A molecular analysis of the ostrich 'Struthio camelus massaicus' communal nesting system

Abstract

Seven hypervariable molecular microsatellite markers were isolated and characterised to investigate the colonial nesting of the East African ostrich subspecies Struthio camelus massaicus. The study was based at Nairobi National Park, a 117 km2 park just 7 km south of the city centre. The ostrich breeding system is complex: I estimated that the territorial male defends a territory of 1.15 ± 0.27 (SD) km2 when nesting. Females have larger breeding home ranges that overlap several male territories. From parentage analysis, I estimated that 3-7 females mate with a territorial male and lay up to 66 eggs in a communal nest within the male's territory. One female, the first (major female) to initiate egg laying in a nest, pairs with the territorial male and provides parental care in the form of egg guarding, incubation and escort of the chicks. What makes the communal nesting system of the ostrich unique is that the major female gives free access to other (minor) females to lay in her nest. The major female, who lays a mean of 9.15 ± 2.47 (SD) eggs partitions the clutch into a central clutch comprising an average of 22.9 ± 3.7 (SD) eggs for incubation and peripheral clutch consisting of excess eggs that are not incubated. This study found that the major female may be able to select her eggs for retention in the central clutch. I investigated the possibility that the other central clutch eggs retained were laid by her close relatives. This was not the case. Both the territorial males and the major female had incubated extra pair fertilised eggs. This study found that 71.2% of the incubated eggs were not parented by either the territorial male or the major female. All the major females were found to lay in other nests as minor females, probably before becoming a major female. The conflict arising from communal nesting and biparental care results in the territorial male copulating with his major female and other females venturing into his territory; the female also seeks extra pair matings and selectively favours her eggs for placement into the central clutch where they are incubated by herself and the territorial male. The major female and the territorial male both seek to maximise their individual reproductive success.