Male-biased reproductive effort in a long-lived seabird

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Background: In dimorphic seabirds, the larger sex tends to provision more than the smallersex. In contrast, monogamy and biparental care are often associated with equal effort betweenthe sexes. However, the few studies that have tested sex-specific effort in monomorphic seabirdshave primarily examined the details of foraging at sea. Hypotheses: Parental effort is also sex-biased in a monomorphic seabird mating system forone of two reasons: (1) If females enter the period of parental care less able to invest in care dueto the cost of egg production, male-biased effort may be necessary to avoid reproductive failure.(2) Alternatively, female-biased effort may occur due to the initial disparity in gamete size,particularly in species with internal fertilization. Organism: Leach’s storm-petrel ( Oceanodroma leucorhoa ), a monomorphic seabird with truemonogamy and obligate biparental care. Site: A breeding colony of Oceanodroma leucorhoa at the Bowdoin Scientific Station on KentIsland, Bay of Fundy, New Brunswick, Canada. Methods: Across multiple breeding seasons, we assessed incubation behaviour and chick-rearing behaviour through one manipulative and multiple observational studies. We assessedenergetic investment by inducing feather replacement and measuring the resulting rate of feather growth during both the incubation and chick-rearing phases of parental care. Conclusions: We observed male-biased effort. Males incubated the egg for a greater pro-portion of time than did females and, when faced with an egg that would not hatch, malescontinued to incubate past the point when females abandoned it. Males made a higher per-centage of total food deliveries to chicks than did females, resulting in greater mean dailyfood provisioning by males than by females. During chick rearing, males grew replacementfeathers more slowly than did females, indicating that males were more likely to reduce theirown nutritional condition while raising chicks than were females. These results support thehypothesis that females enter the period of parental care at a nutritional deficit and males mustcompensate to avoid reproductive failure.


Evolutionary Ecology Research



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