This study examines if differences in the extent of fat allocation to developing follicles explains observed variation in offspring size among four populations of watersnakes with access to different sized prey. Offspring size differences among populations suggest disparate investment to reproduction. Environmental variation in prey size likely changes energy allocation patterns by differing caloric intake. Animals using stored resources to invest in reproduction should change fat storage and subsequent fat mobilization during vitellogenesis as caloric intake is altered. Using soxhlet extraction, we measured the extent of fat storage and mobilization among populations by determining differences in fat body mass, percent liver fat, and percent ovary fat of sexually mature snakes across the reproductive season. Fat allocation to reproduction was separated between individual follicle fat content and total fat allocation to ovaries. Increased fat allocation to ovarian follicles reflects larger resultant offspring size. Whereas total percent fat to the ovary reflects overall reproductive investment. We determined differences in the extent of fat storage and mobilization using an analysis of covariance (ANCOVA) with female body mass as a covariate. Percent fat body mass, percent liver fat and percent ovary fat were analyzed independently pre-, mid-, post- vitellogenesis.
Results/Conclusions
Females from populations with access to larger mean prey sizes (LARGE) produced significantly longer, heavier offspring (23% heavier, 4.5% longer) than populations with access to smaller average prey sizes (SMALL). Percent fat body mass significantly differed mid- and post- vitellogenesis between LARGE and SMALL populations. Larger average prey size allowed females to store more fat body mass at LARGE populations. Preliminary data also suggest no difference in percent liver fat mid-vitellogenesis and only weak differences post- vitellogenesis among two of four populations. The liver’s role in vitellogenesis is complex and likely short-term. This pattern is potentially a consequence of variability in timing of vitellogenesis among individuals. The transient role of liver in vitellogenesis makes it a poor indicator of the extent of fat mobilization. Fat allocation to the ovaries, collectively and to each follicle, more accurately reflect the extent of vitellogenesis, as ovaries are final targets of this fat. Data collection of ovary fat content is underway. While data are limited, this study provides a potential physiology mechanism that links environmental variation of prey size with differences in reproduction in capitalist-breeding predators. Ecological factors shifting prey size therefore can alter reproduction in predators by changing fat storage and vitellogenesis.