1 Supplemental Data: Gorilla susceptibility to Ebola virus: the cost of sociality

Ebola virus has first been identified in wild apes in 1994, when 8 individuals of a 43 individual chimpanzee community were affected in Ivory Coast [S1]. Since then, wild ape mortality has been reported during most of human outbreaks in eastern Gabon and north-western Congo [S2]. An animal mortality monitoring network created in 2001 discovered in this area 98 wild animal carcasses from August 2001 to June 2003 [S2], 65 of which were great apes. Among all the carcasses, 21 were tested and 14 (of which 13 apes and 1 duiker) were found to be EBOV-positive. These results suggest that Ebola may severely affect great ape populations. Estimating Ebola-induced mortality in wild apes is however a difficult task. Two types of methods have been used. The first consists in comparing results of pre-epidemic and post-epidemic nest counts along transects or ‘paths of least resistance’. To our knowledge this methodology has been used twice, in Gabon and in Republic of the Congo [S3,S4]. Surveys conducted in 1998-2000 in Minkébé forest block (Gabon) after Mékouka (1994) and Mayibout 2 (1996) human Ebola outbreaks, revealed a decrease in the ape population of more than 90 % since 1990 [S3]. Another study based on this methodology, led in Lossi Gorilla Sanctuary (Republic of the Congo) after 2001–2 outbreak, showed a lower decrease of 56% of a gorilla population [S4], which may be explained by the fact that this area was only partially affected. The second method involves the visual identification of individuals of a population before and after an epidemic. To our knowledge, this approach has been used only once before, in Lossi, where 8 groups of gorillas have been monitored before, during and after the outbreak cited above [S4,S5]. None of these groups has been seen since the end of the outbreak, revealing that group EBOV mortality was probably very high [S4]. However, the mortality rate of solitary males remains unknown. None of these studies has investigated the way EBOV spreads in a population of great apes