Comparative rates of violence in chimpanzees and humans

Abstract

This paper tests the proposal that chimpanzees (Pan troglodytes) and humans have similar rates of death from intraspecific aggression, whereas chimpanzees have higher rates of non-lethal physical attack (Boehm 1999, Hierarchy in the forest: the evolution of egalitarian behavior. Harvard University Press). First, we assembled data on lethal aggression from long-term studies of nine communities of chimpanzees living in five populations. We calculated rates of death from intraspecific aggression both within and between communities. Variation among communities in mortality rates from aggression was high, and rates of death from intercommunity and intracommunity aggression were not correlated. Estimates for average rates of lethal violence for chimpanzees proved to be similar to average rates for subsistence societies of hunter–gatherers and farmers. Second, we compared rates of non-lethal physical aggression for two populations of chimpanzees and one population of recently settled hunter–gatherers. Chimpanzees had rates of aggression between two and three orders of magnitude higher than humans. These preliminary data support Boehm’s hypothesis.

Appendices

Appendix 1

Intercommunity killings, adults and adolescents, up to 2004

Site	Date	Victim (sex)	Sex	Aggressors’ community	Victim’s community	Evidence	References	Certain/suspected
Gombe	1974	Godi	M	Kasekela	Kahama	Attack observed	3	Certain
Gombe	1974	Dé	M	Kasekela	Kahama	Attack observed	3	Certain
Gombe	1975	Goliath	M	Kasekela	Kahama	Attack observed	3	Certain
Gombe	1975	Madam Bee	F	Kasekela	Kahama	Attack observed	3	Certain
Gombe	1977	Sniff	M	Kasekela	Kahama	Attack observed	3	Certain
Gombe	1998	Unknown	M	Kasekela	Kalande	Attack observed	5	Certain
Gombe	1972	Unknown	F	Kahama?	Kalande?	Body found	1	Certain
Gombe	1977	Charlie	M	Kasekela	Kahama	Body found	3	Certain
Gombe	2002	Rusambo	M	Kasekela	Mitumba	Body found	5	Certain
Gombe	1973	Hugh	M	Kasekela?	Kahama	Disappeared	3	Suspected
Gombe	1975	Faben	M	Unknown	Kasekela	Disappeared	3	Suspected
Gombe	1977	Willy Wally	M	Kasekela?	Kahama	Disappeared	3	Suspected
Gombe	1979	Sherry	M	Unknown	Kasekela	Disappeared	3, 7	Suspected
Gombe	1981	Humphrey	M	Unknown	Kasekela	Disappeared	3	Suspected
Gombe	1982	Figan	M	Unknown	Kasekela	Disappeared	3	Suspected
Kibale	2002	Unknown	M	Ngogo	Unknown	Attack observed	6	Certain
Kibale	2002	Unknown	M	Ngogo	Unknown	Attack observed	6	Certain
Kibale	2002	Unknown	M	Ngogo	Unknown	Attack observed	6	Certain
Kibale	2004	Unknown	M	Ngogo	Unknown	Attack observed	9	Certain
Kibale	2004	Unknown	M	Ngogo	Unknown	Attack observed	10	Certain
Kibale	1992	Ruwenzori	M	Rurama	Kanyawara	Body found	4	Certain
Kibale	1998	Unknown	M	Kanyawara	Sebitole	Body found	4	Certain
Kibale	1994	Julian	M	Unknown	Kanyawara	Disappeared	8	Suspected
Kibale	1998	Badfoot	M	Unknown	Kanyawara	Disappeared	8	Suspected
Kibale	2001	Light Brown	M	Unknown	Kanyawara	Disappeared	8	Suspected
Mahale	1970	Kaguba	M	M-group?	K-Group	Disappeared	2	Suspected
Mahale	1975	Kasanga	M	M-group?	K-Group	Disappeared	2	Suspected
Mahale	1975	Kajabala	M	M-group?	K-Group	Disappeared	2	Suspected
Mahale	1978	Kasonta	M	M-group?	K-Group	Disappeared	2	Suspected
Mahale	1979	Sobongo	M	M-group?	K-Group	Disappeared	2	Suspected
Mahale	1982	Kamemanfu	M	M-group?	K-Group	Disappeared	2	Suspected
Mahale	1982	Masisa	M	M-group?	K-Group	Disappeared	2	Suspected

1. References: 1, Wrangham (1975); 2, Nishida and Kawanaka (1985); 3, Goodall (1986); 4, Wrangham (1999); 5, Wilson et al. (submitted); 6, Watts and Mitani (personal communication); 7, Jane Goodall Institute’s Center for Primate Studies; 8, Kibale Chimpanzee Project records; 9, Muller (personal observation), Watts et al. (2005); 10, Watts et al. (2005)

