L3 sexual differentiation

Sexual dimorphism

Differences in body structure or behaviour between males and females of the same species

Sexual selection

A form of natural selection based on differences in reproductive success

Why sexual selection occurs

Males and females invest differently in reproduction, creating different reproductive strategies and behavioural pressures

Why eggs and sperm create different strategies

Eggs are energetically expensive while sperm are cheap, so females are often more selective and males often compete for mating opportunities

Intrasexual selection

Competition within one sex (usually male-male competition) for access to mates

Gorilla example of intrasexual selection

Large male gorillas win fights, control females and reproduce more, so evolution selects for huge male size

Intersexual selection

Mate choice, usually females selecting males with attractive traits

Peacock tail example

Females prefer elaborate male tails, so males with larger tails reproduce more successfully despite survival costs

Honest signals

Costly traits that reliably indicate fitness because weak individuals cannot easily fake them

Examples of honest signals

Bright feathers, antlers, songs and large body size

Sexual conflict (“battle of the sexes”)

Situation where optimal reproductive strategies differ between males and females

Example of sexual conflict

Males may benefit from mating with many females while females benefit from choosing carefully

Mate guarding

Behaviour where a male stays near a female after mating to reduce mating by rival males

Monogamy

Mating system in which one male pairs with one female

Why monogamy evolves

Offspring survival may improve when both parents provide care

Polygyny

Mating system in which one male mates with multiple females

Why polygyny often causes strong sexual dimorphism

Male reproductive success depends strongly on defeating rival males, selecting for larger and more aggressive males

Secondary sexual traits

Traits not directly involved in fertilization but important for mating success, such as antlers, bright colours and songs

Ultimate explanation

Explains WHY a trait evolved by natural or sexual selection

Example of ultimate explanation

Male gorillas evolved large body size because larger males reproduce more successfully

Proximate explanation

Explains HOW a trait or behaviour is produced biologically through hormones, genes or neural mechanisms

Example of proximate explanation

Testosterone promotes muscle growth and aggressive behaviour in male gorillas

Key behavioural endocrinology principle

Behavioural endocrinology integrates ultimate and proximate explanations of behaviour

Costs of sexual reproduction

Requires finding mates, competition, courtship and only half of genes are passed to offspring

Benefit of sexual reproduction

Produces genetic diversity, increasing adaptability to disease and environmental change

Risk of asexual reproduction

Low genetic diversity can make populations vulnerable to disease or environmental change

Asexual reproduction

Reproduction without combining gametes from two parents

Hermaphrodite

Organism with both male and female reproductive capacity

Simultaneous hermaphrodite

Organism functioning as both sexes at the same time

Sequential hermaphrodite

Organism that changes sex during life

Clownfish example

Largest clownfish becomes female; if she dies, the dominant male changes into a female

Key lesson from clownfish

Social environment can influence endocrine state and sexual differentiation

Chromosomal sex

Sex determined by chromosomes (e.g. XX or XY)

Gonadal sex

Sex determined by gonads (testes or ovaries)

Gametic sex

Sex defined by the type of gametes produced (sperm or eggs)

Hormonal sex

Sex defined by predominant hormone profile

Morphological sex

Sex defined by body anatomy and genitalia

Behavioural sex

Sex-typical behavioural patterns

Key concept about sex from the lecture

Biological sex is multidimensional and not determined by chromosomes alone

SRY

Sex-determining gene on the Y chromosome initiating testes development

SOX9

Gene activated by SRY that promotes testes formation

Default mammalian developmental pathway

Female development occurs in the absence of SRY/SOX9-driven masculinization

Sertoli cells

Cells in the testes that produce MIH/AMH

MIH / AMH (Müllerian inhibiting hormone / Anti-Müllerian hormone)

Hormone causing Müllerian ducts to regress during male development

Leydig cells

Cells in the testes that produce testosterone

Müllerian ducts

Embryonic ducts that can develop into female internal reproductive organs

Wolffian ducts

Embryonic ducts maintained by testosterone that develop into male internal reproductive organs

