An ontogenetic perspective on the relationship between age and size at maturity

Abstract

Summary1. Understanding the relationship between age and size at maturity is essential because these traits are pivotal determinants of an organism's fitness. 2. The relationship between age and size is commonly addressed using optimization and quantitative genetic approaches. Here we argue that the value of such studies is often limited by an insufficient consideration of organismal ontogeny. 3. On the basis of a simple conceptual framework of hierarchical resource allocation, we identify key aspects of ontogeny that prove critical to a fuller understanding of the relationship between age and size, and which, to date, have been insufficiently explored. In particular, these include intrinsic variation in growth rate within and among populations, and the physiological nature of the maturation process that co-ordinates growth and reproductive function in an organism. 4. We also provide some guidance to the empirical investigation of these aspects, anticipating that a wider theoretical, but especially empirical appreciation of ontogenetic detail will greatly increase the explanatory and predictive power of life-history studies.

Growth patterns in male rats exposed to concurrent long-term environmental challenges

“…Indeed, the transition from growth to reproduction produces an observable relationship between age and size at maturity (Berner and Blanckenhorn, 2007). In freeliving Norway rats (R. norvegicus) from temperate and tropical populations, both males and females reach sexual maturity at ~200 g (Porter et al, 2015), suggesting this as the critical weight threshold which triggers the physiological processes that underlie the reproductive life of the species (Berner and Blanckenhorn, 2007;Porter et al, 2015). Based on this, the rats fed ad libitum (FR-) reached that threshold between the 7th and 8th week of life, while those with limited resources (FR+) matured almost a month later.…”

“…For instance, larger individuals are better able to compete for resources, usually have higher reproductive output, are less-likely to be predated and experience a lower cost of foraging reductions (Berner and Blanckenhorn, 2007;Muñoz and Bonal, 2008;Preisser and Orrock, 2012). In addition, individuals in a good body condition are better prepared to face and limit an infection .…”

Diaspidid (Hemiptera: Coccoidea) size plasticity as an adaptive life history trait

“…Studies of insect growth and associated physiological changes have in most instances found an observable relationship, either positive or negative, between body size and age at maturity (Nijhout, 2003;Stern, 2003;Berner & Blanckenhorn, 2007). The widely accepted physiological model of insect growth posits that size in insects is determined by the attainment of a size threshold in the final instar, the critical weight (or size), at which an irreversible endocrine cascade is triggered, characterised by a decline in juvenile hormone titres and an increase in ecdysteroid hormone levels, culminating in the initiation of metamorphosis (Nijhout, 2003).…”

“…The widely accepted physiological model of insect growth posits that size in insects is determined by the attainment of a size threshold in the final instar, the critical weight (or size), at which an irreversible endocrine cascade is triggered, characterised by a decline in juvenile hormone titres and an increase in ecdysteroid hormone levels, culminating in the initiation of metamorphosis (Nijhout, 2003). In the interval during which these changes are occurring in the final larval or nymphal instar, the insect continues to feed and grow and may almost double in size (Berner & Blanckenhorn, 2007). This model has been successful in explaining the relationship between size and age over a range of environmental and genetic factors (but not host plant quality, as far as we are aware) for a number of arthropod and vertebrate species, leading to a general conclusion that most multicellular organisms must acquire a specific amount of resources, manifested as size or weight thresholds, before they can enter reproductive life (Berner & Blanckenhorn, 2007).…”

“…In the interval during which these changes are occurring in the final larval or nymphal instar, the insect continues to feed and grow and may almost double in size (Berner & Blanckenhorn, 2007). This model has been successful in explaining the relationship between size and age over a range of environmental and genetic factors (but not host plant quality, as far as we are aware) for a number of arthropod and vertebrate species, leading to a general conclusion that most multicellular organisms must acquire a specific amount of resources, manifested as size or weight thresholds, before they can enter reproductive life (Berner & Blanckenhorn, 2007). The results presented here challenge that assertion.…”