Saturday 1 June 2024

Wombats Moving Home

There comes a time in the life of many young mammals when they have to leave home. There are at least two major reasons for this. Firstly, any given place only has so many resources, so unless your parents die as soon as they've finished raising you (unusual among mammals, although not unheard of), at some point, you have to move elsewhere or there won't be enough food for both of you. Secondly, if the whole family stays together in one place, you'll never meet new sexual partners, forcing you to mate with your siblings - and we all know where that leads.

Understanding how and when animals disperse from their place of birth can be important for conservation as well as, on a broader scale, how new species and subspecies evolve and adapt. Nor is it necessarily something that only applies to young approaching maturity, since older animals may also choose to move from one place to another and often for similar reasons - competition or a lack of suitable mates. Whether a given animal chooses to move home, and how far they travel to do so, can be influenced by several different factors. 

Some of these are external, such as the nature of the habitat and, perhaps, whether it is altering under the effects of climate change or human activity. For example, if you are a forest-dwelling animal and your woodland is being cut up into smaller chunks with grassland or roads in between them, that will inevitably affect both your desire and your ability to move. 

But, even then, the result of such habitat fragmentation isn't straightforward because it depends just how reliant you are on that particular habitat. If you really need woodland and/or don't move very fast, if you're stuck in a small patch of woodland, you'll stay in it, regardless of the cost - that is, dispersal becomes less likely. On the other hand, if you can cope with some other environment temporarily, or you're especially mobile (being able to fly, for instance), you're probably more likely to move long distances if your preferred habitat is breaking up.

Other factors affecting a decision to move may be specific to the individual. The most obvious of these is sex; in the case of mammals, males are more likely to leave home and to travel further than females, especially where males expect to have multiple sexual partners and need to move about in order to find them all.

The common wombat (Vombatus ursinus) is, as its name might suggest, far from being an endangered species. While there are no estimates of their total population, it is thought to be large and not to be declining on a large scale. Things may be different locally, where disease and road traffic accidents may affect specific populations. But, in fairness, it's not a species where conservation is of vital importance - although the two other wombat species are not so lucky, with one having a sizable, but declining, population, and the other being all but extinct in the wild.

Regardless, understanding the dispersal patterns of common wombats may be relevant to ecology more generally in the region they inhabit. That region is essentially southeastern Australia, with the animal being fairly widespread across both Victoria and Tasmania, as well as the eastern parts of New South Wales and the extreme south of South Australia. They are reasonably adaptable animals, inhabiting various types of forest and shrubland, although they don't like the weather to be too hot, so that their only Queensland population lives at high altitude close to the southern border. They also tend to avoid areas that are too rocky and barren, or, conversely, that are covered in especially dense undergrowth.

Wombats are also somewhat unusual animals. They are, of course, marsupials, distinct enough to form their own family, with just three living species; their closest relative is the koala, and that isn't very close at all. They are nocturnal, herbivorous, and solitary but, most significantly, they are among the world's largest burrowing animals. A male badger (either Eurasian or American) has a maximum body weight of around 17 kg (37 lbs), but even the average common wombat weighs 26 kg (57 lbs) and is around one metre (3 feet) in length. While some larger animals do dig burrows, these are often temporary or used only to birth young, whereas wombats can spend 18 hours or more each day in their burrows, which can extend up to 20 metres (66 feet).

recent study examining the dispersal of common wombats took an approach that might not be the first one you'd think of. The obvious approach to finding out when and where wombats travel when they move home would be to... well, watch wombats, perhaps with radiotags, and see when and where they move home. But there is an alternative approach and one that, in this case was cheaper to carry out.

Between 2014 and 2017, researchers had conducted a survey of common wombats across their range, collecting genetic samples from 176 animals to assess how related different populations were to one another. That study, published in 2019, confirmed the three subspecies - one each on the mainland, Tasmania, and Bass Island - as real and genetically distinct. Now, a new team (containing some overlap with the original) went back to the dataset and analysed it on a finer scale.

They looked specifically at the samples collected from the 74 wombats from Tasmania, again using the data to determine how closely related to one another they were, and exactly where each wombat had been living when the initial collection was taken. The reasoning here is that, if wombats travel further in one type of terrain than another, the samples collected from that type of terrain will show that they were more closely related across it since they would all have the same far-travelling recent ancestors. Conversely, where barriers to dispersal existed, the wombats on either side of that barrier should be less related to one another.

The previous study had shown that the further apart two wombats lived on Tasmania, the more distantly related they were likely to be. Not, perhaps, a great surprise, but it was interesting to note that this was less true of wombats on the mainland - that is to say, wombats in places like Victoria seemed to travel further than those on Tasmania. The newer study was able to look at features of the landscape that might explain this.

Tasmania is a fairly mountainous state, but the parts of the mainland that common wombats inhabit are often similarly so, and elevation turned out not the be the major factor one might expect to be. In fact, a 2012 study using GPS collars to establish how far wombats in New South Wales travel in their daily routines showed that those in the most mountainous places travelled furthest - far from being a barrier, the low food quality at high elevations persuades the wombats there to move about more.

The natural vegetation in the area also made little difference, suggesting that wombats are adaptable enough that they're not especially bothered by any of the vegetation types commonly found on the island. But what made the biggest difference wasn't the natural vegetation, but how the land was used.

Specifically, wombats living in parts of the state with large amounts of agricultural land were the least likely to move from place to place. This could partly be due to barriers such as roads, while Launceston (the second largest city in the state) and its associated estuary were also effective blocks to travel. But another possible reason that the researchers advance is that common wombats actually like agricultural land, and are commonly found there. So much so, in fact, that farmers often consider them pests. It could be that there are enough edible resources in agricultural land that the wombats there feel little need to move.

Perhaps more interesting than this is what the researchers found when they checked the genetic samples to determine the sex of the animals from which they had been taken. This showed that female wombats dispersed further and more readily than the males. Quite how far is difficult to determine with a study of this nature, where you're having to guess based on the genetics of offspring rather than watching the animals move directly. Nonetheless, the researchers estimate that female wombats leaving home for the first time typically travelled around 50 km (31 miles). While previous evidence had suggested that this happens in other wombat species, too, the fact is that it's a really unusual thing for a mammal to do - it should be the males that move the most, not the females.

So... why?

This may be to do with the unusual nature of wombats as exceptionally large burrowing mammals. When you're that large, digging a burrow big enough to live in and raise your children is no trivial matter. Once you've found a good place, you're going to want to keep it and not share it with any other females, so it makes sense for it to be far from your mother and sisters. Males don't have to raise young in theirs, so they don't care, and are even willing to share with their brothers on occasion. Conversely, the females of smaller species are quite able to share burrows, and it's probably less effort to dig them in the first place. The researchers point out that either sex can disperse in badgers, depending on the circumstance, and wombats are more extreme than they are.

Unusual lifestyles sometimes lead to unusual behaviour in other respects.

[Photo by Charles J. Sharp, from Wikimedia Commons.]

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