In mammals and birds the external reproductive organs and waste removal organs see to be the same. Is this always the case with other animals such as insects etc...? Are there any good reasons for it?

  • 2
    $\begingroup$ Birds are animals too. $\endgroup$
    – kmm
    May 29, 2012 at 16:39
  • $\begingroup$ @Kevin You're right, fixed it $\endgroup$
    – John Smith
    May 29, 2012 at 17:24
  • 1
    $\begingroup$ I guess I'd also add "some mammals." $\endgroup$
    – kmm
    May 29, 2012 at 17:52

1 Answer 1


The cloaca, which is the common opening of the urinary, excretory, and reproductive systems, is present in birds as well as in non-avian reptiles (and thus presumably dinosaurs), amphibians, and monotremes (e.g., duck-billed platypus). To answer your first question, yes, this condition does seem to be universal for those groups mentioned above.

To answer your second question, evolutionary history is as good a reason as I can think of. The cloacal system has worked well enough for >350 million years (in the case of amphibians).

  • 3
    $\begingroup$ Another nice showcase that we weren’t designed (or that the designer just hated sex …). $\endgroup$ May 30, 2012 at 10:46
  • $\begingroup$ Or perhaps the designer was lazy, extremely short sighted and had no long term plan. Just ship it out as fast as you can before an impossibly short deadline. The designer was overworked, underpaid and had boss that only cared about short term profit. $\endgroup$
    – JayCkat
    Jun 25, 2018 at 15:49

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