We spend much of our lives attempting to figure out the opposite sex in the hopes that it will lead to increased opportunities to attract a mate. But even when in a relationship, looking back to see how it all kicked off can still leave you scratching your head for answers. Understanding attraction and the rules (or lack of them) that govern it are complicated, but there is no shortage of purported help out there to be found in books, cinema and online. Midst the cloud of confusion, we sometimes forget that attraction is not unique to humans, but our instincts to attract a mate are seen throughout the animal kingdom, and scientists have recently figured out how one species of male bat is
standing flying out from the crowd.
Bats are a nocturnal species, so they become active at night-time. To help orientate themselves and to find their prey in darkness they use echolocation which works by the generation of echoes that the bat then uses to orientate itself and to detect flying insects like moths and mosquitoes. Theses frequencies and pulses used by bats have been well studied, and the use of bat and sound detectors are regularly used by bat ecologists to establish which bat species occurs at a particular site, as the audio frequency that bats use to communicate cannot be heard by the human ear. But despite the large body of research that has gone into bat calls, there has been no investigation, until now, into the potential role of bat calls in the role of attraction or sexual selection.
The study, just published in Plos one and led by Sébastien Puechmaille, with affiliations at University College Dublin, the Max Planck Institute for Ornithology in Germany and the Tabachka Bat Research Station, Bulgaria sought to investigate the role of echolocation beyond the traditional ecological role in orientation and foraging. Puechmaille explains that acoustic components of a range of animal calls including birds, frogs, insects, deer and humans are used to attract a mate. Studies have shown that the components within the male call can be related to body size and condition which the female of the species then uses to help choose her mate. In red deer, the same species that can be seen in Killarney National Park, Co. Kerry, the females have been shown to prefer a low frequency call in male deer, which signals a larger body size representing good overall body condition. But as is the case in most species, the call or sound has more than one function, and sometimes the information held within each individual call is not always known. This makes bats a very interesting group of animals to study for this purpose as echolocation is used already known to be used for multiple purposes including orientation and foraging or hunting prey.
Peuchmaille and colleagues set about investigating if female bats selected mates based on the information held within the male bat call. They predicted that the higher pitch male frequency call conveyed information relating to high quality body condition, which would attract females as they were more likely to yield offspring with the highest probability of survival. In order to be test this theory, the males with the higher pitch frequency should have a higher reproductive success, and as a result, such males should have more relatives within their bat group or colony owing to their higher number of offspring.
The team selected a bat species called Rhinolophus mehelyi, a species of horseshoe bat that occurs in parts of Southern and Eastern Europe, North Africa and the Middle East. The distribution of this bat is patchy, making it vulnerable to local extinctions. The species is declining, and according the IUCN, the population may have declined as much as 10% in places like Andalucía in the south of Spain, but elsewhere the decline has not been fully quantified. The preferred roosting site for this species is limestone caves, and while disturbance of cave sites and a lack of suitable underground hibernation sites might explain some factors involved in the population decline, it is largely unknown what is driving it. As a result of the vulnerable status, R. mehelyi is protected by the European Union under Annex II and IV of the EU Habitats and Species Directive.
The study site was located at two cave sites in Bulgaria. The peak frequencies of the bat calls were recorded for both sexes. The bats were also trapped and their body weights and lengths were recorded to inform a measure of their overall body condition. They also captured female bats for a behavioural study, where the bats were placed in a box and played male bat calls that had been sorted into either low or high frequency categories. The box had two compartments each containing a speaker where the different calls were played. When a bat landed in the compartment with either the low or the high frequency call that was recorded as their ‘preference’. Male bats were also tested to see if the response to the call was only seen in females, thus enabling the response to be linked to attraction. Finally, a genetic study was also conducted to test the final part of the theory that the male bats with the higher pitch frequency calls had in fact more relatives within the colony. Tests for relatedness were conducted amongst the male bats which was then compared with the pitch frequency data to test for a correlation.
The results showed that there was indeed a positive relationship between the pitch frequency of the bat call and the overall condition of the male bat i.e. higher pitch frequency calls were related to better overall body condition. The behavioural study also showed that female bats preferred the higher frequency pitch calls made by male bats, as they flew to that side of the box more often.The male bats showed no preference for either call indicating that the female choice was indeed linked to attraction. Finally, the genetic study also supported these results. In one of the cave sites, only about 3% of the overall colony of about 4,500 individuals. In this instance, there was no statistical support for the correlation between pitch and relatedness, but in the other site where 20% of the colony was sampled, and thus providing more statistical power for the test, male bats with higher pitch frequency calls did have more male relatives within the colony.
This species of horseshoe bat, R. mehelyi uses a higher frequency call than other horseshoe bats. However, the higher pitch results in a reduction of the distance that the bat can detect prey items using echolocation. It therefore seems like there might be a trade-off between the different functions using echolocation for differing purposes, and that the role of echolocation is more complex than simply being used for orientation and hunting but also extends into sexual selection, albeit with a cost. Puechmaille explains that this could be driven by evolution as this species of bat overlaps with another closely related horseshoe bat, and in order to distinguish itself from this similar species, the higher pitch male frequency call might have evolved for this purpose. The cause is still unresolved, but this study has shown that the evolution of echolocation could be driven by both natural selection and sexual selection, and this study has opened up a new area for additional studies in this species and others.
Puechmaille SJ, Borissov IM, Zsebok S, Allegrini B, Hizem M, Kuenzel S, Schuchmann M, Teeling EC, & Siemers BM (2014). Female Mate Choice Can Drive the Evolution of High Frequency Echolocation in Bats: A Case Study with Rhinolophus mehelyi. PloS one, 9 (7) PMID: 25075972