THE PROXIMATE MECHANISMS OF NATAL DISPERSAL IN FEMALE HORSES

The aim of this study was to investigate proximate mechanisms of natal dispersal by female horses, Equus caballus (i.e. proximate causes and the factors influencing the timing), and to test predictions from two functional hypotheses: the intrasexual competition hypothesis, and in breeding avoidance. The data concerned 40 individuals born between 197 4-1985 in a closely monitored herd which developed a natural social st ructure during this period. All the females dispersed from their natal groups; none became solitary; 80% transferred to existing harems, the others formed new groups with bachelor stallions. Abduction by stalli ons affected only a quarter of the females whose transfers were observ ed. The results of this study do not support the hypothesis that a fun ction of natal dispersal is to reduce intrasexual competition. The you ng females were not expelled by resident females of their natal groups , and did not, as a rule, experience increased aggression from these f emales before emigration. Their social bonds with members of their nat al groups showed no progressive weakening prior to departure, and ther e was no gradual strengthening of bonds with individuals in the groups to which they transferred. There was no evidence for reproductive com petition between the young females and resident mares of their natal g roups, since the young females always refused the sexual approaches by males of these groups. Finally, age at dispersal did not decrease wit h the number of resident females in the groups they left. In contrast, as predicted by the inbreeding avoidance hypothesis, the primary caus e of dispersal appeared to be sexual attraction to unfamiliar males. W hen in oestrus and before dispersing, the young females accepted matin gs only with males of other groups in spite of frequent sexual approac hes by males of their natal groups (normally close relatives). In addi tion, none dispersed before first oestrus, and most did so during an o estrous period, at or before the conception of their first foal. The m others of most young females interposed themselves when close kin mate s of the natal group approached their daughters sexually; this could c ontribute to the avoidance of close inbreeding. Among the other factor s examined, some did not influence disperser of the young females: the y experienced low levels of aggression by adult stallions of their nat al groups, particularly at the time of departure; their weight and bod y condition had no significant effects on leaving age nor did their mo ther's rank, the number of siblings, or the birth of another. In contr ast, as the number of groups and the breeding sex ratio increased, dis persal age declined, occurring at a median age of 23 months (range 12- 42 months) in the later years, when the herd had developed a natural s ocial system.