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 Biologically, the purpose of an individual is to propel a copy of its genes into the next generation With asexual reproduction, 100% of one’s genes are propagated. With sexual reproduction, 50% of one’s genes are propagated. 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 Promiscuity - males mate with more than one female, females mate with more than one male Polygyny – one male mates with a group of females in a long-standing relationship (elephant seals, gorillas) Polyandry- one female mates with a group of males in a long-standing relationship (jacana) 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…. Kakapo leks Sage grouse courtship dance

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)