Appendix 2

Intercommunity infanticides up to 2002

Site	Date	Infant’s sex	Aggressors’ community	Victim’s community	Evidence	References
Budongo	1991	M	Sonso	N15	Attack observed	5
Budongo	1991	Unknown	Sonso	N15	Body seen	5
Gombe	1971	Unknown	Kasekela	Unknown	Attack observed	1
Gombe	1975	F	Kasekela	Unknown	Attack observed	2
Gombe	1975	M	Kasekela	Unknown	Body seen	2
Gombe	1979	Unknown	Kasekela	Unknown	Attack observed	4
Gombe	1993	F	Kasekela	Mitumba	Attack observed	8
Gombe	1998	Unknown	Kasekela	Kalande	Attack observed	8
Kibale	1995	Unknown	Ngogo	Eastern	Attack observed	6
Kibale	1995	Unknown	Ngogo	Eastern	Attack observed	6
Kibale	2001	Unknown	Ngogo	Northern	Attack observed	7
Kibale	2001	Unknown	Ngogo	Northern	Body seen	7
Kibale	2004	M	Ngogo	?	Attack observed	10
Kibale	2004	Unknown	Ngogo	?	Attack observed	10
Mahale	1970	M	K-group	M-group	Body seen	3
Mahale	1971	M	M-group	K-group	Body seen	3
Mahale	2001	M	M-group	Unknown	Attack observed	9

1. References: 1, Bygott (1972); 2, Goodall (1977); 3, Nishida (1979); 4, Goodall (1986); 5, Newton-Fisher (1999); 6, Watts and Mitani (2000); 7, Watts et al. (2002); 8, Wilson et al. (2004); 9, Kutsukake and Matsusaka (2002); 10, Hogan Sherrow and Sylvia Amsler (personal communication)

Appendix 3

Intracommunity killings, adults and adolescents, up to 2002

Site	Date	Victim	Sex	Community	Evidence	References	Certain/suspected
Mahale	1995	Ntologi	M	M-group	Attack observed	1	Certain
Mahale	1996	Jilba	M	M-group	Body found	2	Certain
Budongo	1998	Zesta	M	Sonso	Attack observed	3	Certain
Kibale	2002	Grapelli	M	Ngogo	Attack observed	4	Certain
Gombe	2004	Vincent	M	Mitumba	Attack observed	5	Certain

1. References: 1, Nishida (1996); 2, Wrangham (1999); 3, Fawcett and Muhumuza (2000); 4, Watts (2004); 5, Tofiki Mikidadi (personal communication)

Appendix 4

Intracommunity infanticides up to 2004

Site	Date	Victim	Sex	Community	Evidence	Attackers	References
Budongo	1967	Unknown	?	Sonso?	Body found		1
Gombe	1975	Otta	F	Kasekela	Observed	Females	2
Gombe	1976	Orion	M	Kasekela	Observed	Females	2
Gombe	1976	Genie	F	Kasekela	Observed	Females	2
Gombe	2004	Tofiki	M	Kasekela	Observed	Male	5
Gombe	1976	MLB2	M	Kasekela	Body found	Unknown	2
Gombe	1994	Rafiki’s	?	Mitumba	Body found		3
Gombe	1976	Banda	F	Kasekela	Suspected	Unknown	2
Gombe	1974	Gandalf	M	Kasekela	Suspected	Unknown	2
Gombe	1975	Patti’s	unk	Kasekela	Suspected		4
Gombe	1976	Little Bee’s	unk	Kasekela	Suspected		4
Gombe	1991	Kenitum	M	Kasekela	Suspected		6
Gombe	2004	Kobe	M	Kasekela	Suspected		5
Kibale	1994	Temba	M	Kanyawara	Observed	Male, female	7
Mahale	1983	Watendele’s	M	M-group	Observed	Males	8
Mahale	1985	TMs	M	M-group	Observed	Male	9
Mahale	1989	Mirinda’s	M	M-group	Observed	Males	10
Mahale	1977	Humbe (Ndilo’s)	M	M-group	Body found	Male?	11
Mahale	1983	Chausiku’s	M	M-group	Body found	Male?	12
Mahale	1990	Betty’s	M	M-group	Observed	Male	10
Mahale	1979	Wakasunga’s	M	M-group	Suspected	Unknown	13
Taï	?	?	?	Northern	Body found		14

