Sunday, 5 December 2010

How to Steal Food and Get Away With It

Merriam's kangaroo rat
Many small rodents hide some of the food they have collected in caches scattered about their local environment. This is a useful thing to do when finding food might be difficult or unpredictable. When you do find some, its best to keep some in reserve in case it gets harder to find fresh food later on. In temperate environments we often associated this with winter - squirrels hoard away nuts so that they can find them once the winter comes and nuts are in short supply.

But it can be common anywhere that food is likely to be scarce, such as desert environments. If you find a sudden bonanza of more than you can eat, it makes sense to keep some of it safe to eat later. But there's a downside to this, in that some other animal might find and eat your secret stash. There really isn't much point in caching food if it's more likely to be eaten by someone else than by you.

One way to avoid this, if you're a really small rodent, is just to keep all the food in your burrow. Larger rodents then can't get at the food, because they're too big to squeeze into your burrow. But you don't want to keep all your seeds in one basket, as it were, so even small rodents often tend to hide caches of food here and there across the landscape. But do different types of rodent living in the same area hide food in different ways, and how good are they at stealing food from other species?

The kangaroo rat family includes around sixty species native to the Americas, most of which live in relatively dry environments. Despite the name, they have nothing to do with marsupials, and aren't actually rats, either, being more closely related to gophers. Several different species can live alongside one another, which suggests that they must have slightly different lifestyles - if they were all identical, one species would presumably be better at it than the others, and drive them to extinction. A recent study looked at three of these species, to see how they protected their food - and how good they were at stealing it off others.

Merriam's kangaroo rat (Dipodomys merriami) is a relatively large member of its family, with long hind legs adapted for hopping. The pale kangaroo mouse (Microdipodomys pallidus) is a fairly close relative, and looks fairly similar except (as you might imagine) for being smaller. The little pocket mouse (Perognathus longimembris), while also belonging to the kangaroo rat family, doesn't have the long kangaroo-like hind legs, and looks like a fairly typical mouse in appearance. The name "pocket mouse" comes from the fact that it has large cheek pouches for carrying seeds about - a feature it shares with other members of the family, and, of course, with the gophers.

 Pocket   Spiny Pocket      Kangaroo     Kangaroo
  Mice        Mice            Rats         Mice
    ^           ^              ^            ^
    |           |              |            |
    |           |              |            |        
    -------------              --------------       Gophers
          |                          |
          |                          |                 ^
           ---------------------------                 |
                       |                               |
                       |                               |

All three species can be found together in the deserts of Nevada. We already know that the pocket mice prefer to get their fresh seeds under the cover of the local shrubbery - which, in this part of the world tends to mean tumbleweed. Or at least it does these days, since tumbleweed is an Asian plant only introduced into America in the 19th century; there would, however, have been plenty of clumps of grass for the pocket mice to hide in before the tumbleweed arrived.

Little pocket mouse
At any rate, the other species are happier out in the open. Which might seem a little odd, since you'd think they're more likely to be eaten by larger animals while there. But it does mean that they can get at seeds that the pocket mice are too timid to reach, and their ability to rapidly leap away from a threat may make this less of a problem than it is for their mouse-shaped relatives.

In the first trial, each animal was given around 12 grams of seeds, and then allowed 24 hours to hide them wherever they wished. Each species showed a noticeably different tactic. The kangaroo rats buried their seeds out in the open, the kangaroo mice buried theirs under the edges of shrubs, and the pocket mice buried theirs as far as they could under cover. This is a little surprising for the kangaroo mice, since they don't normally spend much time around heavy cover, but it may be that the bigger kangaroo rats are more likely to steal food from them if buried out in the open. And, while heavy vegetation probably makes it harder to leap away from a predator without whacking into something, that may well be less of an issue for the smaller kangaroo mice than it is for their bigger relatives.

For the other two species, this still leaves an open question. Do they bury their food in those locations because it's just where they happen to be, and where thy feel safest, or because they don't expect the other species to come in there and steal it? In the second trial, the researchers buried small caches of food at random spots across an enclosure, and gave the animals 24 hours to find it. The question being - where would they look?

While the researchers couldn't find any kangaroo mice to test this way (the originals having been returned to the wild), the pocket mice turned out to look pretty much everywhere. Not having any of their own food with them might have made them hungry enough to venture out into the open, and the fact that their weren't any big kangaroo rats out there to chase them away probably helped to. But it does indicate that kangaroo rats simply putting their food out into the open is no protection against it getting nicked, because the pocket mice will look there if they can.

The kangaroo rats themselves were actually more likely to look under shrubs... and then to cart the food off and go and bury it out in the open. This would suggest that hiding food under a bush is a particularly naff tactic, since that's the first place anyone else is going to look for it. Clearly, the pocket mice are more worried about being eaten by predators - or chased off by bigger rodents - than they are about their dinner being stolen.

On the other hand, if their food keeps getting pinched, why don't they die out? It could be that, as mentioned above, they normally hide most of it in their burrow, which the kangaroo rats are too big to get into. Burying it under bushes may just be a back-up plan that isn't crucial to their survival.

Wandering about looking for another animal's secret stash of food is a bit of an effort, and it may be noteworthy that rodents in more fertile environments, such as chipmunks, often can't be bothered. They know where they put their own hoard, and that's quite enough. In the desert, it seems, you just can't get away from it; if you hide food, some of it will be pinched before you can get back to it. Which makes it all the more important that you're prepared to steal somebody else's to make up for what they stole from you.

Just remember to look in the right place.

[Pictures from Wikipedia and Wikimedia Commons]

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