Angry (Hungry) Birds
Note: This blog post is a joint Fatima Aboul-Seoud/Rob Kurzban production.
For some reason, I recently was interested in the relationship between food and aggression, and so I had a look at an old paper published in 2000 entitled “Effects of short-term hunger and competitive asymmetry on facultative aggression in nestling black guillemots Cepphus grylle.” In the research reported in this paper, the authors, Cook et al. were interested in something called the “food amount hypothesis.” Suppose that two siblings are being provisioned by parents. When times are good, inclusive fitness implies that the chicks will be, more or less, cooperative. When times are tough, however, the logic of parent/offspring conflict kicks in because the marginal benefit of food to me is now greater, so relationships between even full siblings might turn violent. As they put it, the prediction is that “dominant siblings become more aggressive during periods of food shortage, thereby obtaining a disproportionate share of total available parental resources.”
Black guillemots (pronounced: berdz) are a convenient model species because eggs are laid in pairs over a period of a few days. The authors set out to determine if the amount of food delivered was a proximate cue for sibling aggression. To do this, they recorded videos of 20 broods (5 in the control group and 15 in the experimental group). In both groups, they began observation before the eggs were laid and recorded when each egg was laid and hatched. The first chick to hatch was marked and called the A-chick, and the second chick to hatch was called the B-chick.
The experiment looked at what would happen if parents were prevented from feeding their chicks. The experimental manipulation, which is sort of awesome, is described this way: “[P]arental provisioning was prevented… by placing an adult scaring device—either balloons with painted eyes or a fiberglass great black-backed gull—near the entrance of the nest and in view of the returning adult.” So, for six hours out of a twelve hour observing cycle, parents were scared away from feeding their chicks.
In the control group, chicks were generally non-aggressive. (See Figure 1, from the original manuscript.) In the experimental group, during period 1 (before the scaring device was introduced), rates of attacks between experimental groups and controls didn’t differ. Aggression rates were higher during period 2 (when the scaring device was present) and highest during period 3 (just after the device was removed). The researchers also measured provisioning and, indicating the utility of aggression, after the deprivation phase of the study, A-chicks got a relatively larger fraction of feeds from parents relative to controls. The authors conclude (p. 285)”
A-chick aggression in black guillemot broods was elevated only after parental provisioning rates were experimentally reduced. When parental provisioning resumed, adults did not adjust their feeding rate in response to changes in their offsprings’ requirements, and the consequence of the intersibling aggression was a skew in the distribution of food received by siblings in favor of the dominant A-chick.
So, in summary, in these organisms, it appears that having less food causes increased aggression, even in the context of what would otherwise be a close, even cooperative, relationship. These results illustrate that aggression is deployed conditionally, in a principled way, depending on context.
Note that these results do not illustrate, at the risk of repeating myself, that chicks require parental provisioning in order to have the fuel necessary to muster the willpower not to harm their siblings. That would be an obviously silly way to understand these results. Right?
Anyway, part of the reason I looked at this paper was that I recently saw a talk by Johannes Haushofer, who ran studies investigating the effects of direct transfers of money to very poor households in Africa. Very generally, poor households endowed with more resources (compared to controls not so endowed) had (a little) better food security – because additional funds were used to buy food – and, in addition, domestic violence against women went down, as reported in a summary of the work. These results suggest (but of course can’t causally establish) that having less food leads to more aggression in the context what would otherwise be a close, even cooperative, relationship.
Oh, and, speaking of humans, and in completely unrelated other news, the Economist and other news outlets are reporting on a new paper in Proceedings of the National Academy of Sciences that shows a relationship between having less food and increased aggression in the context of what would otherwise be a close, cooperative relationship, the theme illustrated so well by the international “You’re Not You When You’re Hungry” Snickers ads, such as this one.
Cook, M. I., Monaghan, P., & Burns, M. D. (2000). Effects of short-term hunger and competitive asymmetry on facultative aggression in nestling black guillemots Cepphus grylle. Behavioral Ecology, 11(3), 282-287.