Beyond size-number trade-offs: clutch size as a maternal effect

Abstract

Traditionally, research on life-history traits has viewed the link between clutch size and offspring size as a straightforward linear trade-off; the product of these two components is taken as a measure of maternal reproductive output. Investing more per egg results in fewer but larger eggs and, hence, offspring. This simple size-number trade-off has proved attractive to modellers, but our experimental studies on keelback snakes (Tropidonophis mairii, Colubridae) reveal a more complex relationship between clutch size and offspring size. At constant water availability, the amount of water taken up by a snake egg depends upon the number of adjacent eggs. In turn, water uptake affects hatchling size, and therefore an increase in clutch size directly increases offspring size (and thus fitness under field conditions). This allometric advantage may influence the evolution of reproductive traits such as growth versus reproductive effort, optimal age at female maturation, the body-reserve threshold required to initiate reproduction and nest-site selection (e.g. communal oviposition). The published literature suggests that similar kinds of complex effects of clutch size on offspring viability are widespread in both vertebrates and invertebrates. Our results also challenge conventional experimental methodologies such as split-clutch designs for laboratory incubation studies: by separating an egg from its siblings, we may directly affect offspring size and thus viability.

Figure 1

Rate of mass increase (=water uptake from the substrate) by keelback eggs incubated in a range of cluster sizes. Eggs that were incubated in small groups lost mass in the latter part of incubation, whereas eggs in larger groups continued to increase in mass. Squares, 4–5 eggs; circles, 8–10 eggs; triangles, 15–16 eggs; diamonds, 25–27 eggs.

Figure 2

Residual SVLs (corrected for initial egg mass) of hatchling keelback snakes from eggs incubated in clusters of different sizes. Eggs that were tightly packed with many other eggs produced larger offspring than did eggs that were incubated in smaller groups.

Figure 3

Two female keelbacks (T. mairii) ovipositing simultaneously in the same nest on the Fogg Dam wall. Subsequent excavation revealed that a third clutch was already present in the nest. Photograph by C. Beckmann.