Sexual Differentiation & Sex Differences

Intra- (prefix)

Within (Latin).

Auto- (prefix)

Self.

Para- (prefix)

Beside or alongside.

Endo- (prefix)

Within (Greek).

Ecto- (prefix)

Outside of (Greek).

Krínō (Ancient Greek root of -crine)

To separate; also to secrete.

Sex

Based in biological sexed traits.

Gender

Emerges from social constructions and is associated with personal identity.

Butler's definition of gender (Gender Trouble)

Gender is the repeated stylization of the body; a set of repeated acts within a rigid regulatory frame that congeal over time to produce the appearance of a natural sort of being.

Gender as performance (Butler)

Gender is always a doing; it constitutes the identity it is purported to be; though not a doing by a subject who pre-exists the deed.

Amin on gender categories (We are all nonbinary)

Gender categories (trans; cis; nonbinary; binary) are social and interpersonal and not individual; which is what makes them meaningful.

Cisgender

A term for human gender variation (listed on the slide among categories of gender and sex variation).

Transgender

A term for human gender variation (listed on the slide among categories of gender and sex variation).

Non-binary

A term for human gender variation (listed on the slide among categories of gender and sex variation).

Agender

A term for human gender variation (listed on the slide among categories of gender and sex variation).

Transsexual

A term for human sex variation (listed on the slide among categories of gender and sex variation).

Intersex

A term for human sex variation (listed on the slide among categories of gender and sex variation).

Terms used for non-human animals (Goymann et al. 2023)

Biological sex (makes ovum or sperm); sex-typed biology; and sex-typed behavior.

Biological sex (in animals)

Makes ovum or sperm.

Non-human animal gender

The lecturer (and many biologists) argue it does not make sense or add anything meaningful to talk about gender in non-human animals.

Discussion question on measuring sex

How would you measure an organism's sex?

US government definition of sex (2025 executive order)

Sex refers to an individual's immutable biological classification as either male or female and is not a synonym for gender identity.

US government definition of female (White House 2025)

A person belonging at conception to the sex that produces the large reproductive cell.

US government definition of male (White House 2025)

A person belonging at conception to the sex that produces the small reproductive cell.

Binary vs. bimodal

Height is not binary; it is bimodal with overlapping values; the same framing is applied to sex.

Anisogamy

Two dramatically different gamete sizes in a sexually reproducing species (small and large).

Isogamy

Sexual reproduction with similarly sized gametes.

Evolution of anisogamy

Hypothesis that anisogamy evolved independently from isogamy in eukaryotes several times.

Anisogamy in animals

All animals are anisogamous.

Isogamous vs. anisogamous

Isogamous: sex but no sexes. Anisogamous: sex and 2 sexes.

Chromosomal sex

The sex chromosomes (X and Y) make up one of the 23 pairs of chromosomes in humans; the Y chromosome is largely responsible for downstream development of male-sexed traits.

Variations in sex chromosomes

Examples include 46 XX; 46 XY; 47 XXY; and 45 X; chromosomes are uniform throughout the body in these cases.

Chromosomal mosaicism

Different cells in the body have different chromosome makeups (e.g. from Y loss); sometimes but not always associated with downstream differences in sexed traits.

Organizational hormone effects

Long-lasting hormone effects that take place during critical periods in development.

Activational hormone effects

Short-lasting hormone effects that activate the circuits organized by hormones in development.

Gonadal differentiation

The basic anatomical reproductive system develops during gestation (before a fetus is born).

SRY gene

Gene on the Y chromosome responsible for the development of the male reproductive system.

Female reproductive system development

In the absence of hormones (and no SRY) the female reproductive system develops.

Factors that modulate puberty onset

Circadian cycle; season; stress; metabolic factors; pheromones; and feedback from current state.

Puberty onset research

An ongoing area of research (e.g. studies of Mamandur India and studies adjusted for socioeconomic status).

GnRH and puberty onset

The onset of puberty requires both a GnRH surge and a pulsatile release of GnRH.

Spermatogenesis

Production of sperm cells; begins during puberty.

Egg cell

A cell that is competent to further develop when fertilized; egg cells do not reach this state until puberty.

Anatomical sex (organizational effects)

Irreversible effects such as elongation of the larynx and broadening of the pelvis.

Anatomical sex (activational effects)

Effects dependent on hormone levels such as development of musculature.

Anatomical sex (both organizational and activational)

Breast tissue and genital tissue can change in size depending on sex hormone levels; and fat distribution sometimes changes with sex hormone levels and sometimes not.

Plasticity of vaginal tissue (Xu et al. 2022)

Vaginal tissue has androgen receptors; prostatic cells grew in vaginal tissue exposed to testosterone therapy (in 100% of the 22 analyzed).

Breast development in transgender women on hormone therapy

Begins around 3-6 months with maximum growth taking 2-3 years; develops ducts; lobules; and acini similar to biological women; influenced by age of initiation and adolescent stage.

Five features used to study sex differences

Chromosomes; genital development; gonadal development; gametogenesis; and secondary sexual characteristics.

Intersex biology in humans

Many documented forms exist (e.g. androgen insensitivity syndrome; congenital adrenal hyperplasia; Klinefelter syndrome; Turner syndrome; 5-alpha reductase deficiency).

Why sex is not strictly binary

Sex determination is increasingly seen as a flexible and multiply determined system with environmental sensitivity and rapid evolutionary change and not a linear cascade with a binary switch.

Sex-related phenotypes

They are not generated by a single conserved developmental program.

White-throated sparrow plumage polymorphism

Two plumage morphs exist in both sexes: white-striped (WS) and tan-striped (TS); Audubon had mislabeled morphs as males and females.

Disassortative mating

Females seek a different sex AND a different morph.

White-throated sparrow breeding types

4 breeding types (WS male; WS female; TS male; TS female) forming 2 pair types (WS male x TS female and TS male x WS female).

White-throated sparrow singing patterns

Males sing more than females overall; within sex WS birds sing more than TS birds.

WS male x TS female singing

Males sing a lot more than females.

WS female x TS male singing

Females sing slightly more than males.