Lack of parental mate-switching effects on offspring fitness components in a long-lived socially monogamous species

In many species, individuals form socially monogamous pair-bonds lasting multiple breeding seasons, or even whole lifetimes. Studies often suggest social monogamy to be adaptive, but this is usually quantified through the survival and annual reproductive success of the partners. However, beyond the number of offspring produced, parental partnerships may also affect their offspring’s phenotype, health, and ultimately fitness. Using multigenerational data on the Seychelles warbler (Acrocephalus sechellensis), we investigated the impact of parental pair-bond tenure (pair-bond duration) and pair-bond fidelity (pair-bond maintained across breeding seasons) on offspring fitness components. First, we addressed juvenile-stage fitness using indicators reflecting physiological state (haematocrit, telomere length, and body mass). Second, we assessed long-term fitness components using offspring lifespan and lifetime reproductive success (LRS). We found male (but not female) offspring whose parents both died had 73% increased mortality risk compared to male offspring whose parents remained together. We found no evidence of pair-bond tenure- or fidelity-effects on other short-term measures (telomere length, haematocrit, body mass) or LRS. To our knowledge, this is the first study quantifying long-term parental effects of pair-bond tenure and fidelity on offspring fitness components in wild populations. This work provides insights into intergenerational implications of long-term socially monogamous partnerships.