Two behavioural traits promote fine-scale species segregation and moderate hybridisation in a recovering sympatric fur seal population

Abstract

Background: In systems where two or more species experience secondary contact, behavioural factors that regulate interspecific gene flow may be important for maintaining species boundaries and reducing the incidence of hybridisation. At subantarctic Macquarie Island, two species of fur seal breed in close proximity to one another, hybridise at very high levels (up to 21% of hybrid pups are born annually), yet retain discrete gene pools. Using spatial and genetic information collected for pups and adults over twelve years, we assessed two behavioural traits - inter-annual site fidelity and differences in habitat use between the species - as possible contributors to the maintenance of this species segregation. Further, we explored the breakdown of these traits in pure-species individuals and hybrids.

Results: We found virtually complete spatial segregation of the parental species, with only one exception; a single territory that contained adults of both species and also the highest concentration of hybrid pups. The spatial distribution of each species was closely linked to habitat type (pebbled vs boulder beaches), with members of each species breeding almost exclusively on one type or the other but hybrids breeding on both or at the junction between habitats. Inter-annual site fidelity was high for both sexes of pure-species adults, with 66% of females and all males returning to the same territory or a neighbouring one in different years. An important consequence for pure females of breeding on the 'wrong' habitat type, and thus in a heterospecific aggregation, was the production of hybrid pups. Low habitat fidelity of hybrid females facilitated bi-directional backcrossing, resulting in more diverse hybrid offspring.

Conclusion: In a disturbed system where two sympatric fur seal species breed in close proximity, discrete gene pools are retained by extremely fine-scale and strong spatial segregation of the species. Two behavioural traits were found to be important in maintaining this stable population structure, and habitat type was a strong indicator of where species locate and a potentially powerful predictor of future directions of hybridisation. A direct consequence of the breakdown of this trait was the production of hybrid offspring, which may have severe implications if hybrids have reduced fitness.

Figure 1

Fur seal breeding beaches on North Head Peninsula, Macquarie Island, illustrating typical A. gazella habitat (open pebbled beaches, open boxes), A. tropicalis habitat (bouldered coves, grey boxes) and the main territory locations from 1992-2003 (blue circles, A. gazella, red circles, A. tropicalis). X indicates unsuitable habitat for fur seals and dashed lines mark the boundaries of each bay.

Figure 2

Spatial distribution of a) pups and b) adults (squares represent males), showing strong separation of A. gazella (blue for females and pups, dark green for males, Q_Gaz ≥ 0.9) and A. tropicalis (red for females and pups, light green for males, Q_Gaz ≤ 0.1) across the four main breeding aggregations, with habitat type (P = pebbles, B = boulders) in parentheses. The transition zone between habitat types is shaded and illustrates the relatively high density of hybrid pups and females (purple for females and pups, yellow for males). The hybrid threshold is bounded by a horizontal, dashed line and includes all pups with Q_Gaz between 0.1 and 0.9).

Figure 3

Site fidelity of breeding adults, showing frequency histograms of the a) number of years males (white) and females (black) were observed breeding, b) distance (metres) between pupping sites among years for pure-species females, and c) the mean (±2 S.E.) proportion of years pure and hybrid adults were observed returning to the same territory to breed.