Burying Beetle Begging Behavior
I generally avoid posting on research in the non-human literature because I’ve received feedback that – more or less – readers are more interested in People Stuff than Animal (or plant) Stuff. (The frequency of comments on posts provides converging evidence.) Not only that, but the paper I’m writing about today hit the mainstream press, and I usually avoid such papers because I prefer to try to find the cooler parts of the pillow. But I’m breaking with tradition here because 1) the paper is about burying beetles, with whom I feel a special affinity, 2) the research is neat, and 3) I’m giving myself latitude to do as I please right now because I’m still sporting this Verra-be-damned sling on my arm.
The research I discuss below focuses on the burying beetle (Nicrophorus vespilloides), which has a really interesting life history. The females of this species lay eggs inside or near the body of a dead animal, such as a mouse. A few days later, the eggs hatch, and the larvae feed on the carcass. That’s already nifty, but wait, there’s more. After the larvae hatch, mom eats some of them, and regurgitates their bodies to feed the cannibalized larvae’s siblings.
My interest in these critters derives from this cannibalistic behavior, which I will discuss even though it’s really tangential to the point. The reason mom eats her babies can be reasonably easily explained with the usual theoretical tools. If mom allowed all the larvae to survive, she could be left with a large number of small ones instead of a slightly smaller number of larger ones, with better chances out in the world. And, of course, no sense letting good calories go to waste. So, killing some to feed others is fitness-enhancing, and explains the behavior. I and my coauthors used this species as an example in a paper about moral dilemmas. Generally, when organisms are faced with a decision to kill one offspring (or sibling) to save five offspring (or siblings), inclusive fitness theory suggests that they should choose to do so. And that seems to be just how burying beetle brains work, reflecting this calculus, killing some to save more. What’s interesting (to some of us) is that humans, at least in self-report data, radically violate the predictions of the theory, with most people saying they would not push one brother off the famous footbridge to save five brothers, and even more say that it is morally wrong to do so. This illustrates that morality, whatever it is, is a sufficiently strong psychological force that it contravenes kin selected systems under these (granted, hypothetical) circumstances.
All of which is, again, more or less beside the point. Andrews and Smiseth, in their recent paper in Behavioral Ecology, were interested in larvae begging behavior. Larvae beg for food by raising their little heads to their parents and waving their legs indicating their desire to be fed. In the first study, the authors were interested in looking at who is really in control of the transfer of food from mom to larvae. It could be the larvae themselves – big larvae jostle little larvae away from mom – or it could be mom, choosing to regurgitate into the mouths of larger larvae at the expense of the little ones. This difference looms large in the context of why larvae beg: Is begging a way to compete with other larvae – scramble competition – or is begging a signal to parents that they can then use to decide who will be fed?
To get at this, the authors took “junior” (24 hours old) and “senior” (48 hours old) larvae and put them with either a live or dead mother beetle. (The authors note that this independent variable is imperfect: “We obviously acknowledge that dead parents will differ in other respects from live parents, especially with respect to whether they interact with their larvae.”) They then measured how much senior and junior larvae begged for food and how “successful” they were. Here, “success” means making mouth-to-mouth contact with mom. The results of this experiment were that senior larvae did much better than junior larvae when mom was alive, but there was no difference when she was dead. This implies that it’s mom’s behavior, rather than inter-larvae wrestling, than leads to begging success. Mom, not the larvae, seem to be controlling who gets fed. Begging, then, seems to act as some sort of signal.
So, if begging is a signal, what sort of signal is it? Is begging costly? As readers of this blog are likely to already know, the issue of cost looms very large in the context of signals, making this a key question. To measure if begging is costly, they looked at the larvae that mom ate. Did they tend to be the ones who were begging, or not? Indeed, begging seems to draw mom’s fatal ire. As the authors put it: “Thus, the risk that a begging larva fell victim to cannibalism was more than 13 times greater than expected if parents targeted larvae irrespective of their behavior.” Begging for food is hazardous to one’s health. The authors conclude as follows:
These findings suggest that offspring begging increases the parents’ influence over food allocation and that begging is costly by increasing the offspring’s risk of being a target of filial cannibalism. Our results support the assumptions of honest signaling models for the resolution of parent–offspring conflict.
Finally, I want to note that Andrews and Smiseth spend a few lines of their Discussion linking the beetle data to both birds and humans, writing that “evidence from humans argues against a parent-induced cost of begging (crying), whereas evidence from studies on birds is inconclusive…”
Andrews, C. P., & Smiseth, P. T. (2013). Differentiating among alternative models for the resolution of parent–offspring conflict. Behavioral Ecology.