IMPLEMENTACIÓN DE ETHERNET

The dietary composition of gorillas closely refl ects the type of habitat rather than phylogenetic distance. The higher montane forests in equatorial Africa are charac-terized by a lower diversity of trees and fruits (Hamilton 1975 ; Sun et al. 1996 ).

Western and eastern gorillas ( G. gorilla gorilla and G. beringei graueri ) inhabiting lowland tropical forests consume more types of fruit than eastern gorillas ( G. b.

graueri and G. b. beringei ) inhabiting montane forests, who in turn consume more kinds of vegetative food (Table 3.1 ). In particular, the proportion of fruit in the diet is clearly different between gorillas in lowland tropical forests and those in montane forests. Even within a subspecies ( G. b. g ), more than 90 % of plant foods do not overlap and are not available in both lowland (Itebero) and highland (Kahuzi) habi-tats (Yamagiwa et al. 1994 ). Moreover, gorillas seasonally change their diet. Eastern gorillas in Kahuzi show a fruigivorous diet in some months but rely completely on vegetative foods in other months (Casimir 1975 ; Yamagiwa et al. 2005 , 2009 ).

These observations suggest a large fl exibility in the dietary choices of gorillas according to variations in food availability seasonally and locally.

Frugivorous diet is positively correlated with the daily path length (DPL) of gorillas in both lowland and montane habitats (Table 3.1 ; Remis 1997b ; Goldsmith 1999 ; Yamagiwa et al. 2003 ; Doran-Sheehy et al. 2004 ; Ganas and Robbins 2005 ).

Group size also infl uences DPLs, and a larger group tends to travel longer distances in the montane forest of Bwindi (Ganas and Robbins 2005 ). Frugivorous diets may increase scramble competition within groups, and gorillas may respond to this by increasing the number of visited fruit crops. However, such an extension may not result in an expansion of the home range. Based on the general ecological features of primates, a frugivorous species would need a larger home range than a folivorous species of the same group weight (Clutton-Brock and Harvey 1977 ). In contrast to this assumption, smaller home ranges are found for frugivorous western gorillas than for folivorous eastern gorillas (Table 3.1 ), which may be caused by the differ-ences in site fi delity between them. Folivorous mountain gorillas use less than 2 km ² for a monthly range but shift ranges gradually to cover a wider area over the course

Table 3.1 Variations in socioecological features in genus Gorilla G. g. g. G. b. g. G.b.b. Lopé Bai Hokou Mondika Ndoki Lossi Moukalaba Itebero Kahuzi Bwindi Virungas Habitat type Tropical Tropical Tropical Tropical Tropical Tropical Tropical Montane Montane Montane Number (#) plant foods (number of species) 182 (134) 230 (129) 127 (100) 182 (152) 194 (121) 236 (116) 205 (113) a 75 (38) Percent (%) fruit in plant food species

71 60 70 63 40 20 32 a 5 Percent (%) fecal samples including fruit remains

98 >99 100 – 99 89 53 47 – Mean number (#) fruit species per fecal sample

2.7 3.4 3.5 4.1 0.78 1 Insectivory Ant, termites Termites Termite Ant, termite Ant, termite Ant, termite Ant Rare Rare Mean daily path length (m) 1,105 m (220– 2,790) 2,600 m (300– 5,300) 2,014 m (400– 4,860) 1,853 m (300– 5,500) 1,531 m (142– 3,439) 716 m (242– 2,055)

547–1,034 472–1,034 (112– 2,868) Home range (study period) 22 km 2 (10 years) 23 km 2 (2.2 years) 16 km 2 (1.3 years) 11 km 2 (3.2 years) 12 km 2 (1 year) 42 km 2 (8 years) 40 km 2 (3 years) 21–25 km (5–7 years) Annual home range 7–14 km 2 8–13 km 2 15 km 2 12 km 2 13–18 km 2 23–38 km 2 9–12 km Home range overlap Extensive Extensive Extensive Extensive Partly Extensive Extensive Extensive Extensive Extensiv Group cohesiveness High Subgroup Subgroup Subgroup High but fusion High High High High High

