Great Tits: Reproductive Advantages
Last month, Nature covered some work by evolutionary ecologists Michael Coslovsky and Heinz Richner, who reported on some great tits (Parus major). (What did you think I was talking about?) They were studying a natural population of these birds who live in a forest not far from Bern, Switzerland.
Previously, it was found that when barn swallows were exposed to predator models and calls – that is, cues to the presence of predators – the eggs of the presumably stressed out birds had more of a particular hormone, which in turn led to smaller fledglings. This raised the question of why this might be. It could be a byproduct, some type of side effect of the increased concentration of the hormone. Or, it could be by design. Perhaps being small as a fledgling is an advantage in a world in which there are a lot of predators, and the hormone is acting as an intermediary in an adaptive system.
As reported in the paper published in Functional Ecology, Coslovsky and Richner exposed some birds to cues of sparrow hawks (a predator) and some birds to cues of thrushes (not a predator). After hatching, the birds were foster raised by other birds and, just as in the previous work, the offspring of the stressed-out birds were smaller than the offspring of the non-stressed birds. Further, the rate of growth of the wings differed between the two groups as well. The offspring of the stressed birds showed faster growth.
Great Tit. Photo: Nick Ford
In the article reporting the work in Nature, Coslovsky suggests one possibility to explain why this might be the case: “While smaller bodies might seem a drawback, when combined with longer wings they probably increase flight performance by decreasing wing loading and improve overall survival.” In the article itself, the authors of the work make the (tentative) claim that “predation risk may elicit adaptive maternal effects in birds.” In other words, stress doesn’t somehow gum up the works in the next generation; it calibrates the next generation, favoring tradeoffs that are better in a world in which there are more, rather than fewer, predators about. I note in passing that this might remind some of you of the interesting work showing epigenetic effects in rats. Some folks at Utah put together a page – “Lick your rats” –which has very nice pictures.
I like this piece because it illustrates a broader point. Organisms come into the world, which can vary, and the appropriate tradeoffs – morphological or behavioral – depend on the parameters of the world they find themselves in.
Further, while environments vary, there is a certain degree of stability. Consider the case of the great tits. If the concentration of predators tomorrow were uncorrelated with the concentration of predators today (a possibility I don’t rule out), then the idea here would get no traction.
But of course environments do have a certain amount of stability, so the world that mom faces is going to bear certain similarities to the world the chicks face.
This type of effect is seen across a wide range of taxa. Agrawal (1999) investigated if radishes respond to particular predators. As he puts it: “Because plant attackers are not always predictable in time and space, and defenses are thought to impose a cost, it is believed that plants use damage as a cue to induce resistance against subsequent herbivores” (p. 1713). Plants can put resources into the defense against the particular predator that causes damage, allowing it to deploy its energy efficiently. Agrawal showed that plants attacked by a particular caterpillar larva produced more defensive hairs, which in turn defended against subsequent predation by the caterpillar.
Generally, we might expect organisms, especially long-lived organisms, to have mechanisms designed to deploy defenses in a way that efficiently guards against the threats in its present environment, and therefore likely to be present in the near future. Both the bird and the rat data suggest that stress can be thought of this way, as calibrating defenses. In a world in which predators are denser than average, it pays to be vigilant against them, even if this vigilance comes at some type of cost.
It does make one wonder if there could be some similar system in humans. I suppose – and I should emphasize I don’t really know much about this area – that if humans were exposed to extremely high levels of stress, such as frequent attacks, sustaining a heavy wound, seeing a number of wounded and dead friends and allies, and so on – one might expect systematic changes. After such traumas, you might predict that people would experience some sort of stress, not unlike the birds and rats, perhaps designed to recalibrate one’s psychological baselines. You might expect such people to be, for instance, extremely vigilant, perhaps being unable to sleep (during which time one is especially vulnerable). One might see lower thresholds of anger, ready to respond with violence at even mild provocation, befitting a world in which aggression and attack are common. I suppose it’s plausible that such individuals might even think a lot about the traumatic situation, rehearsing what one might do should it arise again. After all, if the world tomorrow is like the world yesterday, one is, under normal circumstances, likely to be faced with the same challenges over time.
Reading that list of symptoms, it does make me wonder if this constellation of responses to traumatic stress – changes in sleeping, irritability, reliving past trauma, etc. – might be viewed as a disorder of some type. In the modern world, in which the world one was in yesterday sometimes doesn’t resemble the one you’re in today – what with airplanes and so on to take you thousands of miles from war zones – someone who had recalibrated psychologically to these extremely stressful conditions would look pretty poorly calibrated in a normal, reasonably peaceful environment.
What we would be seeing, of course, is an adaptive system operating roughly as it was designed to, but doing so in a context that no longer matches that of ancestral environments.
Reference
Agrawal, A. A. (1999). Induced responses to herbivory in wild radish: Effects on several herbivores and plant fitness. Ecology, 80, 1713–172.
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