1 Life in the trees

The title of this course spells out exactly our focus of study; we'll be concerned with mammals that are arboreal - which means that to greater or lesser extents, they live in trees.

You won't be surprised to learn that tree-dwelling mammals don't comprise a single taxonomic group. A few familiar examples illustrate the point.

• The striped possum is a marsupial. It's an agile, grub-eating tree dweller that uses its incisors to tear at wood and gnaw out wood-boring grubs. It has a sharp claw on an elongated finger, which can act as a probe or a skewer; it also has a long tongue, for probing the bark.

• The binturong is the 'yard-long black hearth rug' referred to in Life of Mammals (LoM) p. 215. It's a civet - and therefore a member of the order Carnivora. It's an accomplished climber with a prehensile tail. Living mostly on fruit, it can hang upside down from branches - a lifestyle that is a significant departure from that of the more true-to-type carnivores.

• You'll be familiar with squirrels as sure-footed tree dwellers - their agility is described and beautifully illustrated in LoM pp. 218-222. Squirrels are rodents - the description of squirrels as 'rats with bushy tails' has an element of biological truth.

• The hyrax shows an adeptness at clambering around in trees. The middle area of each foot of the hyrax has muscles that can be retracted, so creating a suction pad that (along with rubbery soles, rich in sweat glands) help provide extra grip. Taxonomically, hyraxes belong to a group called the subungulates, which includes elephants and the aardvark.

What these examples reveal is that different evolutionary lineages have structures and lifestyles well-suited to the opportunities and demands of living in trees. In the remainder of this course, you'll come across numerous adaptations associated with tree dwelling. Many such examples - tails, claws, clinging feet - crop up again and again, revealing convergent evolution at work. Their similarity reflects the fact that the selection pressures that operate in such an environment are likely to be pretty well standard. But of course we'd expect a great deal of diversity too, in that very different types of mammals initially took to the trees. What's more, different species spend varying amounts of time in trees; some spend a good deal of time on the ground, others are on the ground fleetingly, simply to get from one tree to the other. So you'd expect tree dwellers to show varying degrees of adaptation to living in trees.