Results 1 to 21 of 21

Thread: Male mammalian genitalia

  1. #1
    Join Date
    May 2004
    Posts
    1,674

    Male mammalian genitalia

    The MMG of marsupials and placentals are reversed vis-a-vis each other.
    Which arrangement did their common ancestor have?
    How did the arrangement reverse in one of them? How can you evolve half of the change...a degree of arc a millennium?

  2. #2
    Join Date
    Jun 2006
    Posts
    4,664
    Could you give some references for what you are attempting to discuss, please? Links would help.
    Last edited by John Mendenhall; 2018-Feb-12 at 11:30 PM.

  3. #3
    Join Date
    May 2004
    Posts
    1,674
    https://imgur.com/gallery/Axcdx picture 9
    quote:
    Kangaroo Penis
    In Australia, everything is backwards. Even [language].
    Last edited by PetersCreek; 2018-Feb-12 at 10:36 PM. Reason: Language

  4. #4
    Join Date
    Jul 2005
    Posts
    16,258
    They're not reversed. The kangaroo simply has its testes above its penis. Since mammalian testes develop high in the adomen and then migrate down the inguinal canals to hang outside the body, they actually arrive in the outside world more or less level with the penis on either side. In most mammals they then move downwards, to hang below the penis. In kangaroos they move upwards to hang above the penis. This sometimes happens in humans, too, when the testes take a wrong turning and end up in the abdominal wall rather than the scrotum.

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  5. #5
    Join Date
    May 2004
    Posts
    1,674
    But which way was it originally? And how and why did one branch evolve the other way?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

  6. #6
    Join Date
    Jul 2005
    Posts
    16,258
    Given the embryological origin of the testes, it was neither way originally. And many mammals still keep their testes internally - the playtpus, the elephant, the aardvark.

    Marsupials and placental mammals evolved their scrotums independently, from ascrotal ancestors.

    And there are a group of mammals who seem to have backtracked on the scrotum - their testes start to migrate, but remain internal.Seals, whales, rhinos.

    Why one way or the other? We're not even sure why they migrate at all, given that some mammals manage without having external testes.

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  7. #7
    Join Date
    Jul 2005
    Posts
    16,258
    BTW, this thread reminded me of a couple of pleasing words:
    Scrotality - the state of having a scrotum.
    Testicondy - the state of having testes in their original location, beside the kidneys.

    Between those two extremes there are various states of partial descent, including testes that move into the pelvis but remain internal, and those that move through the inguinal canal but remain attached to the abdominal wall.

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  8. #8
    Join Date
    Sep 2006
    Posts
    457
    I recall an explanation, for humans, that the testes are exterior because the temperature that is ideal for sperm production is a bit lower than body temperature. Is that not well supported anymore?

  9. #9
    Join Date
    Jul 2005
    Posts
    16,258
    Quote Originally Posted by Darrell View Post
    I recall an explanation, for humans, that the testes are exterior because the temperature that is ideal for sperm production is a bit lower than body temperature. Is that not well supported anymore?
    That's the story that's commonly given for the evolution of the mammalian scrotum. But why then would elephants keep their testes internal, and rhinos keep them plastered against the abdominal wall? Both have body temperatures within a degree of humans'.
    There's no doubt that human testes produce fewer sperm if they're kept warm, but it seems that rhinos can produce adequate sperm at 38 deg. C - so it seems likely that the testes in each species have adapted to operate best at their ambient temperature, rather than having had to migrate to find a suitable temperature.

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  10. #10
    Join Date
    Sep 2006
    Posts
    457
    Thanks Grant. It is damn inconvenient at times having them on the outside! Seems like there must have been a significant advantage conferred to win out over the apparent disadvantages of apparently being more at risk of damage. But, maybe that is a just-so story concocted by self pitying males and external testes aren't significantly more at risk of damage, at least damage that results in loss of function.

  11. #11
    Join Date
    Jul 2005
    Posts
    16,258
    Quote Originally Posted by Darrell View Post
    Thanks Grant. It is damn inconvenient at times having them on the outside!
    Handy for picking up testicular cancer early, though. Ovarian cancers are notoriously late to present, with an associated high mortality.

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  12. #12
    Join Date
    Sep 2003
    Location
    The beautiful north coast (Ohio)
    Posts
    47,483
    Quote Originally Posted by grant hutchison View Post
    Handy for picking up testicular cancer early, though. Ovarian cancers are notoriously late to present, with an associated high mortality.

