So a paper with the title, “Should you eat your offspring before someone else does?” by Chin-Baarstad and colleagues is nigh irresistible.
From a biological point of view, there are several reasons why a parent might eat its own eggs or young, and this new paper does an good job of summarizing the various theories behind cannibalism. In short, it takes time and energy to look after offspring, and sometimes, it may be better for an animal to cut its losses and try to breed again rather than continuing to pour “money down the drain,” so to speak. Eating offspring that are liable to fail provides a way of partly recouping the energy investment.
The authors studied sand gobies, where males care for eggs. They wanted to test if the mere threat of having a clutch preyed upon by another species would increase the likelihood of a male eating its offspring. They lay out the experiment concisely:
We exposed parental males to three levels of simulated threat from a known sand goby egg predator (the brown shrimp Crangon crangon): (1) no egg predator, (2) only visual cues from an egg predator and (3) visual and chemical cues from an egg predator. We hypothesized that filial cannibalism would increase when the shrimp was present, particularly when both chemical and visual cues from the shrimp were present, as these cues probably represent a greater threat of egg predation.
They placed two female gobies in with one male, and allowed them to spawn. the authors note that females usually depart after laying eggs in the wild, so the females were removed from the tank after spawning.
The shrimp predator could not actually eat the eggs. It was held either in a solid bottle or a bottle with holes, which allowed the goby to both see and smell the shrimp.
They find that there is always a risk of the goby eating the entire clutch (about 30% of the time), even if there are no shrimp predators or cues from them. A visual cue alone is no different from no predator (still about 30%), but the actual presence of the shrimp predator significantly raises the chance of whole clutch cannibalism (to about 50%). Partial clutch cannibalism wasn’t affected by the presence of the shrimp predator at all, which suggests that eating all of your young is a different proposition than eating just some of your young.
There are also various correlations between the size of the gobies and the likelihood of cannibalism. Small males are more likely to cannibalize the entire clutch. Males were more likely to eat some eggs when females that laid them were in good condition, which is again initially paradoxical, as you might expect those to be high quality eggs that you would want to survive. The authors do say that “egg survival is density dependent,” so this may be a factor: the males may be lowering the density to have a high proportion of successful eggs.
For me, a major unanswered question which is why does there seem to be a certain baseline probability of cannibalism, even with no predators? The fish are apparently not long lived, so a one in three chance of the male chucking it and eating an entire clutch seems rather high.
Before I go, one major rant against this paper. Nowhere in this paper is a species name given for the sand goby being studied. This is not cool. I will guess, based on a Google Search, that it is Pomatoschistus minutus (pictured, from this site), but Wolfram Alpha gives an estimate of 200 species in the Gobiidae family. How many of them might qualify as a “sand goby”?
Chin-Baarstad, A., Klug, H., & Lindström, K. (2009). Should you eat your offspring before someone else does? Effect of an egg predator on filial cannibalism in the sand goby Animal Behaviour DOI: 10.1016/j.anbehav.2009.04.022