Costs of reproduction and ageing in the human female

Evolutionary theories of ageing point to reproduction as a significant factor to consider when asking why ageing occurs and why there is inter-individual variation in its progression. Reproduction in human females is costly, in terms of energy, nutrients and metabolic adjustments. Thus, it is expected that women who experienced high reproductive effort resulting from multiple reproductive events will age faster. However, the evidence for long-term negative effects of reproduction is not conclusive. The lack of understanding of whether there are trade-offs between reproduction and ageing in women is partly due to methodological challenges. The costs of reproduction are often calculated based only on parity, while other elements contributing to these costs (e.g. breastfeeding, timing of reproduction) are neglected, which may significantly underestimate the total costs and obscure the all-important inter-individual variation in such costs. Costs must be evaluated in relation to individual characteristics, including developmental conditions, nutritional status and social support that a mother receives during reproduction. Furthermore, ageing and health must be assessed based on comprehensive markers rather than arbitrarily assembled variables. Finally, longitudinal rather than cross-sectional studies and new statistical approaches are needed to reveal how much of a decline in health and progressing ageing can actually be attributed to past reproductive processes. This article is part of the theme issue ‘Evolution of the primate ageing process'.

Selection bias in studies of human reproduction-longevity trade-offs

A shorter lifespan as a potential cost of high reproductive effort in humans has intrigued researchers for more than a century. However, the results have been inconclusive so far and despite strong theoretical expectations we do not currently have compelling evidence for the longevity costs of reproduction. Using Monte Carlo simulation, it is shown here that a common practice in human reproduction-longevity studies using historical data (the most relevant data sources for this question), the omission of women who died prior to menopausal age from the analysis, results in severe underestimation of the potential underlying trade-off between reproduction and lifespan. In other words, assuming that such a trade-off is expressed also during reproductive years, the strength of the trade-off between reproduction and lifespan is progressively weakened when women dying during reproductive ages are sequentially and non-randomly excluded from the analysis. In cases of small sample sizes (e.g. few hundreds of observations), this selection bias by reducing statistical power may even partly explain the null results commonly found in this field. Future studies in this field should thus apply statistical approaches that account for or avoid selection bias in order to recover reliable effect size estimates between reproduction and longevity.

Effects of post-breeding moult and energetic condition on timing of songbird migration into the tropics

Each autumn billions of songbirds migrate between the temperate zone and tropics, but little is known about how events on the breeding grounds affect migration to the tropics. Here, we use light level geolocators to track the autumn migration of wood thrushes Hylocichla mustelina and test for the first time if late moult and poor physiological condition prior to migration delays arrival on the winter territory. Late nesting thrushes postponed feather moult, and birds with less advanced moult in August were significantly farther north on 10 October while en route to the tropics. Individuals in relatively poor energetic condition in August (high β-Hydroxybutyrate, low triglyceride, narrow feather growth bars) passed into the tropics significantly later in October. However, late moult and poor pre-migratory condition did not result in late arrival on the winter territory because stopover duration was highly variable late in migration. Although carry-over effects from the winter territory to spring migration may be strong in migratory songbirds, our study suggests that high reproductive effort late in the season does not impose time constraints that delay winter territory acquisition.

Kingiella chilenica (Bivalvia: Cyamiidae); population dynamics, rates of survival, embryo production and annual recruitment of a semelparous brooding clam

Kingiella chilenica, a brooder mollusc, inhabiting soft bottoms of estuarine tidal flats of southern Chile, is well suited to study larval recruitment. It is a tiny semelparous clam with an annual cycle, and whose recruits must survive an inhospitable winter season prior to growing and reaching reproductive maturity in the following summer season. The population dynamics of this clam was studied through periodic sampling over two successive years to follow fluctuations in its' abundance. Obtained data on embryo production and recruit survival over both periods shows that population abundance varied widely between the two years of study. A high level on recruit survival through the winter of the first year resulted in high numbers of adults in the corresponding summer reproductive season. An inverse situation occurred in the second year, with lower recuit survival and, consequently, fewer reproducing adults. However, the lower abundance of adults in the second year was compensated by their having a higher survival rate over the reproductive season as compared with adults of the previous year. Independently of the abundance of the adults, the number of embryos incubated per female was very similar between both periods. Consequently, we suggest that the net contribution of juveniles produced by females per unit substrate was similar between the annual cycles studied. As predicted for a semelparous species (high reproductive effort), the maximization of the reproductive contribution was clearly reflected by a drop in adult somatic weight, particularly in incubating females. Previous data on abundance for this population suggested inter-annual differences of a high magnitude, with periods (e.g. 1986–1988) with abundances reaching 2–6× that presently reported. Probably, stable environmental conditions (e.g. 1986–1988) promote high abundance and survival of reproducers allowing a large contribution of recruits, which in turn show high degrees of survival through the adult phase. The years reported here (2003–2005), however, represent a period of population decline to lower, perhaps minimum, levels.

Seasonal Decline in Leukocyte Concentrations and Reproductive Output in Female Great Tits (Parus Major)

We studied the relationship between breeding parameters and leukocyte counts (number of cells in 1 mm3 of blood) in Great Tit (Parus major) females. To exclude potential effects on leukocyte concentration of high reproductive effort related to nestling provisioning, we collected blood samples from females at the end of their incubation period. Early-breeding females had greater blood counts of total leukocytes, lymphocytes, and heterophils than birds starting their clutches late in the season. Reproductive success, measured as the number of successfully fledged young, declined over the breeding season and showed no association with total leukocyte concentration. Number of leukocytes was not related to female condition measured as residuals from the regression of body weight on tarsus length. Moreover, the concentration of leukocytes did not predict the probability that a female would survive to the following breeding season. Disminución Estacional en las Concentraciones de Leucocitos y en el Rendimiento Reproductivo en Parus major

Life history strategies, population regulation, and implications for fisheries management

Life history theories attempt to explain the evolution of organism traits as adaptations to environmental variation. A model involving three primary life history strategies (endpoints on a triangular surface) describes general patterns of variation more comprehensively than schemes that examine single traits or merely contrast fast versus slow life histories. It provides a general means to predict a priori the types of populations with high or low demographic resilience, production potential, and conformity to density-dependent regulation. Periodic (long-lived, high fecundity, high recruitment variation) and opportunistic (small, short-lived, high reproductive effort, high demographic resilience) strategies should conform poorly to models that assume density-dependent recruitment. Periodic-type species reveal greatest recruitment variation and compensatory reserve, but with poor conformity to stock–recruitment models. Equilibrium-type populations (low fecundity, large egg size, parental care) should conform better to assumptions of density-dependent recruitment, but have lower demographic resilience. The model's predictions are explored relative to sustainable harvest, endangered species conservation, supplemental stocking, and transferability of ecological indices. When detailed information is lacking, species ordination according to the triangular model provides qualitative guidance for management and development of more detailed predictive models.