Animal Physiology, Development, and Behavior: Study Guide

Endocrine System I

Why Animals Have Both Nervous and Endocrine Systems

Animals require coordinated control over physiology, anatomy, and behavior. Two main control systems—the nervous and endocrine systems—work together to regulate these processes.

• Nervous system: Provides fast, specific, and short-lived signals using action potentials and neurotransmitters.

• Endocrine system: Delivers slower, broadcast signals (hormones in blood) with long-lasting effects.

• The systems are complementary, not redundant.

Comparison:

• Nervous signaling: Milliseconds, point-to-point, stops quickly.

• Endocrine signaling: Minutes to days, affects many cells simultaneously, persists longer.

Example: Stress response requires both rapid nervous input and sustained endocrine signaling.

Types of Chemical Signaling

Chemical signals vary in their spatial scale and target.

• Autocrine: Cell signals itself.

• Paracrine: Local signaling to nearby cells.

• Endocrine: Hormones travel via bloodstream to distant targets.

• Pheromones: Signals between individuals of the same species.

Example: Growth factors act in paracrine fashion; insulin acts endocrinely.

Endocrine vs. Exocrine Glands

Glands are specialized for secretion, but differ in their targets and mechanisms.

• Endocrine glands: Ductless, secrete hormones into blood, effects are often intracellular and long-term, highly vascularized.

• Exocrine glands: Have ducts, secrete onto epithelial surfaces (e.g., skin, digestive tract), effects are extracellular (e.g., digestion, lubrication).

Example: The pancreas is a mixed gland, with both endocrine (insulin, glucagon) and exocrine (digestive enzymes) functions.

Hormones and Receptors

Hormones are chemical messengers that act at low concentrations and only affect target cells with specific receptors.

• Peptides: Hydrophilic, act via cell-surface receptors.

• Steroids: Derived from cholesterol, hydrophobic, act via intracellular receptors.

• Monoamines: Modified amino acids; properties vary (e.g., epinephrine, thyroxine).

Example: Epinephrine can cause vasodilation in skeletal muscle and vasoconstriction in the gut due to different receptors.

Hormone Action Mechanisms

• Peptide/amine hormones: Cannot cross cell membrane, bind to surface receptors, activate second messenger systems (e.g., cAMP), typically fast responses.

• Steroid hormones: Cross cell membrane, bind intracellular receptors, alter gene transcription, slower but longer-lasting responses.

Example: Cortisol (a steroid) alters gene expression for hours to days; insulin (a peptide) acts within minutes.

Endocrine System II: Hypothalamus, Pituitary, and Control Axes

Neuroendocrine Integration

The hypothalamus and pituitary gland form the interface between the nervous and endocrine systems.

• Neurosecretory cells: Conduct action potentials and release hormones (e.g., in hypothalamus).

• Non-neural endocrine cells: Respond to hormones, not action potentials.

Example: Hypothalamic neurons release hormones into the pituitary portal system.

Posterior Pituitary (Neurohemal Organ)

• Composed of nervous tissue, not a true gland.

• Hormones (ADH, oxytocin) are synthesized in the hypothalamus and stored in axon terminals in the posterior pituitary.

Example: ADH regulates water balance; oxytocin triggers uterine contractions and milk ejection.

Anterior Pituitary and the Hypophyseal Portal System

• True endocrine gland, no direct neural connection to hypothalamus.

• Uses portal blood system to receive releasing/inhibiting hormones (RHs/IHs).

• Tropic hormones: ACTH, TSH, FSH, LH—regulate other endocrine glands.

Example: TSH stimulates the thyroid gland; ACTH stimulates the adrenal cortex.

Endocrine Control Axes and Negative Feedback

• Control axes: HPA (stress), HPT (metabolism), HPG (reproduction).

• Negative feedback: Final hormone suppresses earlier steps, maintaining homeostasis.

Example: High cortisol inhibits CRH and ACTH release.

Thyroid and Metabolic Regulation

• Thyroid hormones (TH) increase basal metabolic rate and require iodine for synthesis.

