Sunday, 18 November 2018

How Baby Bats Learn to Fly

Small mammals, on the whole, live fast and die young; a great many of them live for only a single year. And that's assuming they don't get eaten first, which, unsurprisingly, they often are. In order to maintain their numbers, then, it is essential that they breed early and often. In particular, they tend to have large litters, which they then need to get to adult independence as quickly as possible. The common house mouse, for example, can breed at least five times a year, and gives births to litters of typically around five to eight pups at a time, each of which is sexually mature within less than two months.

This is not, however, what we see with bats.

Bats do not give birth to litters, with most species having only a single young at a time. They also tend not to breed quite so frequently, and to care for their young for much longer than other small mammals, typically until they are almost the size of adults themselves. The exact details do vary quite a bit, as one might expect given the huge range of bat species (and some fruit bats, of course, are not exactly what you'd call "small mammals" anyway). But, in general, it's quite a different pattern than we'd see among rodents, rabbits, or shrews, for example.

This, unsurprisingly, has a lot to do with their ability to fly. For a start, it's probably difficult enough to fly when you're pregnant without having to do so while carrying an entire litter. On the other hand, and for much the same reason, young bats do grow rapidly, just as their more earthbound counterparts do. The sooner they can learn to fly, and become independent of their mother, the sooner she is able to stop supplying them with milk, which is nutritionally costly, and, moreover, brings its own weight concerns.

But how does all this happen, and how do baby bats get their start in life? The answer will, of course, depend on the species, but, because, of the difficulties of studying bats, only a handful have been closely looked at. One recent study not only extends this to an additional species, but also includes the first observation of an entirely new behaviour, never before seen.

Peters' tent-making bat (Uroderma bilobatum) is perhaps the best known of the "tent making bats" of Latin America. It's found though almost the entire northern half of South America, and in patchy areas as far north as southern Mexico. It's a member of the "leaf-nosed bat" family, which contains what's probably the most diverse range of species of any of the numerous bat families.

The tent-making bat is, however, in many respects a fairly typical member of the family. Adults weigh up to 20 g (about two-thirds of an ounce), which is middling for bats. They mainly eat fruit (especially figs), but are not strict herbivores, with at least some insects in their diet. Living in tropical lowland forests, they are, as their name suggests, most famous for constructing tents out of large leaves, and clustering on the undersides to shelter themselves from the rain and/or bright sunlight as they sleep.

The bats in this particular study lived in Panama, and they hadn't bothered to construct tents, using the overhanging eaves of village houses. Tents made from banana or palm leaves can apparently last up to two months, so they don't have to be constantly replaced, but it still takes a few days to do so, and if somebody has already provided a ready-made one in the form of a house roof, it makes sense not to bother doing it the hard way.

The bats in the community studied were most likely to give birth in March and July, corresponding with the times that many of the local trees were producing fruit. In addition to the obvious advantage of providing more food for the mother, this may also mean that she has to fly shorter distances to find suitable food after giving birth, allowing her to save up calories for producing milk. Like most bats, they have only a single young at a time.

The very first problem that a bat mother has once her pregnancy comes to an end is how, exactly, she's supposed to give birth. This is not a great challenge for most non-aquatic mammals, which will, after all, be sitting or standing on the ground. A bat, on the other hand, habitually hangs upside down by its feet, and if the baby should fall, they're in deep trouble. It turns out that bats have a number of different approaches to this, with some, for example, managing to manoeuvre themselves to hang feet-downwards and then drop the baby into a basket formed by the wing membranes either side of their tail.

Tent-making bats, as it turns out, don't do this, and do remain upside down throughout the birth, albeit sometimes hanging on by just one foot, instead of two. Unlike some other bats, the young are born head-first, and they seem pretty active once they've managed to get their shoulders free, grabbing onto their mother's fur as soon as they can, and heading straight for the teats (which are roughly where they would be on a primate, rather than further back, as they are on many other mammals). The mother helps them as well as she can, in addition to licking off the goo from the baby's fur.

Like humans, but unlike, say, cats, the young are born able to open their eyes, and, unlike mice, already have fur. They are also capable of gripping onto things almost immediately - they have to be, because the mother can't carry them about with her on foraging flights, and so has to leave them hanging in the roost. She can, however, carry them for short distances, and does so from time to time, moving between different roosts, perhaps to help disguise the true location of her young from predators.

Once all of this has been managed, however, it's important to get the young flying as soon as possible. This process is called "fledging", exactly as it is in birds, although bats do have the advantage that they don't need to develop adult-style feathers in order to do it. The young bats grow rapidly, and their wings grow faster than the rest of their body, indicating how important flight is going to be for them. In this particular species, it takes only around 34 days for the baby bat's wings to grow to their full, adult size, although the rest of the body hasn't quite caught up by that stage.

As the young grow older, the mother naturally decreases the amount of time she spends caring for and suckling them. In many mammals, the mother may encourage the young to stop suckling by swatting them away or otherwise becoming mildly aggressive towards them. In these bats, that doesn't seem to happen. This may well be an advantage of the fact that, once the young are placed somewhere and grip onto the roost with their feet, they really can't follow other bats about until they learn to fly. So it's a lot harder for them to pester their mothers.

It's here, though, that we come to the newly observed behaviour. When the mothers wanted to fly away and find food, but their young was still gripping onto them, she pumped her arms rapidly back and forth. This apparently acted as a tactile signal that it was time to let go, and was normally followed by the young backing off and grabbing onto something with its feet. It's evidently a less aggressive alternative to swatting them away, and is something that has never been seen before in other species.

At some point, of course, the young have to learn how to fly. They start this by practising flapping their wings, something they start doing around day 25. At this point, they are still holding onto solid supports with their feet, but it evidently helps to build coordination and muscle strength, and perhaps bone and joint strength, too. A week or so later, they are able to progress to letting go of the support and taking their first flight.

The first flight are short, and are presumably practice flights, rather than true foraging, because the bats still haven't been weaned at this point. Interestingly, mothers were often seeing carrying their young back from these flights, suggesting that they get the hang of the basics before they've mastered the art of steering sufficiently to be sure of returning home under their own power. (How the mothers manage to find them to do this is another matter, and not yet clear).

By around 40 days, the bats begin to be weaned, and are able to fly properly, finding enough food to support themselves and, when they're ready, leave their mother's roost for good.

[Photo by Brian Gratwicke, from Wikimedia Commons.]

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