(a) Predation events and GC levels
We first examined whether females' GC levels were higher in months when individuals were confirmed victims of predation and months when healthy animals disappeared for unconfirmed reasons (suspected predation and disappear healthy); we found no difference (GLMM: F1,293=0.452, p=0.290). We, therefore, combined all three types of disappearances into a single category (termed predation for ease of discussion) in our overall GLMM.
GC levels were significantly higher in the four weeks following a predation event than when there was no such event (GLMM: F1,610
=0.021; see Engh et al. in press
for more details). GC levels in months when individuals were confirmed or suspected to have been killed by lions did not differ from those in months when individuals were confirmed or suspected to have been killed by leopards (GLMM: F1,200
Lions always attacked in coalitions of three or more, causing the baboons to panic and scatter in different directions. The resulting subgroups often remained separated for several days before reuniting. GC levels were significantly higher in months when lion attacks resulted in the group's separation for several days compared to months when lion attacks did not separate the group (GLMM: F1,133=17.474, p=0.004).
(b) The loss of a close relative and GC levels
Predation appeared to be especially stressful for females whose close relatives were killed. To examine how a relative's death affected a female's GC levels, we matched each of the 22 females who lost a close relative (affected females) with an unaffected control female in the same reproductive state from whom we had obtained faecal samples at the same time (18 affected and control females from the 2003–2004 study, and four affected and control females from the 2002 study). Females who lost a close relative experienced a significant increase in GC levels in the month following their relative's death compared with the month before (Wilcoxon signed-ranks test: Z=−2.678, n=22, p=0.018). By comparison, the GC levels of control females showed no similar increase (Z=−0.438, n=22, p=0.354; ). This analysis included females who lost a juvenile offspring to predation. Results were similar, however, when we restricted the analysis to the 10 females who lost an adult female relative (affected females: Z=−2.293, n=10, p=0.039; control females: Z=−1.070, n=10, p=0.219).
Figure 1 Change in fGC levels of 22 females who lost a close relative (i.e. mother, maternal sibling, offspring) to observed or suspected predation, compared to matched controls whose relatives did not die. Each box encompasses the 25th through 75th percentiles, (more ...)
Although females who lost a close relative to predation experienced a significant increase in GC levels, this effect was only transient. When we compared females' GC levels in the month before their relative's death with their levels in the second month following their relative's death, we found no significant difference (Z=−0.227, n=15, p=0.410; faecal samples during the second month were not available for seven females, either because their relative disappeared towards the end of the study or because they were drawn from the 2002 study, when faecal samples were collected for only four months).
The relatively transient effect of a relative's death on females' stress levels may have occurred in part because bereaved females attempted to cope with their loss by extending their social network. Because females concentrate much of their grooming on close kin (Silk et al. 1999
), females who lost a close female relative might have been expected to experience a decrease in grooming diversity, number of grooming partners and grooming rate. However, the opposite occurred.
There were 14 females for whom we had grooming data during the three months before and the three months after a close adult female relative's death (five females from the 2003–2004 study, and eight from the 2002 study). We used the Shannon–Weaver diversity index to calculate how evenly each affected female's grooming interactions were distributed among all potential female partners over the age of 3 years during each time period, and compared their indices to those of 14 control females in the same reproductive state. There was a significant increase in grooming diversity following a close relative's death (Wilcoxon signed-ranks test: Z=−2.981, n=14, two ties, p=0.010). Affected females also groomed significantly more individuals. On average, each female groomed 1.86 (±0.29 s.e.) females before her relative died, and 3.79 (±0.65 s.e.) after (Z=−2.540, n=14, one tie, p=0.021; ). Finally, there was a significant increase in the rate at which affected females groomed other females in the months after a close relative's death (Z=−2.731, n=14, p=0.016; ). In contrast, control females experienced no significant increase in grooming diversity (Wilcoxon signed-ranks test: Z=−0.804, n=14, one tie, p=0.275), number of grooming partners (Z=−1.489, n=14, three ties, p=0.134; ), or grooming rate (Z=−1.161, n=14, p=0.204; ).
Figure 2 Change in number of grooming partners of 14 females from the three months before to the three months after the loss of a close female relative, compared to matched controls. Each box encompasses the 25th through 75th percentiles, with the median represented (more ...)
Figure 3 Change in the grooming rates of 14 females from the three months before to the three months after the loss of a close female relative, compared to matched controls. Each box encompasses the 25th through 75th percentiles, with the median represented by (more ...)
While the death of a close relative and grooming partner was clearly stressful over the short term, females appeared to compensate for this loss by increasing their grooming diversity, the number of their grooming partners, and the rate at which they groomed other females. Perhaps as a result, their GC levels soon returned to baseline.
We would have liked to assess how grooming diversity and rate changed in the week or two immediately following a close relative's death, but our behavioural data were insufficient to accurately measure those variables over such short periods. Four of the nine females for whom we had both grooming and GC data had more than one female grooming partner before their relatives died, but that did not appear to buffer them from the stress of losing a grooming partner. On average, their GC levels rose by 70.61
(±24.91 s.e.) in the month after their relative's death. The GC levels of the five females with only a single female grooming partner rose an average of 50.04
(±54.90 s.e.) over the same time period.