Almost a no-brainer
How sophisticated behavior would you expect from an animal with a brain as small as a wasp's? Few, if any, female wasps have read David Lack's classic paper on the optimum number of eggs to lay, or even John Dennehy's clear summary of it. This week's paper asks whether they, nonetheless, adjust egg numbers optimally in response to competition from other wasps and resource availability.
"Encountering competitors reduces clutch size and increases offspring size in a parasitoid with female–female fighting" was written by Marlene Goubault, Alexandra Mack, and Ian Hardy, of the University of Nottingham, and published in Proceedings of the Royal Society.
I discussed trade-offs between the size and number of eggs (or seeds, for plants) in one of my first posts. A plant or animal with a given amount of carbon and nitrogen can make a few large eggs or seeds or many small ones. But how does competition change things?
Parasitoid wasps lay eggs in butterfly or moth larvae. The resulting wasp larvae devour the caterpillar and may also fight among themselves, as Carl Zimmer explains in this week's edition of The Loom and the New York Times.
Female wasps may also fight other females for the right to lay eggs in a given caterpillar. The winner kills any existing eggs and replaces them with her own. Bigger females tend to win these battles.
So theorists have previously suggested that, when competition with other females is likely -- in the next generation -- wasps should lay larger eggs, which will develop into larger adults, even though there will be fewer of them. It's better to have a few daughters whose eggs survive than many daughters with no surviving eggs. As the authors put it, "anticipation of the competitive environment of offspring should affect maternal clutch size decisions."
But how can an insect with such a small brain anticipate anything? An evolutionary response to the average level of competition would not require any information-processing ability at all. Egg number could be under genetic control. Those with genes for the optimum egg number in a given environment would out-compete those with lower or higher egg numbers.
But the environment, specifically the availability of caterpillars, is not constant. Can individual wasps adjust egg numbers appropriately? They can't know how much competition their daughters will face, but can they at least adjust egg number based on how much competition they face?
To find out, the authors used wasps guarding a caterpillar which they had paralyzed but into which they had not yet laid their eggs. They made each wasp fight from zero to four other wasps. Then they counted how many eggs were laid and how big the eventual daughters were.
With big caterpillars, competition had no effect on number of eggs laid. This is consistent with wasps being ignorant of trade-offs. However, there was also no effect of egg number on the final weight of daughters, in this case, presumably because they had enough to eat that initial egg size didn't matter. If there's plenty of food, maybe laying as many eggs as possible is the smart thing to do, even if the eggs will initially be smaller than if there were fewer of them.
When they used smaller caterpillars, the number of eggs laid did depend on competition, but also on the size of the wasp and the caterpillar. As expected, daughters emerging from small caterpillars were smaller when there were more of them. There were fewer of them, but they were a little bigger (1.10 versus 0.97 milligrams) if their mother had faced competition, as predicted. (Is this enough size difference to affect competition?)
These results are consistent with the hypothesis that wasps lay fewer and therefore larger eggs when two conditions are met: 1) they faced competition, so their daughters might also, and 2) the available caterpillar is small enough that small eggs may result in small, less competitive, daughters.
Do other animals produce fewer but larger offspring when competition is more severe? Does this pattern disappear when resources are abundant enough that the size of an egg or baby has little relation to adult size? Does brain size have any effect on the ability to make such decisions?