• Disorders: Hypothyroidism and goiter result from iodine deficiency.

Example: Goiter forms as the thyroid enlarges in an attempt to compensate for low hormone production.

Insect Endocrine System and Metamorphosis

• PTTH stimulates ecdysone release, triggering molting.

• Juvenile hormone (JH) determines developmental stage; high JH prevents metamorphosis.

Example: Decreasing JH allows larva to pupate and become an adult.

Animal Reproduction I

Asexual vs. Sexual Reproduction

• Asexual reproduction: Budding, fission, regeneration, parthenogenesis; fast but low genetic diversity.

• Sexual reproduction: Increases genetic variation; slower and more energetically costly.

Example: Hydra reproduce by budding; most vertebrates reproduce sexually.

Gametogenesis

• Spermatogenesis: Produces four functional sperm per meiosis, continuous production.

• Oogenesis: Produces one ovum and polar bodies, arrested at stages, cyclical.

Example: Human oocytes are arrested in prophase I until ovulation.

Gonads and Somatic Support Cells

• Testes: Sertoli cells support sperm; Leydig cells produce testosterone.

• Ovaries: Follicles contain granulosa and theca cells for hormone production and ovum support.

Example: Granulosa cells produce estradiol in response to FSH.

Fertilization Strategies and Challenges

• External fertilization: Less protection, more gametes released.

• Internal fertilization: Greater protection, fewer gametes.

• Polyspermy prevention: Fast block (membrane depolarization), slow block (cortical reaction).

Example: Sea urchins use both fast and slow blocks to prevent polyspermy.

Hormonal Control of Reproduction

• Male system: GnRH stimulates FSH & LH; testosterone and inhibin provide negative feedback.

• Female cycles: Ovarian and uterine cycles coordinated by estradiol and progesterone.

Example: LH surge triggers ovulation in females.

Animal Reproduction II: Timing, Cycles, and Life-History Strategies

Decoupling Steps in Reproduction

• Some animals separate mating, fertilization, and development to align reproduction with environmental conditions.

• Sperm storage: Allows fertilization long after mating (e.g., honeybees, blue crabs).

Example: Queen honeybees store sperm for years, fertilizing eggs as needed.

Embryonic Diapause and Delayed Implantation

• Embryonic diapause: Temporary arrest of development; resumes when conditions improve.

• Delayed implantation: Blastocyst implantation postponed for seasonal birth timing (e.g., Antarctic fur seals).

Example: Bats can mate in fall but delay embryonic development until spring.

Semelparity vs. Iteroparity

• Semelparity: Single reproductive event, then death (e.g., Pacific salmon).

• Iteroparity: Multiple reproductive events over a lifetime (e.g., mammals, birds).

Example: Octopus is semelparous; humans are iteroparous.

Seasonal Reproductive Cycles

• Animals synchronize reproduction with favorable conditions, often using photoperiod as a cue.

• Some mammals show seasonal regression of gonads.

Example: Many birds breed only when days are long.

Animal Development I: Fertilization, Polarity, and Cleavage

Fertilization and Activation of Development

• Egg: Provides cytoplasm, organelles, regulatory factors.

• Sperm: Contributes DNA and centriole.

• Egg activation: Fast block (membrane depolarization), slow block (Ca2+-mediated cortical reaction), increased metabolism.

Example: Sea urchin eggs rapidly depolarize to prevent polyspermy.

Establishment of Polarity

• Body axes established early, depending on egg structure and sperm entry point.

• Amphibians: animal vs vegetal hemispheres, gray crescent formation.

• Drosophila: maternal mRNA/proteins localized during oogenesis.

Example: The gray crescent is essential for normal frog development.

Experimental Embryology and Cytoplasmic Determinants

• Spemann’s experiments showed specific cytoplasmic regions are required for proper development.

• Only embryo halves with the gray crescent develop normally.

Example: Cytoplasmic determinants influence cell fate before cleavage.

