Multitasking and the evolution of optimal clutch size in fluctuating environments

Abstract

Adaptive studies of avian clutch size variation across environmental gradients have resulted in what has become known as the fecundity gradient paradox, the observation that clutch size typically decreases with increasing breeding season length along latitudinal gradients, but increases with increasing breeding season length along elevational gradients. These puzzling findings challenge the common belief that organisms should reduce their clutch size in favor of additional nesting attempts as the length of the breeding season increases, an approach typically described as a bet‐hedging strategy. Here, we propose an alternative hypothesis—the multitasking hypothesis—and show that laying smaller clutches represents a multitasking strategy of switching between breeding and recovery from breeding. Both our individual‐based and analytical models demonstrate that a small clutch size strategy is favored during shorter breeding seasons because less time and energy are wasted under the severe time constraints associated with breeding multiply within a season. Our model also shows that a within‐generation bet‐hedging strategy is not favored by natural selection, even under a high risk of predation and in long breeding seasons. Thus, saving time—wasting less time as a result of an inability to complete a breeding cycle at the end of breeding season—is likely to be the primary benefit favoring the evolution of small avian clutch sizes during short breeding seasons. We also synthesize the seasonality hypothesis (pronounced seasonality leads to larger clutch size) and clutch size‐dependent predation hypothesis (larger clutch size causes higher predation risks) within our multitasking hypothesis to develop an integrative model to help resolve the paradox of contrasting patterns of clutch size along elevational and latitudinal gradients. Ultimately, our models provide a new perspective for understanding life‐history evolution under fluctuating environments.

Ashmole's hypothesis and the latitudinal gradient in clutch size

“…Finally, Liu et al . (2018) also supported the positive association between seasonality and clutch size while suggesting that improved energetic efficiency, and not bet‐hedging, may be another benefit of small clutches and might explain why clutch size typically decreases (instead of increases) with increasing seasonality along elevation gradients (Badyaev & Ghalambor, 2001). Therefore, modelling and simulation studies have consistently supported the role of seasonality in generating the latitudinal gradient in clutch size while showing how the incorporation of additional mechanisms, such as allocation trade‐offs, may provide a more‐complete understanding of the multiple selective pressures shaping clutch size variation.…”

“…The same study suggested that, while latitudinal gradients in seasonality may be sufficient to generate a latitudinal gradient in clutch size, the clutch size gradient may be further enhanced by latitudinal variation in nest predation risk and breeding season length, which favour a bet-hedging strategy at low latitudes more than at high latitudes (Griebeler et al, 2010). Finally, Liu et al (2018) also supported the positive association between seasonality and clutch size while suggesting that improved energetic efficiency, and not bet-hedging, may be another benefit of small clutches and might explain why clutch size typically decreases (instead of increases) with increasing seasonality along elevation gradients (Badyaev & Ghalambor, 2001). Therefore, modelling and simulation studies have consistently supported the role of seasonality in generating the latitudinal gradient in clutch size while showing how the incorporation of additional mechanisms, such as allocation trade-offs, may provide a more-complete understanding of the multiple selective pressures shaping clutch size variation.…”

“…Others confuse the hypothesis for the pattern it was formulated to explain; for example stating that Ashmole's hypothesis is that, “…clutch size for a bird species is larger in northern than in southern latitudes,” (Guthery, 2008, p. 18). Others invert the pattern and the hypothesis to explain it: “According to the “Seasonality Hypothesis,” Ashmole (1963 a , b ) predicted that clutch sizes will be larger at higher latitudes” (Liu et al ., 2018, p. 8804). Multiple studies associate or blend Ashmole's hypothesis with Lack's day‐length hypothesis (Kuitunen & Suhonen, 1991; Li & Lu, 2012; Zima et al ., 2017).…”

Reuse of nests in the Great Reed Warbler Acrocephalus arundinaceus: A behavior to save time and energy and to deter nest parasites?

“…There is also a possibility that the female was absent from the nest for longer time periods during the incubation for some reason (e.g., presence of predators). Furthermore, incubation of eight eggs in the nest is probably less effective in a passerine which lays 3–5 eggs on average (e.g., Bensch, 1996; Mérő et al, 2014, 2015), as the incubation of larger clutches requires more energy for maintaining at incubation temperature (Liu et al, 2018; Reid et al, 2002). Thus, we suggest that it is possible that two of the new eggs did not receive constant heat during incubation, and therefore, may have failed to produce nestlings (Potti & Merino, 1996).…”