Biology 463/563 Ornithology
Dr. David Swanson, Office: CL 180
** ENDOCRINE SYSTEM **
- In general, similar hormones (and actions) to mammals.
ENDOCRINE GLANDS = ductless glands; secretion diffuses directly into blood vessels which are closely associated with secretory units of the gland.
Secretion = hormones - act in small amounts, usually at a site different from the site of origin.
1. Pituitary = located at base of brain, connected to the hypothalamus region of the brain by a stalk (infundibular stalk). Divided into two lobes.
- A) Anterior Pituitary secretes:
- (1) Gonadotropic Hormones (FSH, LH) = regulate growth of gonads and production of gonadal hormones; also regulates production of gametes.
- (2) Adrenocorticotropic Hormone (ACTH) = controls function of adrenal cortex
- (3) Thyrotropic Hormone = stimulates activity of thyroid gland
- (4) Prolactin = inhibits gonadal activity, increases fat storage, possibly induces brooding behavior.
- (5) Growth Hormone (GH) = stimulates growth; also stimulates lipolysis and glycogen sparing.
- B) Posterior Pituitary secretes (Hormones are actually produced by cell bodies in the hypothalamus, but they are released from axon terminals in the posterior pituitary):
2. Adrenal - located at cranial pole of kidneys; 2 distinct regions occur: Interrenal (cortex) and Chromaffin (medulla); these tissues intermingled in bird adrenal.
- (1) Arginine Vasotocin (AVT) = regulates blood pressure by serving as vasoconstrictor; antidiuretic mechanism - decreases glomerular filtration rate by constricting arterioles leading to glomerulus and increases reabsorption of water from collecting ducts of mammalian type nephrons in kidney.
- (2) Oxytocin = stimulates expulsion of eggs from oviduct.
3. Thyroid = located alongside trachea; secretes thyroid hormones which increase metabolic rate and influence feather growth among other actions.
- A) Interrenal = regulates carbohydrate and lipid metabolism, salt metabolism, regulates gonadotropins.
- B) Chromaffin = secretes norepinephrine and epinephrine, regulate blood pressure and heart rate.
4. Parathyroid = closely associated with thyroid; secretes parathyroid hormone which acts to mobilize Calcium ions from bone.
5. Ultimobranchial Bodies = located posterior to parathyroids; secrete calcitonin which inhibits release of calcium ions from bone. Homologue to C-cells of mammal thyroid.
6. Pancreas = secretes:
7. Gonads = secrete:
- A) Insulin = promotes cellular uptake of glucose, decreases blood glucose.
- B) Glucagon = promotes glycogenolysis, lipolysis, increases blood glucose.
- A) Androgens = in males; promotes development of secondary sex characteristics, general growth, courtship behavior.
- B) Estrogens = in females; promotes development of secondary sex characteristics, breeding behavior, stimulates substrate mobilization for egg production.
- Photoperiod regulates activity state of gonads so that birds can:
- Rowan (1925, 1929) did pioneering work with the Dark-eyed Junco. Found that inactive testes in winter could be activated by exposure to long daylength.
- 1) predict optimal time for reproduction
- 2) accommodate annual variations in weather
- 3) synchronize reproductive function in males and females
- 4) terminate reproductive function after the breeding season
- Subsequent research has shown that:
- Photorefractoriness = period of time after breeding season during which gonads cannot be activated by daylength.
- 1) Sexual cycle initiated by daylength: GnRH (hypothalamus) --> FSH, LH (anterior pituitary) --> stimulates gonad development
- 2) Eyes are not essential for photoperiodic response. Eyeless birds respond to changing photoperiod in the same fashion as untreated birds. Response appears to be mediated by hypothalamic receptors.
- 3) Endogenous circannual rhythms may control breeding cycle; reset (synchronized) in some fashion by long days.
- 1) In some manner, gonadotropins from anterior pituitary are blocked.
- 2) Prolactin may be involved.
- 3) Short days of winter somehow act to reset photostimulatory phase.
- Photoperiod probably also regulates other seasonal cycles as well (e.g., migration, winter fattening).
POSSIBLE MECHANISMS (of Photoperiod Sensitivity)
I. Circadian Cycles exist in different hormones (e.g., prolactin, corticosterone, melatonin [from pineal, important to circadian rhythms]).
- Phases of circadian hormone cycles relative to other hormone cycles may influence sensitivity to photoperiod (e.g., prolactin injections give differing results depending on the phase of corticosterone).
II. Circadian Cycles in photosensitivity relative to the amount of daylength present may also be important.
- With longer days there is an increased chance of the photostimulatory period occurring during the day + a longer duration of overlap between the photostimulatory phase and daylight.
- The amount of overlap may allow measurement of daylength. SEE PG. 271, GILL.