The relationship between eye darkness and behavior can be considered at different levels of analysis. Comparing entire orders obscures much, but is useful in getting an idea of “the big picture.” Following are all orders of land vertebrates for which there are 10 or more species in the sample. This “picture” is based on 32 orders, containing an average of 174 species each for a total of 5,584 species in this sample. “A.E.D.” indicates average eye-darkness. Orders of birds are shown in blue; mammals in green; reptiles in red; and amphibians in orange.
A.E.D. ORDER SAMPLE SIZE
------------------------------------------------------------------1.00-------------------------
.933 Swifts & Hummingbirds, Apodiformes (93 species)
.824 Rodents, Rodentia (17 species)
.775 Bats, Chiroptera (10 species)
.773 Shearwaters, etc., , Procellariiformes (75 species) Dark-eyed
.713 Nightjars & Relatives, Caprimulgiformes (54 species)
.673 Sandgrouse, Pteroclidiformes (13 speciies)
-------------------------------------------------------------------.667-----------------------
.658 Trogons, Trogoniformes (19 species)
.642 Sandpipers & Relatives, Charadriformes (263 species)
.638 Passerine Birds, Passeriformes (2,197 species)
.611 Kingfishers, etc., Coraciiformes (140 species)
.570 Gamebirds, Galliformes (194 species)
.540 Tinamous, Tinamiformes (37 species)
.538 Cattle, Deer, etc.Artiodactyla (20 species)
.512 Cuckoos, etc., Cuculiformes (103 species) Mid-range
.490 Woodpeckers & Relatives, Piciformes (290 species)
.477 Ducks & Screamers, Anseriformes (151 species)
.465 Carnivores, Carnivora (57 species)
.435 Parrots, Psittaciformes (324 species)
.387 Primates, Primates (62 species)
.383 Penguins, Sphenisciformes (15 species)
.369 Cranes & Relatives, Gruiiformes (139 species)
-------------------------------------------------------------------.333-----------------------
.322 Pelicans & Relatives, Pelicaniformes (52 species)
.300 Salamanders, Urodela (45 species)
.280 Hawks & Relatives, Falconiformes (284 species)
.262 Owls, Strigiformes (80 species)
.258 Pigeons, Columbiformes (249 species) Light-eyed
.233 Grebes, Podicipediformes (15 species)
.215 Snakes & Lizards, Squamata (302 species)
.161 Herons & Relatives, Ciconiiformes (109 species)
.150 Turtles, Chelonia (20 species)
.118 Crocodilians, Crocodelia (19 species)
.050 Frogs, Anura (136 species)
-------------------------------------------------------------------.000-----------------------
One can get an idea of the pattern involving eye darkness and behavior of land vertebrates by considering the above table. Following are several observations of hypothetical relationships that involve apparent correlates of eye darkness. They are not, of course, independent of each other.
1. DIFFERENT CLASSES: Birds and mammals are darker-eyed than are amphibians and reptiles.
2. MOBILITY: Dark-eyed animals are more mobile in the land/air environment than are light -eyed animals. In the history of land vertebrates, darker eyes and greater mobility appear to have evolved together.
3. FEEDING: (a) Ambush hunting is very common among animals in the lightest-eyed orders of land vertebrates, but is uncommon among darker-eyed orders. Ambush hunting is most common in amphibians and reptiles, but it is also seen in light-eyed mammals and birds. For mammals, that is not so clear from the above table. Carnivores and primates both have families that ambush prey, but there are other large families in both those orders that do not feed in that way and have darker eyes. With birds, the picture above is clearer. The three orders of birds probably most identified with hunting by means of stealth-- herons, etc., owls,etc., and hawks, etc., are all in the light-eyed category. [Predators that ambush prey tend to be light-eyed, but some light-eyed animals (e.g. pigeons) are not predators of any kind.]
(b.) Animals that feed in-the-open and on-the-wing are grouped at the dark-eyed end of the distribution. Among the four darkest-eyed orders are the only aerial mammals and probably the two most aerial orders of birds. Most species in the next darkest-eyed order of birds, nightjars, are also aerial feeders.
(c.) Animals that mostly eat immobile food (vegetable or animal) are found at all levels of eye-darkness, but the midrange of eye-darkness is associated with that type feeding more than is any other level. [The fact that the mammal order of predators, Carnivora, also falls near the midrange of eye darkness follows from the fact that the order is about equally divided between light-eyed and dark-eyed families. No family of carnivores has average eye-darkness near the midrange. With diverse orders such as this one the underlying pattern becomes clear only at lower levels of analysis (e.g. between families)].
(d.) Feeding in total darkness is associated with dark-eyes. Of the six darkest-eyed orders, three--rodents, bats and nightjars--are associated with night feeding. To feed in total darkness, an animal needs to have senses other than vision that are very keen to aid in finding food. For that reason, differences in eye darkness are probably associated with differences in keenness of some non-visual senses. Evidence at other levels of analysis for a relationship between dark eyes and feeding in total darkness are discussed in the annotated bibliography at the reference for Johnsgard (1988).
July 29, 2006
The Big Picture
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