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.