I've been intrigued by the tiny primates we call "tarsiers" ever since reading, long ago, a speculative essay in the now-defunct popular science magazine Omni that posed the question, "If humans hadn't evolved, what other animal might have taken our 'lords of the Earth' top billing?" After considering the usual suspects — dolphins, bonobos, octopi (which have the the largest invertebrate brains) — the author proposed that, absent humans, the tarsier would have been a prime evolutionary candidate. Other than their diminutive size (65 million years ago, our own mammalian ancestors were about as big as tarsiers), they have a lot going for them:
VISION: One look tells you that tarsiers have great nocturnal eyesight, with eyeballs nearly as big as their entire brain. Curiously, they lack the reflective tapeta lucidum ("bright tapestry") retinal layer common to most nocturnal vertebrates, which you can see as "eyeshine" if you point a flashlight into the eyes of a cat, for instance. The purpose of the tapeta ludicum is to reinforce night vision, but those huge tarsier eyeballs more than compensate. In addition, they can probably see in the ultraviolet (they have the necessary "S" cone cells at the periphery of their vision), giving them another night-hunting advantage.
HEARING: The tarsier auditory cortex is large and discreet, indicating high sensitivity to sound. While they vocalize around 70 cycles per second, they can hear (and communicate) at ultra-high frequencies approaching 100,000 cycles per second — another useful adaption for locating prey at night.
DIET: Unlike all other primates, they are entirely carnivorous. They catch insects, birds, lizards and bats by leaping at prey using their long hind limbs. Since ingesting protein is far more efficient than synthesizing it (which herbivores do), the tarsiers' diet is about as streamlined as you can get.
AGILITY: Tarsiers get their name from the elongated tarsus bones of their feet, which give them great ability to both cling to vertical tree trunks and to leap several feet when hunting. Flexible neck muscles allow them to turn their heads nearly 360 degrees, so they can watch and wait silently for their prey with virtually no body movement.
Long contentious, the evolutionary history of tarsiers finally appears to be settled in favor of their line being a sister group to anthropoids, which includes primates like us (according to a paper in the April 30 Scientific Reports). The case was settled by noting that both simians and tarsiers lack the ability to synthesize vitamin C, which would not have been true for competing evolutionary paths.
I recently saw my first tarsier in the wild on a night walk along the banks of the Kinabatangan River in the Malaysian province of Sabah in northeast Borneo. Our local guide, who had sharper eyes than any of us tourists, suddenly stopped dead and pointed his flashlight toward a tree trunk a few feet away. There was this little critter, appearing quite unfazed by the interruption of his night hunting program, staring back at us, unmoving but alert. In that moment, it wasn't hard to imagine his ilk taking over the land if our species were to become extinct — and perhaps doing a much better job of stewardship than we've managed so far.
Barry Evans (email@example.com) worries about the future of tarsiers, whose Southeast Asian jungle habitat is fast giving way to human encroachment.