Songs help birds attract mates by advertising their fitness or territory to potential mates. Because the sexiest songs are assumed to be energetically expensive, or in some way related to a certain attractive attribute, potential mates can judge the singers to learn more about their qualities and potential as an eligible mate. In much the same way that birds use song, some bat species also use complex songs either to boast about their strength or to lure mates into their territory. The New Zealand lesser short-tailed bat (Mystacina tuberculata) is one of only two bat species where courting males congregate together – forming a lek – where they perform serenades within each other's earshot in the hope of attracting females. To be considered sexy, the quality of the bats’ songs should be costly or related to the beauty of their territory. Yet, if the songs were expensive to produce – somehow revealing the singer's inner strength – bats might hypothetically try to cut corners in other expensive areas of their lives, such as the body temperature they maintain. Kathleen Collier from the University of Auckland, New Zealand, along with collaborators Zenon Czenze from the University of New England, Australia, and Stuart Parsons from Queensland University of Technology sought to estimate the costs of singing for lek-breeding bats, using skin temperature, to estimate how much energy singing uses.
To study this unique phenomenon of communal singing in bats, the researchers attached temperature-sensitive radio transmitters to the backs of 14 male New Zealand lesser short-tailed bats at their singing roosts. The researchers then recorded the bats’ skin temperatures while the mammals quietly went about their usual routine and also when they were singing. The researchers then used the skin temperatures collected by the radio transmitters to estimate the metabolic rate of the bats during both activities.
Throughout 100 nights of observation, the researchers found that the bats were not significantly warmer or cooler when singing. And when the team compared the bats’ estimated metabolic rates, they also were not significantly different. The bats were not exerting themselves while singing out their hearts for a mate, implying that their singing may not be as much of an energetic sacrifice as was previously thought. However, it is possible that using skin temperature to estimate metabolic rate may have missed some other important energy consuming activities – such as panting to keep body temperature close to normal – and may have hidden the animals’ actual metabolic rates while singing. Yet, even though these data are just rough estimates of the bats’ actual energy use, they do allow us to compare between serenading and non-serenading behaviours, which had not been possible before.
This study is also the first to have investigated the energetic costs of singing in a mammal and suggests that the costs may be less than previously thought. If these mating rituals are less energetically expensive, then what messages could these displays actually be sending to potential mates? In fact, are potential mates even getting what they thought they were investing in? Even though that isn't clear right now, it isn't deterring male New Zealand lesser short-tailed bats from getting together and serenading in the hope of attracting an eligible date.