Behavioural Indicators of Stress
The impact of social structure on self-directed behaviour.
Self-directed behaviours, including scratching, auto-grooming, body shaking,
and yawning, have all been shown to reflect some level of stress, anxiety,
or uncertainty in baboons and macaques (Maestripieri et al 1992; Castles &
Whiten 1998). In long-tailed macaques, scratching and autogrooming are more
frequent when an individual is within one meter of the alpha male than when
alone or in passive contact (Pavani et al 1991; Troisi & Schino 1987). Troisi
and Schino (1987) suggest that this can be explained as an internal conflict
between two opposing social motivations: the motivation to approach the
alpha male and groom him and the motivation to avoid him in case he attacks.
Among female olive baboons, a behavioural index reflecting a combined
measure of scratching, autogrooming, self-touching, body shaking, and
yawning was forty percent higher when the female's nearest neighbor was
higher-ranking than when it was lower-ranking than herself (Castles et al
1999).
In this study we measured the occurrence of self-directed behaviours in the 'Cape Point' troop of chacma baboons. This troop inhabits the Cape of Good Hope section of the Table Mountain National Park in the southern tip of the Cape Peninsula of South Africa. The aims of the study were to quantify SDB rates in the study troop, compare them to other available data on female baboons, and investigate the relationship between SDB rates and (a) female dominance rank, (b) reproductive state, and (c) herding by baboon monitors. The study troop, consisting of 13 adult and subadult females and 4 subadult males, was unusual in two ways: (1) it lacked adult males and (2) it was regularly herded away from tourist areas by park staff employed as baboon monitors (see ‘Baboons and Humans’ sidebar on the BRU home page). Fifteen-minute continuous focal samples were conducted on females, in which the occurrence of SDBs (scratching, self-grooming, yawning, and body-shaking) was recorded. Overall rates of SDBs, rates of scratching and self-grooming, and durations of self-grooming bouts were then calculated and compared across females and conditions.
We expected the unusual social structure of this troop to have an effect on their SDB rates. In particular, the absence of adult males in the troop and the consequent lower average rank of a female's nearest neighbor might have resulted in lower overall rates of self-directed behaviour compared to a chacma troop with adult males. While data for a chacma troop with adult males were not yet available for comparative analysis, rates of SDBs in this troop were in fact significantly higher than those in olive baboons (Castles et al. 1999). This may reflect greater levels of stress experienced by this population, or may be a consequence of greater ectoparasite loads.
In addition, certain categories of females might be expected to engage in
higher rates of self-directed behaviour, e.g., females with young infants
(who stand to lose when a new male immigrates into the troop) and estrous
females (who are in a state of sexual arousal). We found support for the
former, but not the latter: rates of SDBs among the females in this study
were significantly higher in pregnant and lactating females than in cycling
females, which may indicate greater stress in the former as a result of
greater perceived risk of infanticide. We found no relationship, however,
between SDB rates or self-grooming bout duration and female dominance rank.
Finally, we found that self-grooming bout duration was significantly shorter when the troop was being herded by baboon monitors compared to when baboon monitors were absent. This result suggests that herding by baboon monitors may interrupt female grooming bouts.
Results of this study are indicative of several potential factors influencing stress in the Cape Peninsula population. To greater explore the role of stress and its causes and consequences in this population, we are following up this study with a more comprehensive comparison across three troops using both behavioural and hormonal measures of stress (see Stress, Sociality, & Reproduction).
