Comprehensive Biology and Endocrinology: Evolution, Neural Development, and Hormonal Communication

Natural Selection

Evolution proceeds by differential success in reproduction

Darwin's Hypothesis

Reproduction will increase a population rapidly unless factors limit it, individuals are not identical, some variation is inherited, and not all offspring survive

Peacock Problem

Why do male peacocks have large elaborate tails; explained by sexual selection

Convergent Evolution

Similarities in behaviors or structure among unrelated animals due to adaptations to similar environments

Homology

Resemblance or structural similarity based on common ancestry

Analogy

Similar physical traits or functions that evolved independently in unrelated species, not inherited from a common ancestor

Mutations

Spontaneous changes in genes; a rapid form of evolution

Phylogeny

The evolutionary history of a particular group of organisms, represented by a family tree

Ecological Niches

Evolved specific behaviors and neural mechanisms that allow animals to exploit specific sets of environmental conditions and opportunities

Complicated Lives Require Complicated Brains

Animals with more complex environments and social lives evolve more complex neural mechanisms

Outstanding Features

Some species have extreme traits linked to specialized neural structures (ex. owls have great auditory features)

Convenience

Lab rats are fast breeders, cheap, and have short life spans

Comparison

Studying different species allows comparison of brain and behavior

Preservation

Studying species helps with conservation

Economic Importance

Some species have direct economic relevance

Treatment of Disease

Animal models help develop treatments

Curiosity

Scientific interest drives study of certain species

Neural Tube

Develops three subdivisions: forebrain, midbrain, and hindbrain

Forebrain

Develops into the telencephalon and diencephalon

Telencephalon

Part of the forebrain that develops from the neural tube

Diencephalon

Part of the forebrain that develops from the neural tube

Mesencephalon

The midbrain division of the neural tube

Metencephalon

Hindbrain subdivision that becomes the cerebellum and pons

Myelencephalon

Hindbrain subdivision that becomes the medulla

Cerebellum

Part of the brainstem derived from the metencephalon

Pons

Part of the brainstem derived from the metencephalon

Medulla

Part of the brainstem derived from the myelencephalon

Brainstem

Refers collectively to the cerebellum, pons, and medulla

Advantages of Bigger Brains

Increased survival, ability for group interaction, innovative behavior, success in new environments

Costs of Bigger Brains

Long gestation period, prolonged dependence on parents, high metabolic cost, complex genes vulnerable to mutation

Behavioral Endocrinology

The study of the relationship between hormones and behavior

Hormones

Chemicals secreted by one cell group that travel through the bloodstream to act on target tissues to produce physiological effects

Endocrine Glands

Release hormones within the body

Exocrine Glands

Use ducts to secrete fluids such as tears or sweat outside the body

Endocrine Communication

A hormone is released into the bloodstream to act on target tissues

Autocrine Communication

A released chemical acts on the releasing cell itself; involved in early development, regulates pain and inflammation, can perform apoptosis

Paracrine Communication

Released chemicals diffuse to nearby target cells; elicits quick responses that last a short amount of time; involves neurotransmitters and hormones

Allomone Communication

Chemicals released by one species to affect the behavior of another species

Pheromone Communication

Hormones released into the environment to communicate between members of the same species

Neuroendocrine Cells

Found in the hypothalamus and pituitary gland

Gradual Action

Hormones act in a gradual fashion

Probability of Behavior

Hormones act by changing the probability or intensity of a behavior, not causing it directly

Bidirectional Relationship

Hormones change behavior, and behaviors change hormones

Different Effects on Different Cells

A hormone can have different effects on different cells, organs, and behaviors

Small Amounts

Hormones are produced in small amounts

Rhythmic Variation

Hormone levels vary rhythmically throughout the day

Hormone Interactions

Effects on one hormone can influence those of another

Receptor Requirement

Hormones can only affect cells with a receptor protein for that hormone

Protein/Peptide Hormones

A string of amino acids

ACTH (Adrenocorticotropic Hormone)

