Sperm storage reflects within- and extra-pair mating opportunities in a cooperatively breeding bird

Abstract

In passerine birds, storage and maturation of sperm takes place in the cloacal protuberance (CP), an external swelling of the reproductive organ. The considerable variation in CP size among species is presumed to be a consequence of varying levels of sperm-competition, but whether individual variation in CP size within a species also reflects sperm competition is not well established. Here, we study temporal variation in male CP size in relation to within-pair and extra-pair mating opportunities and cuckoldry risk in purple-crowned fairy-wrens Malurus coronatus. This is a socially monogamous cooperatively breeding passerine that can breed year-round and has low levels of extra-pair paternity (in 6 % of broods). We show that male CP size sharply increased a few weeks before, and rapidly regressed after his partner laid eggs, consistent with a cost of its maintenance and/or sperm production. Surprisingly, despite low levels of extra-pair paternity, CP size of nonbreeding and pre-breeding males was positively correlated with the number of breeding females in the population, suggesting that CP size is sensitive to extra-pair mating opportunities. However, CP sizes do not seem to reflect cuckoldry risk: CP size of dominant males was unaffected by the presence of a subordinate that was unrelated to the dominant female, although those subordinates occasionally sire offspring, and had a larger CP than subordinates living with their mother. Our results suggest that, even in a species with low levels of extra-pair paternity, individual investment in sperm storage reflects both within-pair and, albeit to lesser extent, extra-pair mating opportunities.

Extra‐pair paternity in relation to breeding synchrony in ground tits: an individual‐based approach

“…Then, how is the synchronization in breeding date achieved among individuals involving in an EP mating event? According to the temporal change of male cloacal protuberance (the site of sperm maturation and storage) in the purple‐crowned fairy‐wren Malurus coronatus , Kingma et al () showed that male fertility is determined primarily by the breeding cycle of his social partner, typically around the fertile period of the female. Based on this finding, Kingma et al () thought that incestuously mated female in this species might adjust egg‐laying time to match the peak fertility of their preferred (genetically dissimilar) EP males to avoid fitness loss from inbreeding depression.…”

Rapid plastic breeding response to rain matches peak prey abundance in a tropical savanna bird

“…Body mass (to nearest 0.1 g) and tarsus (to nearest 0.1 mm) were measured. Cloacal protuberance (CP, site of sperm storage) was measured in males ( n = 132 captures of 59 individuals), and volume was calculated following Kingma, Hall, and Peters ().…”

“…The influence of climate variation was analysed on (a) timing of initiation of breeding attempts (date of laying of the first egg of a clutch), (b) body mass (corrected for structural size), (c) male sperm storage (cloacal protuberance size, a proxy for readiness to breed; Kingma et al, ) and (d) food availability (monthly arthropod abundance). For all four biological responses, we set out to determine (a) the phenological critical time window: the time period of a climate variable (rainfall or temperature) that explains most of the variation of these biological responses, and (b) the nature of this relationship (e.g.…”

“…Breeding in this population can occur in every month of the year, with individual females producing 1-6 clutches per year (x ± SE = 2.3 ± 1.2). There are two peaks of breeding: during the wet season (December-March) and, in some but not all years, a smaller peak (September-October) in the dry season ( Figure S7 captures of 59 individuals), and volume was calculated following Kingma, Hall, and Peters (2012).…”

More than kin: subordinates foster strong bonds with relatives and potential mates in a social bird

“…Territories are aligned linearly along the rivers, and boundaries and groups are stable year-round and across years (Hall and Peters 2008;Kingma et al 2011). Breeding can take place year-round but peaks in the monsoonal wet season (December to March) (Hall and Peters 2009;Kingma et al 2012). A strength of our study system is that only the dominant breeding pair engages in duets Peters 2008, 2009), providing a reliable cue to assign breeder (dominant) or subordinate status to each individual independent of submissive or aggressive interactions.…”