Behavioural patterns among female African savannah elephants: the role of age, lactational status, and sex of the nursing calf

摘要

For social mammals, phenotypic factors, such as age and reproductive state, and environmental factors, such as competition and requirements for offspring, shape individual resource needs and cause animals to display behavioural patterns most useful in resource acquisition. Female mammals trade off investment in growth and survival against complex and resource-intensive reproductive challenges; thus, they alter behaviours according to changing priorities. To maximize net gain females increase resource intake or limit its expenditure. Behaviours that involve resource acquisition, such as foraging, are potentially rewarding, yet lead to competition, especially in group-living species; whereas behaviours that do not provide resources, such as resting or grooming, encourage sharing and buffer competition. In cooperative species with linear dominance hierarchies such as female African elephants (Loxodonta africana africana), rank is often determined by age and size, which are highly correlated. When compared with younger, competitively disadvantaged individuals, higher ranked animals attain greater access to resources, but other phenotypic and environmental factors may influence their needs. Hence, we examined how lactational status and sex of the nursing offspring influenced time spent on resource acquisition, and we assessed how these factors affect rates of aggression related to age. We conducted the study at Addo Elephant National Park in South Africa from June to December 2010, using 20 min focal animal sampling on reproductively mature females (N = 109). Adult females (N = 64) exhibited aggression more frequently and socialized in a non-aggressive manner less frequently than subadult females (N = 45). Lactating females (N = 75) spent more time foraging, displayed aggression more frequently, socialized in a non-aggressive manner less frequently, and exhibited chemosensory behaviours less frequently than non-lactating conspecifics (N = 34). Mothers of female calves (N = 39) spent more time foraging than mothers of male calves (N = 36). The latter spent more time nursing and resting. We show how behavioural patterns, permitted or limited by age (correlated with size and rank) and reproductive condition, pertain to resource needs in female elephants, in contexts not directly related to survival or starvation avoidance. We discuss our findings in the context of female social relationships, for a long-lived, cooperative species.