Episode Eight: Life in the Trees

An emergent tree in a tropical forest can grow to over forty metres high. The first branch may be twenty metres from the ground. A slip from this height would almost certainly be fatal. To make matters worse, branches may break without warning, or the tree may blow over. But, though life may seem precarious here, for those mammals which have made this three dimensional world their home the rewards are great; trees provide food, security from ground living predators and a refuge from the elements. To reap these benefits, however, some very specialised adaptations are needed.

Rock hyrax are not your typical tree dweller. They look more like ground hugging guinea pigs than accomplished climbers but, surprisingly, they are well adapted to walking around the low level branches of the acacia trees on which they feed. The soles of their feet are moist and rubbery creating a slight suction which allows the hyrax to almost stick to the branches. But this adaptation would not be sufficient to negotiate much taller trees -- for that, tree dwelling mammals have evolved other more unique adaptations.

Clearly a good grip is a basic requirement for moving around at height -- sloths and slender lorises may have very different looking mechanisms for gripping (claws on one, fingers and thumbs on the other) but both can grip tightly with all four limbs.

If, however, you require both your hands for feeding, like the tamandua, another adaptation is necessary -- a prehensile tail. This gripping tail allows the termite eating tamandua to hang on while keeping its front limbs free for breaking into the hard mounds of its prey.

Some tree dwelling mammals spend little time actually hanging on to branches. A grey squirrel's agility is legendary -- their light body, balancing tail and sharp claws allows them to move around the tree tops at an astonishing speed. But evolution hasn't stopped there. Flying squirrels don't just leap, they glide -- as much as 90 metres. Fruit bats, or flying foxes, let go of the trees all together. They, along with their insectivorous cousins, are the only group of mammals to have developed true flight. For the flying foxes, this ability has enabled them to travel large distances looking for fruiting trees.

Across the globe, mammals have evolved to exploit every conceivable type of forest. In one special place -- the island of Madagascar -- an ancestral tree dweller diversified into an astonishing range of species. Lemurs have now filled almost every niche -- the sifaka is perhaps the most spectacular, leaping as much as fifteen metres between branches. But the lemurs don't have the trees all to themselves. Living alongside them is the predatory fossa -- a sort of giant mongoose -- which can match any lemur for agility.