Published eye colors for mammal species were not easy to find. Whereas many field guides for birds include iris color, field guides for mammals typically give no information on species iris color. My sample includes only 179 species out of a population of more than 4000. Even with this small sample, I can point out the underlying pattern which I believe is present in all the classes of land vertebrates.
Mammals, like birds, and unlike amphibians and reptiles, include large taxonomic families all across the range of eye darkness. That aids in recognizing the pattern involving eye darkness and behavior. Following are the four families of mammals with 15 or more species in the sample. An * will be used to designate families that I believe to be primarily carnivorous predators as opposed to being omnivores/herbivores or animals that primarily take immobile animal matter. The families will be rank ordered on average eye darkness according to my 5-point scale. Dotted lines at .333 and at .667 will be used to divide the families into one of three levels of eye darkness.
Families of Mammals Ranked by Average Eye Darkness (AED)
_______________________________1.00______________________
AED Family
.983 *Weasels, etc (15 species) DARK-EYED
-----------------------------------------------.667----------------------------------
.472 Old World Monkeys (27 species) MID-RANGE
.467 Cattle, Goats (15 species)
-----------------------------------------------.333-----------------------------------
.033 *Cats (15 species) LIGHT-EYED
________________________________.00________________________
Ideally, I like to look at average eye darkness for a family based on at least 15 species. Although in this small sample only four families met that criterion, those four serve to illustrate the feeding pattern as I see it in land vertebrates. Two families are carnivorous predators. The one in the light-eyed category (cats) hunts primarily by means of ambush. Success depends on either just waiting or slow stalking until close enough for a decisive pounce.
The family in the dark-eyed category (weasel, etc) hunts primarily by means of speed, persistence and immediate direct pursuit--even into the den of the prey. Two quotations (on page 11) from P.L. Errington’s 1967 book, Of Predation and Life, relate how two species in the weasel family seek to flush prey rather than stalk prey. This provides a stark contrast with cats: (1) “I have been impressed with the noisiness of a weasel’s hunting in dry vegetation as it bounded amid leaves and rushes and weeds.” (2) “From my observations of hunting minks, like a weasel, it too, can be noisy enough to raise doubts that it cares whether prospective prey hears it coming or not.” Both families, cats and weasels, are very successful predators, but they differ in their approaches to hunting. Each uses its own skills to best advantage.
The two families with eye-darkness near the mid-range are omnivores/herbivores.
The pattern is this: families of predators seldom have average eye darkness near the mid-range. Those that are light-eyed engage in ambush hunting and those that are dark-eyed engage in immediate direct-pursuit hunting. Families of non-predators may have average eye darkness anywhere from very light to very dark, but many will fall near the mid-range of eye darkness.
Next, I want to show average eye-darkness for all families for which I found eye colors for five or more species. Of course, the smaller the number of species, the less reliable the average. With that caution about the unreliability of individual averages, one can still, by viewing the total group, get a better sense of the pattern involved and perhaps derive some testable hypotheses.
Families of Mammals Ranked on Average Eye Darkness
* denotes predators AED= Average Eye Darkness
_______________________________1.00_____________________
AED Family
1.00 True Mice (6 species) DARK-EYED
1.00 *Evening Bats (5 species)
.983 *Weasels, etc. (15 species)
.875 Squirrels (6 species)
.775 *Raccoons (5 species)
________________________________,667_____________________
.550 Old World Fruit Bats (5 species)
.472 Old World Monkeys (27 species) MID-RANGE
.467 Cattle, Goats (15 species)
.417 New World Monkeys (9 species)
________________________________.333_____________________
.281 * Lorises (8 species)
.222 * Dogs, Foxes (9 species)
.219 * Civets (8 species)
.200 Large Lemurs(5 species)
.033 * Cats (15 species) LIGHT-EYED
________________________________.00_______________________
To me, the pattern here is the same as that which I described when considering only the four largest families. The seven families of predators are grouped at either end of the eye darkness scale with the seven families of omnivores/herbivores spread out across the distribution with four families found near the mid-range (i.e. ,50).
Five of the 14 families are in Order Carnivora. All are predators. The two most extreme of the five on average eye darkness (cats and weasels) also have the most exclusively carnivorous diets. Civets and canids both do some ambush hunting, mixed with tentative direct attack (less bold or direct than what is seen in the weasel family) and foraging for vegetation and carrion. Raccoons, in the dark-eyed category, often use sensitive touch to locate prey when hunting. They also eat a lot of vegetation.
The two families of bats exhibit the same pattern seen in birds. Bats that take insects in-the-open and on-the-wing have very dark eyes. Bats that eat fruit have average eye darkness near the mid-range.
Four families are primates. Lorises are light-eyed predators that ambush prey by means of a slow approach followed by a sudden grab with their hands. Of the three families of primates in the above list that are not predators, two are in the mid-range category.
With all the limitations of a small sample of mammal species, the underlying pattern is still evident. Within-order comparisons of carnivores, bats and primates all reflect the same pattern.
June 20, 2006
Large Families of Mammals
IRISPIGMENTATION.COM
