No abstract

Feeding rates influence reproductive output in many kinds of animals, but we need to understand the timescale of this influence before we can compare reproductive energy allocation to energy intake. A central issue is the extent to which reproduction is fuelled by long‐term energy stores (“capital” breeding) versus recently‐acquired resources (“income” breeding). Our data on free‐living aspic vipers show that there is no simple answer to this question: reproductive frequency is determined by long‐term energy stores, offspring size is influenced by maternal food intake immediately before ovulation, and litter size is influenced by both long‐term stores and short‐term energy acquisition. Thus, offspring size in free‐living vipers reflects the mother's energy balance over the preceding year (via a trade‐off between litter size and offspring size) as well as her energy balance in the current breeding season. Hence, different components of a given reproductive output (litter) are not only functionally linked, but also respond to different temporal scales of prey availability. A female's body size has little effect on her reproductive output. Attempts to quantify reproductive energy allocation must take into account the fact that different reproductive traits (such as offspring size versus number) may respond to energy availability over different timespans. Thus, although the aspic viper is a typical “capital breeder” in terms of its reliance on stored reserves for maternal “decisions” concerning reproductive frequency, it is to some degree a facultative “income breeder” with respect to the determination of offspring size and litter size.

One major goal of physiological ecology is to seek links between life history traits. Identification of a body condition threshold for breeding (e.g. critical level of body reserves) provides a link between the physiological process involved in storage of body reserves and the ability to reproduce. One hundred and twenty-nine freeranging adult female Vipera aspis, a viviparous snake, were marked with electronic identification tags and/or by scale clipping, weighed, and measured at the onset of vitellogenesis, and immediately released in the field in western central France (47°03'N; 02°00'W). The 129 snakes were recaptured 2-6 months later between ovulation and parturition, and individual reproductive status was then determined. Eighty-four females (65%) captured at the onset of vitellogenesis became vitellogenic, 45 did not. There was no difference in mean body length between reproductive and non-reproductive females. Initial body condition influenced reproductive outcome: we found a precise threshold in body condition necessary for the induction of vitellogenesis. Almost all females (88%) with a body condition greater than the observed threshold became vitellogenic, 12% did not, and no female with a body condition under the threshold became vitellogenic. Body reserves were estimated in the 129 living females using data gathered on 69 autopsied females. Females which became vitellogenic had large body reserves, but females which did not were not particularly emaciated (whilst postparturiant females had few body reserves remaining). This precise condition threshold for breeding is discussed in terms of the reproductive ecology of this species.

Female aspic vipers (Vipera aspis) are ''capital breeders,'' and delay reproduction until they have amassed large energy reserves. Data from an eight-year markrecapture study on free-ranging vipers suggest that potential costs of reproduction were high for these animals, in terms of survival as well as growth and energy storage. Females that reproduced experienced higher mortality rates than nonreproductive females and, hence, exhibited a tendency toward semelparity, grew less, and devoted most of their energy stores to reproduction. Both the depletion of body reserves and the low survival of reproductive females translated into significant costs (decrements of lifetime reproductive success [LRS]). However, the cessation of growth during pregnancy had no detectable effect on LRS. Most females produced only a single litter during their lifetimes. A female's ''costs'' in energy terms were not negatively correlated with her future reproductive output, probably because female vipers vary considerably in the rate at which they can accumulate energy. This notion is supported by the observations that (1) females with higher initial body reserves expended more energy during reproduction, and (2) females that accumulated energy more rapidly after parturition were more likely to survive and to breed again. This kind of variation among females masks any underlying trade-off between current reproductive effort and probable future reproductive success. Despite this complication, a strong link between rates of survival and postreproductive mass recovery suggests that changes in body reserves govern reproductive effort in this species.

Bonnet, X., Naulleau, G., Shine, R. and Lourdais, O. 2000. Reproductive versus ecological advantages to larger body size in female snakes, Vipera aspis. -Oikos 89: 509-518.Body size can influence an organism's microevolutionary fitness either via ecological factors (ecological selection) or changes in reproductive output (sexual or fecundity selection). Published studies on sexual dimorphism in reptiles have generally focussed on sexual-selective forces on males, under the implicit assumption that the intensity of fecundity selection in females (and hence, overall selection on female body size) is likely to be relatively consistent among lineages. In this paper, we explore the degree to which larger body size enhances ecological attributes (e.g., food intake, growth, survival) and reproductive output (reproductive frequency, litter size, offspring size, offspring viability) in free-ranging female aspic vipers, Vipera aspis. The less-than-annual reproductive frequency of these animals allows us to make a direct comparison between females in years during which they concentrate on ''ecological'' challenges (especially building energy reserves) versus reproductive challenges (producing a litter). Because female snakes have limited abdominal space to hold the clutch (litter), we expect that fecundity should depend on body size. However, our data show that larger body size had a more consistent effect on ecological attributes (such as feeding rates and ''costs of reproduction'') than on reproductive output per se. Indeed, total reproductive output was maximised at intermediate body sizes. These results suggest that variation in female body size among and within species (and hence, in the degree of sexual dimorphism) may be driven by the ecological as well as reproductive consequences of body size variation in both sexes.