Alternative life-history styles in reptiles

Reptiles are gonochorists (with the exception of a few parthenogenetic lizards and a single snake) and have relatively few young. Hermaphroditism is absent, possibly due to the existence of internal fertilisation and the complexity of the associated genitalia. Environmental sex determination (ESD) is known in 2 lizards, most chelonians and all crocodilians so far studied. Its significance is obscure. Previous explanations, based on presumed selective advantages of sex-related hatchling size differences, seem improbable due to both groups slow attainment of sexual maturity. It is proposed that a more likely advantage is the ability to produce highly skewed sex ratios by behavioural selection for nesting site choice. Both orders are the most fecund reptiles and suffer high juvenile mortality. The direction of sex determination differs between crocodilians and chelonians (high incubation temperatures produce males and females, respectively.) The evolution of heteromorphic sex chromosomes has occurred independently many times in squamates, and in one chelonian, and this may be correlated with the undesirability of ESD in short-lived species with low fecundity. Parental care is almost completely absent, reaching its highest expression among crocodilians and certain anguid lizards. All young, including those of species showing parental care, are precocial and immediately feed exogenously. The absence of altricial young is possibly due to the difficulty of food handling between adults and young. Viviparity is found only among squamates, where it has evolved independently on numerous occasions. Its evolution is usually associated with cold climates, although certain other traits (e.g. the possession of venom and/or the existence of maternal brooding) may serve as pre-adaptations. Analysis of the effects of size and phylogeny on patterns of co-variation in certain squamate life-history traits reveals that although many variables scale allometrically with body size, significant variation in clutch size and age at maturity remains at order, family and generic level. The size and mass of females and their clutches may vary between populations and years, reflecting phenotypic plasticity to proximate environmental factors. Relative Clutch Mass (RCM — the ratio of female body mass to clutch mass) is correlated with foraging mode and predator escape tactics in lizards (but not in snakes) and with reproductive mode (i.e. viviparity) in snakes.