Binocular vision was key to efficient leaping for ancient ancestors Free

Forward-facing eyes with pinpoint accurate binocular vision are the hallmark of a top predator, and primates are well equipped. Their eyes can focus on objects with exceptional depth perception. But how did this fantastic skill originate? ‘Two adaptive scenarios focus largely on pressures exerted by foraging demands’, says Addison Kemp (University of Southern California, USA), explaining that forward-facing eyes may have evolved to help ancient primates locate fruit or catch insects. Another contributory factor to the evolution of forward-facing eyes might have been the need to accurately locate handholds as the animals leapt between trees. But without a convenient time machine to nip back ∼65 million years, Kemp turned to one of our ancestors’ near living relatives: fat tailed dwarf lemurs (Cheirogaleus medius). ‘This small species is among the best models for understanding the locomotion of the earliest primates’, says Kemp, who travelled to the Duke Lemur Centre, USA, home to a troop of the mini primates, to find out how important forward-facing eyes are for the leaping creatures.

Constructing an arena with a soft foam floor and randomly arranged vertical poles, Kemp found that the lemurs were keen to leap between the poles in return for tasty grape rewards. However, the nocturnal animals preferred leaping after lights out, so Kemp filmed the animals in the dark under infrared light. Once the lemurs were successfully leaping from pole to pole across the arena, Kemp gently fitted a helmet to each animal, leaving one eye completely clear, while a small blind located near the bridge of the lemur's nose reduced the view from that eye to a narrow tunnel, limiting the visual overlap between the two eyes to impair their binocular vision and depth perception. Then, she filmed the tiny mammals as they attempted to leap across the arena with their partially obscured vision.

While the lemurs mostly managed to launch themselves and land successfully, the animals’ manoeuvres were more cumbersome when their binocular vision was disrupted. And 1 in 20 landings verged on the disastrous: sometimes the lemurs overshot the pole, grasping hold too late and swinging before coming to a stop; other times they misjudged the distance and collided unceremoniously with the pole they had aimed for. On some occasions, the lemurs almost managed to grab hold, but didn't quite get a grip, sliding down the pole before reaching a halt, and sometimes they even misjudged the distance entirely, falling to the floor. ‘These errors all required additional compensatory movements upon landing’, says Kemp. And when she analysed how impaired vision had impacted the lemurs’ leaps, Kemp noticed that they all chose to jump over shorter distances, launched themselves more slowly and spent more time adjusting their handhold before making a leap, to ensure they had a solid hold for a powerful push off. The lemurs also wove their heads more often from side to side before making a leap, using the relative motion of the poles to help them to judge the distance with more precision.

‘While binocular vision is not as critical for leaping as it is for hunting insects, improvements that helped an animal to navigate its habitat were likely a benefit to early primates’, says Kemp. So, judging distance with a pair of forward-pointing eyes for a safe landing when leaping through the treetops gave our forebears a head start. And, although we no longer leap through the forest canopy, we definitely make the most of this advantage each time we catch a ball or drive a car.