By Pavitra Muralidhar
We often think of natural selection as the “survival of the fittest,” but survival is only part of the equation. Unless you’re immortal, you must reproduce in order for your genes to survive into the future. Besides staying alive, the individuals in a population have to compete for the chance to mate, and this competition leads to sexual selection. This process includes competition within one sex for mating opportunities (intra-sexual selection) and mate choice by the opposite sex (inter-sexual selection). The concept of sexual selection often evokes flashy images, like the tail of a peacock or antlers of an elk, but sexual selection extends far beyond morphological (“form”) traits, such as these, and can even include behavior. Traits involved in signaling and communication, in particular, can have a dual role in survival of the individual (natural selection) and attracting mates for an individual (sexual selection).
Echolocation has a long history of scientific study.
A new paper by Puechmaille et al. (2014) examines the role of auditory signaling in intersexual selection in a species of Mehely’s horseshoe bat, Rhinolophus mehelyi. While echolocation is a fascinating and well-studied phenomenon in bats, previous research has generally focused on how bats use this ability to navigate and hunt prey – in other words, how natural selection shapes this trait to enhance individual survival. Puechmaille et al. observed that, in the Rhinolophidae family of bats, individuals with larger body size tended to have lower peak frequency calls, which indicates a physical constraint controlling the pitches emitted by each species. This is similar to the way that instruments with larger chambers, like the bassoon, create lower-pitched sounds than those with smaller chambers, like the piccolo.
The Mehelyi bat (Rhinolophus mehelyi), however, has a higher frequency call than expected based on its body size. This relationship led the authors to suspect that the increase in emitted frequency in this species could be a result of sexual selection pressure. Natural selection and physical constraints would promote a lower-frequency call for echolocation in the Mehelyi bat, based on the size pattern seen in other bats, but sexual selection could be optimizing for higher frequency calls in males in order to attract mates. If sexual selection was operating, the authors also wanted to test whether higher echolocation frequencies indicate higher quality males and serve as an “honest signal” of mate quality, or whether there was no relationship between call pitch and mate quality, in which case the signal is not honest. While both males and females use echolocation in this species, researchers only tested for female mate choice based on male calls because previous research showed that females are the “choosy” sex in this species.
First, the researchers had to determine if the female Mehelyi bats preferred high or low call frequencies. Females were allowed to pick between lower-pitched calls and higher-pitched calls, played by speakers on opposite sides of her, by flying towards them. If they chose higher-pitched calls at a probability greater than chance (in this case, that would be 40%, the male preference in this), this indicates that they actually prefer higher frequencies, consistent with the sexual selection hypothesis. A few male bats were given the same test to determine whether the species as a whole prefers higher or lower frequencies. Most males did not make a choice between calls, and the few that did tended to pick lower-pitched calls, flying to them 60% of the time. However, the female Mehelyi bats of all ages were twice as likely to fly toward a high frequency call (66%) than a low frequency one (33%), a far greater preference than the males displayed, consistent with the sexual selection hypothesis.
The next step was determining whether call frequency advertised a male’s quality as a mate. High quality males should be selected as mates more frequently, leading to more offspring (higher reproductive fitness) and more related individuals in the population. Therefore, the authors calculated average relatedness of each male to the colony as a proxy for his reproductive fitness and his prospects as a mate. In one of the two colonies studied, the researchers did find a relationship between high pitch calls and average relatedness, indicating that higher frequency calls really are honest signals of a male’s quality (the other was inconclusive). Furthermore, there was no correlation between call frequency and quality for females in either population, which is what we would expect if higher pitch is only a signal of male quality. So, it seems that the falsetto is not incidental nor false advertising, but truly an indicator of the male’s quality.
Because echolocation is such a crucial sense for bats, scientists have long implicitly assumed that the trait is subject solely to natural selection. After all, how could such a finely tuned process be subject to the whims of fashion? Sexual selection and natural selection are usually presented as a dichotomy: traits that are under natural selection are thought to be so essential that tinkering via sexual selection would result in a highly disadvantaged organism. This study gives the lie to that assumption by showing that important survival strategies like echolocation can be under strong sexual selection as well. If male bats with higher frequency calls do much better in the mating game, then a great increase in reproductive success makes up for a slight decrease in survival. (In fact, the risk could be part of what makes some sexually selected traits so sexy.) Since echolocation is used for communication as well as navigation, further studies might tell us more about how signaling traits can be co-opted as indicators of mate quality. The authors suggest that future investigation of the diverse communication calls in bats may elucidate this intersection between natural and sexual selection and teach us more about how traits evolve under their combined influence.
Pavitra Muralidhar is a PhD student in the Department of Organismic and Evolutionary Biology at Harvard University.