Biology Reference
In-Depth Information
Testis size and breeding
system
The heaviest primates, the gorilla (
Gorilla
gorilla
) and orang-utan (
Pongo pygmaeus
)
have breeding systems that involve one
male monopolizing mating with several
females, and have testes that weigh
30 and 35 g, respectively (average weight
of both testes). The smaller chimpanzee
(
Pan troglodytes
), by contrast, has a
breeding system where several males
copulate with each oestrus female and
this species has testes weighing 120 g! It
seems likely that the marked differences
in testes weights are related to differences
in breeding system. In single-male
breeding systems (gorilla and orang-
utan) each male needs ejaculate only
enough sperm to ensure fertilization.
In multimale systems (chimpanzee),
however, a male's sperm has to compete
with sperm from other males. Selection
should, therefore, favour increased sperm
production and, hence, larger testes.
Harcourt
et al
. (1981) tested this
hypothesis by comparing 20 genera of
primates, varying in body size from the
320 g marmoset (
Callithrix
) to the 170 kg gorilla. Figure 2.7 shows that, as expected,
testes weight increases with body weight. For a given body weight, however, it is clear that
genera with multimale breeding systems have heavier testes than genera with single-male
or monogamous breeding systems. The data points for the former group lie above the line,
and those for the latter lie below ('single-male' indicates that there is only one breeding
male although, as in the gorilla, there may be more than one male in the social group;
'monogamous' indicates that there is just one male and one female in a group). These data
therefore support the sperm competition hypothesis.
250
100
+
10
1
0.2
1
10
100
200
Body weight (kg)
Fig. 2.7
Log combined testes weight
(g) versus body weight (kg) for different
primate genera. Solid circles are
multimale breeding systems. Open
circles are monogamous. Open
triangles are single-male systems (one
male with several females). The cross is
our own species,
Homo
, for
comparison. From Harcourt
et al
.
(1981). Reprinted with permission from
the Nature Publishing Group.
Larger testes in
multimale groups
Using phylogenies in comparative
analysis
Since the mid 1980s, there has been a further major advance in comparative analysis.
This is to use phylogenies, firstly to identify independent evolutionary transitions and,
secondly, to elucidate the order in which traits have evolved (Felsenstein, 1985; Grafen,
1989; Harvey & Pagel, 1991). Before we describe
how
this is done, we must first expand
on
why
it needs to be done.
