Saturday, 31 March 2012
The Life of an Endangered Porcupine
Of all the porcupines, though, one of the strangest is the thin-spined porcupine (Chaetomys subspinosa) of eastern Brazil. Although it was first described way back in 1818, for most of the twentieth century, no scientist had even seen one, until a survey in 1986 confirmed their continued existence. Much smaller than other porcupines, and more bristly than spiny, there has been considerable debate as to whether it even is a porcupine. Instead, it's been argued that it might actually be a type of animal called a spiny rat, another fairly odd family of rodents which it does certainly resemble.
The question was settled in 2009 when genetic and chromosomal studies proved, quite conclusively, that it is a porcupine, albeit an unusually small and rat-like one. Indeed, its ancestors seem to have separated from those of all other American porcupines as far back as the early Miocene, around 20 million years ago. In comparison, the next branch we know of within the family is the split between the North American porcupine and its assorted South American cousins around 10 million years ago, with all the other branches being considerably more recent than that.
N. American S. American Thin-spined
Porcupine Porcupines Porcupine
| ^ | Guinea pigs,
| | | etc.
| | | ^
--------------- | | Old World
| | | Porcupines
| | | ^
----------------------- | |
| | |
| | |
Thin-spined porcupines live in the coastal forests of central eastern Brazil, an area that is particularly prone to logging. This is a problem for the porcupines not just because the overall area of forest to live in is declining, but because the patches that do remain are becoming increasingly isolated from one another. Since the porcupines won't travel between separate patches of woodland, that means that surviving populations become increasingly inbred. As a result, while the species is not officially classified as endangered, it is considered 'threatened', and is clearly vulnerable to habitat loss.
There have been some studies to learn more about the animal since its rediscovery in the 1980s, but not very many, and it remains somewhat enigmatic, and certainly not very well known to the outside world - compared with photogenic big cats and pandas, there isn't much love for rodents. But the most recent study, by Pedro Oliviera and colleagues of the Pontifical Catholic University of Minas Gerais, gave new insights into how these unusual porcupines live their lives.
First, find your porcupine. This has been something of a stumbling block with previous studies, since they seem to be quite good at hiding (remember, they aren't very large or colourful, and live up trees in the jungle). Searching in the relatively open restingas forest of Espírito Santo, they managed to find three female porcupines, and sent their most agile field assistant to fetch them down from the tree so that they could be radio-collared. On the not unreasonable assumption that porcupines probably don't like being secured by their tails (the only bit that doesn't have spines) and taken down from wherever they've been sleeping, they then waited three weeks before tracking them, to give them time to start behaving normally again, and get used to the collars.
One of the things we can tell from this kind of study is the size of the animal's home range - that is, the area of land over which it habitually travels. This will vary, depending largely on the availability of food, but it gives us some idea of how much land an animal needs to feel comfortable and stay healthy, which can be important in conservation efforts.
It turns out that the home ranges of the thin-spined species are unusually small for porcupines, with this study showing an average of about 2 hectares (5 acres). That's probably at least in part because they are smaller animals, and is likely also related to their relatively low calorie diet, which consists almost entirely of leaves. When your food isn't very nutritious, and isn't in short supply, it makes sense to spend as much time as possible eating it, and relatively little expending energy travelling about. Indeed, while they have been previously described as fairly athletic, leaping about between trees, the only published studies show them moving about slowly and cautiously, perhaps conserving their energy. (As an aside, contrary to the current wikipedia article on this species, these porcupines not only have gripping claws, but also prehensile tails, which help them move through the branches).
It may be relevant that all of the porcupines used in this study were female, the researchers having failed to find any males to track. Males of many mammals have larger home ranges than females, often so that they can meet more members of the opposite sex on their travels. Indeed, one of the females in this study turned out to have been pregnant, and was looking after an infant while the study was ongoing. Perhaps unsurprisingly, she was the one that travelled the least.
The home ranges weren't exclusive, as they are for some animals. In other words, the porcupines don't fight each other for space or drive neighbours away from 'their' trees; their home ranges overlap, and not just between different sexes. As is typical, within the home range, they visit some locations more than others, and it turns out that they use specific trees for one of at least three different purposes.
Thin-spined porcupines are nocturnal, and spend the day snoozing among branches. Some of the trees, then, are their favoured roosts, of which there will be several within their range. Although they don't seem to build nests, they do select trees for the amount of vines and heavy foliage they contain, roosting in the most densely covered patches. Which may well explain why they've proven so difficult to see!
The only time they are moving about, then, is during the night, and even then, they almost never come down to the ground. Most of the trees they visit at night are the ones they use for feeding, and its significant that these are rarely the same ones that they sleep in. Evidently, the ones with the tastiest leaves - Pera and Tapirira - are not the ones with the best cover and camouflage for using during the day.
The third main use to which they put trees is as a latrine site. Each porcupine had just one or two trees from which it defecated every evening after it got up. They climbed down to the lower branches, aiming at a dense patch of spiky bromeliads on or close to the ground. That they consistently used the same sites may suggest that this helped to advertise their presence to other porcupines, perhaps by scent marking.
Indeed, the animals seem to have scent glands, a fact that had previously been suspected, but never formally published. As the animals moved about in the trees, they were often seen sniffing the branches ahead of them, and frequently used the exact same routes while doing so. That suggests that they may be able to create scent trails to follow, perhaps even helping out close relatives or sexual partners sharing the same territory.
At the end of the study, the surviving animals were re-captured, had their tags removed, and were allowed to return to their normal lives. One had died of unknown causes in the interim, and that happened to be the one with the infant. The infant, however, survived; having been weaned at the age of around five months, it had left home not long before its mother's death, carrying on the legacy of its species.
Understanding how these porcupines live, and how they use their landscape may be important for future conservation efforts of a unique animal that few have ever heard of.
[Image from sosespecies. Cladogram adapted from Vilela et al, 2009]