Bats are very social mammals, and it is difficult to do justice to a topic as broad as this without writing reams and reams of text. As such, I will cover bat behaviour and social structure briefly here and urge anyone interested in finding out more to check out the Bibliography for a list of excellent bat books.
Bats are often renowned for forming huge colonies. In his book, Bats, Phil Richardson gives the record for most bats in a single roost as being held by the Mexican free-tailed bat (Tadarida brasiliensis) with an estimated 50 million individuals during the 1960s. Although the numbers of this bat have declined significantly over the last few decades, roosts can still be found in the U.S. containing some 5 million bats. Roosting in such large numbers probably provides some protection from predators and the all-important social interaction that mammals and many other species seem to appreciate. Bats also tend to opt for roosts in some of the most inaccessible places – the reason for this is probably a reflection of how bat hands have evolved into wings. Wings provide little defence against predators and, although bats have sharp claws, these cannot be used for kicking or scratching while the bat is resting, so they choose roosts that are as difficult as possible for predators to access. This inaccessibility of roost choice seems limited to the microbats; fruit bats can often be seen hanging from trees during the day in camps of some 200,000 individuals.
According to former English Nature Conservancy Councillor and now bat worker James Robertson’s 1990 book The Complete Bat, it is the females that gather to form these large colonies, with males remaining largely solitary or in small groups until the breeding season. This is certainly true for common pipistrelles, males of which live a solitary existence, creating and defending mating territories during the breeding season, which the females will visit briefly from August to November. Certain species (e.g. the hoary bat, Lasiurus cinereus and silver-haired bat, Lasionycteris noctivagans) spend much of their time either solitarily or in small groups, coming together only during the mating season. Overall, sociality in bats depends on species and sex.
Follow your nose
Scent plays an important role in the establishment and maintenance of territories and colonies. Bats have scent glands around their throat (called gular glands) and toes, which are used to mark themselves and members of their colony. In the case of the male Gambian epauleted bat (Epomophorus gambianus) of Africa, large white tufts of fur on its shoulders are used to shower a female with his scent. Organs or cavities used for the storage and display of odours are called propatagial sacs. Sac-Winged bats (Saccopteryx, Saccolaimus and Taphozous), found from Africa to South Asia and Australia, use small propatagial sacs located near their shoulders to collect and ferment secretions from their genital region and gular gland, mixing it with urine and saliva – this scent is probably used to attract females.
Male White-lined sac-winged bats (Saccopteryx bilineata) fan the scent from their propatagial sacs onto roosting bats during a complex hovering display. Perhaps the most pungent chiropteran odour is that of male fishing bats (Noctilo spp.) found throughout New World (western Hemisphere) tropical and subtropical lowland regions. So pungent is this odour that you can apparently smell the bats as they fly past. The scent comes from bacteria living on a fatty acid secretion in propatagial sacs under the bats’ arms; it seems that females find this repugnant odour ravishing.
The art of conversation
Vocalization is important for socialising bats – calls have been associated with general social communication and conveyance of threat. In a fascinating study published in the Journal of Zoology in September 2003, Guido Pfalzer and Jurgen Kusch at the University of Kaiserslautern in Germany studied the sounds and social calls of 16 European bat species. Pfalzer and Kusch identified 50 distinct call types, some of which appeared to have similar functions in different species. The researchers also found that the “aggressive” calls varied the least between bats, while the more complex mating calls and isolation calls (used for mother-infant interaction) were very diverse. Communication sounds are well documented in most (if not all) species of European bat. The common pipistrelle (Pipistrellus pipistrellus), for example, is well known to chatter very noisily immediately prior to their emergence from the roost to hunt. Male pipistrelles also emit a loud ‘holler’ at between 18 and 35 kHz during mating flights. By contrast to the noisy chatter of the common pipistrelles, a “sharp whisper” is heard from bat boxes and roosts of the Nathusius’ pipistrelle (P. nathusii). Another species found in Europe, the serotines (Eptesicus serotinus) engage in active squeaking at roosts, emitting a loud, high-pitched chirping or “tsicking” when alarmed.
As its name suggests, the natterer’s bat (Myotis nattereri) also emits sounds – deep chirps and squeaks when roosting, deep humming if alarmed and a high pitched “shrill” during flight. While many bats produce a wide variety of sounds according to their activity and ambience, not all bats are so talkative. The Daubenton’s bat (Myotis daubentonii) doesn’t call whilst in flight and has only occasionally been observed to chirp in roosts.
You scratch my back...
Vampire bats are amongst the most sociable of the chiropterans. Indeed, the extremely rare phenomenon of reciprocity—the idea that favours are done and returned at a later date—is currently known in only a handful of organisms, vampires being one such group. In vampires, this reciprocity is blood-sharing. Gerald Wilkinson at the University of Maryland has demonstrated that for a colony of 200 common vampire bats (Desmodus rotundus) in Costa Rica, blood meals are difficult to obtain and as many as 33% of young bats may fail to feed on any given night; feeding success is apparently linked to age, with only 7% of adults failing to feed.
The rapid metabolism of bats makes failing to feed for several consecutive nights potentially fatal – in the case of vampire bats, starvation can occur after only three nights without a meal. As such, successful vampire bats will regurgitate blood to feed other members of their colony. These sanguivorous microbats are often selective with whom in their group they will feed, however, with mates and direct relatives getting priority. Most interestingly, these bats seem able to spot freeloaders (i.e. those that never go out to feed because it’s easier to sit in the cave and wait for someone to bring dinner to you) and refuse to feed them. Mothers will regurgitate blood for their pups and supplement the diet of their newborn with blood shortly after birth and periodically during their first year.
As with other bats, vampires may travel long distances in search of food and appear to have a homing ability related to their “familiar area”. In a 2000 paper for the International Journal of Mammalian Biology, Horacio Delpietro and a colleague tested the homing ability of 446 common vampire bats, releasing them at various distances from their roosts. Delpietro and Russo found that males were better at finding their way home than females (23% and 8% homing performance, respectively), while females tended to settle in the site of their release. The researchers concluded that the ability of these bats to find their way home was strongly related to whether they were released within the familiar settings peripheral to their roosts.
A later paper by Delpietro and Russo in the journal Mammal Biology looked at the social structure of vampire bats in captivity. It seems that under captive conditions these bats form a “principal colony” (mainly females and their young, with a few males interspersed for good measure). Interestingly, non-resident males (those outside the principal colony) were accepted into the principal colony when it got cold, suggesting social thermoregulation (i.e. the bats group together to share body heat). The paper also documents females suckling young of other females if they lost their own young.