Saturday 26 November 2011

The Secret Life of the Monito del Monte

There are something like a hundred species of marsupial in South America; hardly an insignificant amount. As I've mentioned before, the marsupials were, in fact, in South America before they ever reached Australia. While some headed south over the still green and verdant lands of Antarctica to reach the Australian continent beyond, others stayed behind, becoming the ancestors of the opossums and shrew-opossums that still live in the Americas today - including, of course, one species in the southern US.

But there is an oddity that confuses this simple picture, and that is a curious animal called the monito del monte (Dromiciops gliroides). In evolutionary terms, this gives every indication of being an Australian marsupial, being more closely related to animals like kangaroos than it is to American opossums. Which is a bit odd for an animal that lives in Chile and Argentina. How can this be so? The best guess is that the ancestors of Australian marsupials originated in South America before they crossed over to their new home, and that, for some reason, the monito del monte was the only one to survive in their original homeland. It is, however, also possible that the Australian marsupials really did originate in Australia - or, for that matter, in Antarctica - and that the monito del monte headed back to South America at a later date before the Straits of Magellan opened up.

Kangaroos,     Monito del     Bandicoots,
   etc.          Monte           etc.
    ^              |              ^
    |              |              |
    |              |              |         Shrew-
    ----------------              |        Opossums
           |                      |            ^
           |                      |            |        Opossums
           ------------------------            |            ^
                     |                         |            |
          "Australian" Marsupials              |            |
                     |                         |            |
                     ---------------------------            |
                                  |                         |
                                  |                         |

As you can see from the above, the shrew-opossums are also closer to kangaroos than they are to regular opossums. That, however, isn't an issue, because the Australian marsupials could easily have left for their continent after diverging from the shrew-opossums. The real problem is that the monito del monte is closer to kangaroos than bandicoots are, and the latter are uniequivocally Australian. Some analyses have even shown that the Australian carnivores (such as the Tasmanian devil) may have branched off before the monito del monte did.

Whichever way it happened, the monito del monte has been isolated from its Australian kin for many millions of years, and is, as a result, quite a unique little animal. It is not just the only member of its family, but the only member of its order - a higher level grouping, intended to be on a par with, for example, "primates" or "rodents". Yet, because it's relatively rare, is nocturnal, and lives only in remote forests in the Andes mountains, we don't know all that much about it.

The animals are moderately sized - about a foot long - and apparently spend most of their lives in the trees, preferring southern beech forests, stands of bamboo, and anywhere with plenty of vines and other tree-creepers. They're quite picky about their habitat, which is a problem, considering the advance of agriculture in the area. They are considered "near-threatened", which is to say that they may not be in danger yet, but that could easily change. However, given how little they've been studied, it's not easy to know how well they're really doing.

One of the first studies of population trends in this species was recently published by Marcela Franco, of the Southern University of Chile, and co-workers, who also took the opportunity to examine several other features of the animal's lifestyle.

Over four years, they found a total of 163 individuals in a 60 hectare (150 acre) patch of woodland north of Valdivia, in Chile. That's rather more than previous studies suggested would be found in such a small area, which suggests either that they were better at finding the animals (which is perfectly possible, given the limited number of prior studies), or perhaps that the animals were becoming crowded into relatively small forests as the growth of agricultural land prevented them from leaving. At least as importantly, there was no real change in the population over the four year period of the study, which at least suggests that the animals are holding on reasonably well.

By examining and weighing the animals, they found that, while females are generally bigger than males anyway, they particularly put on weight towards the end of summer. This makes sense, given that the monito del monte hibernates when the weather gets cold (indeed, the scientific name "gliroides" means "like a dormouse"), so animals will want to put on weight as the summer draws to a close, to prepare for the long winter snooze. This was, as expected, most notable around the tail, where these animals seem to store most of their fat reserves, using it rather like a camel's hump.

In addition to confirming that the animals are nocturnal, being active mostly between midnight and dawn, they also looked into how they sleep. It had previously been reported that the animals huddle together in spherical nests during the day, and one possible explanation for this would be to keep each other warm. Small animals lose heat more readily than larger ones, so it would not be surprising if the monito del monte, an animal that we know doesn't like the cold, would be among the many that do this.

But, apparently that's not the reason. Because the study showed that the animals were less likely to sleep together during the winter, and, even when they did, the huddles were, on average, smaller. If you cuddle up mainly to keep warm, it would make no sense to do it mostly during the summer, and hardly at all when the weather was at its coldest. Instead, they are huddling together the most during and immediately after the breeding season, which suggests its more a matter of looking after young, and of being relatively social animals that prefer to stay with their relatives, something that's presumably less important in winter when they're all asleep anyway.

All of this is important if we want to prevent the monito del monte from becoming endangered - something that it has, so far, avoided. Understanding when animals are active helps us know what other animals might eat them, and their social behaviour, habitat preferences, and typical population densities are all important to working out how they will fare as their forest environment becomes more fragmented. As with so many animals, it's not just their own well-being that's at stake; they play an important role in the ecology of their local habitat. For example, they are only known animal to spread the seeds of a local form of mistletoe that is, itself, a key part of the overall health of these particular forests.

And a little more light has been shed on a rather curious corner of the world of mammals.

[Picture by Jose Luis Bartheld, from Wikimedia Commons. Cladogram adapted from Asher et al. 2004]

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