration and coconut carrying? Not necessarily. Maybe all coconut-carrying bird species are descended from a recent ancestor that just happened to by nonmigratory. On the other hand, if both coconut carrying and migration are widely dispersed across the bird family tree, yet these two traits never occur together, then it's more reasonable to conclude that traits that give a bird the ability to carry a coconut may also reduce the ability (or perhaps the need) to migrate.

Using a large data set, they scored the contribution of each parent, from building the nest to feeding and defending chicks. They also scored the climate where each bird species lives and the degree of polygamy (from none to common) for each sex.

They found a negative relationship, overall, between care provided by mothers vs. fathers. To some extent, this seems like an inevitable consequence of the way care scores were calculated. If males did 75% of the nest building, for example, then females did 25%. It would be interesting to see the relation between male and female parental care where each was measured as hours spent or calories consumed, but it would be hard to get such detailed data for so many species. Still, the percent of care provided by males ranged from near 0 to well over 50%. They did note that "total care is more closely correlated with [more] male care." What explains these differences?

In species where checks are fairly self-reliant (precocial), fathers tended to provide less care to their existing young when there were more opportunities to have chicks with someone else. So did mothers, when they had similar opportunities. But neither was true when the chicks were relatively helpless (altricial).

Climate was not a significant predictor of male vs. female care. I wonder about the most extreme climates, like Antarctica, where it may take extreme efforts by both parents to keep an egg and chick alive.

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