Nonlinear continuum of egg size-number trade-offs in a snake: is egg-size variation fitness related?

Abstract

The relationship between offspring size and offspring number is crucial to life history evolution. To examine how these two life history variables are coupled and whether an altered balance between them will result in changes in maternal fitness, we manipulated clutch size of the Chinese cobra (Naja atra) by using the techniques of hormonal manipulation and follicle ablation. Females receiving exogenous follicle-stimulating hormone produced more but smaller eggs, and females undergoing follicle ablation produced fewer but larger eggs. Neither body size (body mass and snout-vent length) at hatching nor egg mass at oviposition had a role in determining hatchling survival and growth. Female hatchlings were more likely to die in early post-hatching days and grew more slowly than male hatchlings. Our data show that: (1) there is a nonlinear continuum of egg size-number trade-offs in N. atra within which there is a single inflexion where the rate at which egg size decreases with increasing clutch size, or clutch size increases with decreasing egg size, is maximized; (2) there is a fixed upper limit to egg size for a given-sized female, and the limit is not determined by her body volume; (3) egg size has no role in determining hatchling survival and growth; and (4) the extent to which females may enjoy reproductive benefits in a given reproductive episode depends on how well egg size and egg number are balanced.

Reproductive traits of the gray ratsnake Ptyas korros from three geographically distinct populations

“…In the present study, clutches produced by JS females more completely filled the available abdominal space than did those produced by CZ and DH females. This conclusion is supported by the geographic variation in egg shape, as egg shape is indicative of crowdedness of eggs in the uterus and more rounded eggs are always associated with larger or heavier clutches (Castilla et al, 1992;Ji and Braña, 2000;Ji and Wang, 2005;Ji et al, 2006Ji et al, , 2009. More rounded eggs in the JS population implied that the oviducts of females in the population were more tightly packed when they were gravid.…”

“…The upper limit of reproductive investment is ultimately determined by the abdominal space available to hold eggs (Ji et al, 2006(Ji et al, , 2009. In the present study, clutches produced by JS females more completely filled the available abdominal space than did those produced by CZ and DH females.…”

“…Evidence from diverse animal taxa shows that females can adjust the size of their offspring by assessing the environment that their offspring will face based on their own experience (Boersma, 1997;Johnston and Leggett, 2002;Jordan and Snell, 2002;Bashey, 2008), or offspring size can vary in response to variation in total reproductive investment or maternal size (Sargent et al, 1987;Winkler and Wallin, 1987;Reznick et al, 1990;Caley et al, 2001;Ji et al, 2006Ji et al, , 2009Wang et al, 2011). In the present study, larger females generally devoted more energy to the production of eggs, as revealed by the fact that clutch mass was positively correlated with female SVL in each of the three populations.…”

An experimental test of body volume constraint on female reproductive output

“…Offspring size may thus be affected by the total reproductive investment as well as the trade-off between clutch size and egg size. Clutch size modification (hormonal manipulation or follicle ablation) may lead to change in egg size of some lizards and snakes (Sinervo and Licht, '91;Sinervo and DeNardo, '96;Sinervo, '99;Olsson et al, 2002;Ji et al, , 2009. In T. septentrionalis, however, clutch size manipulation did not result in reduction or enlargement of egg (offspring) size (Du, 2003;Ji and Diong, 2006).…”

Geographical variation in reproductive traits and trade-offs between size and number of eggs in the king ratsnake, Elaphe carinata

“…Larger offspring are believed to be better able to survive than smaller offspring because of their better performances (Ferguson & Fox, 1984;McGinley, Temme & Geber, 1987;Sargent, Taylor & Gross, 1987;Mousseau & Fox, 1998;Janzen, Tucker & Paukstis, 2000a, b; but see also Ji et al, 2009). However, as the resources available for reproduction are finite, females cannot increase the size of their offspring without a concomitant reduction in the number of offspring produced (Stearns, 1992;Bernardo, 1996;Einum & Fleming, 2000;Agrawal et al, 2001;Roff, 2002).…”