Geographic Variation in Age and Size at Maturity in a Small Australian Viviparous Skink

Age and size at maturity are pivotal life-history traits through their effects on other key traits, such as annual and lifetime fecundity. We used skeletochronology to investigate the relationships among size (snout–vent length), age, and maturity in two populations of a small viviparous skink, Niveoscincus ocellatus, from Tasmania, Australia. The species occupies a wide geographic and climatic range within the temperate zone, and we chose populations from the climatic extremes of this range. Growth in N. ocellatus is rapid early in life but slows considerably after maturity in both sexes. Within sites, we found no difference in growth patterns or length at maturity between the sexes. However, there were large differences between sites. At our “warm” site, lizards were mature at three years of age at a relatively small size. Lizards from the “cold” site typically delayed maturity until their fourth year (although some males were mature at the end of their third year); as a result, they were significantly larger at maturity and thereafter remained larger for any age than did warm-site lizards. These patterns are consistent with predictions from models of the proximate influence of the thermal environment on growth and maturity patterns in squamate reptiles. Lizards from the cold site are born later in the season and have a shorter activity season prior to obligatory winter hibernation, and conditions for growth are less favorable in any particular month than at the warm site. Because delaying maturity is costly to current fecundity, we suggest that in N. ocellatus lifetime fecundity is enhanced at the cold site by additional growth and gains in future fecundity through the relationship between body length and reproductive output.

Fig. 1.

Mean monthly maximum air temperature (C) for 1992–1997 at the cold (Central Plateau) and the warm (East Coast) study sites (courtesy of the Tasmanian Bureau of Meteorology) and typical period of Niveoscincus ocellatus winter inactivity

Fig. 2.

Size frequency distribution in snout–vent length for mature Niveoscincus ocellatus at the warm and cold sites for the period 1994–1997

Fig. 3.

Relationship between age and size in Niveoscincus ocellatus in the warm site (A) and the cold site (B). Age (in months) is estimated from the number of LAGs, the capture date, and the assumption that all individuals were born in January at the warm site and in February at the cold site

Fig. 4.

von Bertalanffy growth curves based on data from male and female Niveoscincus ocellatus from the warm site (A) and the cold site (B). Juvenile data are included in the construction of curves for both sexes. Parameters for these curves are presented in Table 1

Fig. 5.

Combined male, female von Bertalanffy growth curves for Niveoscincus ocellatus from the warm site and the cold site. Juvenile data are included in the construction of curves for both populations. Parameters for these curves are presented in Table 1

Accepted: 14 Dec 2000