Thumb - Evolution - Other Animals With Opposable Digits

Other Animals With Opposable Digits

Many animals also have some kind of opposable thumb or toe. An animal species is said to have opposable thumbs if the thumb is capable of bending in such a way that it can touch all the other digits on the hand or foot. Most species do not have opposable thumbs. Opposable thumbs are a signature feature of the primate family, and played a large role in the ancient humans' invention and use of tools.

Primates

• Primates fall into one of four groups:
  • Nonopposable thumbs: tarsiers and marmosets
  • Pseudo-opposable thumbs: all strepsirrhines and Cebidae
  • Opposable thumbs: Old World monkeys and all great apes
  • Opposable with comparatively long thumbs: gibbons (or lesser apes)

The thumb is not opposable in all primates — some primates, such as spider monkey and colobus, are virtually thumbless. The spider monkey compensate this by using the hairless part of its long, prehensil tail for grabbing objects. In apes and Old World monkeys, the thumbs than can be rotated around its axis, but the extensive area of contact between the pulps of the thumb and index finger is a human characteristic.

Darwinius masillae, an Eocene primate fossil often described as a missing link between prosimian and simian, had hands and feet with highly flexible digits featuring opposable thumbs and halluces.

Other placental mammals

• Giant Pandas — five clawed fingers plus an extra-long sesamoid bone beside the true first digit that, though not a true digit, works like an opposable thumb.
• In some Muridae the hallux is clawless and fully opposable, including arboreal species such as Hapalomys, Chiropodomys, Vandeleuria, and Chiromyscus; and saltatorial, bipedal species such as Notomys and possibly some Gerbillinae.
• The East African maned rat (Lophiomys imhausi), an arboreal, porcupine-like rodent, has four digits on their hands and feet and a partially opposable thumb.

Additionally, in many polydactyl cats, both the innermost and outermost ("pinky") toes may become opposable, allowing the cat to perform more complex tasks.

Marsupials

• In most phalangerid marsupials (a family of possums) except species Trichosurus and Wyulda the first and second digits of the forefoot are opposable to the other three. In the hind foot, the first toe is clawless but opposable and provides firm grip on branches. The second and third toes are partly syndactylous, united by skin at the top joint while the two separate nails serve as hair combs. The fourth and fifth digits are the largest of the hind foot.
• Similar to phalangerids though in a different order, koalas have five digits on their fore and hind feet with sharp curved claws except for the first digit of the hind foot. The first and second digits of the forefeet are opposable to the other three, which enables the koala to grip smaller branches and search for fresh leaves in the outer canopy. Similar to the phalangerids, the second and third digits of the hind foot are fused but have separate claws.
• Opossums are New World marsupials with opposable thumbs in the hind feet giving these animals their characteristic grasping capability (with the exception of the Water Opossum, the webbed feet of which restrict opposability).
• The mouse-like microbiotheres were a group of South American marsupials most closely related to Australian marsupials. The only extant member, Dromiciops gliroides, is not closely related to opossums but has paws similar to these animals, each having opposable toes adapted for gripping.

Reptiles

• The bird-like dinosaur Troodon had a partially opposable finger. It is possible that this adaptation was used to better manipulate ground objects or moving undergrowth branches when searching for prey.
• The small, predatory dinosaur Bambiraptor may have had mutually opposable first and third fingers and a forelimb manoeuvrability that would allow the hand to reach its mouth. Its forelimb morphology and range of motion enabled two-handed prehension, one-handed clutching of objects to the chest, and use of the hand as a hook.

• Nqwebasaurus — a coelurosaur with a long, three-fingered hand which included a partially opposable thumb (a "killer claw").

Most birds have at least one opposable digit on the foot, in various configurations, but these are seldom called "thumbs".

Amphibians

• Phyllomedusa, a genus of frogs native to South America.