On The Trails: Claws for grooming (with a note on rictal bristles)
- Mary F. Willson
- 2 hours ago
- 3 min read
By Mary F. Willson
Some time ago, I wrote about animals preening, mostly emphasizing the physical and social functions of that activity. Now I focus on how the digits (fingers and toes) are used for grooming and preening.
Humans run their fingers through their hair, creating order or creating deliberate disorder. The flexible fingers of humans and other primates are used for picking off parasites and bits of debris. Other animals have digits that are modified in various ways that are useful for grooming.
A well-known example is the special claw on the hind feet of beavers. Beavers are known for their big, webbed back feet. They also have an apparently unique toe anatomy. It’s often referred to as "split claw," but that’s a poor description. The inner two toes on the back feet are different from the other toes. The innermost toe is small, and the nail, according to some reports, can be raised or lowered, making it possible to grasp small things—the analogy is "like a bird’s bill." I have not found a description of its use, but perhaps it can grab debris or even ectoparasites.
Beneath the nail of the next toe is an unusual big horny bump with a serrated upper surface. This toe, sometimes called the "grooming claw," is used to clean the fur and dress the fur with waterproofing oil from the scent gland near the base of the tail (the giant beaver of the Pleistocene time was not closely related and did not have this groomer). This grooming claw is unique to modern beavers, begging the question of Why.
Other mammals have special grooming claws too. For example, rock hyraxes in Africa have hoof-like toes except on the inner toe of the hind feet; that toe has a long claw used for grooming and scratching. Koalas in Australia have hind feet with two fused digits that bear a double claw, used like a comb. Lemurs and their close relatives have a grooming claw on one toe.
Many birds (e.g., herons, grebes, terns, nightjars, cormorants, barn owls, etc.) have a specialized middle toe bearing a row of small, tooth-like projections. These pectinate (comb-like) claws are used in a variety of ways. They are used to arrange the feathers, remove debris, and in herons to distribute the powder down in the plumage. The down breaks up into a keratin dust, which helps to clean the feathers, remove fish oil (for fish-eating birds), provides some waterproofing (e.g., cormorants). It is even reported to be used to straighten rictal bristles* of nightjars, if the bristles had become misaligned.
*Some birds have rather whiskery bristles on the side of the face. Located near the back of the gape (or "rictus"), where the lower jaw joins the upper jaw, they are called "rictal bristles." As in mammalian whiskers, they are made of the structural protein keratin, but they are modified feathers. They generally consist chiefly of the feather shaft, with few or no barbs. They are tactile sensory organs, capable of feeling air currents and bits of debris and are not present immediately when the bird hatches. Although they were long thought to serve as a net for insect capture, this now seems to be highly unlikely, although they might help guide a captured bug to the bird’s mouth. Their principal functions seem to be protecting the eyes from flying debris and bits of an insect that is struggling in the bird’s bill, helping the bird navigate in tight spaces in low light, and sensing air currents and the bird’s speed. They vary in length up to almost forty millimeters long.
*"This bird was captured, banded and safely released under federal research permit."
• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent. For a complete archive of Mary Willson’s “On the Trails” essays, go to https://onthetrailsjuneau.wordpress.com.