Stimulates the adrenal cortex

Prolactin

Stimulates milk production

Insulin

Stimulates glycogen formation and storage

Oxytocin

Stimulates contraction of uterine muscles and release of milk

Vasopressin

Stimulates water reabsorption by the kidneys and constriction of blood vessels

Amine Hormones

Modified amino acids; also called monoamine hormones

Epinephrine

Prepares the body for action

Norepinephrine

Prepares the body for action

Melatonin

Regulates seasonal changes and puberty

Steroid Hormones

Composed of 4 rings of carbon atoms

Estrogens

A steroid hormone

Progestins

A steroid hormone

Androgens (Testosterone)

A steroid hormone

Glucocorticoids (Cortisol)

A steroid hormone

Sex

Typically biological, determined by a combination of genes and hormones

Gender

The social expression of masculinity and femininity

Sexual Differentiation

The process by which individuals develop bodies, brains, and behaviors that are either XY-typical or XX-typical; begins before birth and continues into adulthood

Autosomes

The 22 non-sex chromosome pairs humans have

Sex Chromosomes

The 23rd pair of chromosomes that determines biological sex

SRY Gene

Produces the Sry protein which signals to "make testes" and inhibits X chromosome from promoting ovary development

Indifferent Gonads

The developing fetus contains tissues capable of making either testes or ovaries before hormonal influence

Gonadal Sex

The presence of either ovaries in females or testes in males

Gonadal Hormones (Male)

Testosterone

Default Sex (Without Hormones)

Without hormonal influence ovaries develop and testes tissue disintegrates; XX is the default

With Hormonal Influence

Testes develop and ovary tissue disintegrates

Gonads (Female)

Ovaries

Gonadal Hormones (Female)

Not active until puberty

XXY

Klinefelter Syndrome; typically XY appearance, less body hair, wide hips, long arms, infertile

XYY Syndrome

XY appearance, low intelligence and fertility, increased criminal activity

Turner Syndrome (X only)

Short stature, lack of ovaries

XXX Syndrome

XX appearance, low fertility and verbal skills

Androgen Insensitivity Syndrome

SRY gene leads to testes development but no functional receptors for hormones; external genitalia reverts to XX default; no ovaries, no menstruation, infertile

5-alpha Reductase Deficiency

Genetic mutation preventing function of the enzyme; early life resembles XX external genitalia but at puberty develops into typical XY genitalia

Organizational Hormones

Steroid hormone actions in early life that permanently and irreversibly shape neural structure, reproductive physiology, and behavior

Activational Hormones

Hormones that transiently promote certain behavioral effects; during puberty hormone production increases and activates brain structures organized during development

Critical Period

Early developmental timeframe where androgens and estrogens permanently shape neural structure and behavior

Sexual Attraction

Brings males and females together, synchronized with physiological readiness; may involve learned associations such as appearance

Appetitive Behaviors

Behaviors that establish, maintain, or promote sexual interaction; species specific

Copulation

One or more intromissions in which the male penis is inserted into the female vagina; followed by ejaculation

Intromission

Insertion of the male penis into the female vagina during copulation

Ejaculation

Release of semen following stimulation during copulation

Postcopulatory Behavior

Includes a refractory phase, possible copulatory lock, and parental behaviors

Refractory Phase

Period following copulation where mating does not occur; length varies by species

Copulatory Lock

Penis swells temporarily and cannot be withdrawn from the female

Parental Behaviors

Postcopulatory behaviors to nurture offspring

Sexual Receptivity

If a female is willing to copulate she is considered sexually receptive

Lordosis

Posture females adopt that allows intromission when stimulated properly

VMH (Ventromedial Hypothalamus)

Clusters of neurons in the lower-middle hypothalamus; acts as an "on switch" for mating behavior when activated by estrogen

PAG (Periaqueductal Gray)

Midbrain relay station; receives signal from VMH and passes it to the medullary reticular formation

Medullary Reticular Formation

Brainstem structure involved in motor control and copulatory behavior; receives signal from PAG

Reticulospinal Tract

Carries descending signal from the medullary reticular formation down to the spinal cord