It is acknowledged for long time that there are trade-offs between different life-history traits/components such as growth or reproduction and lifespan (for review, see Speakman [
1]). Trade-offs may occur at functional levels ranging from cellular, tissue and individual (physiological) to ecology (e.g., increased risks of predation when individuals engage in mating or parental activities, or foraging while acquiring energy and nutrients for the reproduction event) and population/evolutionary (genetic) (cited by Plumel et al. [
2]). For instance, current reproduction entails physiological costs that may reduce future reproductive success and/or survival of reproductive adults. But current reproductive effort may also involve trading-off the size against number of offspring (for review, see Speakman [
1]); trading-off the beneficial effects of pleiotropic genes (genes that control for more than one phenotypic trait in an organism) on reproduction against their harmful effects on survival at later ages [
3]; or trading-off, also in a antagonistic pleiotrophic manner, the beneficial effects on reproduction of the hypothalamic-pituitary-gonadal (HPG) axis early in life against its negative effects on cellular function and senescence at later ages when the HPG-axis becomes dysregulated [
4]. However, most human demographic data, particularly those on natural fertility populations, find no relationship or even a positive association between fertility (total number of offspring born) and longevity (for a systematic review, see Hurt et al. [
5]; for reviews, see Le Bourg [
6] and Mitteldorf [
7]). Negative impacts of childbearing on female longevity appear to be mostly associated to lower income groups in which general health is poor or pre-industrial groups that had not adequate access to health care (for references, see Ricklefts and Cadena [
8]). Noteworthy, Ricklefts and Cadena [
8] analyzed 18 species of mammals and 12 species of birds housed in favorable-condition zoo environments. They found that under these favorable zoo conditions, number of offspring produced up to a given age and age at first reproduction did not affect lifespan of females. This study, however, was criticized by other authors who claimed that zoo data are not ideal to investigate life-history trade-offs because of sample size and data quality issues [
9].
Larke and Crews [
10] suggested that the reason by which studies in women fail to find a trade-off between parity/fertility and longevity lies in the fact that the relationship between these traits is irrelevant. Instead, they proposed that it is the amount of “somatic reserve capacity” (reserves accumulated during earlier life) left over after reproduction that determines longevity. According to this hypothesis, greater somatic reserve capacity after reproduction may be obtained through greater cultural and less somatic parental investment (i.e., less physiological costs derived from the production, gestation, post-natal care, feeding and protection of young) into any one offspring. Of note, the concept of “somatic reserve capacity” is ambiguous and cannot be quantified or tested. In contrast, the “physiological costs of reproduction” can be measured and analyzed at least in small mammals such as the mouse (for review, see Speakman [
1]).
According to Speakman [
1], the physiological costs of reproduction can be either direct or indirect. The direct costs in turn can be divided into (i) those derived from satisfying the demands of pregnancy and lactation including energy, macronutrients (like protein), essential amino and fatty acids, vitamins, calcium and other micronutrients; and (ii) the physiological and anatomical modifications necessary to achieve these demands including growth of mammary tissue during late pregnancy, and increase in the sizes of both the liver and the pancreas, absorptive surface of the intestinal mucosa and length of the intestinal tract during lactation. Indirect costs can also be divided into 2 sub-types: (i) those related with optional compensatory adjustments in other components of physiological functioning to save energy to meet the direct physiological costs including reductions in thermoregulation, physical activity and immunocompetence; and (ii) obligatory costs that are an inevitable physiological consequence of the reproductive event, including hyperthermia, disruption of sleep patterns and bone loss. Note that Speakman [
1] included oxidative stress as an obligatory consequential cost of reproduction but the degree to which oxidative stress affects reproductive costs remains under debate [
2,
11‐
13].
The present study aims to ascertain whether there is a trade-off between fertility and longevity in mice, first mated at the same chronological age and kept under controlled housing conditions in the presence of a fertile male/female until the end of reproductive life in females or natural death in males.