REPRODUCTIVE ENERGY ALLOCATION AND LONG-TERM ENERGY STORES IN A VIVIPAROUS LIZARD (EULAMPRUS TYMPANUM)

Abstract

When animals reproduce, does energy for the brood come from stored reserves (from food eaten months or years before), or from food eaten during the current reproductive season? Variation in the duration of energy storage prior to reproduction is an important, but little-studied, axis of life history variation. We experimentally manipulated the availability of resources to females of a long-lived viviparous lizard (Eulamprus tympanum, Scincidae) in the first year of a two-year study to assess the relative importance of current food intake vs. stored reserves as sources of energy for breeding. Females given greater access to high temperatures (and hence, to resources) in the laboratory amassed larger caudal energy stores than did females with lesser opportunities. Energy intake during gestation slightly influenced offspring size, but the magnitude of energy stores prior to ovulation had a more dramatic effect: females with larger caudal energy stores produced much larger litters in the following year. Growth rates of females were unaffected by basking treatments or by reproduction in either year, but females that reproduced in the second year showed a large concomitant decrease in caudal energy stores compared to females that did not reproduce, indicating a large energetic cost of reproduction. Hence, reproductive output within this species can be influenced by resource availability over 12 months prior to reproduction, and simple comparisons of reproductive output with current resource availability may fail to detect the ways in which phenotypically plastic life history traits are influenced by environmental features. In addition to an appropriate temporal framework, models of resource allocation should also incorporate the possible roles of physiological and morphological constraints (e.g., maternal inability, in some species, to modify postovulation reproductive expenditure) and trade-offs with other life history currencies (e.g., decrements in maternal survival due to high relative clutch masses). The time lag between resource acquisition and expenditure may have significant consequences for a population’s demographic response to shifts in resource availability or predation pressure (via tail autotomy). Finally, this study illustrates the way in which thermal aspects of the environment can influence life history phenotypes in ectothermic vertebrates with long-term energy stores.