The Comparative Approach • Comparing traits among species • Without phylogeny


The Comparative


• Comparing traits among species

• Without phylogeny

– Relate behavior to ecological factors across spp. or populations

• E.g., Eggshell removal

– Problems

• Causation?

• “Cherry picking” examples that support hypotheses

• Confounds, like size, phylogeny


• Body size is an important confound in comparative studies

• Scaling one body part against another is tricky

• Allometry is the study of the relationship between body measurements

• log(Y)= b log (X) + log (a)

• Slope (b) > 1 means Y increases faster than X

– “positive allometry”

• Comparing residuals is informative

Controlling for phylogeny

• Phylogenetic inertia

• Homology

– Common descent

• Homoplasy

– Convergence

• Determining ancestral characters

– Maximum parsimony

• Problem of equal parsimony

Method of Independent


• Looks for relationship between two continuous variables while controlling for phylogeny


• Assumes random change, independent changes in different branches

Is evolution of x correlated with evolution of y (and if so, how)?

Method of Maximum


• Discrete variables

– E.g., duetting and monogamy

• 1 st model: State changes in two variables are independent (a)

• 2 nd model: State changes are interdependent (b)

• Can find most likely direction, order

Hormones and behavior

• Chemical messengers secreted in one part of an organism that affect a relatively distant part of that organism

– Work in conjunction with neurotransmitters

– Work in concert with nervous system to control behaviour

• Relative to nervous system, slower and more general


Feedback mechanisms

• Negative feedback

– Like a thermostat

– The male hypothalamic-pituitarygonadal (HPG) axis

• Gonadotropin (GnRH) in hypothalamus

 follicle-stimulating hormone (FSH) and leuteinizing hormone (LH) in pituitary

 sperm and testosterone (T) in testes

 rising T reduces output of GnRH

• Positive feedback

– Oxytocin release during labor

• Pressure on cervix

 release


Stronger contractions…

Transport and target cells

• Transported in the blood

• Affect remote cells by binding dynamically to receptor molecules

– Protein hormones bind to surface receptors

• Rapidly alter cellular behavior

– Steroids (lipid hormones) pass through the membrane, bind to a receptor and affect transcription

• Generally slow, but fast-acting steroids challenge model

– Mountain chickadees and CORT

Modulating hormone activity

• Hormone levels in blood

• Binding protein concentration

• Receptor density

─ Up- / down-regulation in response to concentration

─ Pulsatile hormone release limits receptor regulation

• Hormone conversion by enzymes

• T  oestrodiol in brain by P450 aromatase

• Chaperones that modify effects of hormones on receptors

Hormones & the brain

• Radioactively labeled sex steroids accumulate at similar brain regions in rats, frogs, and chaffinches

• Preoptic, limbic, & hypothalamus

• Neural firing rates correspond to hormone presence

• T injected directly into male mouse brains

• Median preoptic area

+ vocalization

+ urine marking

+ mounting

• Hypothalamus

+ urine marking

• Other regions

No increase in sexual behaviour

Behavior and environment affect hormone levels

• Hormone secretion is dynamic

– Responds to environmental cues

• The “challenge hypothesis”

– Male green tree frogs

• Human males, coin flips, and T

Chromosomal sex determination

• Gonads (testes and ovaries) develop from bipotential tissues

– The gonads mediate further differentiation

• In most mammals

– XX 

Female, XY


– SRY region on Y is major gene for sex determination

• SRY product leads to a cascade that results in testes

– Otherwise, ovaries

• Other genes on Y involved in spermatogenesis

• Snakes, birds, and some lizards and turtles

– ZZ 

Males, ZW


Environmental sex determination

• Incubation temperature influences the expression of cytochrome-P450

• Cytochrome-P450 converts testosterone and androstenidone into oestrogen hormones

• The amount oestrogens in the gonad directly influence differentiation

Establishment of the HPG

• Critical regulatory pathway

– Growth, stress, sexual behavior, etc.

• Generally, cascade runs




• Develops in reverse

– Lower levels control development of higher levels

• Brains pretty well shuts down

HPG in infancy

• HPG kicks in at puberty

Organizational and

Activational effects

• As they relate to sex and the brain

–Organizational effects of hormones in early life differentiate male and female brains

–Activational effects later in life facilitate expression of sex-specific behaviours

Mammalian examples

• SDN-POA in hypothalamus is up to 5 x larger in males

– Lesions disrupt copulatory behavior in males

– Lesions + female sex hormones cause males to exhibit lordosis

• Dominant female hyenas pass more androgens to their offspring

– Early androgens  aggression

– Masculanization of females pub/web/716_web.jpg

Hormones affect developmental plasticity

• Tree lizards exhibit permanent organizational effects and reversible activational effects

– Males plain orange dewlap are non-territorial

– Males with orange dewlap & blue spot are territorial

• Higher T and progesterone as juveniles

– Critical period

• Adrenal origin suggests association with stress

– Activational effects on spotless males

• When stressed, they go nomadic

– Low T, high Cort

• When not stressed, they are sedentary

– High T, low Cort

• Can go back and forth

Hormones and maternal

• Stress in rats

– Daughters of mothers stressed during pregnancy secrete more Cort when stressed response relative to controls

• Their HPA axis has been sensitized during development

• Potentially adaptive maternal effects in birds

– T helps male offspring grow faster


Hormones and sibling effects

• Embryonic rats are exposed to their sibs hormones

– Females b/t males mount more, have different genital structure

– Males b/t females are more active, less sexual, and respond with less aggression to T injections in adulthood

Parental care in ring

• T stimulates male to court

– Interacting with female increases his T


• Courtship stimulates female to release FSH, stimulating follicle development

– Female’s own “coos” necessary

• Interacting with the nest stimulates progesterone in females

• Increasing LH stimulates female to lay

• Progesterone maintains incubation in both sexes

• Incubation stimulates secretion of prolactin

– Inhibits FSH and LH

– Stimulates crop milk production

• Prolactin decreases while feeding young

– Allows FSH and LH to rebound for next mating