I've been following the story of the Ebola epidemic in wild lowland gorillas, in posts here, covering research by Magdalena Bermajo et al., and here, sampling research on disease spread in wild primates. Now, a new contribution to the story comes from Peter Walsh and colleagues, studying the movements and interactions of gorilla groups with each other and with chimpanzees.
Of particular interest was the fact that overlaps pivoted on a few social units that were frequently present during the mast fruiting event (fig. 1C). One unit visited the Nauclea trees on 26 of the 36 September and October days on which gorillas were observed in Nauclea trees. It accounted for 38% of same-day overlaps, 49% of consecutive-day overlaps, and 56% of 2-day overlaps. It overlapped with nine of the other 15 units in at least one of the three time intervals (four, seven, and nine units, respectively). These observations suggest that some gorilla social units may play a disease-transmission role analogous to that of prostitutes in the spread of HIV in southern Africa (Anderson et al. 1991), rapidly disseminating Ebola among networks of individuals that would otherwise interact little. Such "super spreaders" have recently been shown to play a critical role in the dynamics of explosively emerging diseases, including Ebola in humans (Lloyd-Smith et al. 2005) (Walsh et al. 2007:687-688).
In the next paragraph, the authors discuss coforaging in the same trees by gorillas and chimpanzees in the Goualougo Triangle:
From 2002 to 2004, we observed chimpanzees from four different communities foraging in fruiting trees from the genus Ficus. On 5 of 75 days (i.e., once for every 15 days of feeding observations at Ficus trees) chimpanzees occupied a tree simultaneously with gorillas (see video 3). Co-occupancy lasted for an average of 47 min, with an average of 10.4 chimpanzees and 3.8 gorillas involved. True rates of co-occupancy are probably higher because gorillas appeared to be deterred by the presence of observers. Although shared use of resources has been reported (Kuroda et al. 1996), this is the first study documenting that co-feeding occurs on a regular basis (Walsh et al. 2007:688).
The study doesn't document actual transmission of Ebola, but is intended to provide details of the social context in which transmission could occur. In addition to the cross-specific interactions, they also report one group coming across and inspecting a dead bloody gorilla corpse. Inspection isn't new, but what is interesting is that lowland gorillas live at population densities high enough to run across dead individuals from other groups -- making disease transmission by this mechanism plausible.
Walsh PD, Breuer T, Sanz C, Morgan D, Doran-Sheehy D. 2007. Potential for Ebola transmission between gorilla and chimpanzee social groups. Am Nat 169:684-689. Abstract