UROGENITAL AND ENDOCRINE SYSTEMS
As in all vertebrates the urinary and reproductive systems are closely linked
in development and in function. Birds retain many features in common with
their reptilian cousins, albeit adapted to active homoeothermy:
(1) They secrete uric acid rather than urea. This allows for water
conservation, but also permits storage of embryonic waste in the egg in
solid rather than liquid (and toxic) form.
(2) A rudimentary renal portal system is retained, although this
can be shut down during vigorous exercise when rapid venous return is required.
(3) The reproductive system shows many reptilian features including reductions
in size and mass (for flight), strong seasonality and, of course, oviparity.
Unlike many rodent eggs, however, the avian egg has a large yolk
mass to support homoeothermic development, and a porous shell for gas and
Kidneys and ureters
The metanephric kidneys excrete dilute urate that then gets
concentrated in the cloaca. The kidneys themselves are 3-lobed structures
lying in the dorsolateral abdomen. The ureters arise from the midle lobe
and pass to the cloaca. This itself has three zones from cranial to caudal:
* Coprodeum receiving the faeces
* Urodeum receiving the ureters and gonadal ducts.
* Proctodeum -- a common discharge area.
In some male birds the cloaca may be eversible to form a copulatory organ.
In addition to kidneys that are highly efficient at eliminating nitrogenous
waste and conserving water, marine birds have the problem of eliminating
Birds along with at least three groups of diapsid reptiles (lizards, snakes
and crocodiles) have evolved a system of lateral nasal salt glands.
These are structured in a way reminiscent of the kidney, using a countercurrent
capillary exchange mechanism surounding the secretory tubules of the gland.
The single layer of cells surrounding each tubule actively secretes a fluid
about 5 times more concentrated in salt than the body fluids.
In birds these nasal or supraorbital glands lie over the orbit and
secrete brine into the nasal passages. From here it either drips or is sneezed
Male reproductive system
The males have paired abdominal testes lying cranioventral to the
first kidney lobe. There is enormous variation during the breeding season.
The vas deferens emerges medially and passes caudally to the cloaca
where it has a common opening with the ureter in the Urodeum. The
terminal vas deferens is swollen as a storage organ: the seminal vesicle.
See and complete Fig 5-1
As in mammals, sperm formation is temperature sensitive, and maturation
is assisted by nocturnal drops in temperature, or by the development of
scrotal-like external thermoregulatory swellings holding the seminal vesicles.
In addition, male birds tend to have relatively low extragonadal sperm reserves
and sperm are ejaculated soon after production in the testes. In this way,
it is thought, deleterious aging effects do not have time to accumulate.
Copulation in birds involves apposition of the cloacal region of male and
female following courtship. Courtship itself may be intense and competition
between males for access to females has been major driving force behind
the development of elaborate plumage and song.
Many birds, however, are monogamous and show extreme degrees of cooperation
in caring for young. Probably the most dedicated is the male Emperor Penguin,
who incubates a single egg through the dark of an Antarctic winter.
Fig 5-1 Male Reproductive system (after Proctor
& Lynch, 1993)
Female reproductive system
In most birds only the left ovary and oviduct persist. This
reduces mass and makes for more efficiency as only one developing egg needs
to be supported at any one time.
Atrophy of the ovary in old age, or removal by intervention or disease results
in the development of the vestigial right gonad as an ovotestis.
This secretes testosterone that causes masculinization of the bird.
As in the male, the ovary enlarges greatly during the breeding season. Active
ovaies resemble pale bunches of tiny grapes -- the developing follicles.
The oviduct opens medially to it in a funnel-shaped ostium - the
equivalent of the mammalian infundibulum and fimbriae.
The oviduct receives the developing eggs and has distinct regions in which
differing stages of egg maturation occur. Sequentially, these are the magnum,
the isthmus, the uterus with pigment glands and the
vagina for temporary egg storage.
Details of egg layer formation are given below. See and complete Fig 5-2.
Typically egg formation involves the following steps.
Ovulation releases an egg from a mature follicle on the surface of
the ovary. The egg has extensive food reserves in the form of concentric
layers of yolk. This is immediately picked up by the ostium, where
ciliary currents carry it into the magnum region. Fertilization normally
occurs here if the female has mated: sperm are stored for extensive periods
in glandular regions of the lower oviduct.
Over about three hours the egg receives a coating of albumen.
The egg and albumen then pass into the isthmus, where the shell membranes
are deposited. This takes about one hour.
In the uterine region the egg spends about a day, and pigment gets laid
down in the shell in characteristic patterns. Final hardening of the shell
occcurs here and the egg then passes into the vagina and cloaca for laying.
Fig 5-2 Female Reproductive System (after Proctor
and Lynch, 1993)
The avian endocrine glands are responsible for coordinating the bird's response
to stress, reproduction, metabolic demands and body growth. The basic pattern
is very similar to mammals. See and complete Table 5-1.
Gland Location Hormones Functions
In which birds does an erectile penis develop?
Why haven't birds developed true viviparity?
Many birds' eggs have characteristic pigmentation patterms. How do these
get laid down and what advantage does the colouration give?
What advantage is there to sperm storage in the female?
If birds can get rid of excess salt by means of salt glands, how do marine
Page constructed by Jim
Cummins and updated on April 1 1996.
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