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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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