Experience and trust: The benefits of mate familiarity are realized through sex-specific specialization of parental roles in cassin’s auklet

Maintaining a pair bond year after year (perennial monogamy) often enhances reproductive success, but what familiar pairs are doing differently to improve success is unknown. We tested the hypothesis that endocrine changes mediate improvements in parental attendance in known-age Cassin’s auklets Ptychoramphus aleuticus, which we found limited evidence for. Instead, we found sex-specific parental roles in familiar pairs. Males modulated their nest attendance depending on the attendance of their mate, but the direction depended on mate familiarity.  We suggest his flexibility may be mediated by prolactin. In a historical dataset, females with a familiar mate laid larger eggs that hatched into more robust chicks, but larger eggs correlated with lower female body condition. In study birds, attendance by males and females in good condition predicted chick weight, but attendance by females in poor condition did not, suggesting female-specific energetic constraint. Our findings suggest that ma..., Fieldwork and sample collection: We conducted our study on Southeast Farallon Island (37.6989° N, 123.0034° W; Farallon Islands National Wildlife Refuge, California, USA) in 2019 and 2021. These two years represented a positive El Niño Southern Oscillation (ENSO) event where on average only 8% of followed pairs fledged a chick (2019), and a negative ENSO event (2021) where 65% of pairs fledged a chick [1]. Many individuals of the Southeast Farallon Island Cassin’s auklet population are of known age and have known reproductive histories [2], allowing us to disentangle the confounding effects of age and mate familiarity. Nest boxes were checked every 15 days until an incubating bird was found. If the individual was of known age, it was measured and evaluated for inclusion in the study. All morphometrics are therefore from birds in their first two weeks of incubation. We weighed known-age incubating adults between 13:00 and 15:00 with a Pesola spring scale (+/- 1g), measured wing cord with..., , # Experience and trust: The benefits of mate familiarity are realized through sex-specific specialization of parental roles in cassin’s auklet

[https://doi.org/10.5061/dryad.mcvdnck82](https://doi.org/10.5061/dryad.mcvdnck82)

The data presented here are endocrine, behavior, and fitness metrics from Cassin's auklets collected in 2019 and 2021 on Southeast Farallon Island, California. Endocrine measurements are levels of fecal corticosterone and plasma prolactin. Behavioral measurements were the proportion of nights an individual was present at the nest during both incubation and chick rearing. Fitness metrics include hatching and fledging success, and the maximum weight the chick attained. There is also some basic information about each bird in the dataset, such as its age if known, sex, and body condition. It also includes information on egg volumes from individuals where egg morphometrics were taken between 1992 and 2017, and information about the females that laid those eggs.

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