Adaptive value of same-sex pairing in Laysan albatross

Abstract

Same-sex pairing is widespread among animals but is difficult to explain in an evolutionary context because it does not result in reproduction, and thus same-sex behaviour often is viewed as maladaptive. Here, we compare survival, fecundity and transition probabilities of female Laysan albatross in different pair types, and we show how female-female pairing could be an adaptive alternative mating strategy, albeit one that resulted in lower fitness than male-female pairing. Females in same-sex pairs produced 80% fewer chicks, had lower survival and skipped breeding more often than those in male-female pairs. Females in same-sex pairs that raised a chick sometimes acquired a male mate in the following year, but females in failed same-sex pairs never did, suggesting that males exert sexual selection by assessing female quality and relegating low-quality females into same-sex pairs. Sexual selection by males in a monomorphic, non-ornamented species is rare and suggests that reconsideration is needed of the circumstances in which alternative reproductive behaviour evolves. Given the lack of males and obligate biparental care in this species, this research demonstrates how same-sex pairing was better than not breeding and highlights how it could be an adaptive strategy under certain demographic conditions.

Keywords: Laysan albatross; same-sex pairing; sex-ratio; sexual selection; survival.

Figure 1.

Annual survival rates of female Laysan albatrosses in different pair types based on the previous years’ reproductive outcome. succ. indicates a successful reproductive attempt (chick fledged), failed is a dead egg or dead chick and skipped are birds that skipped breeding that year. Error bars are s.e.

Figure 2.

Probability of female Laysan albatrosses transitioning to a different pair type based on the previous years’ reproductive outcome. succ. indicates a successful reproductive attempt (chick fledged), and failed is a dead egg or dead chick. Error bars are s.e.

Figure 3.

Probability of female Laysan albatrosses in a given pair type skipping breeding in the following year based on the previous years’ reproductive outcome. succ. indicates a successful reproductive attempt (chick fledged), and failed is a dead egg or dead chick. Error bars are s.e.

Figure 4.

Life stage transition probability and annual survival probability for female Laysan albatrosses in different pair types based on the previous years’ reproductive outcome. Arrows indicate the direction of the transition and numbers are the rate of change. Successful indicates a successful reproductive attempt (chick fledged), failed is a dead egg or dead chick and skipped are birds that skipped breeding that year.