Response to predation risk in a cannibalistic herbivore
Feeding can be especially risky for herbivorous animals, which often must trade-off between their nutritional needs and predator avoidance. Prey individuals are commonly expected to decrease foraging activity when facing predation risk. However, multiple exceptions to this general pattern have been observed. Some herbivores can increase foraging and growth rate in the presence of a predator. Understanding the sources of variation in response to predation risk is an essential step to untangle the great complexity of prey-predator dynamics. Nutritional condition is a potentially important factor that may influence the way herbivores respond to predation risk. Colorado potato beetle (Leptinotarsa decemlineata) offers an example of extreme individual differences in nutritional habits and condition. When hatching, some larvae “supplement” their plant-diet by cannibalizing their egg siblings. In this study, we used a split-brood design to assess the magnitude of variation in response to predation risk of individuals from the same population. To investigate if egg cannibalism affects beetles response to predation risk, we performed a two-factorial laboratory experiment in which both cannibalism and predator presence were manipulated.
We found significant variation in the Colorado potato beetle response to predation risk. This variation was first revealed by a significant family x predation risk treatment interaction, with some families increasing and others decreasing foraging and growth when facing the threat of a predator. Interestingly, this data also suggested that families in better condition (e.g. highest growth rate) and under predation risk were more likely to decrease feeding and growth than families in lower condition. Second, we were able to link this variation with the occurrence of egg sibling cannibalism. Egg cannibalism significantly improved larval condition and influenced response to predation risk. When facing predation risk, cannibals decreased feeding while non-cannibals increased growth rate. These results demonstrated that in an herbivorous insect, egg cannibalism can be an important source of variation in response to predation risk. The fitness benefits of being a cannibal in high predation environments are discussed in terms of short and long term probability of being eaten.