When it comes to successfully winning a mate, there are two sides of the coin. Individuals need to both compete against members of their own sex for access to mates and convince members of the opposite sex that they are an attractive mating option. Traditionally, it is assumed that when males compete with one another for access to females, females should find the strongest and more competitive males the most attractive. However, there is no reason that this should necessarily be the case. While more competitive males might father sons that are also stronger and more competitive, strong males might also use coercive tactics against females to gain mating opportunities, which will lead females to resist these suitors. There may also be disadvantages for daughters fathered by these strong males, which could potentially balance out any benefits for sons.
Led by Kensuke Okada, a team of researchers from Okayama University and University of Tsukuba, Japan, and University of Exeter, UK, decided to investigate the two sides of the mating coin in the broad-horned flour beetle (Gnatocerus cornutus). In broad-horned flour beetles, males have enlarged mandibles that they use for fighting with other males. Males with the largest mandibles consistently overpower other males and have the highest mating success. First, Okada and his colleagues assessed whether females find males with larger mandibles the most attractive. As females control when mating occurs in the broad-horned flour beetles, the researchers used the time between introducing a pair and mating as a measure of male attractiveness. Surprisingly, they discovered that despite the tight link between mandible size and competitive prowess, females appeared to take no notice of this trait. Instead, they preferred males that courted more vigorously, and males that performed more courtship bouts enjoyed more rapid mating success. Thus, when given the opportunity to choose a mate, female broad-horned flour beetles prefer lovers, not fighters.
To understand why females might have this preference for lovers rather than fighters, Okada and his colleagues conducted a series of experiments to investigate potential direct and indirect benefits for females to mate with courting males versus competitive males. First, they established that although mandible size and courtship rate are both heritable traits, they are not correlated with one another. The researchers also determined that females gain no direct fitness benefits from mating with their preferred males, as females don't live longer or lay more eggs after mating with an attractive male. The offspring sired by attractive males with high courtship rates developed more quickly, although the benefits of more rapid development were unclear. However, the researchers point out that the genes associated with larger mandibles in males are associated with masculinized females that are less fecund. Further, the more competitive males are also more aggressive, and so females might suffer some injuries by mating with males with large mandibles.
Collectively, these results suggest that females suffer indirect fitness costs from mating with the stronger, more competitive males, as they have less-fertile daughters and may risk injury. On the flip side, the females gain indirect benefits from mating with males that court more vigorously, as they will have sons that also court enthusiastically and are attractive to females. Overall, it is clear from this research that in broad-horned flour beetles, attractiveness and competitive prowess are not two sides of the same coin. While males with large mandibles may win more fights, they are not the males that win female beetles' hearts.