Anatomy Tutorials

What makes a mustelid?

The word “mustelid” or “weasel” may give associations to something slender, smooth, and short-legged. While it is true for some of the more well-known species, properly describing their essence can be a whole different matter, as this is a diverse family of carnivores and omnivores whose members have adapted to a wide range of environments. This drastically affects the way these animals look and behave, so they do not really have many traits in common.

To make things more complicated, many of their traits are found in several other families within the order Carnivora in response to various types of evolution, like sharing similar ecological roles or keeping traits from an early ancestor they branched from. That is not to say they do not have any characteristics that sets them apart!

The basics

Sometimes there can be too much focus on mustelids being long and short-legged, because there are other, often disregarded anatomical features that greater distinguishes them from other animals.

• While not all mustelids share the same anatomy, their build is primarily shaped by curves and gives an even and soft appearance without sharp angles or transitions between body parts. There is no abdominal tuck like in canids, but a slight potbelly can occur.

• Flattened skull with a large braincase and short facial region, elongate from the side and broad on top and from the front. There is little to no dip where the forehead meets the nose bridge.

• Not all mustelids share an identical dentition, but they all possess teeth adapted for eating flesh and have shearing carnassials. The most common dental formula is: They are not rodents and do not have buck teeth.

• Dark brown eyes (not including domesticated varieties or albino) that are widely spaced. All mustelids have horizontal slit pupils, except badgers and otters, which have round pupils. To give some examples, here are the horizontal pupils in the eyes of a North American mink (Neogale vison), eyes of a Japanese marten (Martes melampus), eyes of a wolverine (Gulo gulo), and round pupils in the eyes of a giant otter (Pteronura brasiliensis). If making the pupils visible, this detail is important for anatomical consistency if other animals (e.g., cats, foxes, horses, goats, mongooses, etc.) are given slit pupils in the same depiction with a mustelid.

• Ears that are flat, widely spaced, rounded at the tips, and often low-set. They are also stiff and cannot move freely to express emotion like felids and canids, only to the side or lay against the sides of the skull. Many species have ear pockets (also known as Henry’s pockets, or more formally referred to as cutaneous marginal pouches) on the outermost ridge of the ear rims.

• Short, but robust and muscular limbs.

• Forepaws: Broad, shovel-shaped paws that always have 5 elongate and padded digits for holding and gripping.

• Semi- to non-retractable claws on all digits, on the same level without dewclaws.

• Hindpaws: Always 5 elongate and padded digits. The innermost digit (a.k.a. the smallest digit) should look anatomically connected like the others, rather than like a dewclaw or random attached appendage. Plantigrade to semi-plantigrade gait, with the entire or half of the sole in contact with the ground. Mustelids do not typically walk on their digits, and their heels are not placed as high as in felids and canids.

• An extremely flexible spine that often gives a hunchbacked impression.


  1. King, Carolyn (1984). Macdonald, D. (ed.). The Encyclopedia of Mammals. New York: Facts on File. pp. 108–109. ISBN 978-0-87196-871-5.

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