Mean/median group size 10 7 14 7 10 10 11 Maximum group size 16 13 17 31 23 65 Proportion of multimale groups

0 <10 % 0 0 0 <10 % 0 8 % 46 % 44 % Maximum number (#) SB within a group

1 2 2 1 1 2 1 2 3 7 Female transfer Yes Yes Yes Yes Yes Yes – Yes Yes Yes Male emigration Yes Yes Yes Yes Yes Yes – Yes Yes Yes Male immigration – – – – – – – – – Rare Solitary male Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Solitary female – Yes – – – Yes – – – – All-male group – – Yes – – – – – Yes Yes All-female group – – – – – – – Yes – – Group fi ssion – Yes – – – – – Yes Yes Yes Infanticide – – – Possible – – – Yes – Yes G. g. g., Gorilla gorilla gorilla; G. b. g., Gorilla beringei graueri; G. b. b., Gorilla beringei beringei; SB: Silverback male (mature male) a Data presented from one of three study groups Sources: Lopé, Williamson et al. ( 1990 ), Tutin and Fernandez ( 1992 , 1993 ), Tutin ( 1996 ), Bai Hokou, Remis ( 1997a , b ), Cipoletta ( 2004 ), Mondika, Doran et al. ( 2002 ), Doran-Sheehy et al. ( 2004 ), Ndoki, Nishihara ( 1994 , 1995 ), Parnell ( 2002 ), Stokes et al. ( 2003 ), Lossi, Bermejo ( 2004 ), Moukalaba, Yamagiwa et al. ( 2009 Itebero, Yamagiwa et al. ( 1994 ), Yamagiwa and Mwanza ( 1994 ), Yamagiwa et al. ( 2003 ), Kahuzi, Murnyak ( 1981 ), Yamagiwa et al. ( 1993 ), Yamagiwa et al. ( 2003 2005 ), Yamagiwa and Basabose ( 2006a , b ), Bwindi, McNeilage et al. ( 2001 ), Robbins and McNeilage ( 2003 ), Ganas et al. ( 2004 ), Ganas and Robbins ( 2005 ), Virunga, Schaller ( 1963 ), Weber and Vedder ( 1983 ), Yamagiwa ( 1986 ), Watts ( 1984 , 1998a ), Gray et al. ( 2003 ), Robbins et al. ( 2004 , 2007 , 2009 ), Stoinski et al. ( 2009a, b Fawcett et al. ( 2012 )

of a year (Watts 2000 ). This pattern of ranging may help to avoid overuse of previ-ously trampled areas and allow regeneration of herbaceous vegetation (Vedder 1984 ; Watts 1998b ). Eastern gorillas in the montane forest of Kahuzi tend to shift range seasonally and annually, and no correlation was found between their monthly range size and fruit consumption (Casimir and Butenandt 1973 ; Goodall 1977 ; Yamagiwa and Basabose 2006b ). By contrast, western gorillas tend to revisit the same area within a short period for feeding or nesting at Mondika and Moukalaba (Doran-Sheehy et al. 2004 ; Iwata and Ando 2007 ). The monthly ranges of western gorillas cover a wide area, and the accumulated monthly ranges reach the size of the annual range within a few months at Mondika (Doran-Sheehy et al. 2004 ). The frequency of nest and nest site reuse is positively correlated with fruit consumption of western gorillas at Moukalaba (Iwata and Ando 2007 ). Western lowland gorillas tend to harvest fruit crops effi ciently as fugivorous primates, in contrast to the foli-vorous eastern gorillas in the montane forests.

