set the stage for the evolutionary enhancement of 

 the vomeronasal organ, a chemosensory organ em- 

 bedded in the roof of the mouth. That organ detects 

 heavy, nonairborne molecules taken in through the 

 mouth. It supplements nasal olfaction, which is the 

 ability to smell airborne molecules that enter 

 through the nostrils as the animal breathes. It is dis- 

 tinct as well from taste, which is the ability to analyze 

 chemicals with taste buds on the tongue. In scle- 

 roglossans the enhancement ot both olfaction and 

 vomerolfaction was lite-changing: not only could 

 they rely less on sight to detect prey, but they could 

 also perceive sex and sexual receptivity in their own 

 species on the basis ot chemical cues alone. 



The avid way Gila monsters hunt out eggs suggests 

 that enhanced chemical senses have helped some 

 scleroglossans to find prey particularly rich in energy. 



But perhaps more im- 

 portant, the enhanced 

 sense of vomerolfac- 

 tion also enabled scle- 

 roglossans to identify 

 dangerous prey — prey 

 whose metabolic poi- 

 sons or other defensive 

 chemicals could be 

 deadly or at least costly 

 to digest. Several years 

 ago we combined data 

 we had gathered inde- 

 pendently over our ca- 

 reers and demonstrated 

 that, compared with 

 iguanians, scleroglos- 

 sans eat many fewer 

 ants, other hymen- 

 opterans, and beetles, 

 most of which produce 

 noxious or toxic chem- 

 icals. Most scleroglos- 

 sans are highly active, 

 and so they encounter 

 numerous potential 

 prey; that behavior 

 may afford them the 

 luxury of passing over 

 noxious insects. 



As evolving chem- 

 ical senses enabled 



Male fan-throated lizard from India (Sitana ponticeriana), pic- 

 tured J.3X larger than life size, flashes its blue, black, and red 

 dewlap to defend its territory from others of its species. The 

 animal relies in large measure on camouflage for its own pro- 

 tection as well as for ambushing insect prey, and so it must 

 not cede suitable habitat to another lizard without a fight. 



scleroglossans to locate prey and potential mates 

 without relying on vision, they adopted a more 

 mobile searching strategy. This made them more 

 conspicuous, requiring wariness, speed, and 

 agility — not just camouflage — to escape predators. 

 For the most part they ceased devoting time and 

 energy to defending territories. Their chemical 

 senses also enabled some scleroglossans to enter 

 subterranean microhabitats and others to shift to 

 nocturnal habits, both unavailable to iguanians that 

 rely on vision as their primary sensory system. 



One possible measure of the success of the shift 

 to jaw prehension and chemical senses is that 

 in terms of extant species, scleroglossans outnum- 

 ber iguanians 4.5 to 1. That ratio might overstate 

 an advantage resulting mostly from chance. For ex- 

 ample, scleroglossans could have split into separate 

 groups a bit earlier than iguanians did in their evo- 

 lutionary history, and the present-day diversity 

 might simply magnify that small advantage. But 

 scleroglossan success was real. A more convincing 

 measure is that scleroglossan species outnumber 

 iguanian species at nearly every site where both 

 occur. In most places, scleroglossans dominate the 

 ground, foraging freely over the landscape, whereas 

 most iguanians are confined to rocks, shrubs, tree 

 trunks, or other more restricted habitats. 



Subsequently — about 180 million years ago — 

 scleroglossans split into two groups, Gekkota and 

 Autarchoglossa, which differ in when they are ac- 

 tive and how they use their tongues. Gekkotans in- 

 clude, among others, geckos (after which the group 

 is named) and the snakelike flap-foots of Australia. 

 Most geckos are nocturnal. They have no eyelids; 

 instead, the eye is covered by a "spectacle," or trans- 

 parent scale, similar to the eye-covering of snakes. 

 The tongue serves as a windshield wiper to clean 

 the spectacles (as well as the Hps). Geckos also have 

 a good sense of smell. William E. Cooper Jr., a bi- 

 ologist at Indiana University— Purdue University in 

 Fort Wayne, has shown that geckos detect airborne 

 pheromones of other geckos, and discriminate prey 

 on the basis of chemical signals. 



Geckos are also noted for being able to scale verti- 

 cal surfaces and walk upside down on leaves, rock 

 faces, and trees. They can even jump from tree to 

 tree by catching a leaf on the second tree with a toe. 

 Although some other lizards (anoles and a few 

 skinks) can climb vertical surfaces with their toe 

 pads, none are as adept as the geckos. Kellar Au- 

 tumn, a biologist at Lewis & Clark College in Port- 

 land, Oregon, has shown that gecko feet stick to sur- 

 faces by van der Waals forces, a form of molecular 

 attraction. Their toe pads have ridges, each ridge 



32 



NATURAL HISTORY July/August 2006 



