Peculiar Constraints on Life Histories Imposed by Protective or Nutritive Devices for Embryos

Abstract

SYNOPSIS. A general constraint for sexually produced offspring of multicellular organisms is that they must begin as a single cell (lest they be chimeric) and that the embryos that produce multicellular structures perform many functions less capably than more developed individuals. Beyond this limitation, predictive models have examined optimized tradeoffs between number of offspring and size of offspring. Broadly applicable predictions have been sought, and two general expectations have resulted. (1) Optimal parental investment per offspring should not be affected by total investment per clutch when offspring size is much smaller than clutch size. (2) Selection should usually favor an optimal size of offspring rather than varied sizes within a clutch. However, constraints intrinsic to mechanisms of parental protection or provisioning may change these predicted outcomes of selection on life history traits. (1) Provisioning and packaging of protected embryos may introduce dependence of offspring size on adult size or clutch size, (a) Clutches of embryos are a large mass of metabolizing tissue without a circulatory system but with requirements for gas exchange; when dissolved gases are transported by interstitial flow between embryos, requirements for supply of oxygen may require larger offspring in thicker and hence more massive clutches, (b) Offspring size can become dependent on adult size through capsule size or size of the birth canal, and offspring size is thereby linked to clutch mass when clutch mass increases with adult size. (2) Random placement of protected embryos may increase variation around an optimum size or quality of offspring. Possible examples are (a) determination and packaging of nurse eggs that will be consumed by embryos and (b) hormonal interactions among offspring in a uterus. Constraints that violate general expectations and arise from varied and specific mechanisms of parental protection or provisioning may be common.