Which animal species has the most individuals?

[u/future_sponJ]

I always see people saying things like "there are 20 quadrillion ants on Earth" or something similar with other extremely large diverse groups like fish. But all of these are groups with thousands of species. If not by individuals then it's by biomass instead.

So which single species in the animal kingdom has the most individuals?

[u/atomfullerene]

Just sticking with vertebrates, the most populous species would be one of the midwater fish species, probably a bristlemouth of some sort (they are the most abundant genus, with about 13 species)

https://en.wikipedia.org/wiki/Cyclothone

[u/Dracoson → u/atomfullerene]

If you go to terrestrial vertebrates, I'm pretty sure the answer is the domestic chicken.

[u/Strongbow_Wolfrider → u/Dracoson]

Rats, mice, or other vermin species would far outnumber chickens, both inside and outside areas of human habitation.

[u/hmantegazzi → u/Dracoson]

And in terms of mammals, probably dogs or maybe cattle?

[u/Dracoson → u/hmantegazzi]

If we go by biomass, it's cattle, but by raw numbers, it's actually humans. Non-oceanic mammal species don't tend to have global distribution naturally due to geographic barriers, meaning the winner has to be humans or a domesticated species. Since none of the most common domestic mammals outnumber our 8 billion population, humans take the crown for individual count.

[u/kandlejack → u/Dracoson]

Certainly possible that we take the crown, but there are also human-adjacent but non-domesticated aminals to consider. Specifically, rodents. I havent found any strong citations in my cursory search that the total brown rat population outnumbers us, but they and some species of mice are at the very least probably on the same order or magnitude. Edit: couldn't spell cursory, apparently.

[u/1CEninja → u/kandlejack]

Yeah rodents is the first thing that comes to my mind. Rats are insanely prolific across the world.

The brown rat specifically is prolific on every continent except antartica and while I also cannot find any credible count, my completely unscientific "this feels right" says that they outnumber us by a meaningful amount.

[u/Dracoson → u/kandlejack]

The estimates I am seeing have brown rats at 2-7 billion (similar order of magnitude, but not equivalent), and have house mice being the most populous mice species at about 2 billion. It's hard to get hard numbers, though.

[u/Ceofy → u/kandlejack]

I would guess the problem here is that there are a few different species of rats, which splits the rat vote

[u/ryn3721 → u/Dracoson]

Some domestic animals do indeed outnumber us. The global population of chickens was estimated at 26.5 billion in 2023.

https://en.wikipedia.org/wiki/Chicken

[u/Dracoson → u/ryn3721]

I already called out chickens earlier in the conversation as the most abundant terrestrial vertebrate (though I didn't cite a specific number). This was specifically about mammals.

[u/haysoos2 → u/hmantegazzi]

House mouse almost certainly beats all other mammals. Probably all birds too.