Sunday 27 August 2023

Picking the Right Crevice

It's probably fair to say that bats are commonly associated with in the public mind with caves - there's a reason that Batman's base of operations has been described as a cave since just a few years after his debut. There are good reasons for bats to sleep in caves during the day. Especially in temperate regions, bats keep warm at night by actively flying but, just as it's handy for other small mammals to sleep in burrows, when they sleep they need somewhere that's both secure and has a decent temperature. Even so, to help keep down their energy expenditure many bats in temperate regions enter torpor at night - at a state that's deeper than regular sleep, lowering the animal's body temperature significantly so that it's more like a form of short-term hibernation.

The problem with caves as a habitat, however, is that, in the grand scheme of things, they aren't all that common. Clearly, this depends on the type of landscape you're in, but many places just don't have lots of caves. In the tropics, hanging from a tree branch might well be sufficient, but where the weather is cold, especially in winter, that may not be such a good idea. So bats roost in many other places, too, such as hollows in trees and cracks and crevices in the ground that are similar to, but much smaller than, what we'd normally think of as a "cave". 

One estimate is that over 40% of bats in North America roost in "aboveground rocky habitat" where they can find it, even if they use more vegetated shelters elsewhere. The sorts of rocky spaces that they use vary between species, depending, for example, on whether the bat is sociable enough to want to share, but also on the wider environment. For instance, the local climates may determine whether they want the space to be cooler or warmer than the daytime temperature outside. For that matter, even within a single species, different bats may choose different kinds of roosts depending on their needs at the time - such as whether they have young to care for.

There are a huge number of bat species in the world, making them the second-most diverse mammal order after the rodents. Even in parts of the world where such studies are common, we can't possibly have covered all of them yet, and the detailed behaviour of many is obscure, or comes from anecdotal reports rather than any kind of formal survey. Roosting behaviour is just one aspect of this, but it's one that can be key to conservation efforts. So, for some species, yes, we know a fair amount but for others - well, we just haven't got around to them.

The eastern small-footed myotis (Myotis leibii) is one such example. The myotises, or "mouse-eared" bats are a large group of insect-eating bat species found across the Northern Hemisphere, including many that are common and reasonably well-known. First named in 1842, for much of the 20th century it was considered to live across almost the whole of the contiguous US. In the 1980s it became clear that the western populations represented a different species, leaving the original scientific name with the eastern ones.

While the western small-footed myotis remains common, the same is not true for the eastern one and, for once, while direct human activity has undeniably had some detrimental effect, it's not the primary one. Instead, the real problem is white-nose syndrome, a deadly fungal disease that has been spreading among bats in the US since at least 2006 and to which myotises seem particularly vulnerable. (Unsurprisingly though, humans don't get off the hook entirely; the disease originated in Eurasia where bats have natural immunity to it, and it's probably us that brought it to North America in the first place).

Eastern small-footed myotises live across much of the eastern US, being found along the Atlantic coast from southern Maine to Maryland. Inland, they spread even further, reaching southern Ontario in the northwest and the eastern border of Oklahoma in the southwest. They are considered to be a forest-dwelling species, but they need rocky outcrops to roost in and it's probably noteworthy that their distribution includes the entire Appalachian Mountain range.

They may be widespread, but nowhere are they particularly common. That wasn't a problem before white-nose syndrome came along, with the population seemingly being stable and under no observable threat from humans. In 2018, however, it was reclassified as an endangered species, due entirely to the deadly risk that the epidemic poses to their continued survival.

They are rock-roosting species, but they're clearly not an indiscriminate one. Significantly, females and males don't roost together, picking different places to sleep in even if those places are only a few metres apart - are they, perhaps, not looking for the same thing when they pick them? A recently published study took a closer look at roost selection by this species in the Appalachian Mountains of Virginia and New Hampshire, to see if they could figure out the answer.

The researchers spent nine years watching the bats to see where they roosted, and capturing them in mist nets so that they could be safely examined to determine age, sex, condition, and so on. In many respects, the rock roosts chosen by the bats were similar to those of other related species that have previously been studied, such as spotted bats, long-eared myotises, and canyon bats. That is, they are often spaces in south-facing talus slopes - the collection of loose scree that forms at the base of cliffs as they erode. These are usually devoid of significant vegetation, keeping them in direct sunlight so that the gaps inside are likely warmer than those that may be more shaded or north-facing. Not all roosts fit this description but the great majority did, and, where they were available, the bats also used south-facing "riprap" - essentially artificial scree used to protect the base of dams from erosion.

However, the bats did not select roosts purely for their warmth and comfort. Another feature they seemed to favour was unusually narrow openings, with males, in particular, favouring entrances less a 1 cm (0.4 inches) wide. That's a pretty tight squeeze even for a bat that is, on average, 8 cm (3 inches) long and the obvious explanation is that it's pretty difficult for anything else to get in there, either. We don't know precisely what eats small-footed myotises in this area, but there must certainly be something. Candidates, because they live nearby, would include ravens, and various local lizards and snakes, at least some of which would likely find it difficult to get through such a narrow gap.

It may also stop larger bats roosting in the same slopes from pinching the spot.

Another anti-predator tactic, seen among many other bats (and indeed, burrowing mammals) is not to stick to a single roost once you've found one, but to keep constantly moving lest you become too predictable. In this case, the bats switched roosts almost every day, something that's possible in talus because of the sheer number of crevices available to them. This may also help reduce the risk of parasites or disease from a roost becoming too dirty over time. Individual roosts could, nonetheless, be quite crowded. Males tended to be solitary, but females, at least some of the time, gathered together in groups of up to ten in a single roost, with most of them trying to crowd near the entrance, presumably to get some fresh air.

One consequence of this was that females tended to pick larger roosts which, in turn, meant that they tended to be formed from larger rocks with wider voids between them. Using temperature probes, the researchers were able to show that, as expected, such spaces tend to be more thermally stable than the smaller spaces favoured by males. This may be part of why the sexes stay apart, picking different crevices in the same piles of loose rubble. That's because many of the females would have been pregnant, or nursing young, and the advantages of a stable temperature may outweigh any risks from the space being more accessible, especially for bats that are sharing. 

The bats may be careful about the sort of roosts they choose to spend the day in, but such places are hardly in short supply and are not typically being built on by humans (who wants a house at the bottom of a crumbling cliff?) The risks to this particular endangered species are, at least for today, not coming from a loss of habitat.

[Photos by Virginia State Parks and "Talus.rock" from Wikimedia Commons.]

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