Leptin Manipulation Reduces Appetite and Causes a Switch in Mating Preference in the Plains Spadefoot Toad (Spea bombifrons).

Condition- or context-dependent mate choice occurs when females modify their mate preferences depending on their internal or external environment. While the ecological and evolutionary factors that favor the evolution of such plasticity are emerging, relatively little is known of the mechanisms underlying such choice. Here we evaluated whether leptin, a protein hormone involved in the regulation of appetite, might affect the expression of condition-dependent mate choice decisions. To do so, we administered leptin to spadefoot toads, Spea bombifrons, which exhibit condition-dependent mate choice for males of their own species versus congeneric males of S. multiplicata. In particular, poor-condition S. bombifrons are more likely than are good-condition S. bombifrons to prefer S. multiplicata males, but only in environments where hybridization between the two species is beneficial. We found that our leptin treatment reduced appetite in S. bombifrons adults, as was expected from leptin's known effects on appetite. However, although we predicted that leptin would reduce female preferences for heterospecific males, we found the opposite. In particular, our leptin treatment generated a consistent, repeatable preference for heterospecifics in an environment where females generally prefer conspecifics regardless of condition. These results indicate that leptin has the potential to affect female mate choice, but that it might do so in non-intuitive ways.

Exogenous insulin enhances humoural immune responses in short-day, but not long-day, Siberian hamsters (Phodopus sungorus).

Many animals experience marked seasonal fluctuations in environmental conditions. In response, animals display adaptive alterations in physiology and behaviour, including seasonal changes in immune function. During winter, animals must reallocate finite energy stores from relatively costly, less exigent systems (e.g. reproduction and immunity) to systems critical for immediate survival (e.g. thermoregulation). Seasonal changes in immunity are probably mediated by neuroendocrine factors signalling current energetic state. One potential hormonal candidate is insulin, a metabolic hormone released in response to elevated blood glucose levels. The aim of the present study was to explore the potential role of insulin in signalling energy status to the immune system in a seasonally breeding animal, the Siberian hamster (Phodopus sungorus). Specifically, exogenous insulin was administered to male hamsters housed in either long 'summer-like' or short 'winter-like' days. Animals were then challenged with an innocuous antigen and immune responses were measured. Insulin treatment significantly enhanced humoural immune responses in short, but not long days. In addition, insulin treatment increased food intake and decreased blood glucose levels across photoperiodic treatments. Collectively, these data support the hypothesis that insulin acts as an endocrine signal integrating seasonal energetic changes and immune responses in seasonally breeding rodents.

The glutamate agonist NMDA blocks gonadal regression and enhances antibody response to an immune challenge in Siberian hamsters (Phodopus sungorus).

Seasonal variation in behavior and physiology, including changes in immune function, are common. This variability is elicited by changes in photoperiod and often covaries with fluctuations in both energy reserves and reproductive state. It is unclear, however, whether changes in either variable alone drive seasonal changes in immunity. We investigated the relative contributions of reproduction and energy balance to changes in immune function. To accomplish this, we uncoupled seasonal changes in reproduction from those related to energy balance via daily injections of N-methyl-D: -aspartate (NMDA) in Siberian hamsters (Phodopus sungorus). NMDA is a glutamatergic agonist that blocks short day-induced gonadal regression, while leaving short-day declines in body mass unaffected. In Experiment 1, we examined the effect of differing doses of NMDA on testosterone production as a proxy for NMDA effects on reproduction; a dose-dependent rise in testosterone was observed. In Experiment 2, animals were maintained on long or short days and received daily injections of NMDA. After 8 weeks, all animals underwent a humoral immune challenge. Short-day animals receiving daily injections of NMDA maintained long day-like gonads; however, contrary to our predictions, no trade-off between reproduction or energy balance and immune function was observed. Unexpectedly, NMDA treatment increased immunoglobulin levels in all groups, suggesting that NMDA may provide an immunomodulatory signal, presumably through actions on peripheral glutamate receptors. These results support a previous finding that NMDA blocks reproductive regression. In addition, these findings demonstrate a general immunoenhancing effect of NMDA that appears independent of changes in reproductive or energetic state of the animal.