4.2 Reproductive skipping: adaptive or nonadaptive
Our results also indicate that African penguins are not breeding as often as theoretically possible. This finding is common among long-lived seabirds (e.g., Jenouvrier et al . 2005) and implies African penguins are either making adaptive decisions to avoid reproductive costs some years, and/or that individual-specific constraints are limiting the ability to breed each year in some individuals. Under adaptive explanations, reproductive skipping should be beneficial, increasing survival and/or future breeding probabilities (Williams 1966). On the contrary, our results show that individuals skipping reproduction had a lower probability of breeding the next year and no survival gain compared to breeding individuals. This suggests reproductive skipping in African penguins is driven by non-adaptive individual-specific constraints e.g. higher quality individuals are more likely to breed and remain breeders (Lescroël et al. 2009; Jenouvrier et al. 2015).
Supporting this, inter-individual differences in physiology and behaviour of African penguins have previously been noted. For example, some individuals travel further and dive more often (Campbell et al. 2019; Traisnel & Pichegru 2019), which may indicate inter-individual differences in foraging efficiency. This is a key driver of inter-individual variation in breeding propensity in Adélie penguins (Pygoscelis adeliae ) (Lescroël et al. 2010), and may drive similar trends in African penguins, with more efficient foragers better able to meet the energetic requirements of reproduction.
Variation in other individual-level traits that can influence breeding success, like aggression (Traisnel & Pichegru 2018) or age (Kappeset al . 2021), may also interact with individual quality to affect reproductive skipping. Global declines of African penguins may be driving reduced availability of breeding partners, leaving some (e.g., more experienced or high quality) individuals better placed to retain mates or nest sites, or to find a new breeding partner after divorce or mate mortality (Bruinzeel 2007). However, further individual-level monitoring would be required to determine the drivers of variation in reproductive skipping in African penguins.