The Comparative Approach

• 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

Allometry• 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 Contrasts

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

• Assumes random change, independent changes in different branches

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

Method of Maximum Likelihood• Discrete variables

–E.g., duetting and monogamy• 1st model: State changes in two variables

are independent (a)• 2nd model: State changes are

interdependent (b)• Can find most likely direction, order

Hormones and behavior

Hormones• 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-pituitary-

gonadal (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 Oxytocin 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 Male– 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 Females

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 H P G

• 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

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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 effects• 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 doves• 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