Sunday 19 December 2010

When to be Awake

The activity patterns of mammals fall into three broad categories. Diurnal animals, such as humans, are active during the day, waking up in the morning and going to sleep at night. This has the obvious advantage that you can see where you're going, but does also make it easier for predators to spot you. The opposite is, of course, being nocturnal - active during the night, and resting during the day. The darkness may help hide you from predators, although you'll naturally need some means of finding your own food, whether its by exceptionally good eyesight (as is the case, for example, with owls), or relying on senses other than vision (as in bats). For animals in many areas, the simple fact that humans aren't around much at night may also be an advantage.

The third pattern is crepuscularity. Crepuscular animals are active mainly at dawn and dusk, resting both during the night and during the day. This gives the animals something of the advantages of both the other modes of activity, and it is rather more common than is often recognised. Such animals can keep out of the heat of the day, and find it easier to avoid predators in the twilight, but still have enough light to see by when they're searching for food.

For at least some animals, this is clearly a response to the lighting levels themselves, rather than just an instinctual daily rhythm of getting up twice a day. This is apparent because they also tend to be more active on bright moonlit nights, which give them enough light to see by long after they'd normally go to sleep. In contrast, its generally thought that truly nocturnal animals don't much like the moonlight, which eliminates some of the advantages that the darkness normally gives them.

It's not always as easy as one might think to work out whether an animal is genuinely nocturnal, or just crepuscular. But this can be important if, for example, you want to know how many animals of a particular species are around for conservation purposes. For a start, what exactly do we mean by "active"? Clearly, not being asleep is a good start, but given that, in the wild, you can't generally watch particular animals 24 hours a day, what exactly they're doing when they're awake may make quite a difference.

The North American river otter (Lontra canadensis) is the animal most Americans think of when they think of otters. They're the only species of freshwater otter in North America, and are found throughout much of the US and Canada. Americans often call it the "common otter" as a result, but this isn't terribly helpful, since that's exactly what Europeans call the European otter (Lutra lutra) - which, despite the name, is found everywhere from Britain to Vietnam. Otters form a natural group within the rather diverse weasel family, and there are many species besides these two, found in Africa, South America, and Asia, plus the sea otters of the north Pacific coast.

N. American     S. American     European      Giant
River Otter       Otters       Otter, etc.    Otter
     |              ^              ^            |
     |              |              |            |
     ----------------              |            |
            |                      |            |       Weasels
            |                      |            |          ^ 
            ------------------------            |          |
                       |                        |          |
                       |                        |          |
                       --------------------------          |
                                   |                       |
                                   |                       |

The North American river otter is generally thought to be crepuscular. But it turns out that the story isn't quite so simple. In the recently published study, a number of otters were caught and radio-tagged in the Whiteriver area of Minnesota, then released into the wild and followed for about three and a half months. Crucially, the researchers looked at two different measures of the animals' activity. Like many mammals, male otters consistently wander across a far wider area than females. Indeed, in this case, males occupy home ranges of around thirty square kilometres (twelve square miles), and females only around a third of that. Since the males' ranges overlap with those of several of the females, this gives them plenty of mating opportunities, and this is also a fairly common pattern amongst mammals. On the other hand, otters are pretty social animals, and don't tend to fight each other when they meet or defend their territories aggressively.

So, one measure of activity is to see when radio-tagged animals are travelling between different parts of their home range, and their locations are being picked up by different receivers. This is the usual way that these sorts of studies are done, and an animal is fairly obviously active if it's moving from one area to another. The results showed that, as expected, the otters were most likely to move about just after sunset and just before dawn. Males moved about the most, especially during the breeding season, when they were actively searching for mates.

However, just because the otters aren't moving between different areas that doesn't necessarily mean they're asleep. So another possible measure is to look at the signal strength picked up by individual receivers. If that's changing a lot, then the otters presumably aren't sleeping in their dens, and must at least be doing something. When the researchers looked at this measure, they found that the otters were active throughout the night. So this means that the otters are basically nocturnal, and just travel the furthest during the hours of twilight - the rest of the time they're presumably foraging for food in specific locations.

In fact, when looked at in this way, both males and females are equally active; it's just that the females tend to stick to familiar hunting grounds within the river, rather than moving longer distances. Perhaps surprisingly, both sexes tended to be more active the colder the weather got. Most likely, they needed extra food to keep themselves warm, rather than staying at home out of the snow as a human might. Moonlight, on the other hand, made no difference to them at all. If you're swimming about underwater in a muddy river, it really doesn't matter how much moonlight there is anyway, and otters mainly seem to find fish using their sense of touch, rather than vision.

With nocturnal animals, you'd normally expect that they'd actually be less active during the full moon, something that's the case with many small mammals. This is largely because they're more likely to be spotted by predators when the moonlight is relatively strong, but that would be less of a concern for otters, which don't have that many predators, since animals like pumas and wolves tend not to follow them into the water.

However, its interesting to note that some smaller nocturnal animals don't seem to care about the moon, either. Another recent study showed that giant kangaroo rats (Dipodomys deserti) are, unlike most of their relatives, also equally active when the moon is full as when it is new. In their case, the suggestion is that, rather than just hoping predators don't see them, they are quite good at looking out for danger and then escaping at high speed before they can be eaten. For any animal that's good enough at doing that, the fact that they can see the predator coming when the moon is full might cancel out the increased odds of the predator spotting them, so that, all in all, it makes little difference.

Such exceptions to the general rules mean that animal behaviour isn't always as clear cut as one might think, even when it comes to something as basic as when they're awake and when they're asleep.

[Picture from Wikimedia Commons]

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