Bats in the Megafire

A little over ten years ago, I wrote on this blog about the effect of forest fires on bats in Florida. It turned out that the bats in question, which were big brown bats (Eptesicus fuscus), one of the most common bat species in the US, were actually quite happy when forest fires tore through the sandhills where they lived. This is because it improved their environment, in particular making it easier to fly about through what might otherwise be a mass of tangled vegetation and thick undergrowth. 

That was a decade ago, and further studies since that date have broadly confirmed the findings of the one I was referring to in that post. A 2019 study, for example, showed that the same was true for a number of species in Australia. Overall, it seems that, at worst, forest fires have no overall effect on bat populations and, in general, they're actually a benefit.

Big Bat Mothers

Myotis sodalis, the Indiana bat

A great many species of mammal exhibit some degree of sexual dimorphism, whereby one sex is larger than the other as a an adult. Usually, this is the male, something we see in our own species, where men are, on average, taller, heavier, and more muscular than women. In many animals, of course, the differences between males and females are more dramatic than they are in humans, as we can see, for instance, by comparing stags and does, or male and female seals.

The typical reason for this appears to be that larger males are better able to drive of rivals for their mates' affections, and are also able to demonstrate their fitness and ability to obtain food - and thus, hopefully, the quality of their genes - to those same mates. But it isn't always this way round, since there are a number of species where it is the female that is larger, perhaps since it makes it easier for them to care for and defend their young. (Indeed, outside of mammals and birds, this is by far the more common arrangement). Clearly, which influence wins out, and which sex ends up the larger, depends on the species concerned.

But is there some pattern to this, some rule of thumb we can use to predict which sex will be larger, and, if so, by how much? Working in the aftermath of World War II, German ornithologist Bernhard Rensch thought that there was. In 1950, he formally proposed what has since become known as Rensch's Rule. This states that, if the males are larger, sexual dimorphism will be most extreme in the largest species in any given group, whereas, if the females are larger, it will be most extreme in the smallest species.

News in Brief #4

A Rat's Idea of 'Vermin'

Arguably the most important factor in the success of humanity as a species is out ability to modify the environment around us, to take somewhere inhospitable, and not only make it habitable for our species, but enable it to support far more humans than any natural environment possibly could. The existence of cities, and of the great swathes of agricultural land around them are clear testaments to modify the world to the point of creating entirely new environments.

We may be the most spectacular example of a species that can do that, but many others do so, albeit on a smaller scale. They're not all mammals, or even vertebrates - consider termite mounds, of example. Often, the way that one species modifies it environment also helps others, whether intentionally, as in humans providing farmland for cattle, or as an unavoidable by-product, such as humans creating the perfect habitat for rats. This process is called "facilitation", and it seems to have been more studied in botany than it has in zoology.

However, Andrew Edelman, of the University of New Mexico, has been looking at the interaction between two animal species in the Chihuahuan Desert. Banner-tailed kangaroo rats (Dipodomys spectabilis) are, like many rodents, seed-eating herbivores. They make large mounds in the desert in which to live, and, while they're generally pretty anti-social, not letting other rats near their homes during their lifetime, the mounds can nonetheless be passed down from rat to rat for generations, sometimes surviving for over fifty years.

Why Forest Fires Are Good For You (if you're a big brown bat)

Big brown bats

Forest fires are both destructive and spectacular. Not quite up there with volcanoes or tsunamis, perhaps, but nonetheless pretty dramatic, wreaking havoc on the local environment. Human-caused fires, when they get out of control, can be very damaging to the ecosystem, which may take years to recover. Yet forest fires have been around since long before humans, and, for at least some forests, they are just a natural part of the cycle of life. Indeed, there are some plants whose seeds germinate specifically after a local wildfire. But how does this affect animal communities?

One such environment is that of sandhill forests. Sandhills are so named because of their sandy, well-drained soils, often dominated by ash, but (unlike the much more barren sand dunes) they support significant plant communities. In the southeast USA, between Virginia and eastern Texas, the dominant tree in these forests is the longleaf pine. Frequent fires are essential for this species to do well, and, left without fire for too long, the nature of the forests change dramatically, in this case being replaced by denser oak woodland.

Bat Swarms of Colorado

When we think of bats, the environment that first springs to mind is that of a cave. While there are a number of bats that roost in trees during the day, a great many are, indeed, cave-dwellers. Because caves aren't all that common in most places, bat colonies within them can be huge, which creates something of a problem when they all try to fly out in the evening. But, if you watched a bat cave throughout a year, in most cases, the activity level would change radically as the seasons progressed.

This is largely because most bats eat insects. In temperate climates, insects aren't around much during the winter; in many cases they've died off and left their eggs to hatch the following spring when the weather has improved. Any animal that relies heavily on eating insects to survive has to deal with this scarcity somehow. Insect eating birds, such as swallows and thrushes, cope with the problem by migrating in the winter, heading to warmer climes where insects are still common. There are some bats that do exactly this - for example, the hoary bat migrates annually between Canada and the southern US - but more commonly, they deal with the absence of food by hibernating.