Ecological factors influencing sexual segregation and group size in fallow deer (Dama dama).

Abstract

Grouping patterns in fallow deer Dama dama were studied between 1986-9 by direct observation from transects of eight populations in the New Forest and adjacent agricultural land in southern England. Study populations were chosen on the basis of pronounced differences in population density, sex ratio, and dispersion, and differences between the sites in habitat availability and patterns of habitat use.
Within both forest and agricultural study sites, a large majority of female deer was observed in single sex female groups throughout the year. The lowest levels of female segregation occurred during the autumn reproductive period. Males generally exhibited lower levels of sexual segregation than females, particularly during winter and spring. In the forest sites, winter and spring mixed-sex groups were largely the result of young males joining female feeding aggregations. In contrast, mixed-sex groups in agricultural sites contained mature males throughout the year and were not exclusively based on habitats used for feeding. There was a negative correlation between the extent of winter and spring male segregation and population sex ratio, suggesting that males join female groups when male densities are relatively low. Female group size varied seasonally in all sites, attaining maximum values in winter or spring and minimum values in summer or autumn. This was not a reflection of changing patterns of habitat use; group size varied seasonally in both open and closed habitats. Female groups were larger in open habitats than in closed habitats throughout the year. The size of female groups varied between the study sites but there were no clear differences in overall female group size between the forest and agricultural sites. There was a tendency for open habitat group size to be largest in the forest sites and closed habitat group size to be largest in the agricultural sites. Female group size overall was unrelated to the proportion of each site occupied by closed habitat. However, an increase in the amount of closed habitat was reflected by a decrease in closed habitat group size. Female group size overall was negatively related to the use of closed habitat; not surprisingly, an increase in use of open habitats resulted in increased overall group size. Female group size overall was not related to density; large groups occurred in both high and low density sites. However, in high density populations, open habitat group size was larger and closed habitat group size smaller than in low density populations.