Comparing cat and fisher to show difference in eye size w.r.t. pectoral insignia in mustelids

I have suggested that a common theme among Carnivora with white insignia on the chest of an otherwise all-dark body is poor vision, and an unusual reliance on olfaction.
I would like to illustrate this by comparing the domestic cat (Felis catus) with the fisher (Martes pennanti, and
I have chosen the fisher, among all the Mustelidae, for two reasons:

  • it has about the same body mass as the domestic cat, and
  • it is among the mustelids that have whitish insignia on the chest, the white patches being extremely variable individually, and always asymmetrical.

When one looks at photos of the fisher, the first impression is that there are no insignia on the chest, because by far the majority of individuals are plain dark.

However, as the photos below show, some individuals do indeed have whitish insignia on the chest.
Both Felis catus and Martes pennanti have adult body mass of about 4 kg, the difference being that the mustelid is far more sexually dimorphic in body size than is the cat.

The similarity in body mass is important in any comparison of eye sizes. This is because eye size is hypoallometric: other things being equal, species differ predictably in the relative size of their eyes, with the eyeballs becoming smaller and smaller as the body gets bigger and bigger.
My interpretation of the photos presented here:
Although domestic cat and fisher are like-size, they differ greatly in eye size.

This helps to explain why it is that in small, wild felids there is no such thing as pectoral insignia, whereas in the fisher this pattern is well-exemplified.

It is, of course, true that some individuals of small felids in the selectively bred form of the domestic cat do show colouration that is nearly all-black, with an asymmetrical, irregular white patch on the chest.

However, the important things to realise are that

  • such a pattern is completely unknown in any wild felid of any body size,
  • the appearance of white on the chest is usually accompanied by other irregularities such as an odd white toe, in the case of black individuals of the domestic cat (which occur in 22 breeds, but can hardly be bred into a consistently all-black breed), and
  • seems to be mainly ontogenetic, with all-blackness restricted to adults.

So, what I see here is a situation in which felids, which are typically large-eyed, have virtually no incidence of pectoral insignia, whereas mustelids, which are typically small-eyed, have many such incidences, including that seen in a species most similar to the domestic cat in body size.

The pattern seems to make sense, because

  • relatively poor vision means that bold insignia on the front of the animal are helpful for individual recognition, and
  • such insignia in felids would compromise the crypsis/camouflage necessary for their particular reliance on stalking of prey, something not true for mustelids. 

The following ($_86.JPG) illustrates three features of interest.

Firstly, the large size of the eye relative to body size is typical for felids; indeed this expression of the eyes is a major reason why the domestic cat is so popular.

Secondly, extremely few individuals of the domestic cat actually show this colouration, which is extreme for the species and has no function whatsoever in any wild felid, including the species ancestral to the domestic cat. Note that at least one toe is also white.
The following ( is typical of the relative size of the eye in the domestic cat, a species which weighs – like the fisher below – approximately 4 kg.
Now let us look at the fisher:
The following ( is either a bit smaller or a bit larger than the typical domestic cat, depending on whether it is male or female. I do not know its sex. However, one can see, at a glance, how much smaller the eye is relative to body size than in the cat.

This individual, like most individuals of the fisher, lacks any whitish insignia on the chest. No individual of the fisher has odd white toes; that pattern simply does not exist in this species.
The following ( once again shows the small size of the eyes relative to body size.
The following ( is an individual with irregular whitish insignia, clearly asymmetrical, on the chest and extending in this case also to the front-of-neck, and even the inner foreleg.

Again, nobody could confuse this animal with a cat, even if only because its eyes are so small.

The fisher resembles the wolverine in its insignia, although a smaller percentage of individuals show these insignia in Martes pennanti than in Gulo gulo (

The following ( shows the small size of the eyes well, and also the individually variable whitish insignia on the chest and front-of-neck. A good way to think of the fisher is as a smaller version of the wolverine, somewhat specialised on a diet of Erethizon dorsatum (
The following individual ( again exemplifies the insignia, and again clearly shows how small the eyes are.

Not even the largest felids, such as the lion (Panthera leo), have such small eyes relative to body size. In other words, if this was a felid, it would need to be not 4 kg but 400 kg to show this kind of relative size in its eye.
Once again, if you half-close your eyes and imagine that you are looking at a domestic cat and not the fisher, can you see how this could never fool you as a cat, if only because you would instantly notice that its eyes are far too small?

This individual (, like most, seems to lack any whitish markings. 

The following ( individual again possesses white markings, although in this case they are confined to the chest.

The following ( further illustrates the extreme irregularity of the whitish insignia in the fisher.

Posted on September 02, 2022 09:45 AM by milewski milewski


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