Sunday 17 November 2013

Hold On to Your Nuts

American red squirrel
The squirrel family is a particularly large one, with almost 300 recognised species. About half of these are ground squirrels of various kinds, including such animals as prairie dogs and marmots. The remainder are tree squirrels, which can be found on every tree-bearing continent bar Australia. Tree squirrels aren't a single group in evolutionary terms, because chipmunks are closer to marmots than to, say, fox squirrels, but they do at least have a fair bit in common.

Life for tree squirrels isn't too bad if they happen to live in the tropics, where trees provide abundant food year round. (The Indian palm squirrel has the added advantage of being a sacred animal in Hinduism, but that's another matter). It's all a bit tougher when they live somewhere with proper winters. By and large, tree squirrels don't hibernate - although many ground squirrels do - which means that those in the north need some way to keep eating throughout the year.

The solution for many, as is well known, is to hide nuts, seeds, and other easily preserved food sources in caches, and return to them when the weather gets bad. This can be important in seed dispersal, should the squirrel forget where it put the hoard, or die from some other cause before it can eat it all.

In Britain, the native tree squirrel, and, indeed, the only native squirrel of any kind, is the red squirrel (Sciurus vulgaris). Since the 1870s, however, it has been under increasing pressure from larger, foreign squirrels introduced from the United States. These are eastern grey squirrels (Sciurus carolinensis), simply called "grey squirrels" in Britain, on the grounds that there's no other sort. Over the last 140 years or so, they have almost entirely extirpated red squirrels from England and made a good start on Wales and Ireland, too.

However, it's not just in Scotland and the western half of Ireland that red squirrels remain reasonably common, because they're a very widespread species. They remain common throughout most of continental Europe (although grey squirrels have recently launched an assault on Italy), and across Asia as far as eastern Siberia and Hokkaido in Japan. They're also not the only squirrels that happen to be red.

American red squirrels (Tamiasciurus hudsonicus) are, it has to be said, not very red in comparison with the Eurasian sort, but at least they're not a bland grey, and they live perfectly happily alongside their cousins in and around New England, parts of the Midwest, and the Appalachian Mountains. They're also considerably more widespread than that, being found in the western mountains from Washington down to Arizona and New Mexico, and across almost the whole of Canada and Alaska apart from the north coasts and a few islands, such as Newfoundland.

And, while the winter is undoubtedly pretty noticeable in, say, Maryland, it's even more so in Canada. Which means its really important for American red squirrels living there to maintain a good supply of food. And to remember where the heck they put it.

They do this by keeping a single stash in one location - unlike the red squirrels of Eurasia, which hide smaller caches all over the place.  Like many mammals, each American red squirrel has its own territory, and visits those of its neighbours only to mate. So far as possible, they keep the same territory throughout their lives, so obviously, it's of interest to the squirrel to try and pick the very best one they can, with the best available food supply.

They don't seem to fight over territory very much, and adopt a "finders, keepers" strategy, where smaller squirrels stand up for their rights even against larger ones, and everyone tends to stay where they started. This is probably because it's easier to hide and escape from predators in territory that you know really well. Young squirrels get their territories in the first place by one of two methods. Firstly, it seems that some mothers break the usual rule by bequeathing their territory, and its existing cache of food, to one or more of their offspring, and heading off somewhere else. This gives their young a head start in life, and increases their overall chances of survival.

The second alternative, assuming that no new forest is magically appearing from somewhere, is to wait until another squirrel dies, and then move in to their old home. Being fairly short-lived animals - only two or three years, on average - this isn't necessarily a very long wait.

However, while squirrels may be wary of pinching territory off living competitors, it's a different matter when the territory is empty and there for the taking by anyone in the neighbourhood. So we might expect that younger, smaller, squirrels have territories with less available food than their older neighbours - whether because the territory itself is smaller, or because it's less fertile. And that, we'd assume, is why barely more than 20% of young squirrels make it through their first winter. Essentially, all the best places have already been taken.

But just how much of a difference does it really make? How much does the size and quality of a squirrel's territory affect the size of the food hoard it bothers to collect, and how much does that affect its chances of reaching the next spring?

A recent study, led by Jalene LaMontagne, looked into these questions for American red squirrels living in southwestern Yukon. In that area, the primary food source for the squirrels is Canadian spruce. The squirrels clip and store the cones from these trees in their caches, husking them to get the seeds out through the winter and early spring. For four years, the researchers monitored the number of cones available to the squirrels, how many ended up in their caches, and what that meant for the squirrels' survival.

The years in question were what botanists call "non-mast" years, when the spruce produced less cones than normal, making access to food resources particularly important. As expected, younger squirrels tended to have smaller territories, and in areas where the trees had fewer cones. Naturally, this had an effect on their survival, but not as much as might have been expected.

In fact, it seemed that, on average, the number of cones available on the trees within a squirrel's territory had nothing much to do with the size of their horde. Presumably, the trees produced more cones than the squirrels could reasonably harvest, and what really mattered was how many cones the squirrels had managed to, well, squirrel away. The two don't correlate because production of cones varies so much from year to year that a good territory one year might be a bad one next winter. All in all, since the squirrels stay put from year to year, it just seems to even out over time.

There was a slight effect of larger territories meaning a better chance of survival, regardless of the size of the cache. The researchers assume that the squirrels must have been eating something else besides the stored food, and larger territories meant there was more of it. Mushrooms are a likely possibility here, although they're probably only something the squirrels eat when they get hungry.

The upshot of this is that the quality of a squirrel's territory is less useful to them than you'd think: it changes so much from year to year, at least in this part of the world, that it really doesn't help them much. What does matter, and matters a lot, is the size of their horde, perhaps accumulated over a long period of time.

In short, squirrels really do need to hold on to their nuts. Or cones, as the case may be.

[Picture by "Connormah", from Wikimedia Commons]

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