Stinging spines protect slug caterpillars (Limacodidae) from multiple generalist predators

Predators have played a significant role in the evolution of herbivorous insects, and we can observe a wide variety of larval defense mechanisms in nature, especially among members of the Lepidoptera. Slug caterpillars (Limacodidae) are known for their unusual morphologies, including various types of protuberances and stinging spines on their dorsal surfaces, which suggest that their evolution has been strongly shaped by their interactions with predators. We tested the hypothesis that limacodid larvae with stinging spines would suffer less predation from generalist predators than larvae that either did not possess stinging spines or were more lightly spined. In a series of behavioral bioassays, we tested the preferences of 2 different invertebrate predators (assassin bugs and paper wasps) for "spined" or "unspined" larvae. We found that all of the predators preferred the unspined or lightly spined prey species over the heavily spined limacodid species Acharia (=Sibine) stimulea. Our results also indicate that at least one of the predators that we tested, the paper wasps, showed a form of aversion learning as indicated by a decreased number of inspections of A. stimulea after previous experience. We conclude that limacodid larvae that are heavily armored with stinging spines are well defended against attacks from invertebrate predators and are significantly more likely to survive predator encounters than are unspined or lightly spined larvae. Copyright 2009, Oxford University Press.

Optimal individual positions within animal groups

Animal groups are highly variable in their spatial structure, and individual fitness is strongly associated with the spatial position of an animal within a group. Predation risk and food gains are often higher at the group peripheries; thus, animals must trade-off predation costs and foraging benefits when choosing a position. Assuming this is the case, we first use simulation models to demonstrate how predation risk and food gains differ for different positions within a group. Second, we use the patterns from the simulation to develop a novel model of the trade-off between the costs and the benefits of occupying different positions and predict the optimal location for an animal in a group. A variety of testable patterns emerge. As expected, increasing levels of satiation and vulnerability to predators and increasing predation risk result in increased preferences for central positions, likely to lead to increased competition or more tightly packed groups. As food availability increases, individuals should first prefer center positions, then edge, and returning to central positions under highest food levels. Increasing group size and/or density lead to more uniform preferences across individuals. Finally, we predict some situations where individuals differing in satiation and vulnerability prefer a range of different locations and other situations where there is an abrupt dichotomy between central and edge positions, dependent on the levels of monopolization of food by peripheral individuals. We discuss the implications of our findings for the structure of groups and the levels of competition within them and make suggestions for empirical tests. Copyright 2008, Oxford University Press.

Sex and the selfish herd: sexual segregation within nonmating whirligig groups

The fitness costs and benefits at different positions in fish shoals, bird flocks, and insect swarms can be asymmetric; a group's edge may provide more feeding opportunities, but also greater predator risk. Animals make trade-offs between these selection pressures based on individual differences in traits including satiation level, ability to avoid predators, and sex. Previous studies did not evaluate the impact of sex on group positioning in these types of nonhierarchical, nonmating groups called congregations. A controlled laboratory experiment was conducted, using marked whirligig beetles (Coleoptera: Gyrinidae), to test for sexual segregation and why different sexes might choose different positions. Soon after a disturbance, males often were found at the periphery and females at the center of groups. There was also an overlying influence of feeding on position; satiated individuals moved toward the center and hungry individuals toward the periphery. Several minutes after a disturbance, sexual segregation disappeared, but segregation due to hunger persisted. Sexual segregation in this study was best explained by the predator avoidance hypothesis, not the energy needs hypothesis. Females weighed less than males; this may make them more at risk to predation because of reduced swimming speed or less mechanical protection from their exoskeleton. No difference between the sexes was found in the volume of their defensive chemicals. This is one of the first studies to show that sex influences position of individuals within simple nonmating groups (congregations) and suggests that more attention should be given to positional sex differences within shoals, flocks, herds, and swarms. Copyright 2007, Oxford University Press.