1. References: 1, Suzuki (1971) (occurred before long-term study and hence not used in calculation of death rates); 2, Goodall (1986), Jane Goodall Institute CPS; 3, Anne Pusey (personal communication); 4, Goodall (1986, p.284) (“Three other mothers, etc.”); 5, Carson Murray (personal communication); 6, JGI-CPS unpublished data (observers thought that victim was hurt during an attack on his mother); 7, Arcadi and Wrangham (1999); 8, Takahata (1985); 9, K. Masui unpublished observation (1986), cited in Hamai et al. (1992); 10, Hamai et al. (1992); 11, Norikoshi (1982); 12, Nishida and Kawanaka (1985); 13, Kawanaka (1981); 14, Boesch and Boesch-Achermann (2000, p 33) (infant found being eaten by females, and young mother without infant seen in border region)

Appendix 5

Number of known, inferred or suspected adult or adolescent victims of intercommunity aggression at long-term study sites up to 2002

Site	Attack observed	Body found	Disappearance	Total
Budongo	0	0	0	0
Gombe	6	3	6	15
Kibale	5	2	3	10
Mahale	0	0	7	7
Taï	0	0	0	0
Total	11	5	16	32

1. Data are from Appendix 1. “Disappearance” means that an individual disappeared without suffering from ill health or old age

Appendix 6

Number of known or inferred victims of intercommunity infanticide at long-term study sites up to 2002

	Attack observed	Body found	Total
Budongo	1	1	2
Gombe	5	1	6
Kibale	5	1	6
Mahale	1	2	3
Taï	0	0	0
Total	12	5	17

1. Data are from Appendix 2. Although observers sometimes consider infants that disappeared to have been possible victims of infanticide, no attempt is made to list them because infants often die from other causes also

Appendix 7

Number of known, inferred or suspected adult or adolescent victims of intracommunity aggression at long-term study sites up to 2002

	Attack observed	Body found	Total
Budongo	1	0	1
Gombe	1	0	1
Kibale	1	0	1
Mahale	1	1	2
Taï	0	0	0
Total	4	1	5

1. Data are from Appendix 3

Appendix 8

Number of known or inferred victims of intracommunity infanticide at long-term study sites up to 2004

	Attack observed	Body found	Suspected	Total
Budongo	0	1	0	1
Gombe	3	2	6	11
Kibale	1	0	0	1
Mahale	4	2	1	7
Taï	0	1	0	1
Total	8	6	7	21

1. Data are from Appendix 4

Appendix 9

Average community composition for study communities

Site	Community	Years	A–M	A–F	Adol–M	Adol–F	IJ	Total	References
Budongo	Sonso	1996	12.0	11.0	4.0	6.0	17.0	50.0	1
Gombe	Kahama	1973–1977	4.2	1.8	0.6	1.0	2.4	10.0	2
Gombe	Kasekela	1966–1998	8.7	12.2	1.8	3.5	20.1	46.3	2
Gombe	Mitumba	1996–2002	3.1	7.4	1.6	1.9	8.0	22.0	2
Kibale	Kanyawara	1988–2000	10.0	13.2	2.7	3.2	15.0	44.1	3
Kibale	Ngogo	1999	24.0	47.0	15.0	9.0	49.0	144.0	4
Mahale	K-group	1966–1983	3.8	10.7	0.8	2.6	7.8	25.7	5
Mahale	M-group	1980–1999	8.5	28.6	7.6	7.7	26.6	79.0	6
Taï	Northern	1982–1996	6.7	21.5	3.0	4.2	25.8	61.2	7

1. References: 1, Reynolds (1998), Newton-Fisher (2002); 2, Jane Goodall Institute’s Center for Primate Studies (unpublished); 3, Kibale Chimpanzee Project (unpublished); 4, Watts et al. (2002); 5, Nishida et al. (1985); 6, Nishida et al. (2003); 7, Boesch and Boesch-Achermann (2000)

Appendix 10

Duration of study periods relevant to observation of lethal aggression

Site	Community	Study begin	Study end	No. of study years	No. of years poorly known	No. of dData years
Budongo	Sonso	1991	2004	14	4	10
Gombe	Kahama	1973	1977	5	0	5
Gombe	Kasekela	1960	2004	45	6	39
Gombe	Mitumba	1994	2004	11	2	9
Kibale	Kanyawara	1987	2004	18	4	14
Kibale	Ngogo	1994	2004	11	3	8
Mahale	K-group	1966	1983	18	2	16
Mahale	M-group	1968	2004	37	3	35
Taï	Northern	1979	2004	26	3	23
Total				185	27	158

1. “Study end”, ongoing studies are considered through 2004; “# Study years”, number of years when community was demographically well-known; “# Years poorly known”, period when demography uncertain, and/or observers rarely visited territorial boundaries and did not observe contacts with neighboring communities; “No. of dData years”, estimated number of years when intercommunity aggression could be observed