Jiggling on the Ecotone

It might leave a slightly different message if a human did it, but leaving piles of droppings in the territory around your home can be an important signal for many mammal species. Although making such piles visible may help other animals find them, the primary signal is, as one might expect, the smell. And not just the smell of the faeces, of course, but complex chemicals mixed in with it from urine or the secretions of anal scent glands. These can allow an animal with a sense of smell more subtle than our own to glean a lot of useful information about who left the deposit - and why.

In many species, this takes the form, not of solitary deposits, but of latrines. In the zoological sense, this refers to a single location for defecating shared by many animals of the same species. The animals who use the site may belong to a particular pack or herd, all using the same communal site, but they could equally well be rivals or neighbours leaving messages for one another. How the latrines are distributed can give researchers significant clues about what those messages might be.

Vishnu's Otter in Bavaria

Otters are members of the weasel family, something that has been recognised since the dawn of taxonomy in 1758, when Linnaeus placed them in what was then the weasel genus (taxonomic families not yet being a concept). Their adaptations to the water, including their powerful muscular tails, were long thought significant enough that they were separated out into a distinct subfamily, a position they still retain, although it turns out that they are more closely related to the weasels proper than to some other members of the family such as, say, pine martens or wolverines.

Perhaps because rivers are a good environment for forming fossil-bearing sites, we know of a significant number of fossil otters and, most of them would doubtless have been instantly recognisable as otters were we able to see them in the flesh, there is perhaps more variety amongst them than we might at first expect. For example, a number of them are quite large - in some cases, larger even than the "giant otter" (Pteronura brasiliensis) of today's Amazon, the largest living member of the weasel family. (So, yes, it's bigger than a wolverine).

Giant Wolverines of South Africa

Modern wolverine

Africa is the world's second-largest continent, containing about 20% of Earth's total land area. Despite this, it's probably fair to say that its the least palaeontologically explored continent - with the obvious exception of Antarctica. But this is not to say that there aren't several very good fossil sites in Africa, many of which have contributed significantly to our knowledge of animal evolution.

The most famous of these sites are those connected with the evolution of our own species, but there are a number of sites that have yielded, for example, significant dinosaur fossils. When it comes to mammals other than humans (and whatever else was living alongside them), Kenya and South Africa have proved particularly rich sources - although, of course, there are others.

Miocene (Pt 17): A Diversity of Dogs

Eucyon

As North America dried out in the Late Miocene, and slowly transitioned towards the deserts and open grasslands that became dominant across the central parts of the continent, it wasn't just the herbivores that were forced to adapt. Carnivores may not have been affected so directly, but, if their food source was changing, they had to do so too.

There were two particularly significant changes among the North American mammalian carnivores that took place around this time. One was the extinction of the bear-dogs, a group with a long history on the continent that was, by this time, typified by relatively large animals that would have looked more like bears than dogs (although they were neither). Over in Europe and Asia, the bear-dogs survived for rather longer, although even there they struggled...  but in North America, the changes were too extreme for them to cope with, and they died out as the Late Miocene dawned.

The Giant Chinese Badger-Otter

Otters and badgers are both members of the weasel family, the Mustelidae. As a result, while they are visibly quite different, they have a number of anatomical similarities, reflecting their shared ancestry. Indeed, some of the similarities, such as those in the precise shape of their teeth, are rather greater than one might expect simply from them belonging to the same family. On the basis of this, it was being suggested as recently as the 1990s that otters were essentially aquatic badgers - descendants of early badger species that had entered the water, developing webbed feet, a long muscular tail, and so on, in the process.

We now know, from various genetic and molecular studies, that this isn't so. The closest living relatives of otters are probably the weasels themselves and/or the zorillas and their kin, with badgers representing a rather earlier branch in the mustelid family tree. Given this, the apparent strong similarities between the two are either a case of parallel evolution, perhaps due to the fact that, by the standards of weasels, they're both fairly large animals, or, perhaps more likely, that they are ancient features of the group that happen to have been lost in their other relatives.

Ancient Otters of Idaho

The common river otters of Eurasia and North America are remarkably similar animals. If you placed them side by side you might notice that the Americans ones are (on average) slightly larger, and have a slightly more bland colour to their coat... but, really, it's not easy. I, for one, wouldn't claim to be able to tell which species a particular photograph was of without additional evidence (most obviously, where it was taken).

It was therefore assumed for a long time, not unreasonably, that they had to be very close relatives. As early as 1843, they had been placed in separate genera, as Lutra lutra and Lontra canadensis, respectively. However, it's worth noting that this is 16 years before the publication of On the Origin of Species, so, when he did so, it's isn't obvious that John Edward Gray was necessarily thinking about how "related" they might be. Indeed, for much of the 20th century it was common to ignore Gray and place all of the fairly typical looking clawed otters together into the genus Lutra.

Growl Whistle Squeak

While scent marking, for example, is great for leaving long-lasting messages, for other purposes, vocal communication has a number of advantages. However, since non-human animals can't talk, this necessarily imparts less information than it does in our own species. But how much less? Or, to put it another way, how complex can animal vocalisations get?

The number of different sounds an animal can make depends on a number of factors. Many of these are physical, due to the way that their larynx and vocal cords are set up, and to their ability to modulate the sounds it produces with their mouth, lips, and so on. For instance, while there has been at least some success in getting chimps and gorillas to use human sign language, they can't actually speak because of purely physical limitations in their upper respiratory tract. Added to this is the matter of just how much complexity they need to get across anyway.

