Island living

Madagascar is the world’s fourth largest island. It is thought to have reached its present day location in relation to the African continent about 130 million years ago. Today, 400km separate Madagascar from the rest of Africa. Lemuriformes are though to have colonized Madagascar roughly around 65 million years ago, so how did the ancestors of sifakas, ring-tailed lemurs, and others arrive on the island? Are lemurs great endurance swimmers? Not exactly.

Madagascar's hissing cockroaches

What determines whether a species colonizes an island? Is colonization of an island something scientists can predict? The principles of island biogeography lend us a hand in determining whether or not an island will be successfully colonized by new species.

For starters, the closer the island is to the mainland or another large land mass, the more likely that island is to be colonized. The older the island is the more likely the island is to be colonized. The larger the island is the more species are likely to colonize it. All of these fancy rules are pretty simple once you think about them. The older the island is, the more time has passed, allowing species to reach the island. The larger the island is, the more space there is for all of those new species. And the closer the island is to other landmasses, the less distance between the two landmasses makes for an easier crossing for new species.

In terms of the genetics behind island biogeography, the following rules apply. As time passes and the island has been settled by those species that were able to the island and been successfully live there, genes are less likely to be exchanged between the original population from the mainland and the population on the island. As even more time passes, the two populations become genetically different.

Ancestral lemurs likely reached Madagascar by a series of rafts. This would have occurred not over a summer holiday but over an extended period of time in a series of events. As time passed, ancestral lemurs diverged from their original population and became genetically distinct. They thrived on the island of Madagascar and became the species we know today.

Girl power

Sifakas don’t experience a lot of feeding competition, from what I’ve seen so far, which isn’t surprising given their diet. If any feeding competition between a male and a female were to occur though, I bet you’d be surprised at who would come out on top. The female. Sifakas, like almost all of their primate relatives on Madagascar, have a social system in which females are dominant over males. Meaning the females can and do push the males around when it comes to any sort of feeding competition. It’s an unusual system to be sure, and no one is entirely positive as to how female dominance came to evolve.

Ring-tailed lemur sunning in early morning

One of the main theories is that female dominance evolved in response to the unpredictable environment of Madagascar. Females have feeding priority because it is advantageous for them to be healthy so that they can successfully produce offspring. Remember an earlier post about the term “fitness” in the field of biology? If females aren’t eating enough or are eating poorly, and they’re offspring aren’t surviving, no one’s fitness is increasing. It doesn’t matter if you’re a male or a female: the offspring need to survive. 

Like I said, I haven’t seen a lot of aggression in my time here watching sifakas. There seems to be plenty of leaves and flowers to go around. Every now and then, you will hear a rustle in the trees and some squealing, and it’s usually a female kicking a male out of his spot. Otherwise, everyone seems to get along rather peacefully, eating and sleeping all day long. If you’re a fan of girl power, these primates certainly have it. Other species of lemurs, those with choice food items, may experience higher rates of aggression within the troop, but sifakas are pretty laid back.

Lactating mother, Sarah Louise, with infant

Critical thinking: Can you think of a scenario in which female dominance would be beneficial?

Critical thinking: With unlimited funds and resources, how might you test your hypothesis as to what scenario might make female dominance beneficial?

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?