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Why do mammals generally have two testes?

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    $\begingroup$ My guess would be symmetry and redundancy. :-) $\endgroup$
    – MCM
    Commented Sep 25, 2012 at 4:15
  • $\begingroup$ I guess the same could be asked of ovaries $\endgroup$
    – harpalss
    Commented Sep 25, 2012 at 8:14
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    $\begingroup$ And, you could probably ask the same about: ears, eyes, nipples, etc. $\endgroup$
    – Steve Lianoglou
    Commented Sep 25, 2012 at 17:03
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    $\begingroup$ Two eyes and ears is to do with stereo receptivity, having two sensors of the same type in different places helps to gather much more information about distance and position (triangulation) of the source. One cool example of this is the hammerhead shark. It's believed that it has smell sensory organs in either end of its head. The further apart they are the better the sharks are at finding prey. It is also why predators have the eyes on the front of their head - so they work in tandem. news.bbc.co.uk/earth/hi/earth_news/newsid_8376000/8376740.stm $\endgroup$
    – rg255
    Commented Nov 7, 2012 at 13:48
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    $\begingroup$ Mammals inherited them, testes are paired way back in fish. $\endgroup$
    – John
    Commented Aug 5, 2017 at 3:59

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EvoDevo is not my field, but I will try to give you some pointers.

Mammals as vertebrates, start out as fertilized oocytes, transform into a rapidly dividing lump of cells with eventual polarity through numerous gradients of numerous chemical messengers, and form a neural tube. The forming body is ever more segmented through the use of gradients and thresholds (see french flag model) and (dorsoventral) axial symmetry is maintained throughout development, and as such two testes represent the default development.

Therefore, it may be more interesting to ask for singular organs which require more information, and thus maintenance throughout evolution of this information. But when considering the energy requirements, the need for synchronization, the need for a separate blood supply and thus the introduction of additional faults, it is easy to see why we only have one heart.

Wikipedia; author: Zephyris Wikipedia; author: Zephyris, title: "The first few weeks of embryogenesis in humans", DOR: 25/09/2012

See:

I hope this helps as a starting point.

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    $\begingroup$ I might further speculate that the female reproductive system has strong axial symmetry and the testes are probably following the conservative development pattern of the female reproductive system. $\endgroup$
    – shigeta
    Commented Sep 25, 2012 at 18:18
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Short answer, they didn't. It evolved long before mammals and we just inherited it.

Becasue the testes form from the mesoderm, all mesoderm organs are bilaterally symetric or start so developmentally in chordates. Since they do not form on the midline like the spleen, they are paired by default. The testes NOT being paird would requires further explination and some evolutionary function to develop. Since testes is essential to chordate reproduction, a reduction in their function and redundancy would need a very strong evolutionary pressure to happen.

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Because mammals are bilaterians, which have bilateral symmetry (as opposed to cnidarians' radial symmetry).

Bilateral symmetry means your left side should be symmetrical to the right side - two eyes, two ears, two hands, two feet, two testes.

Edit: also adding @John's comments below:

Those develop from the endoderm, which does not form paired organs, everything from the mesoderm and ectoderm, testes included is paired or starts paired. – John

The heart is a paired organ, they are just joined, the gut is part of the endoderm which is not paired. Why do we have two kidneys but one liver? – John

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  • $\begingroup$ By your logic, why then are there not two (or one symmetric) stomachs, livers or hearts? $\endgroup$
    – Domen
    Commented Aug 3 at 18:42
  • $\begingroup$ Vertebrates have a generally bilaterally symmetrical body-plan, but this symmetry is broken by the consistently asymmetric placement of various internal organs such as the heart, liver, spleen, and gut, or the asymmetric development of paired organs (such as brain hemispheres and lungs). - link - generally bilateral symmetry is the ancestral state, while left-right assymetries are derived features that evolved subsequently $\endgroup$
    – sashoalm
    Commented Aug 4 at 11:35
  • $\begingroup$ @Domen because those develope from the endoderm, which dos not form paired organs, everything fromed from the mesoderm and ecoderm, testes included is paired or starts paired. $\endgroup$
    – John
    Commented Aug 4 at 21:07
  • $\begingroup$ @sashoalm the heart is a paired organs, they are just joined, the gut is part of the endoderm which is not paired. biology.stackexchange.com/questions/100928/… $\endgroup$
    – John
    Commented Aug 4 at 21:08
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    $\begingroup$ @John, I don't need an explanation – it should be provided in the answer, that's why I made the comment and downvoted this answer. $\endgroup$
    – Domen
    Commented Aug 4 at 21:09

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