Role of testosterone in development

Maintains Wolffian ducts and promotes male internal reproductive structures

DHT (5α-dihydrotestosterone)

Potent androgen derived from testosterone that masculinizes external genitalia

5α-reductase

Enzyme converting testosterone into DHT

Role of DHT

Masculinizes penis, scrotum and prostate

Important distinction between testosterone and DHT

Testosterone mainly masculinizes internal reproductive structures while DHT masculinizes external genitalia

Accessory sex organs

Reproductive organs developing from Wolffian or Müllerian duct systems

Bird sex chromosomes

Females are ZW and males are ZZ

Female heterogametic sex

In birds females possess two different sex chromosomes (ZW)

Environmental sex determination

Sex differentiation influenced by environmental conditions rather than chromosomes alone

Reptile temperature-dependent sex determination

Incubation temperature influences hormone balance and sexual differentiation

Aromatase

Enzyme converting testosterone into estradiol

How temperature influences reptile sex differentiation

Temperature affects aromatase activity, altering testosterone/estradiol balance during development

Important concept about hormones

Hormone effects depend on species, tissue, developmental timing and receptor distribution

Organizational effects

Permanent developmental effects of hormones on the nervous system and body

Activational effects

Temporary hormone effects activating neural circuits that were organized earlier

Important distinction between organizational and activational effects

Early hormones organize circuits permanently, while adult hormones activate existing circuits

Masculinization

Development of male-typical neural circuits, anatomy or behaviour

Defeminization

Suppression or prevention of female-typical neural circuits or behaviour

Important concept about masculinization and defeminization

They are related but separate developmental processes

Rodent brain masculinization

In rodents testosterone is converted into estradiol in the brain, which masculinizes neural circuits

Why estradiol masculinizes the rodent brain

Estradiol activates masculinizing developmental pathways in the developing rodent brain

α-fetoprotein

Protein in female fetuses that binds estrogens and protects the brain from masculinization

Why α-fetoprotein is important

Prevents maternal estrogens from entering and masculinizing the female fetal brain

Dimorphic behaviour

Behaviour differing between sexes, not necessarily directly related to mating

Dog urinary posture experiment

Early testosterone exposure permanently altered adult urinary posture, supporting organizational effects of hormones

Conclusion of dog urinary posture experiment

Hormones during development permanently organize sexually dimorphic behaviour

Squirrel dispersal experiment

Early hormone exposure altered male/female dispersal behaviour, supporting organizational effects

Main conclusion from squirrel experiment

Sex differences in behaviour can be permanently organized early in development

Sexual behaviour

Behaviours associated with reproduction and mating

Appetitive behaviour

Sexual motivation and searching for mates

Consummatory behaviour

Execution of sexual acts during mating

Consummatory sequence in male sexual behaviour

Mounting → intromission → ejaculation

Preoptic area (POA)

Brain region important for male sexual motivation and copulatory behaviour

Sexually dimorphic brain regions

Brain regions differing structurally or functionally between males and females

SDN-POA

Sexually dimorphic nucleus of the preoptic area involved in male sexual behaviour

BNST

Bed nucleus of the stria terminalis involved in reproductive and social behaviour

AVPV

Hypothalamic nucleus often larger in females and involved in reproductive endocrine regulation

Medial amygdala

Brain region involved in social and reproductive behaviours

Olfactory bulbs and vomeronasal organ

Structures involved in detecting pheromonal and reproductive cues

Japanese quail

Model species for studying avian sexual behaviour

Zebra finch example

Hormones influence development of sexually dimorphic bird song systems

Testosterone and adult male sexual behaviour

Testosterone activates male sexual behaviour if neural circuits were organized earlier during development

Why adult testosterone alone is insufficient

Activational hormones cannot fully create neural circuits that were not organized during development

Key behavioural endocrinology chain from the lecture

Evolution and environment influence hormones, hormones organize the brain and body, and the organized brain produces sexually dimorphic behaviour

Most important overarching idea of the lecture

Sex differences in behaviour arise through interactions between evolution, genes, hormones, brain development and environme