Zbyszek Boratyński, Esa Koskela, Tapio Mappes, Suzanne C. Mills and Mikael Mokkonen
In bank voles (Myodes glareolus), like in many mammals, male mating success, i.e. number of sexual partners, depends on males’ dominance status. Dominance of males is often determined by their level of testosterone and dominant males display traits that are preferred by female partners. However, production and maintenance of high level of testosterone and dominance status in males could be detrimental for their sisters’ fitness, as both inherit the characteristics of their dominant fathers.
Metabolic rate is often used as a universal physiological currency in organismal biology. The minimum, or basal, rate is the absolutely necessary amount of energy (absorbed nutrients) needed for self-maintenance of the organism’s body.
Thanks to selective breeding experiments researchers can artificially manipulate organismal phenotypes, for example, like we did here for dominance status of male bank voles. The promise of such a procedure is that one can study responses to selective breeding not only in the trait under selection, here male dominance status, but also in putatively associated characters, like physiological self-maintenance costs.
We discovered that, while artificial selection for male dominance resulted in increased male dominance and enhanced reproductive success, such selection also resulted in increased self-maintenance costs in females, as measured by their basal metabolic rate. Moreover, males that were selected to be subordinate, but were nevertheless phenotypically dominant, also suffered very high self-maintenance costs.
We also released bank voles, from both dominant and subordinate selection lines, to fenced field enclosures where we monitored their mating and reproductive successes. This field experiment indicated that high self-maintenance costs resulted in low reproductive success of bank voles selected for high dominance. Both males selected for high dominance and their sisters produced the highest number of pups, but only if they had low level of self-maintenance (i.e. low level of basal metabolism).
Our results support the theory of sexual antagonism as sisters of dominant brothers suffer from elevated energetic self-maintenance. In natural conditions such an increase in energetic costs requires increased absorption of nutrients, but more time spent foraging could reduce the time that they can allocate to reproduction.