Size Matters: Individual Variation in Ectotherm Growth and Asymptotic Size

Abstract

Body size, and, by extension, growth has impacts on physiology, survival, attainment of sexual maturity, fecundity, generation time, and population dynamics, especially in ectotherm animals that often exhibit extensive growth following attainment of sexual maturity. Frequently, growth is analyzed at the population level, providing useful population mean growth parameters but ignoring individual variation that is also of ecological and evolutionary significance. Our long-term study of Lake Erie Watersnakes, Nerodia sipedon insularum, provides data sufficient for a detailed analysis of population and individual growth. We describe population mean growth separately for males and females based on size of known age individuals (847 captures of 769 males, 748 captures of 684 females) and annual growth increments of individuals of unknown age (1,152 males, 730 females). We characterize individual variation in asymptotic size based on repeated measurements of 69 males and 71 females that were each captured in five to nine different years. The most striking result of our analyses is that asymptotic size varies dramatically among individuals, ranging from 631–820 mm snout-vent length in males and from 835–1125 mm in females. Because female fecundity increases with increasing body size, we explore the impact of individual variation in asymptotic size on lifetime reproductive success using a range of realistic estimates of annual survival. When all females commence reproduction at the same age, lifetime reproductive success is greatest for females with greater asymptotic size regardless of annual survival. But when reproduction is delayed in females with greater asymptotic size, lifetime reproductive success is greatest for females with lower asymptotic size when annual survival is low. Possible causes of individual variation in asymptotic size, including individual- and cohort-specific variation in size at birth and early growth, warrant further investigation.

Sunning themselves in heaps, knots, and snarls: The extraordinary abundance and demography of island watersnakes

“…Interpreting this pattern is complicated by extensive individual variation in the asymptotic size of Lake Erie Watersnakes (King et al., 2016) such that larger individuals are not necessarily older. Consequently, it is unclear whether high survival leads to increased body size, large body size leads to increased survival, or some other mechanism underlies variation in both size and survival.…”

“…Temporary marks alerted field workers that a snake had been recently processed and that it needs only be scanned before release. Watersnakes were classified as adults if SVL ≥ 430 mm (males) or ≥590 mm (females) (King, 1986; King, Queral‐Regil, & Stanford, 2006; King, Stanford, Jones, & Bekker, 2016). Processing was carried out in the field, at the F. T. Stone Laboratory, our research base during censuses of sites on South Bass, Middle Bass, North Bass and Gibraltar island, or at “South Bay,” a rental property used as our research base during censuses of sites on Kelleys Island.…”

“…However, there is also much individual variation in the asymptotic size of Lake Erie Watersnakes (King et al., 2016) so larger individuals are not necessarily older. Consequently, it is unclear whether body size might show an association with survival, and if it does, whether high survival promotes increased body size, large body size promotes increased survival (e.g., by making snakes difficult for predators to subdue; King, 1993), or some other mechanism underlies variation in both.…”

Among‐individual heterogeneity in maternal behaviour and physiology affects reproductive allocation and offspring life‐history traits in the garter snake Thamnophis elegans

“…For example, Lake Erie watersnakes Nerodia sipedon insularum exhibit significant among-individual heterogeneity in asymptotic size. In turn, the fitness benefit of larger size is dependent on annual rates of survival, which may provide a mechanism to maintain growth polymorphisms within the population (King et al 2016).…”

Growth Models and Analysis of Crustacean Growth Data

“…Each individual owns a specific genetic factor, resulting in genotypic variation. For example, King et al (2016) found a significant difference in terminal sizes of Lake Erie watersnakes, Nerodia sipedon insularum. Kirkwood and Sommers (1984) proposed an IBM by setting L ∞ to be random.…”