CONICET | Buscador de Institutos y Recursos Humanos

The greater rhea (Rhea americana) is a threatened ratite that has a complex mating system, in which several females lay eggs in communal nests and males incubate and care for chicks. We assessed relatedness within and among broods to clarify the genetic mating system. Feathers of 145 chicks and three incubating males in the wild belonging to 5 nests were sampled. Genotyping was determined for eight microsatellite loci developed for rheas, and sibship analysis was conducted using the program COLONY. The analysis revealed multiple progenitors per nest (14.8 females and 11.2 males). On average, a given female laid eggs in 1.9 nests and had 3.7 chicks, and each male had progeny in 1.75 nests and produced 4.5 chicks. Two males were the major male progenitor in the clutch they incubated (28.6% and 34.5% of chicks), while the third genotyped male was not the progenitor of any chick, neither within the brood in his nest nor in the other sampled nests. On average, each male mated with 3.4 females (range: 1-8), while females had 2.8 partners (range 1-5). Most adults (mean= 81%) sharing progeny in the same nest were potentially unrelated, and females were always less related than males. Although in general full-sib chicks occur within nests, both full-sib and half-sib relationships represented only 5.5% of all paired comparisons among all chicks; this means that 94.5% of paired comparisons revealed unrelated chicks. Our study showed that: (i) there were several progenitors within a clutch; (ii) no incubating male was the progenitor of the majority of the chicks produced in a nest; (iii) incubating males could have no progeny; (iv) females did not comprise a cohesive group laying their eggs in the same nests. This strategic promiscuity results in considerable genetic variability rates in this species.