Growth and Survivorship in a California Population of Gerrhonotus coeruleus, with Comments on Intraspecific Variation in Adult Female Size

Abstract

Gerrhonotus coeruleus is a viviparous lizard that ranges into high latitudes and altitudes in western North America. Females in a population on the N coast of California (Mendocino Co.) reach sexual maturity in 32 months and produce a single litter each season. Relatively low clutch sizes (X = 3.8 i 0.2) contribute to low fecundity. Annual survivorship was approximately 73 % for adults and 54 % for juvenile lizards. Female survivorship exceeded male survivorship during 1974 and mean female size (87.7 i 1.0 mm) was significantly larger than mean male size (83.6 i 1.2 mm) the following year. Growth of females in a second coastal population (Monterey Co.) was more rapid and these lizards were larger at sexual maturity than Mendocino County females. Females in a montane population (El Dorado Co., Calif.) reached reproductive maturity when larger and older (44 months) than females in either coastal population although they attain a size in 32 months equal to Mendocino County females at the same age. Litter size, newborn size and relative clutch mass were significantly higher in the El Dorado County population. The evolution of delayed maturity and larger adult size in this species may result from selection for both larger litters and larger young. INTRODUCTION Viviparous reproductive habits superficially suggest such a fundamentally different pattern from egg-laying modes of reproduction that dramatic differences in life history components might be expected. The existence of a pattern of life history traits associated with reproductive mode and the relative position of viviparous species within this general pattern were first proposed for lizards by Tinkle (1969) and Tinkle et al. (1970). Life history traits predicted to be associated with viviparity include later maturity, a single litter per reproductive season, larger eggs, greater life expectancy and reduced reproductive risk. Studies of viviparous lizards (Alcala, 1966; Zweifel and Lowe, 1966; Bustard, 1970; Ballinger, 1973, 1979; Lee, 1975; Pilorge and Barbault, 1981) reveal that these species do not all share a unique complex of life history characteristics. Viviparity occurs commonly within squamate reptiles and the diversity of lineages with viviparous species is evidence for the polyphyletic evolution of this reproductive mode from oviparous species (Packard et al., 1977; Tinkle and Gibbons, 1977; Shine and Bull, 1979; Blackburn, 1982, in press). A variety of selective advantages of viviparity have been suggested (Fitch, 1935; Weekes, 1935; Neill, 1964; Packard, 1966; Goin and Goin, 1971; Packard et al., 1977; Tinkle and Gibbons, 1977; Shine and Bull, 1979). Comparisons among viviparous species representing diverse genera should be useful for the identification of general features associated with viviparity, but an intrinsic risk to this method of analysis is that it may be insensitive to the life history attributes that favor the evolution of viviparity in a particular lineage. If the selective advantages that culminate in the acquisition of viviparity are as diverse as the number of ecological benefits predicted for viviparous reptiles, comparisons with oviparous congeners will be most useful in characterizing the distinctive attributes and evolutionary trends within each lineage (Ballinger, 1973; Guillette et al., 1980; Guillette, 1981). Twenty-four genera of lizards are known that contain both oviparous and viviparous species (Blackburn, 1982, in press). Unfortunately, life history studies of