Biology Reference
In-Depth Information
(a)
(b)
q
12
0,0
0,1
0,1
q
21
q
12
q
24
q
42
q
21
q
13
q
31
q
24
q
42
1,1
0,0
q
43
q
31
q
13
q
34
q
34
1,0
1,0
1,1
q
43
Fig. 2.13
(a) The correlated evolution of discrete traits. Consider two discrete traits in
primates: the first (x) is absence (0) or presence (1) of oestrus advertisement by sexual
swellings; the second (y) is a single-male (0) or multimale (1) mating system. The eight
arrows indicate the possible transitions between the four states. Statistical methods are used
to quantify the evolutionary rates of these transitions in a primate phylogeny and to find the
model which best describes the evolutionary transitions. For example, if q
1,2
= q
3,4
, then this
implies that the transition to a multimale mating system was independent of the absence
or presence of oestrus signals. (b) Flow diagram, showing the statistically most probable
evolutionary routes in the primate phylogeny, from an ancestral state of no sexual swellings
and a single-male/monogamous mating system (0,0) to a derived state of sexual swellings
and a multimale mating system (1,1). Thinnest arrows correspond to transition rates with
a high (
94%) posterior probability of being zero. Thickest arrows are the most frequent
transitions, with thinner arrows less frequent. The combination of arrows indicates that
the mating system changes firstly in evolution (state 0,1) and this then selects for a change
in female display to sexual swellings (1,1). The alternative hypothesis, that swellings evolve
first (1,0) and this then selects for a multimale system (1,1) is not supported. From Pagel &
Meade (2006). With permission of the University of Chicago Press.
>
may gain from copulating with the dominant male because he is likely to be the best
genetic sire and the one best able to protect her and her offspring from predators, and
from harassment by other males in the group. On the other hand, if it was clear to all
males that the dominant male had certain paternity, then subordinate males might
harm or even kill her offspring (Hrdy, 1979). Charles Nunn (1999) suggested that
sexual swellings might provide a graded signal, which both enables the female to bias
paternity chances to the dominant male while at the same time enhancing opportunities
of mating with subordinate males, too, at times when she is still potentially fertile. This
would give each male sufficient paternity chances that they would all protect her and
her offspring, or at least not be a threat.
In support of the graded signal hypothesis, ovulation is most likely during a female's
period of maximal swelling, which is when the dominant male guards her and copulates
most frequently (Fig. 2.14). During this period, he is likely to use olfactory cues, too, to fine-
tune his assessment of female fertility (Higham
et al
., 2009). However, individual females
may ovulate over several days around maximal swelling. Furthermore, females of species
with swellings are sexually active, and hence attractive to males, for about twice the length
of time per ovulatory cycle (11 days) compared to species without swellings (five days;
