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We've gotten from a wolf to a small breed such as a chihuahua. So the question is: can we go even further and have a dog as small as a thumb (adult dog)?

If not, what are the factors that limit the smallest size of a dog breed? Is it limitations in genetics, or some physiological reasons which would prevent such small dog to survive?

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    $\begingroup$ I'm not sure answers to this question would need to be opinion based, so I'm voting to reopen it....It asks for a biological explanation and not conjecture. Even if the answer is "we don't know", the question could still receive a valid, and informed response. $\endgroup$ Jan 8 '19 at 15:51
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    $\begingroup$ alternatively, I'm assuming you could rephrase your question to ask specifically what genes control dog size or perhaps a question that asks about a specific physiological limitation. $\endgroup$ Jan 8 '19 at 15:51
  • $\begingroup$ You may be interested in this essay. $\endgroup$
    – NatWH
    Jan 8 '19 at 16:04
  • $\begingroup$ How long do you have, and how many individuals? The problem seems to be that very small dogs (and purebred dogs in general) tend to have lots of health problems, so it would take many generations of many individuals to weed those out. $\endgroup$
    – jamesqf
    Jan 8 '19 at 19:38
  • $\begingroup$ Whales ans elephants come from small fish, so yes, deffo, itll take 200k years only. $\endgroup$ Apr 10 at 1:09
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As mentioned above the inbreeding required to select smaller and smaller dogs would be problematic because of the myriad of health problems that would be associated with it (see inbreeding depression).

The question about physiological limitations is interesting. The main reason biological organisms often don't scale is the fact that length, area, and volume don't scale in the same way. E.g. if something is twice as big L to 2L the surface area is L to L^2 and the volume is L to L^3 which means that many systems no longer work or require more energy (see Allometry) . For example in insects the distance for oxygen to diffuse to cells is short enough that simple diffusion is possible, in humans that is no longer the case so we have whole circulatory and respiratory systems to, in effect, help reduce the diffusion distance for oxygen.

I haven't used this kind of analysis in a while so am not very confident applying it here but I would suggest that some issues that could arise due to things scaling with length versus area or volume are:

  • required heart rate
  • maintaining body temperature - many small mammals deal with this by falling into periods of cold torpor (sort of like hibernation) where metabolic rate is suppressed
  • fragility of bones (as Cort Ammon mentions this is actually not a problem: https://jeb.biologists.org/content/207/9/1577)
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  • $\begingroup$ I believe bones are actually more fragile on large creatures, thanks to the square cubed law. However, maintaining body temperature is definitely a VERY hard challenge for a small creature. $\endgroup$
    – Cort Ammon
    Apr 10 at 1:23
  • $\begingroup$ Inbreeding will defeat the goal of breeding smaller animals because genetic diversity will decrease, which will, in turn, decrease the likelihood of further change. The goal would be to select for smaller size while maintaining genetic diversity. $\endgroup$
    – kmm
    Apr 10 at 16:15
  • $\begingroup$ Compare to the smallest mammals, like bumblebee bats and pygmy shrews. $\endgroup$
    – John
    Apr 11 at 5:18

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