Transcript Evolution of brain and behaviour

Sex: Evolutionary, Hormonal and
Neural Bases
Chapter 12
Why Sex?
-sex is not necessary for reproduction
-fission: reproduction by simple cell division
(unicellular organisms
-parthenogenesis: reproduction by
multicellular duplication (‘cloning’)
Asexual reproduction in human
beings
The Argument Against Sex
1. Biologically, the purpose of an individual is to
propel a copy of its genes into the next
generation
2. With asexual reproduction, 100% of one’s
genes are propagated.
3. With sexual reproduction, 50% of one’s genes
are propagated.
4. Sex is dangerous and expensive
What good is sex?
The Red Queen Hypothesis
1. Sometimes a trait that is adaptive for one generation is
NOT for the next
2. Sexual reproduction could promote rapid change in traits
through reshuffling of genes
3. One likely requirement for rapidly changing adaptation has
to do with developing resistance to parasites
4. Parasites like bacteria can adapt very rapidly compared to
us because they have short lifespans
EXAMPLE:
Penicillin was introduced in 1943.
By 1946, 16% of staph strains were resistant to penicillin
Today, about 90% are resistant
Sex kills germs
5. Sexual reproduction may help us to stay afloat in
the race against pathogens
EVIDENCE:
Some animals can reproduce
sexually or asexually
In a species of freshwater snail,
the rate of sexual reproduction in
a population (I.e. the # of males)
correlates with the incidence of a
kind of parasite (trematodes)
Why have two sexes?
Directional selection
frequency
Amount of some kind of trait
Why have two sexes?
Disruptive selection
frequency
Amount of some kind of trait
Sperm and Ova
These two types of sex cells likely represent two
extremes resulting from disruptive selection
Sperm: many, small, cheap and easy to produce
but the likelihood of each one producing an
offspring is very low
Ova: few, large, expensive to produce but the
likelihood of each one producing an offspring is
higher
Sexual selection
Many of the structural and behavioural differences
between males and females follow naturally from the
differences between male and female sex cells
All other considerations aside, the optimal mating
strategy for a male is to distribute sex cells as widely
as possible (male competition)
All other considerations aside, the optimal mating
strategy for a female is to select the highest quality
mates that are available (female choice)
Sex differences in physical
appearance
The challenge for the male is to be chosen
-males compete against one another for access to females
(sex differences in physical size) and to be chosen by females
(sexual display and the handicap principle)
The challenge for the female is to choose wisely
-females need to assess the fitness of the male – will their
male offspring compete successfully? The function of
courtship.
Parental investment
Males
Because males have enormous capacities for producing
and distributing sex cells, the optimal strategy is to end
relationship with offspring quickly
Females
Because females have much lower capacity for producing
offspring, the optimal strategy is to maintain relationshp
with offspring until they have a reasonable chance of
survival
Mating systems
1. Promiscuity - males mate with more than one
female, females mate with more than one male
2. Polygyny – one male mates with a group of
females in a long-standing relationship (elephant
seals, gorillas)
3. Polyandry- one female mates with a group of
males in a long-standing relationship (jacana)
4. Monogamy – one female mates with one male in a
long-standing relationship
How we all stack up
Does this all apply to us?
Sociobiology and ‘is vs. ought’
Sexual selection: physical and behavioural
differences between men and women in
relationships
Mating systems: follow ecology, more likely to
find monogamy or polyandry in sparse
environments (eg. Polyandry in Northern Tibet)
Sexual behaviour
The four stages of reproductive
behaviour
1. Sexual attraction
-this is what is necessary to bring the two sexes
together
-could include appearance, movements, even
construction of artifacts (leks)
Stayin’
alive….
Sage grouse courtship dance
Kakapo leks
2. The appetitive stage
-behaviours that establish, maintain, or promote sexual interaction
-females engaging in such behaviours are said to be proceptive
-proceptive behaviours often include movements, postures,
vocalizations
-male appetitive behaviour is most often maintaining proximity
3. Copulation
Usually involves a series of penetrations (intromissions),
followed by ejaculation
Refractory phase and the Coolidge effect
4. Postcopulatory behaviour
Quite varied, can range from simple quiescence and
continued maintenance of proximity to the more dramatic
‘copulatory lock’
Rat sexual behaviour
•Sexual attraction is largely olfactory
•proceptivity includes ‘bunny hops’ and ear wiggles
•copulation involves a series of extremely short
intromissions followed by ejaculation
•females exhibit lordosis (arched back and tail moved to
side)
•postcopulatory behaviour - self-grooming and the
postejaculatory song
•no copulatory lock but there is a copulatory plug
•testosterone is important to male response
•female receptivity requires estrogen and progesterone
•carrying pups to term requires prolactin
Neural mechanisms of sexual
behaviour - males
Neural mechanisms of sexual
behaviour - females
Sexual differentiation
“Nature’s impulse”
Pathways to sexual
differentiation
Contrasting organizational and
activational effects of hormones
Organizational effects: Structural changes that typically
take place during development and which are usually
irreversible.
Activational effects: Transient effects of hormones on
systems, usually in the mature organism (in brains,
much like neurotransmitters).
Organizational and activational
effects of hormones on sexual
behaviour in rats
• Male rats castrated at birth show female receptive
behaviour as adults if primed with estrogen and
progesterone
• Female rats treated with testosterone around birth will
NOT show female receptive behaviour as adults regardless
of the hormonal milieu, but will mount other females and
even achieve intromission and a kind of ejaculation
• Testosterone has an organizational effect on the behaviours
(and presumably brains) of young rats.
Human sexual behaviour
Notoriously difficult to study
-Kinsey’s pioneering studies
-Watson’s disastrously misguided efforts
-Masters and Johnson
Human sexual behaviour is wildly diverse
-we are the only animals to use variety in sexual postures
Hormones play a permissive role in
our sexual behaviour
• There must be some testosterone for male sexual
behaviour to occur, but there is no correlation
between testosterone levels and sexual interest or
activity
• There have been many attempts to find differences
in sexual interest or activity in women depending
on hormonal milieu (eg. During different times in
the menstrual cycle), but no convincing effects
have emerged
Things you may be interested in
reading about, but which won’t be
on the final exam
• Pathologies of sexual differentiation (pp.
384-387)
• The aromatization hypothesis (pp. 389-390)
• Sexual differentiation in the spotted hyena
(p. 391)
• Social influences on sexual differentiation
(p. 394)
• Guevedoces (p. 395)