Experimental evidence for a causal effect of pair-bond duration on reproductive performance in oystercatchers (Haematopus ostralegus)

Abstract

Many studies have suggested that reproductive performance improves during the pair-bond, which might explain why individuals remate with the same partner in many species. However, discussion exists about whether the association between reproductive performance and pair-bond duration that is reported in these studies reflects a causal relationship. Usually it is unclear whether a positive association is caused by pairs improving during their pair-bond or by high-quality pairs staying together for longer. Furthermore, reproductive performance often also depends on the age or breeding experience of parents, which all covary with pair-bond duration. A much needed experimental approach is lacking so far. We investigated the effect of pair-bond duration on reproductive performance in a long-lived monogamous bird species based on natural as well as experimental variation. The duration of oystercatcher (Haematopus ostralegus) pair-bonds, which were followed for 21 years, strongly affected reproductive output, even after controlling for effects of age and breeding experience. Pairs improved during their pair-bond, and there were no indications of selective disappearance of low-quality pairs; however, pairs that stayed together for very long performed badly. Experimental removal of one partner showed that the reproductive cost of divorce depended on the pair-bond duration with the old partner. In addition, after remating, the newly formed pairs strongly improved again, independent of the age and breeding experience of the remated pair members. As such, this study provides the first experimental evidence of a causal effect of pairbond duration on reproductive performance.

No phenotypic plasticity in nest-site selection in response to extreme flooding events

“…As the frequency of flooding events increases we would expect directional selection to favour higher nest elevation, working via the fitness component of nest success. Encouragingly, our study population has shown an increase in mean nest elevation over time at half the rate of change seen in maximum high tide [35], providing evidence that changing nest elevation as a response to flooding may be possible.…”

“…(d) Alternatives to phenotypic plasticity As H. ostralegus is a long-lived species, we initially hypothesized that phenotypic plasticity in nest-site selection would provide the most likely explanation for the increasing mean nest elevation observed within our population [35]; however, our current analyses provide no evidence to support this prediction, compelling us to consider alternative mechanisms. Micro-evolution in nest elevation is possible, although the long generation time of our study species makes this explanation unlikely.…”

“…Although flooding events are known to cause high reproductive failure in H. ostralegus [3] alternative selective pressures, such as those discussed above, may be inhibiting any plastic nest elevation response. This may be further exacerbated by the infrequent nature of ECEs, with higher elevation nest sites potentially favoured in flooding years but selected against in non-flooding years [10,30,35]. Although there is currently no clear selection for higher nests, as flooding events become more frequent the selective landscape may begin to change.…”

Context matters: female aggression and testosterone in a year-round territorial neotropical songbird ( Thryothorus leucotis )

“…Female aggression towards male intruders may function to defend their mates against eviction by prospecting males, which would be advantageous if staying with current partners confers some benefit, such as increased reproductive success with a familiar mate (e.g. van de Pol et al 2006). Similar to non-territorial females, encounters with non-territorial males should be more prevalent during pre-breeding, leading to the prediction that female aggression should be more intense then than during breeding.…”

“…Indeed in male vertebrates, territorial aggression is differentially hormonally regulated depending on the seasonal context (breeding versus nonbreeding seasons, e.g. Caldwell et al 1984;Soma et al 2000;Canoine & Gwinner 2002;Wingfield & Soma 2002;Buck & Barnes 2003;Hau et al 2004). Evidence is scanty, but different endocrine mechanisms may regulate breeding and non-breeding territorial aggression in females as well (Schwabl 1992;Hau et al 2004;Scotti et al 2007).…”