Penile morphology in primates
- Wide variety of penile morphology, even among closely related primate
- Why are there differences? A-E has al different galago species. Why?
- Can use penile morphology alone to tell galago species apart. Just by penis
shape they can tell what species they are looking at. Penis are more
differentiated than the body shape
Why diversity in penile morphology in primates?
1. Lock and Key hypothesis – complex genitalia ensure that different species are
prevented from inter-breeding
o Male and female genitalia should be structurally complementary
within a species
o Only a few examples from the genus Macaca support this hypothesis
FIG. 9.2 UO (urethal opening where semen comes out) they all
fit pretty well. But this is the only example. If you look at other
species such as galago you don’t see one-to-one
o Problems with this hypothesis:
In some species with diverse penile morphology (i.e. galagos),
male and female genitalia do not “match”. Matching between
different primates species can occur. i.e. captive galagos
Other mechanisms keep different species from inter-breeding.
Species-specific vocalizations, behaviours, olfactory cues, etc.
Strong support for this hypothesis is lacking.
2. Genitalic recognition hypothesis
- Variant of the lock and key hypothesis – females’ bodies “recognize” males of
their species by the stimulation they receive during mating makes more
sense for species that are induced ovulators
- Induced ovulators
o Induced ovulation: ovulation only occurs in response to the act of
- Spontaneous ovulation
o Spontaneous ovulation: ovulation occurs cyclically, and it NOT
dependent on mating
- No strong support for this hypothesis
3. Pleitropism hypothesis:
- Pleiotropy: when a change to one gene affects unrelated phenotypic traits.
o Changes in male genitalia are caused by genetic changes in other
systems acted upon by natural selection.
o No strong support for this hypothesis 4. Mechanical conflict of interest hypothesis
- Male genitalia evolve to bypass female “barriers” to reproduction, or to
damage the female reproductive tract/genitalia
- i.e. if penis damages female’s vagina and decreases her inside so she cannot
mate with anyone else at the cost of hers .
not genetically applicable to primates
no support for this hypothesis in primates, but outside of male it does
- 5. Cryptic female choice hypothesis – macaques mating
- Male genitalia have evolved to function as an “internal courtship device”
females may unconsciously select the sperm of certain males over others
- Predicts that the most elaborate male genitalia should be found where
females mate with >1 male while they are fertile.
o Dispersed and polygynandrous mating systems
- Speculative hypothesis at present
- In many studies they have to kill females to see how far the sperm has
travelled. with priamtes it is hard so it is