What If Cats and Dogs Had Opposable Thumbs Day

What the day is

What If Cats and Dogs Had Opposable Thumbs Day is observed on March 3. The premise is a joke about pets opening the fridge, and the underlying question is a genuine one in comparative anatomy.

Origin and background

An opposable thumb is one that can rotate to touch the other digits, allowing a precision grip. Humans have one, and so do most primates, but the trait is not unique to us. Opossums have opposable thumbs on their hind feet. Koalas have two opposable digits on each front paw. Giant pandas famously manipulate bamboo using an enlarged wrist bone that functions as a sixth digit, which is not a true thumb at all but does the same job, and is a much-cited example of evolution improvising with available parts.

The important point is that the hand is not the bottleneck. Dexterity requires the neural machinery to control it, and primates devote a large area of the motor cortex to fine hand control, with dense direct connections to the spinal circuits driving individual fingers. A thumb grafted onto an animal without that wiring would mostly be a thumb it could not use precisely.

Cats already demonstrate the point. Many cats open doors, drawers and cupboards without thumbs at all, using paws, claws and persistence, and the limiting factor is clearly not anatomy. Some cats are polydactyl, carrying extra toes including thumb-like ones, and they are not noticeably more capable.

Dogs went the other way. Their forelimbs are specialized for endurance running, with the digits fixed for stability rather than movement, and the dewclaw is the vestige of a digit that lost its grip function.

How to take part

• Watch what your pet already does without thumbs. It is usually more than you credit.
• Look up the panda's false thumb, which is a wrist bone doing a finger's job.
• Tape your own thumb to your palm for an hour and notice what becomes impossible.
• Check whether your cat is polydactyl, which is common in some regions.
• Secure the bin properly either way.