Cleavage and Blastula Formation

• Cleavage: Rapid mitotic divisions without growth, producing smaller cells (blastomeres).

• Blastula: Hollow or partially hollow structure with blastocoel.

Example: Cleavage partitions cytoplasm for later differentiation.

Cleavage Patterns and Developmental Modes

• Cleavage can be complete or incomplete.

• Radial cleavage (deuterostomes), spiral cleavage (protostomes).

• Mosaic development: fate fixed early; regulative development: fate flexible.

Example: Deuterostomes (e.g., vertebrates) are typically regulative developers.

Animal Development II: Gastrulation, Germ Layers, and Organogenesis

Gastrulation and Germ Layer Formation

• Gastrulation reorganizes the embryo into three germ layers: ectoderm, mesoderm, endoderm.

• Establishes the basic body plan.

Example: The blastopore forms during gastrulation.

Gastrulation in Different Animals

• Sea urchins: vegetal pole invagination, archenteron formation.

• Frogs: gray crescent initiates cell movement, dorsal lip and blastopore formation.

• Birds/mammals: primitive streak and Hensen’s node.

Example: The primitive streak in birds is functionally equivalent to the blastopore in amphibians.

Coelom Formation

• Deuterostomes: Mesoderm forms as outpocketings of the archenteron.

• Protostomes: Mesoderm arises from specific blastomeres.

Example: The coelom provides a hydrostatic skeleton and space for organ development.

Germ Layer Fates

• Ectoderm: Epidermis, nervous system, pigment cells.

• Mesoderm: Muscles, bones, circulatory and urogenital systems.

• Endoderm: Digestive tract lining and associated organs.

Example: The mesoderm is crucial for complex organ systems.

Neurulation and Neural Crest Cells

• Notochord: Induces ectoderm to form neural tube.

• Neural crest cells: Migrate extensively, form sensory neurons, skull bones, pigment cells.

Example: Neural crest cells are multipotent, giving rise to diverse tissues.

Mesoderm Differentiation and Segmentation

• Somites: Repeating mesodermal segments; form vertebrae, ribs, skeletal muscles, dermis.

• Intermediate mesoderm forms kidneys/gonads; lateral plate forms body cavity/circulatory structures.

Example: Segmentation is key for vertebrate body organization.

Positional Information and Morphogens

• Cell fate depends on position, not just lineage.

• Morphogens form gradients that direct development (e.g., Sonic hedgehog in limb development).

Example: The ZPA in limb buds secretes Shh, specifying digit identity.

Extraembryonic Membranes

• Yolk sac: Nutrient transfer.

• Allantois: Waste storage and gas exchange.

• Amnion: Fluid-filled protective sac.

• Chorion: Gas exchange and protection.

Example: Amniotic eggs enabled vertebrates to reproduce on land.

Excretory Systems: Osmoregulation

Internal Environment and Osmoregulation

• Animals must maintain stable water and ion concentrations in body fluids.

• Osmotic pressure: Determined by solute concentration.

• Isosmotic: Same osmotic concentration as environment.

• Hyperosmotic: Higher solute concentration than environment.

• Hypoosmotic: Lower solute concentration than environment.

Example: Marine invertebrates are typically isosmotic with seawater.

Freshwater Animals

• Body fluids are hyperosmotic to environment; water enters by osmosis, ions diffuse out.

• Adaptations: produce large volumes of dilute urine, actively transport Na+ and Cl- into body through gills.

Example: Freshwater fish excrete much urine and use active transport for ion uptake.

Ocean Invertebrates

• Most are isosmotic with seawater (~1 Osm), so little energy is spent on osmoregulation.

Example: Jellyfish and sea stars are osmotic conformers.

Ocean Bony Fishes

• Body fluids are hypoosmotic to seawater; risk dehydration and salt overload.

• Adaptations: drink seawater, excrete excess salts via chloride cells in gills, use significant metabolic energy.

Example: Marine bony fish must drink seawater and actively excrete salts.

