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

Oecologia (2009) 159:689696 DOI 10.1007/s00442-008-1252-2

PHYSIOLOGICAL ECOLOGY - ORIGINAL PAPER

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

Xiang Ji Wei-Guo Du Yan-Fu Qu Long-Hui Lin

Received: 7 July 2008 / Accepted: 26 November 2008 / Published online: 9 January 2009 Springer-Verlag 2008

Abstract The relationship between oVspring size and oVspring 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 Wtness, 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-oVs in N. atra within which there is a single inXexion 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 Wxed 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 beneWts in a given reproductive episode depends on how well egg size and egg number are balanced.

Keywords Naja atra Chinese cobra Manipulating clutch size OVspring survival Growth

Introduction

Maternal Wtness depends on oVspring size and oVspring number that are mutually constrained by a trade-oV resulting from common dependence on the limited reproductive resources (Stearns 1992; Bernardo 1996; Einum and Flemming 2000; Agrawal et al. 2001; RoV 2002). The relationship between oVspring size and oVspring number is crucial to life history evolution, and has therefore attracted considerable scientiWc interest over the past decades (Clutton-Brock 1991; Stearns 1992; Williams 1994; RoV 2002). However, how these two variables are coupled and whether an altered balance...