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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
Why can squirrels run straight down trees? And could my cat do the same thing if it had to?
Mike Follows
Sutton Coldfield, West Midlands, UK
Only a few animals possess the specific adaptations that allow them to move head first in a controlled way down a tree trunk, including squirrels.
Squirrels have a highly flexible subtalar joint – the joint between the talus (ankle bone) and calcaneus (heel bone). In most terrestrial mammals, this joint is flat and tightly interlocking, limiting side-to-side movement and stabilising the joint. However, in squirrels, the articular surfaces are rounded and obliquely angled, making the joint more mobile, allowing the foot to rotate independently of the leg.
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Squirrels have a highly flexible subtalar joint, allowing the foot to rotate independently of the leg
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Birds such as nuthatches can also walk head first down trees, searching for insects in crevices that other creatures miss. They have strong legs and large hind toes with curved claws that allow them to gain purchase on the bark, and they use their tails for balance.
Geckos have special toe pads with setae – microscopic hairs – that allow them to use electrostatic forces to grip vertical and inverted surfaces, so running down a tree poses no difficulty for them.
Some monkeys are also capable of moving head first down trees. While they lack the specialised foot rotation of squirrels, they rely on strong grasping hands and feet, excellent balance and agility. Tamarins, in particular, have claw-like nails that improve grip on vertical surfaces, allowing them to descend in a controlled way.
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