Papper Ecologia

462

Opinion

TRENDS in Ecology & Evolution Vol.17 No.10 October 2002

The physiology/lifehistory nexus
Robert E. Ricklefs and Martin Wikelski
The rate of reproduction, age at maturity and longevity vary widely among species. Most of this life-history variation falls on a slow–fast continuum, with low reproductive rate, slow development and long life span at one end and the oppositetraits at the other end. The absence of alternative combinations of these variables implies constraint on the diversification of life histories, but the nature of this constraint remains elusive. Here, we argue that individual and adaptive responses to different environments are limited by physiological mechanisms. Although energy and materials allocations are important results of physiologicaltradeoffs, endocrine control mechanisms can produce incompatible physiological states that restrict life histories to a single dominant axis of variation. To approach the problem of life-history variation properly, studies should integrate behavior and physiology within the environmental and demographic contexts of selection. Published online: 19 August 2002

individuals to environmental stressesand perceived risks. As a result, the organization of behavioral and physiological control mechanisms might constrain individual (and evolutionary) responses and limit life-history variation among species.
What do we mean by physiology?

Robert E. Ricklefs* Dept of Biology, University of MissouriSt Louis, 8001 Natural Bridge Road, St Louis, MO 63121-4499, USA. *e-mail: ricklefs@umsl.eduMartin Wikelski Dept of Ecology and Evolutionary Biology, Princeton University, 303 Guyot Hall, Princeton, NJ 08544, USA.

Biodiversity reflects in part the diversification of life histories. However, many potential combinations of life-history traits do not actually occur in nature [1,2]. Indeed, the few major axes of life-history variation stand in stark contrast to the variety of selectivepressures on life histories: physical conditions, seasonality and unpredictability of the environment, food availability, predators and disease organisms, and relationships within social and family groups. Most life-history thinking has been concerned with constrained evolutionary responses to the environment. Differences among the life histories of species are viewed commonly as having a geneticbasis and reflecting the optimization of phenotypes with respect to their environments. The optimal balance between parental investment and adult self-maintenance is also influenced by the life table of the population, particularly the relative value of present and future reproduction [1,3–5]. Constraints on adaptive responses are established by the allocation of limited time, energy and nutrientsamong competing functions [6,7]. Relatively less attention has been paid to nongenetic responses to the environment, such as adjustment of parental investment in response to perceived risk, except for the study of phenotypic flexibility (the reaction norm) as a life-history character itself [4,8,9]. Here we argue that physiological function, including endocrine control mechanisms, mediates therelationship of the organism to its environment and therefore is essential to our understanding of the diversification of life histories. Much of the variation in life histories, particularly variation in parental investment and self-maintenance, reflects phenotypic responses of
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When referring to physiology, we include aspects of organism function, such as metabolism,nutrition and thermal relationships. These have been considered traditionally in the context of the physiological ecology paradigm, which focuses on particular systems (e.g. the lungs, blood and muscle with regard to the respiratory metabolism of activity) or the integration of systems within the organism (symmorphosis) [10]. With the development of population biology, evolutionary ecology and...