Body size and lifespan are condition dependent in the mealworm beetle, Tenebrio molitor, but not sexually selected traits
Traits under strong directional selection are predicted to be condition dependent and thus increase in development when an organism acquires more resources. This prediction has been tested for a variety of traits, particularly those under precopulatory sexual selection. However, few studies compare the condition dependence of a variety of phenotypic traits, potentially subject to different selective forces. Here we examine the condition dependence of several important life history traits, including those under both pre- and postcopulatory sexual selection, in the mealworm beetle, Tenebrio molitor. We manipulated condition by randomly assigning larvae to one of high-, medium- or low-quality diets. For males reared on the three diets, we measured the following: (a) adult body size and lifespan, (b) pheromone attractiveness, (c) weight of their ejaculate transferred to females and (d) fecundity of their female mates. Males raised on a high-quality diet were larger and lived longer than males raised on medium- and low-quality diets. Although females were significantly attracted to male pheromones, there were no significant differences amongst condition treatments in male attractiveness nor ejaculate transfer. Furthermore, mates’ fecundity was also not affected by male condition. We discuss these results considering previous work on trade-offs and condition dependence of life history traits.
Theory suggests that traits that are under strong selection should be sensitive to the amount of resources an organism has available (i.e. dependent on condition), such that when those resources are plentiful, then these traits should increase disproportionately to other traits that are not so important to relative fitness. Much research has been done on the condition dependence of precopulatory traits under strong sexual selection, but not as much on postcopulatory traits. Our research focused on measuring the effect of resources on both pre- and postcopulatory traits in male mealworm beetles as well as their effect on their mates’ relative fitness. Interestingly, although we found that important life history traits such as body size and adult lifespan were condition dependent, precopulatory traits such as pheromone attractiveness and postcopulatory traits such as ejaculate transfer were not. Our study highlights the complicated nature of tradeoffs in resource allocation faced by individuals under sexual selection.
KeywordsCondition dependence Sexual selection Direct benefits Pheromones Lifespan Tenebrio molitor
We would like to thank Darryl Gwynne, Mark Fitzpatrick, Andrew McDonald, Edyta Piascik, Laura Robson, Jill Wheeler, Bill Wagner, Mike Stock, Dave Hosken and three anonymous reviewers for helpful comments on previous versions of this manuscript. Stephanie Smith helped with data collection.
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