Monkeys get around in trees with a remarkable grace that belies the fact that any false moves can lead to a fatal plummet to the ground below. Sure, they have four limbs to orchestrate the balancing act, but in addition they boast a nimble tail. Jesse Young and a team of researchers from universities in Cleveland, Meridian and Austin, USA, wanted to take a closer look at what the tail does to ensure safe passage in trees. Previous studies have noted that monkeys that live and move in trees have longer and heavier tails than monkeys that mainly live on the ground – this led the researchers to believe that bigger tails may have evolved to work as a counterweight, a bit like a tightrope walker's balance pole, to save them from toppling when they overbalance and ensure an overall smooth journey through the canopy.
To understand how the tail works as a counterbalance, Young and his colleagues filmed squirrel monkeys – a small species about 0.5 kg – as they traversed horizontal poles set up in a laboratory. The poles had decreasing widths of 5, 2.5 and 1.25 cm, mimicking the challenges that the monkeys encounter in their natural habitat. From these videos, the team calculated how much the tail counterbalanced the movement of other body parts, taking into account the tail's weight – a heavy tail is a better balance pole than a light one – and the speed as the monkey snapped the tail from side to side – a faster whip of the tail means more counterbalance.
The team found that the elongated shape of the tail – equivalent to 167% of body length – helps it to counterbalance all the monkey's other limbs despite its meagre size, which only amounts to 5% of the animal's entire body mass. Consistent with their expectation, Young and the team found that the tail worked harder at counterbalancing when negotiating progressively narrower supports. This suggests that the tail is not just a passive appendage but plays an active role in ensuring stability.
Building on this result, the team turned their attention to videos of monkeys recorded in the wild in Ecuador and Costa Rica. These videos showed that monkeys hold their tails more upright and move it more vigorously when they are on narrower branches. This demonstrates the importance of the tail in the real world. The team also found that the counterbalancing movements – perky whips of a tail cocked high – increased with longer tail length, suggesting that the species with longer tails take full advantage to gain the most stability.
Young and his colleagues have shown that monkeys’ tails play a key role in balance when managing treacherous tree branches. The results suggest that the role of long tails in tree-dwelling monkeys is to save them from toppling, which may be considered one of the fundamental adaptations for a life in trees. What is certain is that when you live high up in the trees, evolving a biological balancing pole with the ability to provide a life-saving jolt in response to a bad step is a smart move.
