Reproduction

Avian Reproduction

Chapter 15 in Gill.

I. Sexual reproduction

In breeding season, birds compete with each other for the chance to breed

mates

resources

balance between cooperation and competition determine breeding system

monogamous

polygamous

polyandry

polygynous

helpers at the nest

territorial

colonial

clutch size

altricial vs. precocial

We’ll get to this but first the nitty gritty:

Gonads

paired testes

single (usually) ovary

[overhead]

exceptions are

Brown Kiwis

many raptors

some pigeons, gulls and passerines

testes expand 4-500 xs in mass, nonbreeding season usually a couple of mm.

Ovary looks like cluslter of grapes

500- several thousand primary oocytes

Hormones

Follicle-stimulating hormone regulates gamete production in both sexes

Luteinizing hormone (from Pituitary)

controls breeding physiology in both sexes

regulates hormone production in testes

stimulates maturation of ova

controls some secondary characteristics

Testes and ovary produce estrogen and testosterone, which are both present in both sexes. Different ratios and effects.

Secondary characteristics

mostly controlled by estrogen/testosterone ratios

male breeding plumage

bill color

wattles

Sex determination

females heterogametic

ZW = female

ZZ = male

gynandromorphs

II. Sperm production

Testes

Produce sperm

secrete hormones (notably testosterone)

internal - implications?

sperm produced at night

stored in seminal vesicles in cloacal protruberence

sperm mostly typical of vertebrates - 3 parts

head - acrosome and nucleus - the payload

midpiece - fuel

axial filament & tail

oscine passerines have spiral head and helical tail membrane

III. Maturation of the ovum/embryo

parts of the egg

ovum/embryo

food supply

protective layers

yolk added to ovum

in follicle

in layers - most in week before ovulation

prior to ovulation

follicle ruptures

ovum released into infandibulum/ostium - funnel end of oviduct

now ready for...

IV. Fertilization

Copulation via ‘cloacal kiss’

some birds have penis-like structure

most waterfowl (copulation in water?)

seems taxonomically haphazard - 1 passerine

female overts cloaca, exposing end of oviduct

male ejaculates 1.7 to 3.5 billion spermatazoa

Sperm swim toward infandibulum in 30 minutes

may fertilize an ovum immediately

but remains viable

for weeks

domestic fowl can lay fertile eggs 72 days after copulation

Laysan Duck

unfertilized eggs can develop

most die

what sex would they be?

Egg production

birds are all oviparous - body too hot for embryonic development?

glandular epithelial cells of oviduct add

albumen-in magnum region - 3 hrs

shell membranes-in isthmus region - 1 hr

shell and pigmentation - 20 hrs.

takes about 24 hrs

3-4 hours in oviduct -

albumen - part of albumen anchors yolk

shell membranes - porous

19-20 hrs in uterus where shell is formed

mostly calcium carbonate (CaCO₃)

where pesticides can affect reproduction

Diversion on why yolks are yellow.

Carotenoid pigments

Must be incorporated from diet-only by plants some bacteria and fungi

Conspicuous in sexual signals of many animals and may be related to health.

Powerful antioxidants and immunostimulants

Protect vulnerable tissues from damage by free radicals

Developing embryo relies on oxidation of fatty acids from lipid-rich yolk for almost all its energy. This metabolic pathway normally produces free radicals and antioxidants reduces the damage to cell membranes and cell function that these metabolites can cause. Female transfers lots of antioxidants to embryo (in egg formation) and many are transferred from yolk to embryo prior to hatching (stored in liver).

(Reptile eggs use protein as the main energy source--this is not as high in energy as lipids--and doesn't produce as much metabolic water.)

V. The Amazing Avian Egg.

usually 1-2 days between eggs, can be 4-8

eggs range from 0.2 g to 9 kg of elephant birds

usually 2-11% body mass - (Kiwis an amazing 40%!)

clutch size

depends on

nutrition

daily requirements range from

40-50% basal metabolic rate

125-180% bmr

greatest need during yolk formation

greatest for precocial young

genetics

varies within species

across species 1 - 23

determinant vs. indeterminate layers

shapes and color - dictated by nest structure in part

variation inspired oologists

huge collections

19th century heyday

indian story

structure - cleidoic

porous - gas exchange to embryo

water loss

CO₂ content of airspace

freed reptiles from aquatic reproduction

reptiles eggs leathery - can absorb water

increased calcification

adds protection

cost is loss of ability to absorb water

compensation is high water content of albumen-principal energy storage is fat yields more H20 than protein energy storage of reptiles

embryonic needs

O₂

get rid of CO₂

H₂O - provided by albumen (90% water)

get rid of nitrogenous wastes

uric acid again is important-water soluble forms of N would be intolerable inside egg

high, constant temperature

physical protection - shell very strong

biotic protection - pores too small for viruses or bacteria to pass through - interior of egg one of the most sterile environments

extra-embryonic membranes

shell membranes

vitelline membrane around yolk

stomach & intestines

rich in lipids (21-36%) and protein (16-22%)

chorion

allantois

‘chorioallantois’

site of gas exchange

blood vessels develop - candling eggs

shell structure

cuticle - mostly protein, part of biotic shield

palisade layer

mammilary cone

shell structure an exquisite compromise

extremely strong from outside

easy to break form inside

more pores -> easier gas exchange but unacceptable water loss

air space

gets higher in CO₂ concentration

fills lungs of young at hatch

embryo mobilizes calcium from shell, which increases porosity as embryo needs more O₂

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