Evolutionary History of Osmoregulation

• Ocean invertebrates: ~1 Osm; freshwater/terrestrial vertebrates: ~0.3 Osm.

• Ocean bony fishes descended from freshwater ancestors, explaining their hypoosmotic state.

Example: Evolutionary transitions influenced osmoregulatory physiology.

Air-Breathing Marine Vertebrates and Salt Glands

• Descended from terrestrial vertebrates; ingest large salt loads when feeding.

• Adaptations: salt glands in head region excrete concentrated salt solutions (e.g., seabirds, sea turtles).

Example: Marine iguanas sneeze out salt from nasal glands.

Animals in Variable Salinity Environments

• Migratory species (e.g., salmon) switch between freshwater and saltwater osmoregulatory strategies.

• Brackish environments: some invertebrates are osmotic conformers, others are regulators.

Example: Endocrine regulation is essential for migratory fishes.

Mammalian Excretory System: Kidneys and Nephrons

• Kidneys: Primary excretory organs; functional unit is the nephron.

• Nephron components: Glomerulus (filtration), Bowman's capsule (collects filtrate), proximal tubule (reabsorption), loop of Henle (urine concentration), distal tubule & collecting duct (fine-tuning).

Example: Urine concentration is vital for water conservation in terrestrial animals.

Nephron Types and Water Conservation

• Cortical nephrons: Short loops of Henle.

• Juxtamedullary nephrons: Long loops, enable highly concentrated urine (e.g., kangaroo rat).

Example: Desert mammals have a high proportion of juxtamedullary nephrons.

Animal Behavior: Mechanisms, Evolution, and Ecology

What Is Behavior?

• Behavior is any action performed by an animal via the nervous system, including movement, communication, foraging, and mating.

Example: Bird song, ant foraging, and courtship displays are all behaviors.

Proximate vs. Ultimate Causes of Behavior

• Proximate causes: Immediate mechanisms (stimuli, hormones, neural circuits).

• Ultimate causes: Evolutionary explanations (adaptive value).

Example: Herring gull chicks peck at a red dot due to neural wiring (proximate) and increased feeding success (ultimate).

Neural Basis and Innate Behavior

• Specific behaviors rely on particular brain regions.

• Some behaviors are fixed action patterns (innate, stereotyped, triggered by specific stimuli).

Example: Goose egg-rolling is a fixed action pattern.

Behavior Evolves

• Genes influence neural tissue, which influences behavior.

• Behavioral traits can evolve under natural selection.

Example: Fruit fly circadian clock mutations alter activity patterns.

Learning and Behavioral Flexibility

• Learning modifies behavior based on experience, enhancing survival in variable environments.

• Imprinting: Occurs during a critical period, leads to irreversible behavioral changes.

Example: Ducklings imprint on the first moving object they see.

Navigation and Movement

• Trail following: Pheromone-based navigation (e.g., ants).

• Path integration: Internal navigation without retracing steps.

• Redundancy: Multiple navigation systems (sun compass, magnetic fields, landmarks).

Example: Migratory birds use sun, stars, and Earth's magnetic field.

Social Behavior and Group Living

• Equal-status groups: Flocks, herds, schools; increased vigilance.

• Hierarchical societies: Dominance structures, unequal reproductive success.

Example: Wolf packs have dominant breeding pairs.

Territoriality and Space Use

• Territories: Defended areas with exclusive access.

• Home ranges: Areas used but not defended.

Example: Songbirds defend nesting territories during breeding season.

General Study Tips

• Focus on mechanisms, not memorization alone.

• Practice tracing hormone pathways and explaining effects.

• Compare systems (e.g., nervous vs endocrine, male vs female, insect vs vertebrate).

• Understand processes and transitions (e.g., blastula → gastrula → organogenesis).

• Explain how structure influences function at each stage.

• Compare different taxa and environments.

• Trace cause-and-effect relationships (e.g., morphogen gradients → tissue identity).

• Connect physiology and behavior to ecology and evolution.