Nevertheless, frugivorous western gorillas do not show territoriality against neigh-boring groups or the fi ssion–fusion features observed for chimpanzees. Similar to folivorous mountain gorillas in the Virungas (Schaller 1963 ; Watts 1998a ), both east-ern and westeast-ern gorillas in other habitats tend to overlap ranges with neighboring groups irrespective of dietary characteristics (Tutin 1996 ; Yamagiwa et al. 1996 ; Bermejo 2004 ; Remis 1997b ; Cipoletta 2004 ; Ganas and Robbins 2005 ). Intergroup encounters of mountain gorillas occur aggressively and occasionally involve fi erce fi ghts between silverbacks of different groups and infanticides by outside group males (Fossey 1974 , 1983 ; Harcourt 1978 ; Watts 2003 ; Harcourt and Stewart 2007 ; Yamagiwa et al. 2009 ). By contrast, intergroup encounters of western gorillas may occur more calmly and have no effect on extending their DPL s at Mondika (Doran- Sheehy et al. 2004 ). At Lossi, an intergroup encounter resulted in the fusion of two groups with peaceful intermingling and co-nesting (Bermejo 2004 ). Frugivorous diets may not increase feeding competition between groups through territorial defense of home range. Although frugivorous western gorillas tend to spread widely and sometimes to separate in different subgroups while foraging, most subgroupings likely occur in multimale groups , and females usually associate with silverback males (Goldsmith 1999 ; Doran-Sheehy et al. 2004 ). Feeding competition possibly induced by frugivorous diets may have no effects on association between females and males.

However, group composition is clearly different between mountain gorillas ( G. b.

b. ) and the other subspecies ( G. b. g . and G. g. g .), and this difference may be infl u-enced not by ecological factors but by social factors. In the Virunga mountain gorilla population, many episodes of infanticide have been reported so far (Fossey 1984 ; Watts 1989 ). Infanticide is regarded as a male mating tactic of killing suckling infants to stimulate their mothers to resume estrus (Hrdy 1979 ; van Schaik 2000 ).

Female gorillas are vulnerable to infanticide in the absence of a protector male, and infanticide has great infl uence on a female’s decision of movement (Watts 1989 , 1996 ; Harcourt and Stewart 2007 ; Yamagiwa et al. 2009 ). Female mountain gorillas tend to transfer alone to avoid competition with other females over protection from the silverback male, and they generally transfer into multimale groups where they can get more protection (Watts 2000 ; Robbins and Robbins 2005 ). These female decisions may in turn infl uence the males’ choice of movement around maturity.

They may change their reproductive strategies, from emigrating out of their natal group and attracting females from other groups during a solitary life, and thus estab-lishing their own reproductive group, to remaining in the natal groups to share the reproductive opportunity with their putative fathers and brothers (Robbins 1999 ; Robbins and Robbins 2005 ; Harcourt and Stewart 2007 ; Stoinski et al. 2009a , b ).

However, kin-relatedness is not always necessary for coexistence among mature males. Unrelated males sometimes form all-male groups, and some of them con-tinue to stay together in a group after accepting reproductive females (Yamagiwa 1987 ; Robbins 1995 ). Demographic history and an analysis of paternity of the habit-uated groups in the Virungas indicated that association among males was not limited to father–son pairs or half-siblings and that the multimale group structure of moun-tain gorillas may permit multimale mating (Robbins 2001 ; Robbins and Robbins 2005 ; Bradley et al. 2005 ; Stoinski et al. 2009a ).

Infanticide has rarely been reported for other subspecies of gorillas. Female eastern and western lowland gorillas tend to transfer with other females to small groups or solitary males, and maturing silverbacks take females to establish new groups through group fi ssion (Yamagiwa and Kahekwa 2001 ; Stokes et al. 2003 ). Most of the groups contain only one silverback, and infanticide may not have appeared as a male mating tactic in the population of eastern and western lowland gorillas. Recent observations of infanticide in the population of eastern lowland gorillas at Kahuzi suggest that rapid changes in gorilla social units and their relations following drastic environmen-tal changes caused by recent human disturbances may increase the probability of infanticide, which possibly increases birthrate (Yamagiwa et al. 2009 , 2011 ).