Ž .
of recumbency behaviour of cows having two to four farm mates 44.0 minrbout was longer than the ones having other numbers of farm mates. Social and maintenance behaviours of cows having
16 farm mates were like those having zero or one farm mates, which suggested that the social bond among them was weak. It is concluded that a group of cows having two to four farm mates,
that is, a group size of three to five cows, may be optimal for a stable life in a communal pasture. q
2000 Elsevier Science B.V. All rights reserved.
Keywords: Cattle; Optimal group size; Allogrooming; Social bond; Grazing behaviour
1. Introduction
Communal pastures into which cattle from various farms are turned out are very popular in Japan. Stress can be a problem on communal pastures. Being with unfamiliars
Ž .
increased the heart rate and changed behaviours in deer Pollard et al., 1993 , sheep Ž
. Ž
. Baldock and Sibly, 1990 , and cattle Kenny and Tarrant, 1987 , which was interpreted
to mean that social mixing may be stressful. In addition, there is a big problem in communal pastures where the body weights of some animals are reduced during an early
Ž .
grazing period. Susaki and Hirota 1976 reported that there was a slight positive correlation between the body weight gains of calves during the grazing period in a
communal pasture and the number of their farm mates reared together on each farm during the non-grazing period. They suggested that the social experience of indoor-group
rearing in winter reduced social stress in a communal pasture in a grazing season.
Ž .
Warnick et al. 1977 reported that calves group-reared in their early days quickly learned competitive feeding after weaning.
Social stress in a communal pasture may be reduced by not only early social experiences, but also by having familiar farm mates, which are defined in this paper as
cows from the same farm. Familiar conspecifics can lower the stress responses of Ž
. subjects Gunnar et al., 1980; Coe et al., 1982; Boissy and Le Neindre, 1990 . Arnone
Ž .
and Dantzer 1980 reported that the presence of a congener with which social bonds had been established played a protective role against behavioural and physiological
consequences of frustration. It is well-known that calves reared together in the same pen Ž
or on the same farm form a subgroup in outdoor rearing Ewbank, 1967; Broom and .
Leaver, 1978; Hayasaka, 1987 . Members in a subgroup allogroom each other, and graze Ž
. and rest together Bouissou and Hovels, 1976; Sato et al., 1993 . It is recognized that
¨
Ž .
Ž allogrooming has a calming effect on cattle Sato and Tarumizu, 1993 , horses Feh and
. Ž
. De Mazieres, 1993 , and monkeys Schino et al., 1988; Boccia et al., 1989 . Thus, the
presence of farm mates may have a buffering effect against multiple stressors. In addition, the suppressing effect of farm mates on multiple stresses may depend on the
strength of their social bonds and the number of their farm mates. Cows having no or only a few farm mates may be more stressed than ones with many farm mates.
Individual vigilance behaviour decreases and individual eating behaviour increases with
Ž .
group size in many species of animals Krebs and Davies, 1995 . The trade-off of those benefits and costs of grouping, such as competition and infection, decides the optimal
group size. A further consideration may be the size of subgrouping in a cattle herd.
In the present study, we tried to investigate if farm mates affiliate with each other and if the number of farm mates influences social and maintenance behaviours of cows in
communal pastures.
2. Materials and methods
