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ANTC23H3 Lecture Notes - Baculum, Orgasm, Pleiotropy

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Joyce Parga

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Announcements: Final exam August 23rd, 7-10pm, MW160
Penile Morphology in Primates
Wide diversity of shapes, even among closely
related primate species
Can have different sizes, shapes, appendages, etc.
Can use penile morphology alone to tell
galago species apart
A-E on the slide are all different galago species
Hypothesis requires that male and
female genitalia should be structurally
complementary within a species
Only a few examples of 'lock and key'
evidence - some species of macaque
have complementary structures of the
penis and vaginal lumen
Mating between different primate
species can actually occur
The structural complementary
genitalia is only found in a few
Vocalizations, olfactory
cues, behaviours, etc. are
Other mechanisms keep species
from interbreeding
Strong support for this hypothesis is
Lock and Key hypothesis
Variant of the lock and key hypothesis
Females' bodies "recognize" males of
their species by the stimulation they
receive during mating
Induced is contingent on
Spontaneous ovulation
occurs cyclically and is not
dependent on mating (All
primates are spontaneous
Induced Ovulators v. Spontaneous
Makes more sense for species that are
induced ovulators
Genitalic recognition hypothesis
Pleiotropy: when a change to one gene
affects unrelated phenotypic traits
Unsupportable hypothesis
Pleiotropism hypothesis
Males have evolved genitalia to bypass
female 'barriers' to reproduction or to
damage the female reproductive
Not generally applicable to primates,
however it does apply in other areas of
animal and insect reproductive
Mechanical conflict of interest hypothesis
Cryptic female choice - mate choice
during or after copulation (not
necessarily females being conscious of
this behaviour; more of a physical
reaction like a change in Ph level,
physical contractions, etc. that
facilitates fertilization)
Male genitalia have evolved to function
as an internal courtship device
Females may then unconsciously select
the sperm of certain males over others
Dispersed and polygynandrous
Speculative hypothesis at present;
extremely difficult to test in primates
Cryptic female choice
Baculum or os penis
Bone inside the penis of some animals
Plural bacula
Many primate species have them
May provide structural support during copulation
Tends to be near the distal end, where structural support
might be most needed
Adaptive function
The longest intromission
Eg. The pair-sit of stump-tailed macaques
Maintenance of intromission after ejaculation has occurred
Species with elongated bacula tend to be those having
Possible indication of prolonged intromissions during
In the Eocene, Adapids (similar to modern-day lemurs) had an
extremely long bacula
Human males lack a baculum, yet prolonged copulation is possible
because of increased vascularisation providing the structural rigidity
necessary for copulation
Penile Spines
Made of keratin
When the male intromits and thrusts, the spines point
ventrally (towards the proximal end of the penis), acting
as anchors to pull the plug out
May help to dislodge or move copulatory plugs
May stimulate the female (cryptic female choice)
May aid the male in intromition, providing sensory feedback
to the male
Adaptive function?
Males without spines took longer to intromit and showed an
increased duration of intra-vaginal thrusting
Could have been a result of chemical interaction from the
depilatory cream
Experimental removal of penile spines with depilatory cream
Prosimians tend to have spines, and larger ones than anthropoids
Penile Morphology RTL
Penis is distally concave ("plunger-shaped")
Shaped to help displace copulatory plugs via suction/negative
Shape may be functionally adaptive
Penile Morphology in Pan
Long, thin, tapering at the end
No other primate species have this penile shape
Chimps and bonobos have a filiform penis
Penis may pierce through other male's sperm plugs (Dixson
Allows for deep penetration when females have sexual
Hypothesized that the shape is functionally adaptive
Rhesus macaques film
Macaca mulatta
Male dispersal as a mating strategy
Function in inbreeding avoidance
Choice for novel males
Female mate choice
Consortships,increased affliative behaviours
Sneak copulation as a mating strategy
Hip-push invitations to mate
Sexual signals
Indication of sperm competition
Polygynandrous mating system
Large relative testes size
Clutching reaction (orgasm?)
Lip smacking
Looking back and making eye contact
with males
Female sexual behaviour
Genital Morphology
1:06 PM
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