Cannibalism is abhorrent to us when we contemplate eating others of our own species; the plight of the Donner party and the rugby-team survivors of the 1972 Andes flight disaster make chilling tales. Yet cannibalism is widespread in the animal world. There are obvious benefits, both short term and long term: cannibals gain calories and reduce competitors. The nutritional boost might decrease development time and increase survival, and the cannibals might reach maturity at a larger size, which could increase fecundity. However, there may be inclusive fitness costs as well if kin are the victims, and if the victim fights back there is the risk of injury. The latter cost may be greatly reduced by cannibalizing eggs, which are defenceless. Eggs may also be richer in nutrients and easier to digest than juveniles and, for herbivorous insects, eggs can supply a critical source of protein. On the other hand, hatchlings consuming eggs are likely to be consuming siblings.
The many experimental studies of the benefits of cannibalism have yielded mixed results. In some insect species, larvae fed other larvae develop faster and have lower mortality but in other species there is a negative effect of eating conspecifics. Fewer studies evaluate the costs of cannibalism. In this issue Karyn Collie, Sung Jin Kim and Mitchell Baker of Queens College of the City University of New York estimate the quantitative fitness benefit from faster development and simulate the necessary development time gain to balance the inclusive fitness costs of egg cannibalism by the Colorado potato beetle, Leptinotarsa decemlineata (Fig. 2). Normally, if the consumed individual has an equal probability of surviving to reproductive age as the cannibal, consuming a half sibling must increase the cannibal’s fitness by 25% and a full sibling must increase it by 50% for cannibalism to be favoured. But as the probability of hatching for the egg to be cannibalized goes down, the required fitness gains are reduced.
Figure 2. Larvae from a single clutch of the Colorado potato beetle that are consuming their own chorions. The larger larva in the top right corner also consumed a viable sibling egg and is now larger than its siblings, demonstrating the immediate size benefit of cannibalism. Photo: Karyn Collie.
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