Reproductive biology

The ancestral condition was egg laying, but live bearing has arisen repeatedly among squamates in at least nine different families of lizards as well as among many snakes—many times within some genera. Nearly 20% of all lizard species are viviparous, representing at least 56 independent origins. At least 35 additional origins exist among snakes. In a few species, some populations are oviparous, and others are viviparous. Viviparity arises via egg retention. A female that can "hold" her eggs can bask, warming them and enhancing development as well as protecting them from nest predators. Live bearing and egg retention have allowed squamates to invade cooler regions and to live at higher elevations and higher latitudes. Live bearing allowed transcontinental migrations of some squamates across cold, high-latitude land bridges. Examples include New World natricine snakes and Boinae, among others. Several oviparous squamates (some anguids, skinks, and a few snakes) "guard" or attend nests, protecting developing eggs; a few, including skinks in the genus Eumeces and snakes in the subfamily Pythoninae, enhance development by providing water or heat to developing eggs.

Within Iguania, viviparity clearly arose in connection with invasion of cold habitats. Iguanian eggs simply are held in the oviducts until they hatch and neonates are laid or extruded (ovoviviparity). The most complex known squamate placenta is found in the South American skink, Mabuya heathi. Within scleroglossans, viviparity has not arisen in teiids, varanids, or helodermatids, and it occurs in only one species of lacertid. An ability to seek out good nest sites, thereby increasing juvenile survivorship, could be an alternative to viviparity. Carrying offspring for long periods of time, as is necessary in viviparous species, probably increases predation risk to females, because gravid females burdened with embryos cannot run as fast or escape as well. Nearly all cordylids are viviparous, as are many anguids and skinks. Cordylids as a group are ecologically more similar to iguanians than to other autarchoglossans, and evolution of viviparity may reflect a long history of high levels of predation or predictable predation on nests of their ancestors, likely from other autarchoglossans. Complex placentas, as seen in some skinks and all xantusiids, do not occur within Iguania.

Because iguanians rely on crypsis to evade predators, filling their body cavity with eggs does not negatively affect de-tectability; hence, many iguanians, such as ctenosaurs and horned lizards, produce massive clutches and are extremely fecund. At the low end of relative clutch mass are those of anoles, unusual iguanians that lay a single egg at one time but produce many clutches. The single evolutionary event that resulted in the small and genetically fixed clutch size of the more than 300 Anolis species may have been related to arbo-reality. Alternatively, high numbers of deaths in the nest could have driven small clutch sizes, providing an advantage to individuals that distributed their eggs in time and space (bet hedging). Nevertheless, production of a single egg does not interfere with crypsis provided by the elongated, twiglike morphologic features of Anolis. For unknown reasons, probably historical, all members of the clade Gekkota have a fixed clutch size of one or two. This, too, may reflect evolution of smaller clutch size associated with mass-related maneuverability on vertical surfaces in a gekkotan ancestor. As a consequence of their more active lifestyle, autarchoglossans typically are streamlined; there are no horned lizard counterparts, although some cordylids that have reverted to ambush foraging approach a spiny tanklike body form. As a consequence, relative clutch mass is constrained and usually smaller, on average, than in non-Anolis iguanians.

Reduced clutch volume in scleroglossans and its attendant reproductive consequence (lower investment per reproductive episode) might at first glance appear to be costly. Iguanians and autarchoglossans use space in fundamentally different ways. Iguanians are territorial and live most of their lives in a relatively small area. Clutches usually are deposited within their territories or nearby; thus, nest site selection is limited by iguanian behavior. Moreover, chemical cues play a minor role in nest selection, because most iguanian species have poorly developed vomerolfaction systems. In contrast, most autarchoglossans are not constrained in their use of space by terrioriality, and they have well-developed chemosensory systems. Using their superior vomerolfactory abilities, they can seek out and choose the best possible nest sites. Many teiid nests in exposed sandy areas along streams and skink (Eume-ces) nests in rotting logs are examples. Both Tupinambis and Varanus deposit clutches in termite nests, which provide heat, humidity, and protection. A survey of squamates using social-insect nests for egg-deposition sites found no iguanians; however, scleroglossans in five lizard families and three snake families use them regularly. Among iguanians, communal nesting occurs in a few iguanines, an herbivorous clade with well-developed chemosensory abilities, and a few species in which limited nest sites appear to be the foci of territories. Thus, scleroglossans could have an advantage in nest site selection that offsets any cost resulting from reduced clutch volume.

Because of dramatic differences in the use of space between iguanians (territorial) and scleroglossans, particularly autar-

choglossans (nonterritorial and free ranging), social systems are drastically different. Iguanians typically have polygynous mating systems centered on defendable resources in which social signals are visual. Autarchoglossans typically have polygy-nous mating systems centered on sequential female defense in which a combination of visual and chemical cues predominate as social signals. A few have monogamous mating systems. Among cordylids that have reverted to ambush foraging and territoriality, the social system has also switched to resource defense polygyny.

Parthenogenesis (reproduction without fertilization) has evolved many times among lizards and in at least one snake. In such all-female unisexual "species," a female lays eggs that develop into exact genetic clones of herself. Many such parthenoforms have arisen via hybridization of sexual parental species. Because no energy is invested in making males, parthenogenetic forms have a much faster rate of increase than do sexual species. Without sexual reproduction, however, they cannot evolve. If heterozygosity itself confers fitness, asexual reproduction maintains existing heterozygosity (acquired in hybridization), whereas in sexual reproduction recombination will disrupt heterozygosity.

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