Biology Reference
In-Depth Information
(a)
(b)
110
0.3
100
0.2
90
0.1
80
0.0
70
-12
-11-10 -9
-8 -7 -6
-5
-4 -3 -2 -1
0
1
2
3
456
-10
-8
-6
-4
-2
0
2
4
Days relative to ovulation
Days relative to ovulation
Fig. 2.14
Sexual swellings in a group of wild chimpanzees studied in an evergreen forest in the Tai National
Park, Côte d'Ivorie. (a) Swelling size in 12 females (mean
SE) aligned to the day of ovulation (day 0).
Swellings measured from photographs and ovulation determined by enzyme immunoassays from urine samples.
The shaded area indicates the fertile phase, when fertilization is most likely. (b) Alpha male copulation rate
(mean
±
1SD) during the phase of maximum swellings. Data from 10 females, aligned to the day of ovulation
(day 0). Fertile period shaded. From Deschner
et al
. (2004). With permission from Elsevier.
±
Nunn, 1999). This makes it less likely that the dominant male can monopolise a female for
her entire fertile period and so increases her chances of copulating with several males.
A further factor influencing subordinate male access will be the synchrony of fertile
females in the group. Subordinate males will have more chances to mate if the dominant
male shifts his attention to other attractive females. This is the likely explanation for
why swellings tend to occur most in multimale species that are non-seasonal breeders
(e.g. baboons) and not in species which are strongly seasonal in their reproduction
(e.g. vervet monkeys). In seasonal breeders, all the females come into oestrus within a
few weeks, so females are more likely to overlap in their receptivity. This means that a
dominant male is less likely to attempt to monopolise a female, so subordinate males can
more readily get access. Hence, sexual swellings are not such an advantage to females.
In this context, it is interesting to note that the evolutionary transition from swellings to
no swellings within a multimale system involved an increase in breeding seasonality
(the
sinica
group of macaques; Fig. 2.12; Nunn, 1999).
Breeding
synchrony may
also be important
The comparative approach reviewed
The statistical approach we have described is certainly a major improvement on the first
applications of the comparative method. To summarize, the main improvements are:
Main
improvements in
modern studies
(1)
Different aspects of social organization are treated independently.
(2)
Confounding variables are dealt with in a rigorous manner.
