with their tongues. The tongue then deposits the 

 chemicals on the vomeronasal organ. Among the 

 autarchoglossans are tegus and whiptail lizards, 

 which march around in the hot sun in deserts and 

 open tropical habitats of the New World, sticking 

 their faces under surface objects to find insect larvae, 

 digging into termite nests to feast on hundreds of 

 termites, and even finding and eating carrion, all 

 without relying primarily on vision. Other promi- 

 nent members of the group are anguids, such as alli- 

 gator lizards; Gila monsters; lacertids, such as jeweled 

 lizards; monitor lizards; skinks; and all snakes. 



The long, thin tongues of monitor lizards and 

 snakes take chemical sensing to its most sophis- 

 ticated level. Not only can they discriminate chem- 

 icals, but by comparing chemical dosages picked up 

 on each fork of the tongue, they can detect edges as 

 well as the direction of a chemical source. Just as 

 chameleons are the logical endpoint to the evolu- 

 tion of a sit-and-wait foraging strategy, snakes might 

 be considered the logical endpoint within a group 

 of organisms that have highly flexible skulls and a 

 well-developed chemosensory system for locating 

 and discriminating prey. 



And just as scleroglossans seem to have elbowed 

 iguanians out of terrestrial habitats, so too autar- 

 choglossans seem to have pushed gekkotans off the 

 ground — or at least into a nocturnal lifestyle. Al- 

 though one group of autarchoglossan lizards, the 

 snakes, includes many nocturnal species (most from 

 warm regions), autarchoglossans are generally ac- 

 tive by day. In fact, their high activity levels usually 

 require a high body temperature. Gekkotans oper- 

 ate at lower body temperatures, but their nocturnal 

 habits have limited their distribution to regions that 

 are warm enough at night. Most geckos, for in- 

 stance, live in the tropics or in deserts, and only a 

 few live in colder climates. 



Autarchoglossans also continued another tradition 

 ot their scleroglossan ancestors. Whereas chemical 

 senses had enabled the scleroglossans to expand their 

 ranges and relax their need to defend territory 

 where they were camouflaged, the continuing evo- 

 lution of those senses enabled the autarchoglossans 

 to forage in places where they could not see. And in 

 several families of autarchoglossan lizards, species 

 evolved that dug burrows and lived underground. 



In many cases the subterranean lifestyle led to the 

 evolutionary loss or reduction of limbs and to 

 shortened tails. In some species, eyes were reduced 

 to simple light-detecting organs — the animals had 

 essentially shifted to a world dominated by chemi- 

 cal signals. Other lineages evolved elongate bodies 

 and tails, even as limbs were lost or reduced, u;iving 



rise to what are known in the United States as glass 

 lizards (genus Ophisaurus). (The common term 

 "glass lizard," however, can refer to species in sev- 

 eral families of lizards that have evolved indepen- 

 dently on different continents.) Neither limbless- 

 ness nor subterranean activity has ever evolved in 

 iguanian lizards. 



That brings us to snakes, which are characteristi- 

 cally limbless. Although they figure large in 

 people's lore and imagination, snakes are simply one 

 evolutionary group (Serpentes) of scleroglossan 

 lizards. They are closely related to lizards that belong 

 to the family Varanidae, which comprises the Ko- 

 modo dragon and other monitor lizards. They may 

 have descended from a varanidlike ancestor, and a 

 common assumption is that they evolved from some 

 burrowing terrestrial lizard. Another theory, how- 

 ever, is that the first snakes were aquatic, having 

 evolved from the mosasaurs, an extinct group of 

 large marine reptiles that were also closely related to 

 the varanids [see "Terrible Lizards of the Sea," by 

 Richard Ellis, September 2003]. But even if terrestrial 

 snakes descended from an aquatic ancestor, herpetol- 

 ogists generally agree that today's species ot aquatic 



34 



natural HISTORY July/August 2006 



