WS and TS birds

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.

ZAL2 and ZAL2m

ZAL2 is the standard form of autosome #2 (a non-sex chromosome) and ZAL2m is the rearranged version found in the white-striped morph.

TS birds genotype

ZAL2/ZAL2 homozygous.

WS birds genotype

ZAL2/ZAL2m heterozygous.

Offspring of a WS x TS pair

50% homozygous (TS) and 50% heterozygous (WS).

WS x WS pairs

About 0.8% of breeding pairs; roughly 1 in 500 offspring are ZAL2m/ZAL2m homozygotes.

Horton et al. 2013 finding

A bright aggressive hatch-year female (a ZAL2m homozygote) was the most aggressive bird tested; suggesting the ZAL2m chromosome confers an aggressive phenotype.

Genes in the ZAL2m rearranged region

Parts of the structures of LH and FSH; LH and FSH receptors; 2 serotonin receptors; the enzyme that converts testosterone to DHT; estrogen receptor 1 (ESR1); and vasoactive intestinal peptide (VIP).

ESR1

Estrogen receptor 1; widely expressed in the brain and related to aggression in songbirds.

VIP (vasoactive intestinal peptide)

A neuromodulator important in both aggression and parenting behavior in songbirds.

ESR1 mRNA in the medial amygdala

Expression differed so much between TS and WS birds that it could identify the morph; the difference was present by day 7 post-hatching and in the non-breeding season so it was not caused by behavior.

ESR1 as a mediator

Statistically ESR1 expression mediated the relationship between morph and song rate.

Behavior differences vs. morph

Differences in aggressive behavior and singing were NOT large enough to identify the morph by that trait alone.

Causal evidence for ESR1 (Merritt et al. 2018 and 2020)

Exogenous estradiol increased aggression in WS but not TS birds; knocking down ESR1 mRNA in WS birds to TS levels removed the relationship between estradiol and aggression.

VIP in the anterior hypothalamus

VIP gene expression there mediated the relationship between morph and aggressive behaviors and song production.

Life history strategy tradeoff

Birds trade off between parenting behaviors (e.g. provisioning) and mate-access behaviors (e.g. territorial aggression).

TS female behavior

Basically no singing and high provisioning.

WS male behavior

Sings a lot with low provisioning.

TS male behavior

Low levels of singing and high provisioning.

WS female behavior

Sings more than a TS male with low provisioning.

VIP pathway to parenting

A hypothalamic neuron releases VIP; VIP stimulates the anterior pituitary to release prolactin (PRL) into the body; prolactin promotes parental provisioning.

VIP expression and morph

TS birds have more VIP expression in the hypothalamus than WS birds.

VIP and provisioning

VIP expression predicted parental provisioning in white-throated sparrows even controlling for morph.

Prolactin and provisioning

Administration of prolactin in zebra finches stimulates parental provisioning.

Medial pre-optic area

Part of the hypothalamus associated with parenting behavior in birds; has many prolactin receptors and many estrogen receptors (ESR1).

ESR1 in the hypothalamus vs. amygdala

TS birds have lower ESR1 expression in the amygdala (singing and territorial behavior) but higher ESR1 expression in the hypothalamus (provisioning and parental behavior).

Hypothesis for ESR1 differences

Driven by changes in genetic regulation of ESR1 expression due to differences in the 2m chromosome.

2m chromosome and SRY analogy

Like the Y chromosome with SRY leading to typically male development; the 2m chromosome codes for different amounts of estradiol receptors (ESR1) in different brain regions and different amounts of VIP neurons.

Why white-throated sparrows are said to have 4 sexes

Substantial genetic variation exists by chromosomal sex AND by chromosomal morph (2/2 or 2/2m); looking at only chromosomal sex would miss half the story.

Studying life history tradeoffs in sparrows

Both sex and morph must be studied.

Traits typically sex-related in sparrows

They are being sorted by something other than sex.

Lesson of the sparrow work (big idea)

It does not break a sex binary; it illustrates how we can move past binary categories and toward understanding mechanism.

Approach to finding mechanism (big idea)

When a morph difference in physiology corresponded with a morph difference in behavior researchers looked for predictive relationships between the continuous physiological variable and the continuous behavioral variable while holding morph constant.

Sex and gender as variables (Pape et al. 2024; Maney et al. 2025)

They are often included in analyses as explanatory variables but do not themselves constitute concrete measurable features capable of directly influencing outcomes.

ESR1 mRNA in the medial amygdala (conclusion)

Both necessary and sufficient to explain the behavioral difference between morphs.