Simple answer is domestic cats have fewer nephrons because they are very much smaller than humans and dogs.
But what you really want to compare would be nephrons per kg. Which for cats would be around 46,000/kg (4kg cat), around 20,000kg for dogs (20kg dog), and around 13,000/kg for humans (70kg human). I used the lower bound nephron estimate and some middling values for species weight. Obviously dog size varies considerably but I would assume chihuahuas have very much smaller kidneys and correspondingly smaller nephron counts.
What you really see is that cats have a lot of nephrons for their size.
Edit: Cat kidneys are efficient enough that they can derive most of their water from their food when they have a principally meat diet. And in the absecence of fresh water wild cats are seen to supplement with saltwater and can tolerate it for a fair while https://journals.physiology.org/doi/abs/10.1152/ajplegacy.1959.196.3.633. I recall while at university this was explained to me as a result of domestic cats’ prior evolution as a desert dwelling wild cat, but I don’t know how corroborated that hypothesis actually is.
All that said, domestic cats are fairly susceptible to kidney disease, although their kidneys are efficient for their size they are still very small in absolute terms. As a general rule house cats should be on a fairly low salt diet as a result.
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Simple answer is domestic cats have fewer nephrons because they are very much smaller than humans and dogs.
But what you really want to compare would be nephrons per kg. Which for cats would be around 46,000/kg (4kg cat), around 20,000kg for dogs (20kg dog), and around 13,000/kg for humans (70kg human). I used the lower bound nephron estimate and some middling values for species weight. Obviously dog size varies considerably but I would assume chihuahuas have very much smaller kidneys and correspondingly smaller nephron counts.
What you really see is that cats have a lot of nephrons for their size.
Edit: Cat kidneys are efficient enough that they can derive most of their water from their food when they have a principally meat diet. And in the absecence of fresh water wild cats are seen to supplement with saltwater and can tolerate it for a fair while https://journals.physiology.org/doi/abs/10.1152/ajplegacy.1959.196.3.633. I recall while at university this was explained to me as a result of domestic cats’ prior evolution as a desert dwelling wild cat, but I don’t know how corroborated that hypothesis actually is.
All that said, domestic cats are fairly susceptible to kidney disease, although their kidneys are efficient for their size they are still very small in absolute terms. As a general rule house cats should be on a fairly low salt diet as a result.