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Ecology & Evolutionary Biology
Michael Denny

Lecture 9 Reproduction IntroductionAll vertebrate taxa have members which lay eggs even mammals the Montremes but birds only reproduce this way exclusively oviparousBirds eggs have a solid shell and are particularly strong perhaps explaining why females can produce and carry just one at a timeBirds are also unusual in being iteroparous there are no oneoff semelparous speciesThe bird egg is a selfcontained life support system for the developing embryo o Just add heat and oxygenThe eggs are laid in nests in the open usually there are holenesting species such as owls woodpeckers seabirds some ducks and so the biggest mortality occurs on eggs and nestlings in birdsBirds are also a little unusual in that in most species both sexes contribute to parental care rare in mammalsBut there are interesting exceptions in many tropical frugivores it is the females alone who incubate the eggs and rear the youngperhaps the most extreme reproductive strategy obligate interspecific brood parasitism has evolved independently several times including in cuckoos eg European cuckoo and in blackbirds eg Brown headed CowbirdReproductionFemales lack a functional right oviduct and most males lack a functional penis or at least one capable of intromissionMaleThe male reproductive system consists of paired testis and ductus deferens which transport sperm to the cloacaThe cloaca is a common chamber where urinary reproductive and gastrointestinal products collect before exiting the vent anus Most birds 97 lack an intromittent phallus penis and instead have a swelling on the inside of the vent Copulation in most species comprises cloacal to cloacal contactThe flightless ratites ostriches emus kiwis and others and waterfowl ducks and geese have a penis which is long enough to penetrate the vagina of the femaleHowever the bird penis differs in some key ways from the mammalian penis the bird penis enlarges through lymphatic not vascular pressure the avian penis does not have a ureter and does not transport urine and sperm together and in birds the sperm travels on the outside of the penis not the inside o The penis shares urinary and sperm tractsthe developmental basis for phallus reduction in birds has pinpointed a role of Bmp4 mediated apoptosis in arresting elongation at the distal end of the embryonic phallusbirds lack an intromittant penis because they do not need one for successful matingcan get sperm into vagina without intromissionFemaleIn most but not all species the adult females have a functional left ovary and oviductThe oviduct has five functionally specific regions ostium infundibulum magnum isthmus uterus and vagina Ostium Females store sperm in special sperm storage tubules in the vagina until ovulation of an ovum from the ovary into the ostium where fertilization tales place shortly afterThe ova consist of a single large cell filled with yolk inside vitelline membranesThe yolk is deposited in the ova in the ovaryYolk proteins are synthesized in the liver and transported in the blood to the ovaryThe fertilized egg travels down the oviduct and in each region receives different components Magnum Here the albumen egg white is coated around the yolk membranes a process that takes about 3 hrs in a chicken Proteins are secreted by the magnum and incorporated in the albumenThere are three main coats or portions to the albumen the chalaziferous layer which is attached to the yolk and suspends it in the egg surrounded by a thick dense albumen layer and an outer thin layerWhen you hardboil an egg you will can often see the twisted stringlike chalaza in the eggwhiteIsthmus Here the bilayer shell membrane is laid down
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