Another Perspective on the Slow-Growth/High-Mortality Hypothesis: Chilling Effects on Swallowtail Larvae

The slower-growth/higher-mortality hypothesis proposes that reduced herbivore growth rates benefit plants because slower growing herbivores remain vulnerable to predator attack for an extended time period, resulting in a lower reduction in plant fitness attributed to herbivory. We propose cooling events as an alternative mechanism leading to higher mortality for slower growing larvae. We observed that, in the absence of predators, slower growing pipevine swallowtail (Battus philenor) larvae had higher mortality compared to faster growing larvae during the unseasonably cool spring temperatures of 1998. However, this was not true for the warmer spring of 1999. Laboratory experiments showed that the probability of surviving a chill coma increased with larval mass. We propose that smaller larvae are susceptible to chilling events because they have less energy reserved for metabolism during a chill coma, and that “warm” chill events near the activity threshold may be more lethal than “cold” chill events. Weather station data since 1951 and field data collected from 1976 to 2000 suggest that exposure to chill events below the chill coma threshold is not uncommon for these larvae. We propose that faster growth may be important for larvae because slower growing larvae remain at a chill-susceptible size for an extended period of time. Corresponding Author: S. J. Simpson.