Sunday, 7 May 2017
Compare the Mongoose
(As an aside, the word "meerkat" is Afrikaans... which is a bit odd, since it means something completely different in Dutch).
Even when they aren't singing Hakuna Matata or trying to sell you car insurance, meerkats are common features on wildlife documentaries (at least they are in Britain; I can't speak for other countries) and in zoos across the world. In part, this is because they're rather cute, sociable animals, with complex, telegenic, lives that involve a lot of cooperation. But, while meerkats are probably the most social of all mongooses, they are by no means the only ones. Another example, for instance, is the banded mongoose (Mungos mungo), which lives in groups almost as large as those of meerkats.
However, all the truly social mongooses - those that live in relatively permanent groups with a complex structure - belong to a single evolutionary branch within the wider mongoose family. This probably means that sociality only evolved once amongst mongooses, but it also happens to mean that, compared with other mongoose species, while the meerkat may not be unique, it is unusual. Most species, in other words, seem to be solitary.
It's probably true to say that, while there are certainly exceptions, we know rather less about the majority of solitary mongooses than we do about the social ones. Sociality, especially of the high degree seen among meerkats and their kin, is an interesting thing to delve into, and, furthermore, most of the solitary species are nocturnal, which doesn't help when observing them. But it doesn't make it impossible, and, besides, there are some species that do come out during the day.
Distinguished by a black tip to its tail, the slender mongoose (Galerella sanguinea) is a diurnal, solitary animal that lives across almost the whole of sub-Saharan Africa, save for the heart of the Congo rainforest and southern South Africa. This means that one of the places they are found is the Kalahari Desert of Botswana, Namibia, and northern South Africa... where meerkats also live. While there have been a few studies on slender mongooses before, since they are widespread and common, the most detailed to date was recently published, and specifically compares them with the meerkats living in the same area. Here, after all, are two reasonably closely related animals, living in the same habitat, but responding with quite different lifestyles.
The study was conducted over four years at the Kuruman River Reserve, where the BBC documentary series Meerkat Manor was filmed, and involved examining and watching 130 slender mongooses, as well as analysing their poo to see what they'd been eating.
Compared with meerkats, slender mongooses turn out to give birth to far fewer young over the course of their life. For one thing, litters are much smaller, with no more then three pups being born at a time, whereas this is just about the minimum for the local meerkats. But they also give birth less frequently, most often during a distinct season between October and March (i.e. the summer, since we're in the Southern Hemisphere). In comparison, meerkats in the Kalihari give birth up to three times each year, despite it being a generally harsh environment. Interestingly, smaller litters and less frequent births are also typical of a third mongoose species living in the region, the yellow mongoose (Cynictis penicillata), which is also a solitary species.
Of course, their sociability isn't the only difference between the two species. Meerkats, at up to 2.5 kg (5½ lbs), are significantly larger than slender mongooses, which weigh no more than 0.7 kg (1½ lbs). This almost certainly explains why pregnancy is longer in meerkats, at 70 days, rather than 60, and may well also explain why they live longer, reaching thirteen years, rather than just nine. But if greater size were the most important determinant of their lifespans, we'd also expect other features of their growth to be longer - and they aren't.
While pregnancy may be longer in meerkats, once they are born, the young are dependent on their mother for much less time than are slender mongooses. They study reported that meerkats are weaned at around 42 days, and slender mongooses at around 70, and also begin to forage for their food at an earlier age. This may be because slender mongooses need to be more physically capable and independent by the time they emerge from their birth burrows; meerkats can spare babysitters to look after their young while the mother forages, but slender mongooses have no such luxury. They also need to have absolutely got the hang of finding their own food by the time they leave their mother, since they won't have anyone else to help them once they do.
All of this makes it sound as if solitary living is a bit of a disadvantage. But there must be some benefit to it, or all mongooses would have become social, and another difference between meerkats and the slender species may help explain what that benefit is.
Small invertebrates form a significant part of the diet of both species in the Kalahari, but the details are different, with the meerkats preferring beetles, and the slender mongooses focussing more on crickets. Not only that, but while meerkats eat relatively little that isn't an invertebrate, it turns out that slender mongooses also eat much larger prey - mostly lizards. This means that meerkats spend much of their foraging time rummaging around in the soil trying to find tasty food. Being in a large group may help with that, especially since some of the meerkats keep a lookout for danger while the others are peering down at the ground.
But lizards require a stealthier approach, and, as one might expect, being in a large group does make it easier for your intended food to see you coming and make a run for it. Being solitary therefore allows slender and yellow mongooses to eat a separate food source from the meerkats, allowing both species to live alongside one another in an environment that isn't exactly rich with food to start with. Genetic and behavioural evidence has shown that it's the meerkats who have most likely changed from the ancestral condition, adapting to digging up insects, and even eating scorpions, in a way that has made group living a viable alternative to that of their distant ancestors.
Slender mongooses, like most of their relatives, have apparently found that staying with the old ways can sometimes give you an edge, even if there's a cost to be paid for that lack of cooperation.
[Photo by Sara&Joachim, from Wikimedia Commons.]