Demographic and Life-History Variation in Two Proximate Populations of a Viviparous Skink Separated by a Steep Altitudinal Gradient

1. Life-history parameters (age and size at first reproduction, female age/size distribution, reproductive output) were compared between two populations of the viviparous water skink Eulamprus tympanum in temperate south-eastern Australia. The two populations were in close proximity to each other (6 km) but separated by 1000 m in altitude. 2. Age and size at first reproduction differed significantly between the two populations. Low-altitude females appear to start reproduction at 2 years of age and at a relatively small size (70 mm snout-vent length) whereas high-altitude females start to reproduce at 5 years of age and at a larger size (90 mm snout-vent length). The data suggest that size at first reproduction at high altitude is larger, not only because thermal conditions slow annual growth rates and therefore delay first reproduction, but because small size is a disadvantage for reproducing females under such conditions. 3. Reproductive output also differed significantly between the two populations, with low-altitude females producing relatively large litters with small young and high-altitude females producing relatively small litters with large young. The total offspring mass a female produced in relation to her body size was significantly smaller in high-altitude E. tympanum, suggesting strong environmental constraints on reproduction compared with low-altitude E. tympanum. 4. To test whether reproductive output can be influenced by local environmental conditions, or whether it requires natural selection to produce a change in this parameter, food intake was manipulated in high-altitude females during their pregnancy. Low-diet females produced a significantly smaller total offspring mass and smaller litters than high-diet females. There was no significant difference in mean offspring size between the two treatments. 5. Effects of age on reproductive parameters were examined to determine whether reproductive output increases with maternal age, for example as a consequence of more experience gained by older females, or decreases, possibly as a result of senescence. In neither population was there evidence for an increase or decline in reproductive output with the age of reproducing females independent of the effects of maternal size.