Age‐specific reproduction and disposable soma in an urban population of Common BlackbirdsTurdus merula

Abstract

The mechanism of senescence is an important subject of current research, but our knowledge of the factors influencing the rate of ageing in naturally occurring populations remains rudimentary. Evolutionary theories of senescence predict that investment in reproduction in early life should come at the cost of reduced somatic maintenance and thus result in earlier or more rapid senescence. We use data on the complete reproductive histories of 431 Common Blackbirds (222 males and 209 females) collected during a 19‐year study of the ecology of an urban population of this species to test the main hypotheses addressing the issue of senescence. On average, the birds in this population survived for 3.7 (± 1.9 sd) years. Reproductive success in females peaked at the age of 4, but in males remained stable until the 5th year of life. We observed declines in reproductive success, indicative of senescence, after the peak years in both sexes. The mechanism of age‐related changes in the reproduction of females confirms the individual improvement and selective disappearance hypotheses. In the case of males, the increase in reproductive performance comes as a consequence of the disappearance of poor reproducers. The parental investment associated with early life fecundity (the first two breeding seasons in males and females) impairs the breeding success of females later on. Contrary to expectations, there was no negative impact of high early life fecundity on either mortality or lifespan. Individuals of both sexes with a high early life fecundity had a higher lifetime reproductive success than those in which early life fecundity was low. Hence, the most profitable strategy is to maximize reproductive effort in the early stages of life. This yields the highest lifetime reproductive success, despite the increased impact of senescence, especially in females. These results are consistent with the disposable soma hypothesis.

Survival costs of within‐ and between‐season mate change in the European blackbirdTurdus merula

“…Our study revealed a lower mate fidelity among new adult birds. This is generally in accordance with other studies: partners prefer older, more experienced birds (Choudhury 1995) because the probability of success in such birds is much higher than in young, inexperienced ones (Zhang et al 2015, Jankowiak et al 2017.…”

Egg size variation in the context of polyandry: a case study using long-term field data from snowy plovers

“…However, several of these studies failed to account for selective disappearance (e.g., Cooch et al, 1992;Flint & Sedinger, 1992;Potti, 1993) and thus complicate the interpretation of individual-vs. population-level effects. Similar to our results, recent longitudinal studies document an increase in egg size in early life, followed by peak and then a late-life decline (Bouwhuis et al, 2009;Jankowiak et al, 2018). The early-life increase in egg size may indicate individual improvement through more efficient foraging abilities, better mate choice, or improved predator avoidance (Forslund & Pärt, 1995).…”

“…Life-history theory predicts that fitness trade-offs between current and future reproduction shape inter-specific variation in reproductive strategies observed among iteroparous organisms (Partridge, 1989). To maximize the transfer of their genes to the next generation, individuals adjust their current reproductive investment in light of future reproductive prospects (Jankowiak et al, 2018). The compromise between current and future reproduction enforces tradeoffs between investments in reproduction and other competing functions, such as self-maintenance (Stearns, 1977(Stearns, , 1992Christe et al, 2012).…”

“…In many organisms, reproductive productivity is age-dependent (Bouwhuis et al, 2009;Hammers et al, 2012;Lemaître et al, 2015;Zhang et al, 2015;Salguero-Gómez et al, 2016;Jankowiak et al, 2018;Dingemanse et al, 2020): an individual's performance increases over early life to a maximum and is followed by a decline in older ages. This decline in reproductive performance at old age is considered a sign of senescence -a within-individual age-specific decline in reproductive performance paired with an increase in the probability of death caused by the loss of physiological and cellular function (Medawar, 1952;Williams, 1957).…”

The influence of early‐life allocation to antlers on male performance during adulthood: Evidence from contrasted populations of a large herbivore

“…Therefore, individuals that allocate much to reproduction early in life will accumulate a variety of damages that will ultimately cause a shortened life span or an increased rate of reproductive senescence . The central role played by early-vs. late-life trade-offs in the evolution of ageing constitutes the core of the life-history theory of ageing (Kirkwood, 2017) and has been supported by many empirical studies (Hammers, Richardson, Burke, & Komdeur, 2013;Jankowiak, Zyskowski, & Wysocki, 2018). However, most of these studies have focused on females and there is no consensus yet on whether early-vs. late-life trade-offs are prevalent in males.…”