Endotherms 2 Flashcards
What is viviparity?
Embryo develops inside the mother from whom it gains nourishment and gas exchange this occurs from insects to placental mammals
What is oviparity?
Eggs are laid with little or no development within the mother this occurs from insects to mammals where terrestrial eggs may have shells
What is ovoviviparity?
The embryo develops within the mother and the egg hatches before or just after laying
This is seen from invertebrates to fish with the mother providing gaseous exchange
What are amniotes?
Tetrapods whose living members include reptiles, birds and mammals
Key derived terrestrial include an amniotic egg
Less permeable skin
The ability to ventilate lung using ribcages
A more elevated stance compared with early tetrapods and amphibians
What are the features of the amniotic egg?
This is a major derived character of amniotes
It contains a specialised membranes to protect embryo
Extra embryonic membranes grow out from the embryos
Functions in protection, gas exchange, waste storage and nutrient supply to embryo
Contains an amnion, allantois, chorion, yolk sac
Most reptile, all birds and some mammals eggs are surrounded by shells
What is the allantois?
A disposal sac for metabolic waste and respiratory organ
What is the chorion?
This functions in gas exchange between the embryo and air with O2 and CO2 being able to freely flow through the shell
What is an amnion?
This is a fluid-filled shock protector
What are the features of bird reproduction?
All are oviparous which is unique amongst the vertebrates
Only one ovary or two testes (which undergo periodic atrophy) which is a weight saving adaptation for flight
Birds brood eggs after laying with there apparently being less advantage to retaining them internally
Eggs which are protected by a shell 98% composed of crystalline calcite
How does shell formation occur in eggs?
Inner and outer shell membranes are secreted
Carbohydrate and water are added to albumen and organic granules are added to the shell to form mammillary bodies these are the site of first CaCO3 deposit and grow to form mammillary cones
Formation of the palisade layer of CaCO3 crystals
Formation of surface crystalline layer
Shell penetrated by numerous pores allowing gas exchange
Evaporation of water creates air cell which is used by embryos to ventilate their lungs just before hatching
How does mating and fertilisation occur in birds?
The testes are housed inside the abdomen at body temperature with sperm production occurring at night due to the cooler body temperature and are stored in seminal vesicles
No external genitalia with sperm being passed by cloaca touching
The ovum is fertilized in the upper end of the oviduct with the egg being fertilized within 2 days though the females may store sperm for up to 10 weeks if this is more convenient
How does sex determination occur in birds?
Birds have genetic sex determination with the male being the “default” homogametic sex and females being the heterogametic sex with their W chromosome stimulating the primordial gonad to secrete estrogen with the left gonad to develop as an ovary while the left mullerian duct system to develop into oviduct and egg gland
Despite this there is also some evidence to suggest that the female may have some control over the sex
How does incubation occur in birds?
This is always external and mediated by the hormone prolactin which is high during incubation and brooding in both males and females although in males testosterone inhibits parental behaviour
Incubation usually begins after the last egg laid to allow synchronous hatching (though asynchronous hatching does occur in some owl and raptor species)
Brood patches are formed, these are highly vascularised featherless areas which has its development mediated by estrogen and/or prolactin
How are the three major lineages of mammals defined by reproduction?
Monotremes which are egg-laying mammals found only in Australia and New guinea
Marsupials which are typically mammals with a pouch where there is a small amount of development in the mother before the offspring are born early and relatively poorly developed
Eutherians which are placental mammals with a more complex chorionallantoic placenta, compared to that seen in marsupials
What are the key differences between the mammalian reproductive tracts?
The monotreme tract does not have the ureters connecting directly to the bladder
The monotremes also have larger ovaries as they have to produce higher levels of yolk due to the fact they lay eggs
In marsupials there is a pseudovaginal canal from which the embryo is ejected this is flanked by lateral vaginas in which sperm is transported
