Wednesday, August 28, 2013

Living with others

I spend my days alternating between three troops of Verreaux’s sifakas. Two of my troops contain six individuals and one troop contains seven. Verreaux’s sifaka group size ranges from one individual to up to ten members. Here in Berenty, I’ve seen quite a few groups with six or seven individuals. Although, group size is much smaller in the spiny forest, which is very dry and where food is potentially limited.

Resting midday
Most species of primates live in social groups. While some members may live on their own for some time after they disperse from their natal group to seek new mating opportunities, many primates will spend the majority of their lives living with others. Living in a group has its advantages and disadvantages. Living in a larger group means better protection against predators, such as snakes, birds of prey, and cats. An individual may feed and feel a little safer, knowing that multiple eyes and ears are on the lookout. Larger groups are also better able to defend resources, such as a valuable fruiting or flowering tree. Primates have competition from members of their own species and members of other species, and a larger group size may mean the difference between abandoning a food source or remaining and continuing to feed. This is especially true in instances where the food item is nutritious, such as fruit. Sifakas, consuming mainly leaves, don’t have a lot of competition from other troops of sifakas or from other primate species. However, there can still be instances where one sifaka will displace another sifaka over a valued food item, such as flowers. More members in the group means greater competition within the group.

This is one of the disadvantages of group living. Living in a group also means that primates must get along with each other. Just think of a time when you didn’t get along with a sibling or a roommate. Energy must be invested in maintaining harmony within the group. Sifakas usually get along pretty well with each other, from what I have observed. There isn’t a whole lot of social interaction. I’ve only seen some sort of aggression a few times, usually a quick squabble over food. I’ve seen the sub-adults play wrestling with each other twice now, but otherwise these primates mainly feed, rest, and groom themselves.

Troop clinging to trees
Madagascar is a unique country in that there are no cat species on this island. There are no lions or leopards to worry about. The closest animal this island has to anything resembling a lion is the fossa, which is actually not a member of the felid family at all but a member of the civet family. Fossas are small but they will hunt and kill lemurs. There are no fossas in this part of Madagascar though, so the primates in Berenty need not worry. Their main concern comes from aerial predators. Large group size protects them from predatory birds, as do other strategies, such as feeding lower in the canopy, which I’ve observed both the smaller ring-tailed lemurs and brown lemurs doing. Sifakas are relatively large, which may be part of the reason they feel safe feeding on the tops of trees.
Mother with infant on back feeding on leaf buds

Many variables affect the size of a primate’s group: food availability, space, predation, etc. If the local habitat is reduced drastically by human activity, a primate may not be able to disperse into new territory to find mating opportunities outside of its natal group, causing tension and aggression within the group.

Critical thinking: Can you think of other instances in which human activity may affect the group size of a primate population?

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