Animals are diverse, and some are capable of setting records. For example, in the length of forelimbs. However, there is no definitive answer to this question, as much depends on the method of measurement.
When considering the absolute length of limbs, humpback whales (Megaptera novaeangliae) easily outpace all competitors thanks to their massive flippers, which evolved from the limbs of their terrestrial ancestors and can reach lengths of up to 4.9 meters.
However, when it comes to relative sizes, two types stand out among terrestrial mammals. The first are gibbons, a family of monkeys that live in the canopies of Southeast Asia. They have the longest forelimbs relative to body size. Nevertheless, according to biologists, several species of three-toed sloths may rival gibbons in this aspect.
A key indicator for comparing the limbs of mammals is the intermembral index (IM) — the length of an animal's forelimbs divided by the length of its hind limbs, multiplied by 100. In humans, the IM ranges from 68 to 70, indicating that our arms are approximately 70% longer than our legs.
The IM of both living and fossil animals shows which limbs they use for movement. Bipedal animals, as well as those that climb and jump, typically have an IM ranging from 50 to 80, while quadrupeds have an IM from 80 to 100. An IM above 100 is generally observed in animals that move in a suspended state, such as swinging on tree branches.
Gibbons, whose forelimbs are about 1.5 times longer than their hind limbs, are the main contenders for the highest IM among primates. However, sloths surpass these monkeys: three-toed sloths (Bradypus tridactylus) and brown-throated sloths (Bradypus variegatus) have forelimbs that are on average 1.7 times longer than their hind limbs, corresponding to IMs of 171 and 172, respectively. According to biologists, some individual sloths have been recorded with forelimbs twice as long as their hind limbs. These are the true record holders for forelimb length in the animal kingdom.