Broadly speaking, the more sociable an animal is, the more need it has for complex communication. If you rarely come across other members of your own species, you probably don't need to say much when you do. A simple "go away" is probably about as much as you need, and you can co-opt the same threat against hostile members of other species, too. Beyond that, you may need some kind of mating call, and a means for mothers to find their offspring, and you're pretty well sorted.

What Does the Otter Say?

Animals use a variety of different means to communicate with one another. For we humans, speech is the primary means of communication, but even we also have body language as a useful adjunct. (And writing, of course, although that's arguably just a representation of speech, and it's not, that we know of, something our species has always done). For other mammals, scent is usually at least as important.

The human sense of smell is pretty rubbish, by broader mammalian standards, but for those species where it functions effectively, scent can be an effective and useful means of communication. All you have to do is urinate on a lamppost or do what bears do in the woods, and you've left a long-lasting message that conveys all sorts of handy information to anyone who comes by later. But, on the other hand, it does have its limitations. In particular, it's not much good for rapid signalling, such as alarm calls. So, while we may be the only species to "speak" (in the sense of having complex syntactical rules), it's not surprising that we aren't the only ones to make vocalisations.

Having said that, unless you're a whale, vocal calls are really only useful for communicating with other animals that are at least in your general vicinity. As such, they're much less useful to animals that don't lead rich social lives. If you live all your life on your own, then it may be that the only sound you need to make is something to warn off intruders, and perhaps a little something for the breeding season. Apart from that, scent may well be sufficient to meet your needs.

Weasels at Sea: Sea and Giant Otters

Sea otter mother with pup

Most otters are considered to be 'semi-aquatic' animals. That is, they spend most of their time swimming in rivers or lakes, where they catch the great majority of their food, but they return to dens on the bank in order to sleep and raise their young. However, there is one exception: an otter that is fully aquatic, and is, in fact, the only truly aquatic mammal to have feet, rather than flippers - all the others are seals, whales, dolphins, and the like. This is the sea otter (Enhydra lutris).

Most other otter species will enter salt water on occasion, especially if they live on small islands where fresh water is scarce. The marine otter is unusual in that it habitually hunts at sea, and only occasionally enters rivers, but it still creates dens along the shoreline, and so is considered only semi-aquatic. Generally when otters do swim in the sea, they avoid water that's any deeper than most rivers, and even marine otters won't venture more than about 150 metres (500 feet) offshore. Not so the sea otter.

Sea otters live along the coasts of the northern Pacific. In the south, they reach as far as California, and once even reached the west coast of Mexico, but the majority are now found off the southern coast of Alaska, and there are also sea otters along the coasts of far eastern Siberia and the extreme north of Japan. They are not close relatives of the other American otters (Lontra spp.), but are instead later arrivals on that continent, closer in origin to the various Old World species.

Weasels in Warm Rivers: Otters of the Tropics and the South

Hairy-nosed otter

The body plan of otters is evidently a successful one in evolutionary terms. While the two species of "common otter" are the only ones inhabiting the rivers of the northern temperate zone, there are at least ten species in other parts of the world, and most of them look remarkably similar. They all have long, sinuous, bodies with short legs and webbed feet. Their tails, quite unlike the slender or stumpy appendages of other members of the weasel family, are powerful and muscular, and somewhat flattened to better help propel them through the water when swimming. Their fur is dense and, in most cases, short and sleek.

Furthermore, they are all more or less the same colour. Otters, of all species, are brown over most of their body, and they usually have paler underparts with a particularly noticeable patch of pale fur on the chest, which may extend up onto the chin. Compared with the variety of weasels, or even martens, the visible differences between most species of otter can be subtle, and it's hard tell them some of them apart.

The closest living relative of the familiar Eurasian otter is the hairy-nosed otter (Lutra sumatrana) of South East Asia and western Indonesia; the two species probably diverged during the early Ice Ages. It is also a highly endangered species, and, combined with its remote jungle location, that means we know relatively little about it. This story of threats to otters is, sadly, one I'll come back to a lot in this post. Of all the members of the weasel family, the otters are the ones most at risk, probably because their riverine habitat makes them particularly susceptible to pollution, and may also make them easier to hunt for their valuable fur.

Weasels in Cool Rivers: The Common Otters

Eurasian otter

The otters are, arguably, the least weasel-like of all the members of the weasel family. Although the relationship has been clear for a long time - the first scientific description of an otter, by Linnaeus in 1758, actually placed it in the genus Mustela, to which stoats and weasels belong - so have the differences. As early as 1771, Danish zoologist Morten Thrane Brünnich split them off from their fellow mustelids, erecting the new genus Lutra to describe them.

By 1838, at least three genera of otter had been described, and the group were raised to 'subfamily' status to distinguish them from their terrestrial kin. Other subfamilies were erected later, largely to distinguish the badgers, but the otters have remained as a clearly distinct group within the overall weasel family.

Genetic studies in the last four years have given us a much clearer picture of how the various kinds of mustelid are related to one another, and it turns out to be more complicated than we thought. For example, martens turn out to have been around longer than otters have, which was something of a surprise. The otters, however, survived the revision unscathed, confirmed as a genuine evolutionary group, albeit one that is younger than we might have guessed. Their closest relatives within the family turn out to be the mustelines, the group that, among others, includes the semi-aquatic mink.

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]