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 Avian Reproduction: Anatomy & the Bird Egg

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تاريخ التسجيل : 24/09/2009
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مُساهمةموضوع: Avian Reproduction: Anatomy & the Bird Egg   الأربعاء 24 مارس 2010, 11:09 pm

Avian Reproduction: Anatomy & the Bird Egg
Reproductive Anatomy:

Gonads - paired testes in males & usually a single ovary in females

Ovary

most birds have only left ovary but 2 ovaries are typical of many raptors


contains from 500 to several thousand primary oocytes


Testes & follicles increase dramatically in size as the breeding season approaches.


As day length increases, photic stimulation of the hypothalamus results in the secretion of Gonadotropin releasing hormone (GnRH below). When activated by GnRH, the anterior pituitary secretes two gonadotropin hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH acts on sperm-producing structures in the testes, while LH acts on the interstitial cells of the testes causing them to secrete the steroid hormone testosterone. The pituitary gland monitors the amount of testosterone in the blood, thus creating a negative feedback loop to maintain hormone levels within a set range (Akins and Burns 2001).


Ambient visual cues, such as daylight, activate photosensitive loci in the brain both indirectly, through the eyes, and directly, through the skull. The hypothalamus of the bird brain contains special cells that are sensitive to extremely low light levels, intensities comparable to the amount of light that can penetrate brain tissue (Akins and Burns 2001
).

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From: Akins and Burns (2001)

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The pattern of testosterone secretion in free-living populations of Song Sparrows.
Plasma levels peak in April and May as breeding got underway and then were maintained at a lower “breeding baseline” during the
rest of the breeding season. As prebasic molt ensued, plasma levels of testosterone were basal and remained so throughout autumn and winter.
From: Wingfield and Hahn (1994).



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Biological actions of the steroid hormone testosterone
. The morphological, physiological and behavioral actions of testosterone that are essential
for male reproductive function are given on the right hand and lower sides of the figure. The “costs” of prolonged high levels of testosterone are given on the
left hand side in italics. The patterns of plasma testosterone levels may be a function of secretion patterns to maintain male reproductive function, and “costs”
of testosterone that require that plasma levels be low.
Testosteron increase availability of carotenoids -- Androgens and carotenoids play a fundamental role in the expression of secondary sex traits in animals that communicate information on individual quality. In birds, androgens regulate song, aggression, and a variety of sexual ornaments and displays, whereas carotenoids are responsible for the red, yellow, and orange colors of the integument. Parallel, but independent, research lines suggest that the evolutionary stability of each signaling system stems from tradeoffs with immune function: androgens can be immunosuppressive, and carotenoids diverted to coloration prevent their use as immunostimulants. Despite strong similarities in the patterns of sex, age and seasonal variation, social function, and proximate control, there has been little success at integrating potential links between the two signaling systems. These parallel patterns led us to hypothesize that testosterone increases the bioavailability of circulating carotenoids. To test this hypothesis, manipulated testosterone levels of Red-legged Partridges (Alectoris rufa) while monitoring carotenoids, color, and immune function. Testosterone treatment increased the concentration of carotenoids in plasma and liver by >20%. Plasma carotenoids were in turn responsible for individual differences in coloration and immune response. These results provide experimental evidence for a link between testosterone levels and immunoenhancing carotenoids that (i) reconciles conflicting evidence for the immunosuppressive nature of androgens, (ii) provides physiological grounds for a connection between two of the main signaling systems in animals, (iii) explains how these signaling systems can be evolutionary stable and honest, and (iv) may explain the high prevalence of sexual dimorphism in carotenoid-based coloration in animals.



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Red-legged Partridge (Photo by G. Bortolotti)

Sperm production


occurs in seminiferous tubules of the testes (shown below)


occurs best at slightly cooler temperatures, so spermatogenesis may occur primarily at night when body temperatures are slightly lower


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Light photomicrograph of a section of a testis showing a seminiferous tubule during full
semen production. SG indicates spermatogonia; PS, primary spermatocyte; Ss, secondary spermatocyte;
MS, mature spermatocyte; and L, lumen (original magnification ×800) (Samour 2002).




sperm are stored at the terminal end of the vas deferens (seminal glomus), and this creates a swelling called the cloacal



Male birds have paired abdominal testes lying cranioventral to the first kidney lobe. Testes increase dramatically in size 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 glomus (or seminal vesicle as in the drawing to the right).
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 glomera.
In addition, male birds tend to have relatively low extragonadal sperm reserves and sperm are ejaculated soon after production in the testes.




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Cloacal protuberance

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Longitudinal section of the cloaca of a male budgerigar during the culmination phase of the breeding cycle.
SG indicates seminal glomus; P, proctodeum; and C, cloaca (original magnification ×12) (Samour 2002).



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Sperm competition and testes size -- Comparative analyses suggest that a variety of ecological and behavioural factors contribute to the
tremendous variability in extrapair mating among birds. In an analysis of 1010 species of birds, Pitcher et al. (2005) examined several ecological
and behavioural factors in relation to testes size; an index of sperm competition and the extent of extrapair mating. In univariate and multivariate
analyses, testes size was significantly larger in species that breed colonially than in species that breed solitarily, suggesting that higher breeding
density is associated with greater sperm competition. After cont for phylogenetic effects and other ecological variables, testes size was also
larger in taxa that did not participate in feeding their offspring. In analyses of both the raw species data and phylogenetically independent contrasts,
monogamous taxa had smaller testes than taxa with multiple social mates, and testes size tended to increase with clutch size, which suggests that
sperm depletion may play a role in the evolution of testes size. These results suggest that traditional ecological and behavioural variables, such as
social mating system, breeding density and male parental care can account for a significant portion of the variation in sperm competition in birds.













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Avian Reproduction: Anatomy & the Bird Egg
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