briefly but returned to resume splashing in the river, this time leaning over and splashing vigor-
ously with both hands. Janet returned to do this twice more before joining Nora and Beti playing
in the trees. (Events Book, recorded by Katie Slocombe and Raimond Ogen.)Sex and reproduction
Darwin (1871) suggested that in addition to natural selection (the process of differential
survival based on inter-individual competition) there was also sexual selection based on
the ability of individuals to attract mates and produce offspring. Female chimpanzees
show large sexual swellings around the time of ovulation; normally these signal fecund-
ity and attract the attentions of males. There are two situations in which swellings do
not signal ovulation: first, pre-reproductive females show sexual swellings during
adolescence, and second, non-reproductive females who are pregnant or lactating also
in some cases show sexual swellings. The reasons for these ‘dishonest signals’ have
been much debated and it may be that such swellings in pre-ovulatory females give them
some early experience of sex with resident subadult males (adult males largely ignore
them, however ‘nubile’ they may look to a human observer^48 ) or help them gain accept-
ance if they transfer to other communities; in such cases swellings serve as a ‘social
passport’ (Boesch and Boesch-Achermann 2000: 58). In the case of pregnant or lactat-
ing females they are pseudo-sexual signals that may ‘trick’ males into tolerating them
at times when they are not fertile (see Wallis 1992aand Wallis and Goodall 1993 for
discussions of these and related phenomena).
Turning to males, a notable anatomical feature is the large size of their testes.
Compared with the other apes, or humans, chimpanzee testes are large. Clutton-Brock
and Harvey (1977) showed that relative testis size (i.e. testis : body size) in primates
varies according to social organization. Species living in multi-male societies (including
chimpanzees) have the highest ratio, one-male group species have an intermediate ratio,
and monogamous or pair-bonding species have the lowest ratio.^49 This has been inter-
preted as signifying that sperm competition is important in chimpanzees and other
species living in multi-male societies. Large testes are advantageous when frequent mat-
ing with available females is the norm. The production of large amounts of sperm is
advantageous to males when several males are mating sequentially with each female
around her time of ovulation; the sperm of different males then compete within the
female’s reproductive tract (Dixson 1997).
Seraphin (2000) examined the question of whether increased production of testo-
sterone would be found in higher ranking male chimpanzees but did not find a
straight-line correlation between testosterone (extracted from faeces) and rank.
Perhaps this is related to the fact that most competition for females occurs between
Sex and reproduction 115(^48) In a study of 245 copulations at Sonso, only 15 were with young, nulliparous females (S. O’Hara,
pers. comm.).
(^49) Humans fall into the one-male group cluster, indicating that we evolved as a one-male group
‘polygynous’ or ‘harem’ species.