Thermal acclimation has been frequently found to enhance the performance of whole-animal traits in fish and other ectotherms. Whether these functional benefits to performance after acclimation necessarily enhance organismal fitness has been of significant interest to ecological physiologists in recent years. Our research has combined a traditional physiological ecology approach with studies of behavioural ecology to examine the benefit (or otherwise) of thermal acclimation on multiple whole-animal traits likely to be closely related to fitness. In contrast with the long-held assumption that acclimation is beneficial, our research has shown that the adaptive significance of acclimation is affected by a variety of physiological and behavioural factors.

 Recent studies have examined the effect of short-term acclimation (>3 weeks) to different thermal environments on the locomotor performance and reproductive success of the eastern mosquitofish (Gambusia holbrooki). Male mosquitofish copulate with females solely through ‘sneaky’ copulations and never appear to court or display to females. Typically, male Gambusia will spend much of their time swimming slowly behind the female before rapidly approaching and thrusting the gonopodium (a modified anal fin) into the female's genital pore to copulate. Meanwhile, females never co-operate and resist all attempts, likely due to the high physical costs of male harassment and coercive matings. Or, so it would seem... 

Battle of the sexes: thermal acclimation of male mosquitofish

Male Gambusia holbrooki on the hunt for females

We predicted that acclimation to warm (30°C) and cool (16°C) temperatures would enhance the swimming performance and reproductive success of males in their acclimation environment. This was found to be the case, as males that were acclimated to the test environment achieved greater copulation success, burst swimming and sustained swimming performance than males that were not acclimated to the test environment. e.g. 30°C-acclimated males out-performed 16°C-acclimated males in the three traits when tested at 30°C; however, at the test temperature of 16°C 16°C-acclimated males had greater performance across all traits.

 When males from different acclimation environments were competed against each other for copulations with a single female, 30°C acclimated males were more aggressive and obtained a greater number of copulations than 16°C-acclimated males in both warm and cool environments.
 

What about the girls?

The high rate of male harassment via frequent forced matings is thought to be costly to females and may explain their indiscriminate resistance to mating attempts. We predicted that acclimation to a thermal environment would enhance the ability of females to avoid matings at their host acclimation temperature, relative to females acclimated to another environment.

 Although acclimation enhanced the burst swimming performance of female Gambusia for both warm and cold acclimated fish, we found that females acclimated to 30°C resisted less and allowed a higher rate of ‘sneaky’ matings. However as we predicted, 16°C-acclimated females were better avoiders (enhanced their resistance to males) than 30°C-acclimated females at 16°C. Our results suggest that female Gambusia appear to have greater control of the outcome of mating attempts than previously considered and alter their propensity to receive forced matings following acclimation to thermal environments likely suitable for reproduction.
 

Publications:

Wilson RS, CHL Condon and IA Johnston. 2007. Consequences of thermal acclimation for the mating behaviour and swimming performance of female mosquitofish. Philosophical Transactions of the Royal Society 362: 2131-2139.

Wilson, RS, Hammill, E, Johnston IA. 2007. Competition moderates the benefits of thermal acclimation to reproductive performance in male eastern mosquitofish. Proceedings of the Royal Society of London Series B (DOI: 10.1098/rspb.2006.0401)

Carter, A. & RS Wilson. 2006. Improving sneaky-sex in a low oxygen environment: reproductive and physiological responses of male mosquito fish to chronic hypoxia. Journal of Experimental Biology 209:4878-4884.

Condon, CHL and RS Wilson. 2006. Effect of thermal acclimation on female resistance to forced matings in the eastern mosquito fish. Animal Behaviour 72: 585-593

Wilson, RS. 2005. Temperature influences swimming and sneaky-mating performance of male mosquitofish Gambusia holbrooki. Animal Behaviour 70:1387-1394.

Wilson, RS and CE Franklin. 2002. Testing the Beneficial Acclimation Hypothesis. Trends in Ecology and Evolution 17:66-70