Social Organization of Small Heterosexual Groups of Green Swordtails

The classic works of Noble (1939), Noble & Borne (1940) and Braddock (1945, 1949) have shown that small isosexual and heterosexual groups of Xiphophorus fish form hierarchies based upon dominance submission relations which are established by challenging and fighting. The resulting dominance orders show a tendency toward the straight line type, although incomplete hierarchies and intransitive triangles are common, at least in platies, Xiphophorus maculatus (Braddock, 1945). Once the hierarchy is established, agonistic social behaviour of the fish is less reciprocal than during the formative period. Dominant individuals of either sex butt their subordinates differentially, as individuals; there is no systematic relationship between the difference in rank between individuals and the frequency of chases initiated by the dominant individual toward the subordinate one. The total amount of aggression shown by a fish, however, seems to correspond to his, own rank (Braddock, 1945). With two or three individuals of the same sex, dominance relations tend to be very stable, remaining unchanged for several weeks (Noble, 1939; Braddock, 1945) or even for much longer periods, up to six months in certain cases (Noble & Borne, 1938, 1940; Zayan, 1974). With four to eight individuals, dominance relationships tend to be more frequently reversed, apparently due to revolts of subordinates. Some evidence also indicates that the fish are able to recognize each other's in-dividual characteristics (Zayan, 1974, 1975b). This "individual recogni-tion" would be responsible for the stability of dominance relations within small groups. Poeciliid fish are not considered territorial. These ovoviviparous species have never been reported to establish and to defend individual territories (Braddock, 1942, 1945; Morris, 1955; Zayan, 1974, 1975a, b, c, 1976; Noble & Borne, 1940; Franck, 1964, 1973) as do cichlids, for example. One exception must be made, however, for Heuts (1968) has reported a presumed case of territoriality in two female swordtails. Observations made in the natural environment, although not clearly establishing the existence of territoriality, suggest that males can defend against intruding males, at least temporarily a feeding area or an area surrounding females. Zander (1967) has reported the case of an adult male Xiphophorus helleri patrolling an area of two square metres and chas-ing all intruding males. The same author has also noted that individuals distribute themselves within a pool or river according to size, larger in-dividuals occupying deeper waters and smaller fish living in more shallow zones, near the banks. The existence of a similar gradation in tropical streams is well documented, and has been noted in other poeciliids, e.g., in Poeciliopsis SOCIAL ORGANIZATION OF SMALL HETEROSEXUAL GROUPS OF GREEN SWORDTAILS