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]

Sunday, 12 December 2010

Carnivores, Carnivorans, and Carnassials

Some time ago Irregular Webcomic ran, as one of its occasional polls the question "What do you consider the defining characteristic of a carnivorous animal?" Most of the answers, as you might suppose, centred around the extent to which the animal ate meat. But almost 10% of respondents replied "a member of the order Carnivora."

Now, this has to be interpreted in the context of an earlier poll which, in a game of twenty questions, asked whether the subject of the game was carnivorous or not, it having already been established that the animal was mammalian. If people were interpreting the later question in light of that, rather than answering the actual question as written, we might suppose that they were mentally ruling out anything non-mammalian. Otherwise 10% of IWC readers must think that, for example, crocodiles and sharks are not carnivorous. Which seems a bit unlikely.

Now, in the ecological sense, a carnivore is really any animal that eats other animals. That includes two huge groups within the mammals, namely the bats and shrews. (Although not all bats eat insects - there are plenty of fruit bats and flower bats). For the sake of simplicity, though, let's assume that we refer only to relatively big scary creatures that eat the flesh of other vertebrates. Some people, after all, might not consider insects to be "meat", even if they obviously aren't a vegetarian option, either. So, no bats, shrews, or hedgehogs, among others.

So, if we rephrase the question as "what do you consider the defining characteristic of a carnivorous (meat-eating) mammal?" does the answer "a member of the order Carnivora" make any more sense?

In the Linnean system of classification, on which most modern attempts at grouping animals together are based, an "order" is the next main level down below "class". Mammals, taken as a whole, are a class, and this class is divided into, according to the most common current scheme, 29 living orders. One of the larger orders is the Carnivora, the group to which most carnivorous mammals belong.

The operative word being "most". Because, ignoring extinct animals for the moment, there are at least two other orders containing animals that are fairly obviously carnivorous. One of these contains the carnivorous marsupials. Now, most of these are small animals such as dunnarts, which mainly eat insects and other small invertebrates. But the group does include the quolls, which are about the size of a cat and have a similar diet, and, more famously, the Tasmanian devil. The evolution of this group in some ways parallels that of the Carnivora among the placental mammals, the various species having evolved to fit a similar niche in the Australian ecosystem.

The second group of carnivorous, but not Carnivoran, mammals are the whales. Obviously, some of these eat small invertebrates such as krill, or even really big ones like squid, but most do consume the flesh of other vertebrates. Fish, mostly, of course, but consider the killer whale, which is happy to eat seals.

What of the Carnivora themselves? In terms of the number of living species, they are the fifth largest of the mammalian orders, yet clearly outnumbered by the many mammals that are either mainly vegetarian or insect-eating. This makes sense, since plants are everywhere and are generally easier to catch than animals. Since no animal can ever extract 100% of the energy from whatever its eating (short of some kind of nuclear reactor in its stomach), there will always be less herbivores than plants, and less carnivores than herbivores.

The Carnivorans are considered an order because they all descended from one common ancestor, one they didn't share with the killer whales and Tasmanian devils. The defining feature of the order, despite the name, isn't what they eat, but the shape of their teeth.

Take a look at this picture of a fox's skull. We can see a number of features that suggest this is a carnivorous animal. The teeth are large and sharp, especially the long canine teeth. There is also a thin crest of bone at the back of the skull; this is the saggital crest, and the jaw muscles are attached to either side of it. The relatively large size of the crest suggests that the jaw muscles are big, and able to create a powerful bite.

But neither of these features are specific to the order. Instead, the key feature is the one highlighted by the two arrows. These are called carnassial teeth, and, as you can see, they are much larger than the others at the rear of the mouth. They cut together like a pair of shears, slicing through tough meat and breaking bone. Other carnivorous mammals, such as Tasmanian devils, don't have these distinctive teeth, so their presence can reliably tell us whether we're looking at the skull of a Carnivoran or not.

Early on in their evolutionary history, the Carnivorans divided into two main lineages, which we can call, roughly speaking, the cat-like and dog-like carnivores. The cat-like carnivores include, among others, the cat family, which are perhaps the most truly carnivorous of all land mammals, the mongoose family, and the hyena family. The last may little surprising, since hyenas obviously look like dogs, but it had long been suspected they were closer to cats based on the structure of the skull around their ears, and more modern genetic analysis has shown that they are, indeed, more related to cats than to dogs. The dog-like shape is simply a convenient shape for a hunter, and the longer snout means that they can fit more bone-cracking teeth into the jaw.

Seals        Dog-like         Cat-like
etc.        Carnivores       Carnivores
  ^             ^                ^
  |             |                |           Cloven-hoofed
  |             |                |           animals, etc.
  ---------------                |                 ^
         |                       |                 |
         |                       |                 |
         -------------------------                 |
                     |                             |
                 CARNIVORA                         |
                     |                             |

The dog-like carnivores are, in many cases, rather less carnivorous. The dog family itself is pretty dedicated to eating meat (although not so much as cats), but the group also includes both the bear family and the raccoon family, both of which contain many species that are largely omnivorous, and can do without meat if they must. Indeed one species, the giant panda, although it can eat meat, feeds almost entirely on bamboo. It used to be thought, incidentally, that the giant panda might be a really odd-looking raccoon, but we now know that it really is just what it looks like - a bear.

The dog-like carnivores also include the weasel family, which is remarkably diverse, including such animals as badgers and wolverines along with the more obviously weasely ones. But it also includes a number of aquatic families. Until about the 1990s, the seal family and its relatives were considered to form their own order. After all, they do look quite different from the land-dwelling Carnivorans, just for a start because they have flippers instead of legs. But as the rule came to be accepted that any proper group of animals has to contain a single ancestor and all of its descendants, it became obvious, that, since seals are clearly descended from the Carnivora, either they were themselves dog-like carnivores, or the Carnivora was going to have to be split into two or more. In this case, it was the first option that won out.

There are differences between the skulls of seals and animals such as dogs - for example, the nostrils are set much further back on the top of the snout to make it easier to breathe while surfacing. But otherwise, they do have many of the same features, and its obvious that they are related.

Digging back into the mists of time to look at mammals that are long extinct, we find that, while most mammals have always been either omnivores, insectivores, or herbivores, the evolution of relatively pure carnivores has happened more than once. The group that included Hyaenodon, for example, consisted of some pretty fierce flesh-eaters. They did, in fact, have carnassials, but they were in a different part of the mouth than in true Carnivorans and, while they are related, they aren't the same. And there are many other examples, too, often found among groups that aren't generally carnivorous. A recent report in the Journal of Vertebrate Palaeontology, for example, described the fossil of what appeared to be a mainly carnivorous bandicoot - a group that are normally much more omnivorous.

Flesh-eating is a useful adaptation. The food is scarcer and more difficult to acquire, and it takes much more land area to support a carnivore than a herbivore of the same size (because whatever the carnivore is eating needs room for its own food as well). But, so long as you don't have too many competitors, the food is quite nutritious and its much easier to digest than most plants. Its useful enough that its evolved more than once, and so not all carnivorous mammals are necessarily members of the order Carnivora. Indeed, as the existence of the giant panda shows, not all Carnivorans are necessarily carnivorous.

Indeed, that carnivorous animal in the twenty questions game turns out to be either a bear or a weasel, which means that there could be a 50/50 chance its actually an omnivore. Possibly even one that subsists primarily on marmalade sandwiches...

[Picture adapted from one at Wikimedia Commons. Modified version freely available under the terms of the Creative Commons 3.0 license]

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]