How females allocate resources to each offspring and how they allocate the sex of their offspring are two powerful potential avenues by which mothers can affect offspring fitness. Previous research has focussed extensively on mean offspring size, with much less attention given to variance in offspring size. Here we focussed on variation in offspring size in black ratsnakes, Elaphe obsoleta. We collected and hatched 105 clutches (1283 eggs) over 9 years. We predicted that females should lay larger eggs, or more variable eggs, when the environment is less predictable. We also predicted that females laying early or laying larger eggs should produce mostly sons because adult males are larger than adult female ratsnakes. The largest hatchling was more than twice the length and almost four times the mass of the smallest hatchling. Variation in offspring size was itself highly variable, with CVs in offspring mass among clutches ranging from 1% to 25%. With one exception, the variables we expected should influence variation in offspring size had little effect. We found that clutch size increased with maternal size and that egg size decreased with clutch size, but we found no evidence that variance in egg size among clutches increased as the season progressed or that females increased the mean size of their offspring the later in the season they laid their eggs. Females in better condition after they finish laying their eggs did produce larger eggs. There was no relationship between within‐clutch variation in egg size and laying date or mean egg size. Finally, sex ratio did not vary with mean egg size or hatching date. Given evidence that offspring size in snakes affects survival, selection should reduce variation in offspring size unless that variance enhances maternal fitness and yet we found little support for hypothesized advantages of varying offspring size.
We conducted this study from 1996 to 2004 at the Queen’s Univ. Biol. Stn in eastern Ontario, Canada (45°37'N, 76°13'W). As part of a larger study, all ratsnakes were individually marked with PIT tags when first captured. Most eggs were obtained from nests that we had located by radio tracking gravid females until they oviposited (Blouin-Demers et al. 2004). Females had radio-transmitters surgically implanted in their body cavity using sterile techniques and under isoflurane anaesthesia (Blouin-Demers et al. 2000, Weatherhead and Blouin-Demers 2004). Over the nine years of the study, we followed 72 females, 31 of which were gravid at least once. Collectively these females allowed us to locate 19 nests. Eggs were always laid in decaying organic matter in both natural (e.g. hollow trees) and man-made (e.g. leaf piles) locations and included sites used by single females and by multiple females (Blouin-Demers et al. 2004).
We obtained eggs from nests in several ways. Once a female we tracked to a nest had laid her eggs (determined by capturing and palpating the female after she moved away from the nest), we excavated the nest substrate. If we found a single clutch we assigned it to the female we had tracked. If we found additional clutches, maternity was unknown for all of them. Because females lay all their eggs in a tight cluster, however, we were often able to assign eggs to clutches. When the number of eggs in a cluster exceeded the range of sizes of known single clutches, we assumed more than one female had contributed to the clutch. Once we had found a nest we revisited it every summer after nesting was well underway and excavated it for eggs. In addition to finding eggs at these nests, we regularly encountered gravid females. We brought these gravid females back to the lab, housed them individually, and provided them with a nesting box filled with a mixture of moist sphagnum and peat moss in which they laid within a few days. After laying, females were measured for snout-vent length (SVL) to the nearest 1 cm with a flexible measuring tape run along their body (Blouin-Demers 2003) and weighed to the nearest 1 g with an electronic scale. Females were subsequently released where they had been captured.