Dear Mohammed,

As you know testes must be in scrotum in  all mammalian species. Testes origins from meso-nephrone in embryonic phase and it must be descended in scrotum before birth.   It is needed for thermoregulation otherwise spermatogenesis is detrimentally affected. It is well known phenomenon in reproductive physiology. (I do not talk about thermoregulation of testes)

In birds, testes must be in abdomen because of aerodynamic issue in evolution. And the thermoregulation is arranged by air sacs in male birds. It is well known.The testes in birds is near the air sac in abdomen so it is not need scrotal cooling.

My best wishes,

Umut  

Dear Umut,

im astonishing for your answer,

but do you have any literature in this case?

anyway I think your reason is logical.

I ask this question because I believe that the originally of every thing is so important, i know that what was the resources of reproductive tract but i couldn't find what was their theology!!!

Best regards,

M.M.Ommati 

Dear Dr. Mohammad!

  Appreciating your question and partly supporting Dr. Umut's answers I place here an article for your perusal.

KP Skandhan, A Rajahariprasad. The process of spermatogenesis liberates heat. Scrotum helps in body thermoregulation. Medical Hypotheses 68:303-307, 2007

With Best Wishes

KP Skandhan

The reason of this is that  in scrotal testis temperature (which affects spermatogenesis) much easily regulable. It's an evolutionary step which was thriggered by the endothermy. 

Send you a link on that:

Article The Evolution of the Scrotum and Testicular Descent in Mamma...

Thanks all,

Affectionately,

Mehdi

The issue of thermoregolation necessary for spermatogenesis and so justifying the presence of external (in the scrotum) testis is not totally convincing since several well known common land mammals have internal testis: elephants as well (for obvious reasons) all sea living mammals.

Thank you so much dear coallegues. 

Dear Dr Maurizio,

Whats your idea about this evolution? Do you belive that some mammals such as elefants and sea animals, have spermatogenesis at their body temerature? Beacause as Dr Umut said there are some adaptation in animals such as poultry (air sack) but what is your idea about elephant and some sea mamals?

Regards

The descent of the testes into the scrotal sac is guided by the gubernaculum testis through  a passive movement. The passive movement is brought about by a difference in growth rate between the gubernaculum testis and the rest of the body parts. As the growth rate of other body parts is faster, the gubernaculum testis which is attached to the to the testis will passively draw it into the scrotum. Any abnormalities associated with this arrangement will cause the testis to be retained either abdominally or within the inguinal region. This testicular retention called cryptorchidism usually affect spermatogenesis negatively through the higher core body temperature than what obtains in scrotal testes. However, some animals like elephant, armadillo and birds which naturally have abdominal testes are provided with mechanisms that support spermatogenesis. 

 Hi dear ochio,

Yes, your right, but as you know very well, its a definition of cryptorchidism,that you described very well, but our disscusion is specially in its evolution!

Best wishes,

Mehdi

Dear Mohammad,

As some colleagues have been mentioned, the principal reason of the descent of the testes into the scrotum is to assure the proper environment for the spermatogenesis, for this process in almost all the mammals is required a cooler temperature than the normal body temperature.

The differences with other kind of animals without a doubt, are due to different necessities of biological adaptations for every specific animal, as the example of the birds and their aerodinamic necessities.

There is a very good book that explains this process, the name is "Pathways to Pregnancy and Parturition"

Best Regards

Dear sanches,

Thank you for introducing.

Regards,

Mehdi

Testes are outside the abdomen in bull & most domestic animals so that the mechanism of thermoregulation works properly. In such animals as the body temperature rises, the scrotal sac relaxes and keeps the testes cool and away from the body & vice versa. 

The abdominal testes remain internal because they have a separate inguinal pouch like in elephants.

Dear Mehrajuddin Naikoo,

Thank you so much for your answer, on the other hand, my idea is so similar with Dr Umut Cagin Ari ad Dr Kalanghot P Skandhan who conveyed that: 

In birds, testes must be in abdomen because of aerodynamic issue in evolution. And the thermoregulation is arranged by air sacs in male birds. It is well known.The testes in birds is near the air sac in abdomen so it is not need scrotal cooling.

Hi Mohammad,

The process of testicular descent into the scrotum occurs due to the different rates of growth of body mass and gubernacular growth. this is associated with marked changes in testicular position from inside to outside the abdomino-pelvic cavity. testicular descent occurs at mid-pregnancy in some species, near parturition in some others and during the first few days after parturition in some.

As other colleagues mentioned, this process is to control testicular temperature for normal spermatogenesis.

Dear Rabie,\

Thank you special for your paying attention but i know that your not underestand my question! my question was about which mechanisms are made by poultry during their evolution? not for mammals.

For example, roosters have not spermaticcord (to regulate of testes temperatue) but  their feature is that they have spermatogenesis in their bodies temperate.

I wanted to know that why roosters did not have any change about his testicular position during his evolution like most of mammals?

As my friends, Dr Umut Cagin Ari ad Dr Kalanghot P Skandhan conveyed:

In birds, testes must be in abdomen because of aerodynamic issue in evolution. And the thermoregulation is arranged by air sacs in male birds. It is well known.The testes in birds is near the air sac in abdomen so it is not need scrotal cooling.

Regards,

Mehdi

The scrotal temperature should be about 2 degrees Fahrenheit and 1 degree C lower than the the temperature of the body, so it must hang outside the body.

All Afrotheria (elephant, manatee, earth piglet ...) have internal testicles. In addition to this synapomorphic, "primitive" feature, they also have an underdeveloped thermoregulation (presumably because they come from warm climates, just from Africa).

Dear Rudolf Ritt,

Thank you so much for your cooperation.

That is another marvelous reason.

Regards,

M. Mehdi Ommati