But there are other reasons why animals might choose to disperse, sometimes moving from one location to another once they are already adult. This may be due to some sudden disruption in their original habitat, or to longer-term effects, such as climate change. Either way, while it's not an easy subject to study in the wild, understanding the causes and practices of animal dispersal can be important for issues such as conservation and understanding wider population dynamics.
Another common factor that determines whether animals chose to move from where they live to some new place, and how far they are willing to travel, is how many close neighbours they had in their original home. This has been termed density-dependent dispersal and the usual rule, as one might expect, is that the denser the population in the place that the animal lives, the more likely they are to up sticks and try to find somewhere else. Essentially, when there is only so much of your preferred food to go around, it makes sense to move somewhere where there are fewer other animals trying to eat exactly the same thing.
This is not, however, universal, and there are instances where the opposite is true - the more members of a given species there are in a given area, the less likely they are to leave. The reasons for this are less clear, but it seems to be more common among species whose requirements are difficult to meet, so that once they have found somewhere suitable for them they tend to gather there, rather than taking the risk of looking for a new location that might be hard to find. Another possibility is that the animals in such places are less physically fit, due to the strong competition for resources, and just not up to making a long and potentially perilous journey.
In some cases, animals can switch between the two dispersal patterns - towards or away from where others of their kind are living - depending on conditions or the season of the year.
But these sorts of density-dependent dispersal are typically referring to the population density of the species being studied. It could also be the case that animals might choose to migrate depending on the varying population of some other species and not necessarily just the ones they want to eat. Trying to avoid predators would be one obvious example, although the local presence of parasites could also count.
Another possibility is that the other species eats food very similar to the dispersing species, thus either depleting the local resources and making them leave or actually providing useful clues as to where they should be heading and thus having the opposite effect. While it seems plausible that this sort of thing should happen among mammals, there isn't really any strong evidence for it, although is among, for example, newts. It is, however, possible that this is because nobody has really looked.
In terms of the broader question, however, a recent study examined rodent species living in Algonquin Provincial Park, an approximately 7,600 km² (3,000 square mile) area of protected woodland that lies north of Toronto and west of Ottawa in southern Ontario. The researchers captured, tagged, and released rodents at various points in the woodland, allowing them to determine how the animals were moving about and what their local population densities were. Animals caught at least twice in the same area after previously having been caught elsewhere - and that never returned to that original location - were assumed to have dispersed. The dataset used goes all the way back to 1960 although, clearly, the details of the methods used and the number of locations studied changed during that time, as the data would have been used in multiple other studies examining the abundance and population dynamics of the animals in the woods.
Looking back over the 55 years of data, the researchers identified five species that had been caught frequently enough to be studied in this manner. The single most common was the eastern deer mouse (Peromyscus maniculatus) which is one of the most common species of wild mouse found in the US and Canada east of the Mississippi. The southern red-backed vole (Myodes gapperi) is more restricted to colder environments, being found throughout much of Canada and border regions of the US, and only stretching far to the south along the Appalachian and Rocky Mountains - in which it manages to get as far as Georgia and New Mexico, which isn't bad.
Two species of jumping mouse were also common, including the very widespread meadow jumping mouse (Zapus hudsonius) which is known from most of Canada and the whole of the US east of a line from Montana to Oklahoma. While they tend to frequent open woodlands and marshy fields, the woodland jumping mouse (Napaeozapus insignis) prefers spruce and fir forests, and is much more restricted, being found only in southeastern Canada, the area around the Great Lakes, New England and, again, along the Appalachians.
Finally, we have the eastern chipmunk (Tamias striatus) which is the only chipmunk in the eastern US and Canada and, therefore, one would imagine, pretty familiar to a lot of people.
One of the clear outcomes of this study was that all of these species travel much further than had previously been suspected. This is probably due to its large scale, being able to capture more significant migrations than smaller ones had done. For instance, both the voles and chipmunks had previously been reported to travel up to 600 metres (2,000 feet) and that was only because someone had deliberately carried them that far away from their home to see if they could get back.
600 metres is still a fair distance when you're the size of a small rodent, but the new records of 11 km (6½ miles) for the vole and 6 km (4 miles) for the chipmunk are an order of magnitude greater. Which is useful to know if you want to understand how rapidly the animals would move into virgin territory.
In terms of population density, however, it turns out that the jumping mice, and to a lesser extent the deer mice, tend to move away from areas of low population, keeping them sparsely settled rather than having the species spread out more evenly in suitable habitat. Deer mice are sexually promiscuous, and like to have as many potential mates as they possibly can, so that may well be part of the reason for them; the mating pattern of the two jumping mouse species is apparently still unknown, so we can't say if the same is true of them.
Although the voles are equally promiscuous, they showed a mixed pattern in their movements. The number of voles in the forest as a whole varies from year to year, depending on such things as the weather - how harsh the winter had been, for example. When there were a large number of voles, they spread out, and in years when there were few, they tended to migrate towards one another, as the three species of mice do. So there's probably a balance going on here between competition for resources when overall numbers are high and lack of potential mates when there aren't.
The chipmunks, apparently, don't care how many neighbours they have. A possible reason for this is that, being more territorial than the other species, they can keep all the local food for themselves regardless of how many neighbours they have. But there could equally well be other reasons why they just aren't bothered, especially as they aren't as common as some of the other species anyway.
Nor did the population densities of the different species affect each other, at least so far as the researchers could tell. This is unsurprising for the deer mice, which are by far the most common species in the area, and would be little affected by species that are less frequently seen. But otherwise... well, we still don't really know, although any effect is likely to be small.
Science, it has been said, doesn't know everything. If it did, it would stop.
[Photo by Magnus Manske, from Wikimedia Commons.]
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