Featured paper: barn owl chicks’ spots

Melanic color-dependent antipredator behavior strategies in barn owl nestlings. By Valentin van den Brink, Vassilissa Dolivo, Xavier Falourd, Amélie N. Dreiss, and Alexandre Roulin. Behavioral Ecology, 2011.

I’ve been slacking off on the Featured Papers, since it’s the field season and I’ve been reading almost nothing less than usual; but this paper is too crazy not to mention.

Barn owls have black spots on their feathers, and those spots vary in size. Females usually have larger spots than males, and spot size is heritable, involving both sex-linked and autosomal (non-sex-linked) genes.

Barn owls nest in cavities, and take about 55 days to fledge. For the first two weeks the mother broods the nestlings and helps feed them; after that, the nestlings are capable of keeping themselves warm and of pulling morsels of meat from the carcasses their parents bring them, and the parents no longer sleep in the nest. This means that the nestlings are undefended against predators. The nestlings make matters worse by being noisy, calling to mediate conflicts over the food their parents have left, potentially attracting predators.

So the chicks have some antipredator strategies. They hiss frighteningly, they try to escape, or they pretend to be dead.

Here’s the crazy part: the researchers found that black spot size was associated with variation in these behaviors. Chicks with larger spots hissed more and feigned death more readily and for longer, while chicks with smaller spots tried to escape more. As a professional ornithologist and behavioral ecologist, my reaction to this is: what?!

How can spot size possibly be related to these behaviors? Well, it turns out that we’ve known for a few years now that melanin pigment (black and brown colors) can be associated with responses to stress. More melanin seems to mean a lower stress response. The large-spotted chicks were calmer, hissing and feigning death, while the small-spotted chicks “panicked” and tried to flee.

How the melanin-stress link is mediated is less clear. It may be that it’s a signal of a personality trait—”boldness”—allowing females to choose the optimal personality of their mate given the conditions of the environment they’re breeding in. It might be an accident of genetics: genes affecting stress response might be located near genes affecting melanin on a chromosome, which is known as gene linkage. It may be that more melanin and reduced stress response both improve survival in certain circumstances, so that they have been selected for in tandem. It might be that melanin is costly, so more fit individuals have more melanin and—for whatever reason—reduced stress response.

There’s another strange twist in the barn owl story: spot size and the antipredator strategies are all heritable, but not all of them were inherited from both parents. The tendency for chicks to play dead was inherited from their mother, while the tendency to flee was inherited from their father.

Biology is crazy.

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