possessiveness was a successful strategy of the alpha male only, as was found at Gombe
(Tutin 1975) and Mahale (Hasegawa and Hiraiwa-Hasegawa 1983).
Immigrant females were not shy of mixing with the community males; in contrast,
they spent more time with the adult males than resident females did. This was the case
when they were not in oestrus (when they were in oestrus the difference between resid-
ents and immigrants disappeared). This may be because the immigrant females were
establishing themselves in the community prior to their first birth, gaining familiarity
with the males and hence reducing the risk of later infanticide if and when they gave
birth (see Chapter 7 for details of infanticide).
Oliver (2002) concludes that the commonest tactic used by the Sonso females is to
stimulate competition among the males rather than to actively seek out the high status,
high quality, males. This fits with my own observations. The presence of a female in full
oestrus, with a large pink swelling, has an electrifying effect on the adult males who
gather around her, often displaying erections, looking in her direction, moving and
choosing to feed near her, or sitting on the forest floor watching her every move. From
my own observations I find it hard to be sure whether males other than the dominant
male would like to be able to ‘possess’ such a female. In fact, when several males
including the alpha male are present, only the alpha can do so. In his absence, higher
ranking males can intimidate younger or lower ranking individuals from soliciting an
oestrous female. An alternative strategy for possession, i.e. ‘consortship’, may occur
(see Chapter 5). There is a little circumstantial evidence for this at Sonso^51 but the data
are very scant. To some extent it appears that the Sonso males other than the alpha male
accept that they cannot have an exclusive relationship with a fully oestrous female and
so they move in to mate with her as opportunities arise. For her part she is mostly recep-
tive at any time other than when she is being mate-guarded by the alpha male, perhaps
for a mixture of reasons: reducing the possibility of immediate and future aggression
from them, sexual desire and improving the likelihood of fertilization.
Copulation and time of day
Unlike the situation at Gombe (Wallis 2002a), Sonso chimpanzees do not concentrate
copulations first thing in the morning. Data indicating this were at first thought to be due
to differences in the time that observations were started between the two sites, i.e.
observers at Sonso were missing early morning copulations because they were not pre-
sent to see them (Wallis 2002a). However, newer data, including systematic de-nesting
observations, indicate that this is a true difference. Data, including sexual and feeding
behaviour, were recorded by Zephyr Kiwede, senior field assistant at BFP, and Melissa
Emery Thompson. The results are shown in Table 6.2. Sonso chimpanzees generally
Copulation and time of day 119(^51) For example, over a 12-day period in April 1997, Ruhara (adult female in full oestrus) was seen
occasionally but the dominant male, Duane, was not seen at all. She was not observed to copulate with any
adult male. On 15 November 1997 she gave birth to an infant, Rose. Was she copulating exclusively with
Duane during her April oestrous period? Such sleuthing, based on our records of party composition and
behaviour, is indicative but does not amount to proof. A second case of possible consortship involved Black
and Polly in July 2003.