Mice at the Oak Tree Cafe

Forests, it should come as no surprise, rely on a complex set of interdependencies among the native species. The animals that live in the forests rely on the existence of the trees for shelter or food, or feed on other animals that rely on the trees for food. But the trees also need the animals, or at least some of them.

Obvious examples include the reliance of many plants on insects and other animals for pollination. Another is the fact that plants have edible fruit specifically so that animals will eat them and spread the digestion-resistant seeds in their dung. That doesn't work where the animal obtains nutrition from the seed itself, as is typically the case, for example, of plants that produce nuts. But, even here, the plants may rely on scatter-hoarding.

That is to say, many seed-eating animals in temperate regions store food in caches hidden across the landscape - squirrels being among the better-known examples. Some of those caches won't be found again, or the animal that made them will die from other causes before it has the chance to use them. And then, the seeds can germinate - the great majority won't, but it happens often enough that this simple process is of key importance to the survival and growth of some nut-bearing trees.

Many mammals engage in scatter-hoarding but among the key ones in North America are the deer mouse (Peromyscus maniculatus) and its close relative the white-footed mouse (P. leucopus). These two animals look virtually identical; they are around the same size at 8 to 10 cm (3 to 4 inches) in length and both have brownish fur over most of the body with a distinctly offset white underbelly. Despite the name of the latter species, they also both have white feet. Reliably telling them apart requires either genetic evidence or close measurement of certain aspects of the skull and teeth.

So we'd imagine that they must be similar in other respects, too. Yet their distribution across the continent does differ suggesting that their preferences may not be quite the same. The white-footed mouse lives across most of the eastern and central US, parts of Mexico and the extreme south of eastern Canada. The deer mouse, on the other hand, while also inhabiting many of the same regions in the east and centre, also inhabits the west of the US and the entire southern half of Canada but isn't found on the Atlantic coastal regions south of Boston. (For what it's worth, however, neither lives in Florida or the nearby coastal plain of Alabama and Georgia, where they are replaced by two other, also very closely related, species).

This wider distribution is in part, because deer mice are better able to tolerate colder weather than white-footed mice... and this may be more to do with the type of trees that grow where it's colder than anything directly to do with the climate. If so, perhaps they aren't eating quite the same food. That would not only explain why one can live where the other cannot but why, in much of the eastern US, the two can live side by side in the same forests without one displacing the other.

To see how the two species differ in their reactions to different kinds of tree researchers conducted what's described as a "cafeteria experiment". That is, you let the animals select what they want from a range of self-service options rather than just providing them with a set menu. In this case, the mice got to choose between the acorns of northern red oaks (Quercus rubra) and those of the burr oak (Q. macrocarpa).

The mice were captured from forests in Maine (releasing any that turned out to be pregnant or lactating) and placed in glass tanks with mesh lids to let in air. The tanks were kept outdoors so that the temperature and ambient sounds would feel as close to normal as possible, but under a roof to protect against rain. The base of the tank was packed with soil, but also included a large wooden platform that the mouse could climb up onto and that held the dish in which the acorns would be placed and a drip water bottle to drink from. The animals were kept for four days and then released; if they didn't fancy any of the acorns they were provided with apple slices and sunflower seeds on the following night so that they didn't get hungry.

The question was, what would the mice do with the acorns - ignore them, eat them, or save them for later by caching them in the soil at the bottom of the tank? 

Well, the first finding was that burr oak acorns are apparently tastier than red oak acorns. This is not surprising, because red oak acorns contain high levels of tannins, which can represent as much as 5% of the dry weight. Tannins taste bitter, and are a common component of many plants - they are partly responsible, for example, for the flavour of coffee. This is why, in fact, if you want to make human-edible food from acorns (and you can) you need to soak them in water repeatedly to leach the tannins out first. 

Burr oak acorns also contain tannins, but much less than red oak acorns do, which probably makes them tastier. Smaller amounts of tannin are, after all, not necessarily a problem, and they are found in many human foods - red beans and red wine, for example, or (where they are swamped with sugar) cranberries. Evidence that it really is the tannins that put the mice off eating the red oak acorns came from the fact that, when they didn't finish eating one, it was the tips - where the tannins are most concentrated - that they left behind. When they started on a burr oak acorn, they tended to eat all of it, although the fact that they're smaller than red oak acorns may also play into that.

(Incidentally, if you're noticing a theme here with the colour of tannin-containing foods, it's no coincidence. Tannins are red or dark brown and are so-named because they are a key ingredient in tanning agents).

However, while burr oak acorns are evidently tastier to both species and ended up being eaten straight away, white-footed mice were far more likely to eat or cache the red oak acorns than deer mice were. Yes, they preferred burr oak, but they didn't seem to mind the alternative, either, whereas the deer mice really didn't like the bitter taste. Not only that, but white-footed mice were more likely to cache the red oak acorns - saving the less tasty food for later in case they get hungry and there's nothing better around.

What seems to be going on here, from the perspective of the oak trees, is a trade-off between palatability and dispersal. If you want your seeds to be dispersed without being eaten, you have to get animals to carry them somewhere, and that requires that they be edible enough for this to be worth the mouse's trouble. Red oaks produce acorns that are not especially edible, so mice frequently ignore them, but when they don't, they often cache them for later, giving them a higher chance of germinating. Burr oaks, on the other hand, produce acorns that the mice are more attracted to, but, for the same reason, they are more likely to get eaten rather than carried elsewhere.

There's evidently a balance here that works for both kinds of tree.

Over the last few decades, as winters have become milder, the northern red oak has been declining in the south and simultaneously expanding further into Canada. As it does so, the range of the white-footed mouse, able to eat red oak acorns, has also been shifting northwards. That, one can imagine, is probably no coincidence. Moreover, there's a good reason to care about this and keep an eye on what's happening. That's because this isn't just important for forestry management, but also for public health.

The white-footed mouse, you see, is a significant carrier of Lyme disease.

[Photo by the US Fish & Wildlife Service, in the public domain.]