We studied bullsnakes (Pituophis catenifer sayi) in the Sandhills of Nebraska. Females matured at ca. 90-cm snout–vent length after two seasons (ca. 33 months old), ovulated in early June, and oviposited in mid-June–early July. Frequently, females did not reproduce each year. The proportion reproducing was a function of size of body and warmth of previous summer. Individual eggs averaged 4% of mass of gravid females and size of egg tended to increase with size of body, but did not vary annually. Based on counts of corpora lutea or oviposited eggs, size of clutch averaged 9.5 and was correlated with size of body. Size of clutch varied among years, even after effects of size of body were removed, and tended to be greater in years with warmer temperatures in April–May. Size of clutches and eggs were correlated inversely. Mass of clutch increased with size of body, averaged 37% of mass of gravid females and 59% of post-gravid mass, and did not vary among years. Bullsnakes appear to be capital breeders with respect to frequency of reproduction, but also may respond to resources during vitellogenesis in spring with changes in size of clutch (income breeding). Mass of eggs and clutches were less plastic. Among Pituophis, size of egg and body decrease with increasing latitude; however, mass of egg standardized to size of body does not vary with latitude. Size of clutch standardized to size of body increases with latitude only in P. melanoleucus. Relative mass of clutch does not vary with size of body within or among populations. Frequency of reproduction and size of clutch exhibit considerable variation, apparently based on effects of climate on acquisition of resources, whereas mass of clutch and size of egg vary less (locally and regionally) and appear to be constrained more by natural selection for optimal dimensions.
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Vol. 57 • No. 1