Grumpy Old Voles?

Meadow vole (a related species)

It should go without saying that the behaviour of young mammals changes as they approach, and eventually reach, adulthood. Obviously, there's the development of the mating drive, but there are also social changes, as the animal gains independence, first from suckling, and then more generally from its parent(s), often leaving home, whether to live alone or to find a new pack where its potential mating partners aren't also its own siblings. 

Drawing parallels between these changes and the way that humans develop isn't without risk; the complexity of our society, the existence of culture and so on significantly colour how we behave. But that's not to say that such parallels don't exist, and can't tell us anything, even if it's only how we evolved. We are, after all, still affected by our biology and evolutionary history in at least some respects. For instance, it's notable that exactly how animal pups are raised by their mother can affect how they behave as adults, even if the details are going to be less complex than they are with human child-rearing.

One of the species often used to study the development of social behaviour in non-human mammals is the prairie vole (Microtus ochrogaster). As its name implies, this is native to the grassy plains of the American west, and it's common and widespread across much of the central US. Its range is bounded by the Appalachians in the east and the Rockies in the west, although it's also not found east of a line running diagonally from West Virginia to northern Minnesota. In the south, it reaches as far as Tennessee and northeast New Mexico, while in the north it crosses the border into the Canadian prairie, reaching as far as central Alberta.

Aside from the fact that they are relatively easy to rear in laboratory conditions, prairie voles are of interest in studies of social behaviour because while they live in communal groups sharing runways and a complex of subterranean burrows, they are also monogamous. That is to say, each individual picks a  single partner so that the male can play an equal part in child-rearing; unlike many mammals, young prairie voles are raised by both of their parents. Exactly how they form these pair bonds and switch from the juvenile condition of largely preferring to hang out with others of their own sex to one with an opposite-sex partner has been a common subject of study.

Scientists have studied, for example, how the voles are affected by the stresses of living in a single-parent family, how social behaviour in adults is affected by the amount of parental attention they received as pups, and how social isolation can induce changes that look an awful lot like depression. We even have some idea of exactly what's happening in the neural circuitry of a vole when it "falls in love". 

Much of this is interesting, but it doesn't tell us so much about what happens afterwards. Studies typically tell us whether the voles in question were adults or not but not exactly how old those adults were. The assumption, in other words, is that once they reach adulthood, the voles' behaviour is basically fixed. That's not an unreasonable starting assumption, but is it necessarily true? Does vole behaviour continue to change as they move from being young adults to what we might term "elderly" ones?

An obvious question here is quite how we'd define "elderly" in a vole. We know that prairie voles reach full physical maturity at around 60 days of age, but are sexually mature at 45 days for females, and 55 days for males, so the gap between that and 60 days can reasonably be described as "adolescence". We can also use the weaning age of 15 days as another marker if we want to look at juvenile development. There is no such clear marker for old age, since female voles - like most mammals - do not experience menopause. 

However, the lack of a clear dividing line does not prevent us from comparing older adults with younger ones. In the wild, it is unlikely that many prairie voles live longer than a year and even that much is probably above the average life expectancy. (Thus, the fact that females don't undergo menopause still doesn't mean that they're likely to give birth more than twice - typically in spring and autumn). In the safe environment of a laboratory, they can live much longer than this, with the actual maximum being around three years, but this likely has little relevance to how they live in the wild. We also know that, while nobody has yet studied this in voles, mice begin to show signs of physical decline (reduced strength, agility, and so on) at around six months so that's another possible guideline.

Although it's not quite the first study to look at the social effects of ageing in prairie voles, one published last November is likely the most detailed. It divided the voles into four age groups, from young adults aged 60-80 days to those near the limit of their wild lifespan at 300 to 320 days. Which, to be fair, is probably not literally "elderly", since wild voles probably get eaten before they can reach something that would correlate with that description in humans... but it's probably past the prime of their life.

Each group then underwent a series of five tests to assess their social behaviour. One of the tests involved placing a mysterious object into the voles' cages to see how they would react to the sudden appearance of something weird and unexpected. We'd typically expect that younger animals would be more adventurous in exploring something new, but, if this study is anything to go by, the reverse seems to be the case for prairie voles, with the older individuals being quicker to approach the odd object (actually a small plastic tube with some post-it notes stuffed inside it) and spending more time examining it once they did. Perhaps with age came experience and they were less worried about a plastic tube being a threat.

Or perhaps young adult voles find plastic tubes boring and don't really care about them unless something exciting happens.

Another test involved placing two voles at either end of a tunnel to see which one would back out, accepting the dominance of its apparent rival. Here, it turned out that individuals aged around 150 days had a distinct edge over the very oldest individuals, but otherwise there was little difference. Perhaps these are at the peak of their territorial behaviour, and are best able to intimidate the less physically fit voles to back down and retreat. In other tests, however, age had little if any effect on the willingness of residents to at least try and defend their territory, or how likely they were to approach strangers separated from them by a plexiglass screen.

The final test simply placed two voles together in a neutral environment that neither had any cause to defend, to see how they reacted to one another. While the older voles did not appear any more or less sociable than their younger counterparts in this test, they did become more set in their ways, responding in the same manner regardless of whether the other vole was one they previously knew, a sibling, or a member of the opposite sex. The authors suggest that this could be due to a general lack of motivation, as is also seen in elderly mice; they just stop caring about the identity of the new arrival and started treating everyone the same.

This, they argue, could be due to an increasing importance of social companionship to older voles, leading them to try to form more stable relationships, just as maintaining social engagement has positive health benefits in ageing humans. Perhaps that's reading too much into it, especially given the differing life histories of voles and humans (or even primates). But something is happening, and it seems that even prairie voles may find their priorities change as they get older.

[Photo by Chuck Homler, from Wikimedia Commons.]