Social Organization of Small Heterosexual Groups of Green Swordtails (Xiphophorus Helleri, Pisces, Poeciliidae) Under Conditions of Captivity

Abstract

AbstractSixteen populations, each of four male and four female green swordtail fish, were observed in 54 litre tanks separated into two unequal volumes by a partition allowing swimming from one area to another only at the surface. Each population was observed on 2-4 occasions, making a combined total of 50 observation periods of 2 hours each. Chase-right orders, spatial positions as well as several agonistic and epigamic behaviour units were noted during each period of observation. Males and females apparently form distinct hierarchies which are perfectly linear and transitive in 68% and 42% of the cases respectively. Some dominance structures are incomplete, but no intransitivity is noted. Alpha males are much more aggressive than their isosexual subordinates. In females, the frequency of aggressive behaviour appears to be more proportional to the social rank of the initiator, but the number of chases received from other females is the best indicator of rank in the female hierarchies. Alpha males are responsible for 80 % of all sexual activity and have privilege to behaviour leading to insemination with a high probability in 85 % of the cases. Females are not courted according to their position in the female hierarchy, nor to their size. In the two-compartment situation of the present study, which allowed spacing-out and visual isolation of the individuals, the alpha males occupied the larger area of the aquarium in the company of the four females while the three subordinate males were restricted to the smaller area, unaccompanied by any females. This spacing-out pattern emerged with neat regularity from the data and was apparently caused and maintained by aggressive behaviour, especially by charges, initiated by the alpha male toward male rivals. However, it is not evident that the behaviour of the alpha male should be neatly qualified as territorial defence, since specific-area-linked dominance was never realized in the present study. The social scheme most readily applicable to the present social and spatial organization is the "one male to several females" system or monarchistic male hierarchy, in which one male becomes very dominant over the others, occupies the larger area and is also the sole individual to court the females and to attempt insemination (haremic). These results support the sociobiological theorem that to dominate is to have priority of access to the necessities of life and reproduction.