ESM1: Monitoring Female Chimpanzees’ Estrous Cycles
Inoue and Matsuzawa (2011) tested a female chimpanzee, Ai, on a nine-item sequencing task
presented on a touch-screen computer screen in one of two versions: in the first the chimpanzee
had to selected the nine items in the correct order (‘easy’) while, in the second version, after the
chimpanzee made her first selection the remaining items were masked (‘hard’). Although Inoue
and Matsuzawa (2011) reported that Ai showed high levels of success for both versions of this
sequencing task (>80% correct for the easy task and ~65% correct for the hard task), her
performance was reduced during the ovulation phase of her estrous cycle. Note, although Ai’s
performance showed variation across her cycle, she still never “failed” the task but rather, during
the ovulation phase of her cycle, her performance lowered to around 60% correct responses.
Inoue and Matsuzawa (2011) attributed the female chimpanzee’s reduced performance at the
hard cognitive task to a reduction in her attention or motivation. Specifically, they concluded
that “the cognitive performance of a chimpanzee was disturbed by hormonal changes despite her
long-term experience in the tasks” (p. 104). The luteinizing hormone (LH) surge is associated
with ovulation and co-occurs with maximal tumescence of female chimpanzees’ sexual swellings
(Deschner et al. 2004). From this, it might be predicted that female chimpanzees would show
less success (but not total failure) at a problem-solving task when they concurrently have a
tumescent swelling. Therefore, we recorded each female’s estrous state during testing and
ensured that their testing schedule was counterbalanced across their swelling cycle.
Comparable to humans, an average female chimpanzee cycle lasts for around 30 days
with menstruation lasting three days on average (Deschner et al. 2004; van Esch et al. 2008).
During their cycle, a female’s swelling size and shape will change in ways that can be visually
assessed using a 0-4 scale (Graham 1981). This scale takes into consideration several factors,
including the overall size of the swelling and the turgidity of the “pinkish” area (c.f. Graham
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1981). These factors can vary across individuals and across cycles for the same individual, but
the benefit of this visual scale is that it can be used as an indicator of hormone levels without
necessitating hormonal assays (Deschner et al. 2004). Deschner et al. (2004) concluded that the
absolute size of a female’s swelling is not a reliable indicator of a female’s reproductive quality
and a focus upon female swellings only enables intra-individual changes.
To establish whether each of the 24 female chimpanzees tested in the present study
followed a regular monthly estrous cycle, the appearance of each female’s sexual swelling was
recorded (by S.A.P.) twice per week for three months prior to the commencement of the study.
To ensure reliability, a photograph was taken of the female’s swelling once per week and
assessed by two raters (S.A.P. and L.M.H.) who showed strong inter-rater reliability. In rare
cases (<5%) of rating disagreement, an experienced member of the veterinary staff provided their
opinion. Following Reichert et al. (2002), the females’ swellings were rated on a 0-4 scale,
where 0 represents the most flaccid state of the swelling and 4 the most turgid. This three-month
assessment revealed that nine of the 24 female chimpanzees followed a regular monthly swelling
cycle (Table S1). The remainder showed no cycling patterns with regard to the shape and size of
their swelling and remained classed as 0 throughout the assessment period.
We note that the majority of the females tested in our study were on some form of
contraceptive (Norgestrel/Estradiol, ParaGard®, or Implanon®, see Table S1 for details) and
unfortunately we were only able to test two females that were on no form of contraceptive (and
that had not had a hysterectomy). Whilst acknowledging that those females that exhibited sexual
swellings following a regular cycle may not have been experiencing typical fluctuations in
hormone levels (see also Proctor et al. 2011), we note the work of Bettinger et al. (1997) who
reported that certain forms of birth control do not interfere with regular hormonal fluctuations
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(see also Wagner and Ross 2013). However, we also note that these female chimpanzees would
have experience more constant cycling than wild chimpanzees due to their birth control regimens
which both inhibited pregnancy and controlled their cycling patterns.
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