Age ratio in groups of a social ungulate affects epizoochorous dispersal and diaspore exchanges

Animal‐mediated seed dispersal is a key process in plant population dynamics, species distribution and ecosystem functioning. As long‐distance dispersal agents, ungulates help to maintain native plant populations facing abiotic changes in their habitat and habitat fragmentation or habitat loss. However, they also contribute to the spread of exotic species. Most ungulates live in groups, whose dynamics depend on the social status, personality, sex, age and body size of its different members. But how ungulate sociality affects zoochory remains largely unknown. We designed our experiment with sheep to test the effect of age composition on epizoochorous dispersal. Ewe lambs have stronger social links and respond more strongly to stress than adult ewes, therefore showing higher group cohesion. First, we expected that the more frequent interactions among ewe–lambs would shorten diaspore retention times and dispersal distances as ewe–lamb proportion increases. Second, diaspore exchange should be higher with increasing ewe–lamb proportion. We monitored groups of six domestic sheep Ovis aries (Romane breed) individuals with different age ratios, replicated four times: respectively, 0/6, 2/4, 4/2 and 6/0 adult ewes to ewe lambs. We attached five cocklebur Xanthium strumarium diaspores, colour‐coded for each animal, to the fur on each individual flank, 10 in total. We traced each diaspore fate during six‐hour sessions where we recorded movements, physical interactions among individuals and individual behaviour resulting in diaspore detachment or exchange. As expected, we found shorter retention times and dispersal distances and more diaspore exchanges as the proportion of ewe–lamb increases in the groups. Shaking, then scratching were the main behaviours leading to diaspore loss. Non‐intentional contacts between individuals were the most frequent interactions associated with diaspore exchange. We encourage further research on the effect of group living, be it group size or composition, on the different phases of epizoochorous dispersal.