Sunday, 1 June 2014

Chipmunks, Seeds, and Saliva

Least chipmunk
Chipmunks are a kind of small squirrel, found mostly in western North America. Spending the winter in underground burrows, they are more related to ground squirrels, such as prairie dogs and marmots, than they are to tree squirrels, such as the ubiquitous eastern grey. They share a lot of features in common with other squirrels, and I doubt it's much of a surprise to anyone that this is, indeed, what they are.

One thing they have in common with other squirrels is their habit of hoarding food. In fact, their scientific name, Tamias, is Greek for something like "steward" or "treasurer", implying an animal that carefully looks after its food stores. As with other squirrels, since they don't truly hibernate, this habit helps them to survive through the winter, especially in the colder or drier climates in which many of them live.

There is something of an art to this, since chipmunks (and squirrels in general) are no great respecters of property rights. If they can find somebody else's cache of food, they'll be in there like a shot.

They don't even particularly care if the cache was left by a member of their own species or not. If the owner is the same species as yourself then, well, he's probably a rival, since chipmunks aren't especially sociable. If, on the other hand, he isn't, then that's even less of an issue, since they all eat broadly the same kind of seeds and small nuts. This, however, leads to a problem - sure, you may be able to pinch his food, but what's to stop him pinching yours?

The trick, then, is to try and find somewhere to hide your food that's easy for you to remember, but hard for anyone else to guess. A bit like the password on your bank account, then. Not all chipmunks, it turns out, are equally good at this. Let's take a look, for example, at the two most widespread American species.

There are, in fact, twenty five different kinds of chipmunk, and most of them live in the western half of North America. Indeed, there are only three species that live anywhere else. One of these is the Siberian chipmunk, which actually inhabits much of northern Asia, and as far south as central China. Leaving that aside, we are left with two particularly common species in the Americas.

First, there is the least chipmunk (Neotamias minimus), which, as both its common and scientific names suggest, is the smallest of all chipmunks - they're rarely more than 60 g (2 oz.) in weight. This is another western species, being found more or less throughout the drier and more mountainous bits of the United States. It is also, however, found throughout most of western and central Canada, and (more importantly for our purposes today) roughly between the Great Lakes and the upper Mississippi.

Secondly, we have the eastern chipmunk (Tamias striatus), which is found just about everywhere east of a line between North Dakota and Louisiana, excluding only Florida and the lowland parts of the Carolinas and Georgia. It's also common in southern Canada, from Nova Scotia to the southeastern corner of Saskatchewan. Pretty much, if you've seen a chipmunk in the wild, and you weren't in the western US or Asia at the time, it will probably have been an eastern chipmunk.

Probably, but not certainly. Because there is that bit where they both live together. In the US, that means Wisconsin, Minnesota, and the Upper Peninsula of Michigan. In Canada, its a rough triangle between the north shore of Lake Huron, the south shore of James Bay, and southern Manitoba. In those places, which one you saw depends mainly on how big it happened to be. (If it helps, the least species also has four white stripes between the five black ones, rather than two white and two greyish).

Given that the eastern chipmunk weighs about twice as much as its least counterpart, they'd certainly have no difficulty beating up any that headed for their caches, so you might think that they'd have the edge. But, of course, being well aware of this, least chipmunks are hardly likely to try anything when their eastern rivals are around to see them. In fact, it seems to be the least chipmunks that have the advantage, and they successfully steal far more food from their cousins than the reverse.

This is, at least in part, because they're better at hiding their hoards. Both species tend to look for other animals' caches in the same way - for example, if they find one, they figure that there are likely to be more nearby. But the least chipmunks keep their food stores further apart, and away from the more obvious geographical markers, making them much harder for their rivals to locate. It may also help that their caches are smaller, and likely easier to hide, and that their spatial memory seems particularly good.

But it seems that they may also have another trick up their furry sleeves. Least chipmunks have the unusual habit of sometimes chewing the inedible hulls off seeds before popping them in their cheek pouches and taking them away for caching. Sometimes they make a ball of about ten or so husked seeds, cover them with sticky saliva, and then hack them back up again. After caching, the ball eventually dries out, becoming a solid glob of seeds and hardened goo.

They do this to seeds of plants like sunflowers or pumpkins, where the "seed" is, botanically speaking, a kind of fruit consisting of the actual seed, or kernel, encased in a tough cellulose hull. So far as we know, they are the only kind of chipmunk to do this, although de-hulling at least has also been reported in kangaroo rats, and even in jays. The question is, why bother?

In a recently published experiment, researchers offered chipmunks of both species the chance to find hidden caches of seeds. Some had been de-hulled, some left in their natural state, and others turned into hardened balls - although not by the chipmunk doing the searching. They found that both species overwhelmingly found and stole the natural seeds, mostly leaving the others alone.

This answers one question, at least. If de-hulled seeds are less desirable, or harder to find, they're less likely to get stolen, so it makes sense to do it. But it does rather raise the question of why chipmunks should prefer seeds with the inedible hulls still on them. It's a bit like a hungry man refusing to steal a loaf of bread to make a sandwich because it comes pre-sliced. They're going to have remove the hulls themselves anyway, so they might as well pinch ones where the hard work has already been done.

One possibility is that they're just harder to find. Chipmunks search for seeds as much by scent as by vision, and if the natural smell of the seeds comes mostly from the hull (as well it may, being the outside bit) finding de-hulled seeds may be difficult - unless, of course, you put them there yourself. Rolling the seeds up into a ball might reduce their overall surface area, further reducing the smell of fresh pumpkin seed, especially if you've just gobbed all over them.

Or maybe they just don't like eating food that somebody else has already nibbled. But, whatever the reason, it clearly works for them, and helps least chipmunks steal from their eastern cousins while keeping their own larders safe.

[Photo from naturespicsonline via Wikimedia Commons. In the public domain.]

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