Lek-breeding species are weird. Every year, males aggregate in a particular area of their population’s home range (the “lek”) and produce sexual displays, be them vocal, visual, olfactory, or a combination of the above. Females visit these leks to survey the males on display and select from amongst them the one whose displays are of the highest quality. The males themselves hold no resources and provide no parental care to the resulting offspring; females are making their selection based solely on male quality. This often results in only a small number of males receiving the majority of the matings. Weird.
Because of this bizarre mating system, leks and lek-breeding species have been a natural laboratory for studying sexual selection in free-living animals for over one hundred years. The quality of resources a male guards and/or his ability to provide for his offspring often confound the study of mate selection by females in other mating systems. In lek systems, however, females are (in theory) choosing males solely on the quality of their genes, via the quality of their sexual displays. This makes them excellent systems for the study of mate choice. It also selects for alternative strategies for otherwise-unsuccessful males to secure mating success, such as “satellite” males that may take on the appearance of females to get close enough to real females to mate with them, or “sneaker” males that try to disrupt matings between males and females.
Only two species of bats are confirmed to be lek breeders. The first is the hammer-headed bat (Hypsignathus monstrosus) of equatorial Africa: a large fruit bat that has the largest sexual dimorphism of any mammal (males have larynxes that take up approximately a third of their body cavity and use to call to females). The other is the lesser short-tailed bat (Mystacina tuberculata) of New Zealand: a threatened species, and one of only two land mammals native to the country.
Mystacina tuberculata is a species that possesses several unique or atypical behaviors: they are the most terrestrial bats in the world (they spend upwards of a third of the night foraging on the ground like a mouse), they are the most gregarious tree-roosting bat (a single roost tree can contain thousands of individuals), they are the only bat pollinator of a ground-flowering plant, they are one of the few known singing bats, and they have one of the highest recorded song outputs of any vertebrate. During the summer months, male M. tuberculata will occupy and defend small cracks in trees and vocalize almost continuously for many hours per night, likely to attract females. These “singing roosts” are aggregated around the large communal roost trees used by the population as day roosts – most likely because this increases the chances of females hearing male songs, as females leaving from the communal roost in the evening and returning in the morning must pass through the lek.
Not much was known about M. tuberculata breeding behavior when I began my Ph.D. in 2011, where I studied a population in the Pureora Forest Park in the central North Island. This population contains about a thousand individuals, with a few dozen singing roosts active each year. One of the goals of my project was to understand which males in the population were taking part in the lek, how they were courting females, and which traits predicted successful reproduction. To accomplish this, I – along with collaborators from the New Zealand Department of Conservation, colleagues from the University of Auckland, and volunteers from the community – marked over 700 individuals (both adult and juvenile males and females) with passive integrated transponder (PIT) tags, allowing us to passively monitor the comings and goings of individuals with PIT-tag readers mounted on roost entrances. Further, we collected morphometric information (i.e. forearm length) and a small biopsy of wing tissue for genetic analyses.
I selected 12 singing roosts throughout the lek, and for three nights each, I used PIT-tag readers on the singing roost entrance, as well as infrared video cameras, to document male identity and behavior, as well as the identities of any bats that visited the roost. I also recorded the vocalizations of lekking males while they were in their singing roost.
What we found was surprising. While we presumed that singing roosts were defended by solitary males (as preliminary video footage without PIT-tag information only ever showed a single male in the singing roost at one time), six of my monitored singing roosts contained multiple males (between two to five) in them each night. These males did not overlap at the singing roost (there were as little as three seconds between one male leaving and the next male entering the roost) but rather sang one-at-a-time in roughly the same “order” each night. We dubbed these roosts “timeshares.” Interestingly, timeshare males were significantly larger than solitary males, however, smaller males sang significantly more and had significantly higher lek attendance than larger males. Further, smaller males sired significantly more offspring than larger males.
However, this wasn’t the end of the story. When we investigated the average relatedness of our focal males – that is, calculating how related they were to all other genotyped individuals (0 being completely unrelated, 1.0 being clones) and averaging that measurement for all males, which provides a proxy for reproductive success (more-successful males will have more relatives in the wider population – think Genghis Khan) – there was no difference in relatedness between solitary and timeshare males. That is, solitary males had no more relatives in the wider population than timeshare males and suggests that solitary males – in the long run – were not more successful than timeshare males. How timeshare males are able to recoup reproductive success isn’t currently known.
Lastly, it’s unknown how timeshares form. We hypothesized that timeshare males were likely closely related, and thus would benefit from kin selection, similar to cooperative strategies seen in lek-breeding birds such as turkeys. However, we found no pattern of relatedness between timeshare males: of five timeshare roosts where we were able to assess the relatedness of multiple individuals, two roosts consisted of completely unrelated individuals, two roosts consisted of distantly related individuals, and the fifth consisted of relatively closely-related individuals (perhaps half-brothers).
To see if the relatedness levels of the latter roost could be the product of random chance, we simulated timeshare roost settlement by randomly allocating males (of various relatedness) to five roosts 5000 times. In these simulations, at least one of the singing roosts ended up containing individuals with relatedness levels greater or equal to the one we observed more often than expected by chance. So, kin selection does not entirely explain timeshare roost formation. Weird.
The reason why timeshares form may lie in the fact that timeshare roosts – overall – are occupied longer each night than solitary roosts. When a solitary male leaves his roost to forage the roost remains empty, but when a timeshare male leaves his place can be taken by one of his roostmates, who continues to sing. It may be that the longer a roost is inhabited each night, the greater the chance passing females will hear their songs, resulting in more visits. Thus, timeshares may represent a strategy adopted by large males that are otherwise unable to compete on a one-to-one basis with smaller males, who apparently have energy to spare.
If true, timeshares would represent an indirect byproduct mutualism (the males sing individually, but their combined efforts provide the benefit of an increased lek attendance at the roost level that the individual males are unable to attain solitarily) and would be the first such cooperative display described in mammals. However, much more work on this system is needed before we can say that for sure.
These findings are described in the article entitled Courtship behaviour and display-site sharing appears conditional on body size in a lekking bat, recently published in the journal Animal Behaviour. This work was conducted by C.A. Toth, A.W. Santure, G.I. Holwell, and S. Parsons from the University of Auckland, and D.E. Pattemore from The New Zealand Institute for Plant & Food Research.