The physiological costs of reproduction in small mammals

Abstract

Life-history trade-offs between components of fitness arise because reproduction entails both gains and costs. Costs of reproduction can be divided into ecological and physiological costs. The latter have been rarely studied yet are probably a dominant component of the effect. A deeper understanding of life-history evolution will only come about once these physiological costs are better understood. Physiological costs may be direct or indirect. Direct costs include the energy and nutrient demands of the reproductive event, and the morphological changes that are necessary to facilitate achieving these demands. Indirect costs may be optional 'compensatory costs' whereby the animal chooses to reduce investment in some other aspect of its physiology to maximize the input of resource to reproduction. Such costs may be distinguished from consequential costs that are an inescapable consequence of the reproductive event. In small mammals, the direct costs of reproduction involve increased energy, protein and calcium demands during pregnancy, but most particularly during lactation. Organ remodelling is necessary to achieve the high demands of lactation and involves growth of the alimentary tract and associated organs such as the liver and pancreas. Compensatory indirect costs include reductions in thermogenesis, immune function and physical activity. Obligatory consequential costs include hyperthermia, bone loss, disruption of sleep patterns and oxidative stress. This is unlikely to be a complete list. Our knowledge of these physiological costs is currently at best described as rudimentary. For some, we do not even know whether they are compensatory or obligatory. For almost all of them, we have no idea of exact mechanisms or how these costs translate into fitness trade-offs.

The long-lived queen: reproduction and longevity in female eusocial Damaraland mole-rats (Fukomys damarensis)

“…Animals that reproduce early and frequently tend to have relatively short lives, and this may be, in part, due to resource partitioning; resources that would be allocated to self-maintenance are instead diverted towards the many functions involved in the production of offspring (Williams 1966;Noordwijk & de Jong 1986;Stearns 1989;Kirkwood & Rose 1991;Stearns 1992). Reproduction may also take its toll directly on an animal's survivorship by decreasing immune function, increasing oxidative stress, or impairing other physiological functions (Ricklefs & Wikelski 2002;Speakman 2008). Supporting this hypothesis, the relaxation of the physiological demands of reproduction by inhibiting or reducing reproductive effort can result in an increase in lifespan.…”

Limits to sustained energy intake. XIV. Heritability of reproductive performance in mice

“…During early lactation, food intake increases sharply to meet the increasing demands of the pups; however, during late lactation a plateau is reached (Johnson et al, 2001a;Speakman and Król, 2005a;Speakman, 2008). This asymptotic food intake (FI AS =SusEI) varies considerably between individuals, but the primary physiological or morphological features that drive this variability remain uncertain (Hackländer et al, 2002;Hammond et al, 1996;Johnson et al, 2001b;Król et al, 2003;Speakman et al, 2004a).…”

“…The period of late lactation in small rodents has been intensively studied when investigating limits to SusEI as it is one of the most demanding periods in a small mammal's life (Bergeron et al, 2011;Gittleman and Thompson, 1988;McNab, 2002;Speakman, 2008;Thompson and Nicoll, 1986). During early lactation, food intake increases sharply to meet the increasing demands of the pups; however, during late lactation a plateau is reached (Johnson et al, 2001a;Speakman and Król, 2005a;Speakman, 2008).…”

Diet mediates the relationship between longevity and reproduction in mammals

“…The availability of protein in the diet may be a key factor explaining these differences. Carnivores consume food with significantly higher protein content than herbivores and protein is a key macronutrient for mammalian maintenance and reproduction, including both gestation and lactation (Speakman 2008). The higher protein content in the diet of carnivores may better allow them to simultaneously meet their protein requirements for maintenance and reproduction than herbivores.…”