668 



ECOLOGY AND EVOLUTION 



banded anteater (Marsupialia), aardvark 

 ( Tubulidentata ) , pangoUn ( Pholidota ) , 

 and New World anteaters (Xenarthra). All 

 except the banded anteater have specialized 

 fossorial feet for digging into the nests of 

 ants and termites. 



Convergence is found in the method of 

 support of the body in particular habitats. 

 The tail feathers of tree-climbing birds have 

 separately evolved stiflFened rectrices when 

 the tail is used for support in the wood- 

 peckers (Picidae), woodhewers (Dendro- 

 calaptidae), creepers (Certhiidae), and 

 many swifts (Micropodidae). The shaft of 

 the supporting tail feather is stronger in 

 each of these groups, whether the feather 

 barbs or the tip of the shaft itself comes 

 in contact with the tree trunk (Richardson, 

 1942). In addition, the pygostyle and 

 caudal vertebrae are larger, as compared 

 with the nearest relatives that do not have 

 tails adapted to support, or with other tree- 

 foraging birds that do not so use their 

 tails, such as the nuthatches and some 

 wrens and warblers. The woodhewers and 

 creepers show greater similarity in appear- 

 ance and behavior than do the other tree- 

 climbing birds, and their ecological niche 

 is likewise more similar. 



Friedmann (1946) discusses a number 

 of instances of convergence among birds in 

 separated, but ecologically similar habitats. 

 He says: 



"Perhaps the 'classic' ease of this sort and, 

 indeed, one of the most striking is the amazing 

 similarity in appearance and in general habits 

 of the American troupial genus Sturnella (the 

 meadow-larks) and the African pipit genus 

 Macronyx. Both are 'unusual' members of their 

 respective families as far as their coloration 

 goes— the upper parts streaked blackish, brown- 

 ish, and pale buff; the chin, throat, breast, and 

 upper abdomen, bright yellow with a broad 

 black pectoral band. The similarity is carried 

 even to the white outer tail feathers in the two 

 genera. Both Sturnella and Macronyx inhabit 

 grassy open spaces; both make somewhat 

 arched-over, or semidomed, nests of dry grasses 

 on the ground; both have the habit of turning 

 away (that is, of hiding their brightly colored 

 underparts) from an approaching observer; 

 both spread the tail, showing the white lateral 

 feathers as they fly, and both have a somewhat 

 melancholy whistling note. To make die case 

 even more complete, one species of Macronyx 

 (M. ameliae) has the underparts pinkish red 

 instead of yellow, paralleling the red-breasted 

 near relative of Sturnella, the genus Pezites 



of South America, again a bird of similar 

 habits." 



Witliin the same general habitat, aquatic 

 warm-blooded mammals and birds inde- 

 pendently evolved blubber for insulation 

 from cold water, as may be seen in pen- 

 guins, seals, and whales. In the South 

 American rain forest one may find prehen- 

 sile tails in snakes, opossums, porcupines, 

 anteaters, and monkeys. In the arctic re- 

 gions of North America, white coloration is 

 found in mountain sheep, polar bears, 

 arctic foxes, snowshoe rabbits, ptarmigans, 

 and snowy owls. Benson (1933) records 

 three dark subspecies of rodents (Citellus 

 grammurus tularosae, Perognathus interme- 

 dius ater (Fig. 245), and Neotoma alhigula 

 tnelas) , from dark isolated lava beds in 

 New Mexico, that contrast strikingly with 

 closely related subspecies and species that 

 inhabit light soil and white gypsum sands 

 in the vicinity (see also Blair, 1947, 

 1947a). Radiate evolution of closely related 

 forms toward cryptic coloration matching 

 the background is usual in certain groups 

 of animals (Hardy, 1945; see pp. 610, 627, 

 650). Goodale (1942) has demonstrated 

 how artificial selection can increase or de- 

 crease the areas of white pigmentation in 

 mice. 



Buxton (1923) says: "Any desert crea- 

 ture which is not coloured like its surround- 

 ings is black." Tenebrionid beetles of des- 

 erts have hard, dark exoskeletons, some 

 with elytra fused to the body wall. Ffies 

 (Bombyhinae and Anthracinae) and grass- 

 hoppers (Eugaster guijoni) also have black 

 desert representatives. Kahnus (1941) 

 found that hght-colored mutants in sev- 

 eral species of Drosophila experimentally 

 showed less ability to survive dry atmos- 

 phere than the darker wild type, and that 

 differential survival was not apparent in 

 moist atmospheres. 



Convergence may involve biochemical 

 characters— undoubtedly much more fre- 

 quently than the evidence permits us to re- 

 port. Wald (1942) has discovered that 

 fresh- water fishes, irrespective of relation- 

 ship, contain a retinal pigment (porphy- 

 ropsin) different from that found in marine 

 fishes, terrestrial vertebrates, and in both 

 fresh- water and marine invertebrates (rho- 

 dopsin), and that most euryhaUne fishes 

 have both types of pigment. 



