Quiz: Which animal has the longest arm?

When we think of long-armed animals, we might think of apes, but in fact there is another species that has beaten them.

Which animal has the longest arms?

Based on size alone, humpback whales (Megaptera novaeangliae) will easily crush all other competitors with their giant flippers (fins). The forelimbs, or arms of whales, were modified into flippers during evolution from their terrestrial mammalian ancestors. The flippers of humpback whales can be up to 4.9 meters long.

But in terms of size relative to the body, there are two animals that are said to have the longest arms in the animal kingdom, excluding birds.

A quick search might turn up the small ape (Gibbon), which lives in the forest canopy of Southeast Asia, which has the longest arms relative to body size. However, according to Mary Ellen Holden, a zoologist at the American Museum of Natural History in New York City, it's the three-toed sloth that really ranks first.

White-collar sloth

An important ratio for comparing mammalian limbs is the intermembral index (IM) - the length of an animal's forelimb divided by the length of the hind limb, multiplied by 100. Humans have an IM index in about 68 to 70, which means our arms are about 70% of our legs.

An animal's IM, living or fossilized, is an indicator of which limbs it uses to get around. Animals that use two legs to walk, as well as animals that climb and jump, typically have an IM between 50 and 80, followed by quadrupeds, at 80 to 100.

Animals with an IM over 100 tend to be correlated with hovering movements - including swinging from trees.

Holden says that longer forelimbs may also be correlated with knuckle walking, a method used by great apes such as chimpanzees (Pan troglodytes) and unusual gait, like that of Hyenas.

Small gibbons, with arms 1.5 times as long as their legs, are among the best candidates for the highest IM among primates. The squid (Symphalangus syndactylus), a species of gibbon found in Thailand, Indonesia and Malaysia; and the critically endangered black gibbon (Nomascus concolor), found in China, Laos and northern Vietnam; have an average IM of 140 and 147, respectively. By comparison, a human with that stat would have an arm nearly twice its current length.

"Little apes evolved to feed by grabbing tree branches and swinging, which, over time, led to the selection of growing longer and longer arms. They could actually swing around the shoulder almost," says Holden. 360 degrees."

The sloths beat them, however: the white-collared sloth (Bradypus tridactylus) and the brown-necked sloth (Bradypus variegatus) had arms 1.7 times longer than their legs on average, with IMs of 171, and 172.

Squid lemur.

Sloths and gibbons are very different in the way they use their arms to move through trees. Sloths, despite being excellent swimmers, enjoy hanging from branches and crawling along them at record-low speeds, while gibbons can quickly swing from branch to branch. According to Holden, some sloths have even been found to have arms twice as long as their legs.

"Some three-toed sloths in the genus Bradypus have the longest arms relative to their bodies," says Holden. "Thus, sloths outperform apes when it comes to relative arm length."