Sunday 10 March 2024

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.

What may be the more obvious pattern is for the animals to space their latrines out along the edge of their territory. This is likely serving as a 'keep out' sign, and the fact that animals from two neighbouring territories are using the same site indicates that it is, in some sense, an agreed boundary, perhaps with competitors feeling each other out to see how viable a takeover bid might be. The number of latrines, and the spacing between them along the territorial boundary may be a balance between the effort involved in maintaining them, and the likelihood of a newcomer entering the territory encountering one.

While such boundary marking is probably the more common purpose of latrines, many are often left deep within the territory, where a random intruder is less likely to encounter them. These likely have a different function and it's worth noting that in species such as badgers that use both patterns, they are distinct - that is, latrines are either in the centre of the territory, or on the edge, but rarely in the zone in between. Here, the information being provided may be more complex than "keep out", as the latrines are more likely used by different members of the same social group, perhaps enhancing social bonds or setting out dominance hierarchies. Or, if they are warnings to intruders who have somehow failed to notice the boundary marks, they could be placed in areas of particular importance to the territory's owner, such as where it finds its best food.

Many different kinds of mammal engage in this behaviour, including primates and hoofed animals, but it's particularly well-studied amongst carnivores. Mustelids are no exception, with almost all species having well-developed anal scent glands that can be used to mark even without depositing dung at the same time. (Sea otters lack the glands, spending so little time on land that there would presumably be little point). The use of latrines, however, acts as a visual signal that complements the scent mark and many use it, with the few sociable species, such as European badgers and giant otters, being the most likely to share sites with others of their kind.

There are, however, a great many species of mustelid and even closely related species within the family may behave differently. A 2017 study seems to have been the first to look specifically at the African clawless otter (Aonyx capensis) and revealed that, like some other otters, they not only rub the ground while they scent mark - which presumably helps spread the scent and may use glands other than the anal ones - but they also do a little dance. At the time, the assumption was that the use of the latrines was a means of demarcating the territories occupied by different "clans" of the otters. Now a second study, by different researchers, has taken a closer look.

I described this species back when I did a survey of the world's mustelid species. They live throughout most of Sub-Saharan Africa outside of dense jungles and arid deserts and have clawless, webless, fingers that they use for dextrously feeling about in the mud for tasty shellfish. Significantly, while they are not as sociable as, say, European badgers, they have what's been described as a clan-based social structure, where multiple males jointly defend the same territory. And, as mentioned above, scent-marking is a part of that.

The researchers mapped the locations of otter latrines at two sites, one at the uMlamlazi Nature Reserve and fish farm and the other at a regular farm a few miles away. They then placed camera traps at three of the latrine sites, to more closely observe what the otters were doing when they visited them.

The study revealed that, of the 38 latrines identified across both sites, the otters had placed the great majority of them at ecotones. An "ecotone" is a place where one type of local habitat gives way to another. Most frequently that was at the edge of a patch of woodland, where the trees stopped and were replaced by tall grass, reeds, or mudflats. But, in this case, other ecotones regularly used to place droppings included riverbanks and roadsides.

This suggests, without closely observing their movements, that the otters are placing their latrines at the boundary of their territories. Placing them on riverbanks could suggest that they might also be used to mark areas with the best access to food, given what otters are eating, but that they don't do so very often implies it's not a major consideration if it is - a riverbank could equally well be the boundary of a territory, since the Mlamlazi River is at least 100 metres (330 feet) wide at this point. 

Another factor about the placement of the latrines is that they tended to be in places with limited plant cover. This suggests that the otters want them to be visible, perhaps also in places where the wind might waft the scent around rather than vegetation keeping it concentrated. There's an obvious advantage to doing that if you're scent marking, but it's worth noting that it indicates the otters weren't especially scared of predators coming across the marks, either.

As for what they were doing at the sites, around a third of the time, visiting otters appeared to just be sniffing the site, as one might expect. But those using the site for its more immediate purpose were also performing what the researchers describe as a "jiggle dance". This involves stomping their hind feet and wiggling their back ends about and its exact function remains unclear. If other otters were watching at the time, this could be seen as a visible adjunct to the marking, showing off as a sign of bravado, but that doesn't seem to be what was happening, so maybe it has something to do with distributing the secretions from the scent glands.

That the latrines seem to be on the edges of territories - presumably clan territories, given what we know of the social life of this species - may imply that, in this species they are less important as a means of sending signals to others already living in that territory. Having said that, several otters were sharing the same site, leading to "overmarking" where one otter adds to the scent already left by another. This could indicate such things as relative social status and, while there's no specific evidence of it here, a female indicating that she's ready to mate would also seem entirely plausible.

There's clearly a lot more to be understood about the behaviour here and the exact nature of the clan-based all-male groupings. How the females respond to that is a clear place to start with. But this is a start, and does indicate some differences between these tropical animals and the river otters of Europe and northern Asia. If they're jiggling on the ecotone, they must have a good reason for it...

[Photo by Grant Hillebrand, from Wikimedia Commons.]

1 comment:

  1. I somehow first misread the title as "Jiggling on the Eocene".