The optimal clutch size revisited: separating individual quality from the parental survival costs of reproduction

Abstract

Life-history theory, central to our understanding of diversity in morphology, behaviour and senescence, describes how traits evolve through the optimisation of trade-offs in investment. Despite considerable study, there is only minimal support for trade-offs within species between the two traits most closely linked to fitness - reproduction and survival - questioning the theory's general validity. We used a meta-analysis to separate quality effects from the costs of reproduction using studies of parental investment and survival in birds. Experimental enlargement of broods caused reduced parental survival. However, the effect of experimental enlargements was small and opposite to the effect of phenotypic quality, where individuals that naturally produced larger clutches also survived better. The opposite effects on survival in experimental and observational studies of parental care provides the first meta-analytic evidence for theory suggesting that quality differences mask trade-offs. Fitness projections using the overall effect size revealed that reproduction presented negligible costs, except when reproductive effort was forced beyond the level observed within species, to that seen between species. We conclude that there is little support for the most fundamental life-history trade-off, between reproduction and longevity, operating within a population. We suggest that within species, the fitness landscape of the reproduction-survival trade-off is flat until it reaches the boundaries of the between-species fast-slow life-history continuum. Our interpretation explains why the costs of reproduction are not apparent and why variation in reproductive output persists within species.

Stochasticity explains non-genetic inheritance of lifespan and apparent trade-offs between reproduction and ageing

“…reproduction. Trade-offs between reproduction and longevity are central to life history but when tested experimentally lack empirical support, especially in naturalistic settings 7,40 . As an explanation for this, differences in phenotypic quality, allowing some individuals to show both high reproductive output as well as longevity, has been suggested to mask trade-offs and hence limit selection 40,41 .…”

“…In a population, not all individuals appear to age at the same rate 5 in terms of lifespan and reproduction 6 . Moreover, it is not always evident that these two traits are negatively correlated, as some individuals appear to both show high reproduction and longevity in a population, often termed quality effects [7][8][9] . Understanding reproduction and lifespan is critical to understand ageing at a fundamental level as they are most closely related to Darwinian fitness.…”

“…Mothers that lived the longest produced long-lived daughters. Differences in Darwinian fitness traits are often explained as differences in quality 7,19 . The term quality encompasses differences in local adaptation or long-lasting effects of early environmental effects.…”

Environmental constraints can explain clutch size differences between urban and forest blue tits: insights from an egg removal experiment

“…Low-quality or young individuals that naturally produce smaller clutches may be pushed into the city if free territories in the forest are no longer available (10,106,161). Indeed, laying following egg removal may be more common in the forest if females are more experienced at breeding (60,162,163). Therefore, in the future, we need to determine if the parental quality or the age structure of the city and forest populations differ and if that further explains differences in clutch size between urban and forest habitats.…”

The importance of reaction norms in dietary restriction and ageing research

“…S(sigmoid)-shaped curve, with slow increases in response at low doses, before eventual saturation at high doses (Finney, 2009). The ideas of trade-offs between costs and benefits (Stearns, 1989; Winder et al, 2022) are common in biology and in ageing research (Cohen et al, 2020; Kirkwood & Austad, 2000), with the optimum investment in a certain trait or process traded off against its cost. In the specific case of diet, the phenotypic benefits of a given level of consumption will be determined by the underlying physiological costs and benefits.…”