Seasonality influences cuticle melanization and immune defense in a cricket: support for a temperature-dependent immune investment hypothesis in insects.

To improve thermoregulation in colder environments, insects are expected to darken their cuticles with melanin via the phenoloxidase cascade, a phenomenon predicted by the thermal melanin hypothesis. However, the phenoloxidase cascade also plays a significant role in insect immunity, leading to the additional hypothesis that the thermal environment indirectly shapes immune function via direct selection on cuticle color. Support for the latter hypothesis comes from the cricket Allonemobius socius, where cuticle darkness and immune-related phenoloxidase activity increase with latitude. However, thermal environments vary seasonally as well as geographically, suggesting that seasonal plasticity in immunity may also exist. Although seasonal fluctuations in vertebrate immune function are common (because of flux in breeding or resource abundance), seasonality in invertebrate immunity has not been widely explored. We addressed this possibility by rearing crickets in simulated summer and fall environments and assayed their cuticle color and immune function. Prior to estimating immunity, crickets were placed in a common environment to minimize metabolic rate differences. Individuals reared under fall-like conditions exhibited darker cuticles, greater phenoloxidase activity and greater resistance to the bacteria Serratia marcescens. These data support the hypothesis that changes in the thermal environment modify cuticle color, which indirectly shapes immune investment through pleiotropy. This hypothesis may represent a widespread mechanism governing immunity in numerous systems, considering that most insects operate in seasonally and geographically variable thermal environments.

The influence of male age and simulated pathogenic infection on producing a dishonest sexual signal.

In recent years, studies have shown that reproductive effort decelerates in response to pathogenic infection. If infection substantially reduces a host's residual reproductive value (RRV), however, then an acceleration of effort may instead occur (e.g. terminal investment). Reproductive acceleration would theoretically allow hosts to maintain or exaggerate their sexual signal upon infection. This would create a deceptive message from the perspective of the chooser, who may unwittingly copulate with an infected mate to their detriment. Using the cricket Allonemobius socius, we assessed the potential for reduced RRV to accelerate male reproductive effort and create a dishonest signal. RRV was manipulated through male age and simulated pathogenic insult. Reproductive effort was measured as calling song energetics, mating success, latency to mate and nuptial gift size. We show that males adopted either an accelerated or decelerated reproductive strategy upon infection, and that this decision was probably mediated by RRV. Moreover, males who accelerated their effort produced a dishonest signal by increasing their song energetics while providing fewer paternal resources (i.e. smaller gifts). Our study is one of the few to document the existence of dishonest signals and relate dishonesty to a potential reduction in female fitness, underscoring the conflict inherent in sexual reproduction.