    Grant Hutchison
    Understandable given modern medicine, but I don't understand why that would be an evolutionary driver in pre-medicine humans. So again, if temperature isn't the driver, what is the driver for internal versus external?
    At night the stars put on a show for free (Carole King)

    All moderation in purple - The rules

  13. #13
    Join Date
    Nov 2004
    Posts
    3,960
    Could it be sexual selection signaling?

    "Hey ladies, get a load of these."
    Calm down, have some dip. - George Carlin

  14. #14
    Join Date
    Jul 2005
    Posts
    16,258
    Quote Originally Posted by SkepticJ View Post
    Could it be sexual selection signaling?

    "Hey ladies, get a load of these."
    Yeah, that's one of the hypotheses. But in that case there should be selection pressure for every bigger and more elaborate scrotums. Which we don't see except in a few primates. (Thank goodness.)
    Another option is the "galloping" hypothesis. Four-legged running animals develop high intra-abdominal pressures, and it's hypothesized that these pressures would squeeze sperm out of the testicles before it had properly matured. But gallopers manage to run without squeezing urine out of the their bladders, so it seems an elaborate anatomical adaptation (moving the testes right out of the abdomen), when some sort of sphincter mechanism (as in the urinary tract and gut) would potentially work just as well.
    Another temperature-related hypothesis is that sperm are "activated" by the ... um ... transition from the cool testicular environment to the warmer environment of the female genital tract. But elephants and rhinos and hedgehogs seem to get by fine without such a trigger.

    There just isn't a hypothesis that doesn't immediately invite "Yes, but".

    Grant Hutchison
    Last edited by grant hutchison; 2018-Feb-13 at 08:34 PM. Reason: activation hypothesis
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  15. #15
    Join Date
    Jul 2005
    Posts
    16,258
    Quote Originally Posted by Swift View Post
    Understandable given modern medicine, but I don't understand why that would be an evolutionary driver in pre-medicine humans.
    Sorry, I didn't intend to offer it as such - just offering some comfort to Darrell for all the inconvenience.

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  16. #16
    Join Date
    Jul 2005
    Posts
    16,258
    Seems like there's a research project for someone in this. The original "temperature hypothesis" invokes the fact that aromatase (which converts testosterone to oestrogen) is much more active at body temperature than scrotal temperature. Seems like someone should have looked at the aromatase variants in elephants and hedgehogs, etc, to see what the temperature/activity profiles of their aromatases look like. But I can't find any relevant research. (Albeit, I'm swamped by hits on temperature-dependent sex-determination in reptiles,which involves the same molecule.)

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  17. #17
    Join Date
    Feb 2003
    Location
    Depew, NY
    Posts
    11,253
    Quote Originally Posted by grant hutchison View Post
    Seems like there's a research project for someone in this.
    Cue Deadpool style quote.
    Solfe

  18. #18
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    7,738
    If testes produce more sperm when a bit cooler, would that not be enough to confer an advantage? I believe quite a large number is still held to be "fertile" a few million just will not do. It also seems possible that fecundity would become socially associated too, becoming a partnering advantage. The social aspect would be more strong in language equipped humans than rhinos and elephants.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  19. #19
    Join Date
    Jul 2005
    Posts
    16,258
    Quote Originally Posted by profloater View Post
    If testes produce more sperm when a bit cooler, would that not be enough to confer an advantage? I believe quite a large number is still held to be "fertile" a few million just will not do. It also seems possible that fecundity would become socially associated too, becoming a partnering advantage. The social aspect would be more strong in language equipped humans than rhinos and elephants.
    Why then would a highly social animal like an elephants lack a scrotum, whereas a non-social animal, like a cheetah, has one?

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  20. #20
    Join Date
    Jul 2006
    Posts
    12,140
    I've got a half a dozen stories I could contribute to this thread and not a single one I want to repeat in front of librarians and middle schoolers. The latter more than the former.
    Time wasted having fun is not time wasted - Lennon
    (John, not the other one.)

  21. #21
    Join Date
    Jul 2006
    Posts
    12,140
    Scar 'em for life it would.
    Time wasted having fun is not time wasted - Lennon
    (John, not the other one.)

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •