# What is the most numerous free-living species of mammal outside of humans and rats? **URL:** \ **Category:** Factual Questions\ **Created:** [December 8, 2011, 8:27pm UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555 "2011-12-08T20:27:03Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [December 8, 2011, 8:27pm UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/1 "2011-12-08T20:27:03Z")
I was watching the BBC’s Frozen Planet (You HAVE to see it - gorgeous!) and they talk about certain species of penguins numbering 5 million strong in one location (yikes!). Got me to thinkin’ about mammals. Of course we are the most numerous mammal on earth (aren’t we? Are there 6 billion squirrels? Rats? Mice?) but what other mammal, aside from the rodents just mentioned, still exist in massive numbers? Cats and dogs, of course, but that’s because they live with us. So I guess that cattle and pig numbers are also pretty spectacular for the for the same reason. So let me modify my question: what mammals that are living free from human domestication? And I’ll be more specific still: I’m asking for species that can interbreed, because I’m pretty sure that rodents in general have the highest numbers overall. My guess would be some kind of bats, which seem to only live in shockingly enormous groups. (The world’s largest animal migration is fruit bats, ten million of which congregate in one valley in Zambia for six weeks each year to eat what must obviously be a stunning amount of fruit.) But then again, ten million bats may be nothing if compared to another animal that doesn’t congregate so noticably but still exists in massive numbers spread all over. So, any takers? ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [December 8, 2011, 10:24pm UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/2 "2011-12-08T22:24:50Z")
Some species of bat is a good guess (20% of all mammal species are bats), but I’d bet the Eastern Cottontail rabbit, _Sylvilagus floridanus_, would beat them out. Not sure we would actually be able to know the answer to this question for certain. ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [December 8, 2011, 10:34pm UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/3 "2011-12-08T22:34:14Z")
What about wolves? They have a very large range and they aren’t domesticated. They may not be the top but they are native to large parts of North America, Asia, and Europe. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [December 8, 2011, 10:36pm UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/4 "2011-12-08T22:36:16Z")
My guess is that it’s a domesticated species. Looking in Wikipedia, those to beat are cattle (1.3 billion) and sheep (1 billion). ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [December 8, 2011, 10:39pm UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/5 "2011-12-08T22:39:29Z")
> [@Shagnasty](#): > > What about wolves? They have a very large range and they aren’t domesticated. They may not be the top but they are native to large parts of North America, Asia, and Europe. They have a wide range, but are going to be rare compared to many species of rodent. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [December 8, 2011, 11:30pm UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/6 "2011-12-08T23:30:32Z")
> [@Shagnasty](#): > > They may not be the top but they are native to large parts of North America, Asia, and Europe. The correct answer (if knowable) is more likely to be toward the bottom of a hierarchy. Prey species survive by reproducing in very large numbers, compared to the predator species that eat them. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [December 8, 2011, 11:47pm UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/7 "2011-12-08T23:47:03Z")
> [@t-bonham@scc.net](#): > > The correct answer (if knowable) is more likely to be toward the bottom of a hierarchy. Prey species survive by reproducing in very large numbers, compared to the predator species that eat them. Except that the most numerous mammal species is Homo sapiens, which is a predatory species, but clearly also a very unusual one. ---
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [December 9, 2011, 12:02am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/8 "2011-12-09T00:02:17Z")
> [@Colibri](#): > > They have a wide range, but are going to be rare compared to many species of rodent. > [@t-bonham@scc.net](#): > > The correct answer (if knowable) is more likely to be toward the bottom of a hierarchy. Prey species survive by reproducing in very large numbers, compared to the predator species that eat them. Exactly! Wolves aren’t anywhere in the running…not even a thousand years ago! Land predators are never found in enormous numbers, and certainly not in numbers that outstrip prey numbers - that way lies starvation! The ocean is different, since almost everything in it qualifies as a predator of some kind or another; the ocean has nowhere near enough plant life to support very many vegetarians. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [December 9, 2011, 12:06am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/9 "2011-12-09T00:06:20Z")
> [@Giles](#): > > Except that the most numerous mammal species is Homo sapiens, which is a predatory species, but clearly also a very unusual one. Nonsense! Just in this thread alone, people have mentioned mice, rats, & bats as mammal species that are way more numerous than humans. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [December 9, 2011, 12:07am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/10 "2011-12-09T00:07:50Z")
Note that the wolf species Canis lupus is quite numerous, because it includes domesticated dogs – there are about 400 million of those, vastly outnumbering their wild siblings. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [December 9, 2011, 12:08am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/11 "2011-12-09T00:08:57Z")
> [@t-bonham@scc.net](#): > > Nonsense! > > Just in this thread alone, people have mentioned mice, rats, & bats as mammal species that are way more numerous than humans. Any cites on what their numbers are, by species? ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 9, 2011, 12:13am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/12 "2011-12-09T00:13:12Z")
Ding! Beetles! Oh. NM. ---
**Author:** ![Fubaya](https://avatars.discourse-cdn.com/v4/letter/f/c2a13f/32.png) [@Fubaya](https://boards.straightdope.com/u/Fubaya)\ **Post date:** [December 9, 2011, 12:16am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/13 "2011-12-09T00:16:24Z")
There’s rodents, bats, then everything else. Rodents are said to be around 40% of all mammals and considered to be the most numerous, with bats in second place at 20%. I found [this chart](http://upload.wikimedia.org/wikipedia/commons/c/c7/Mammal_species_pie_chart.png) on wikipedia which doesn’t show anything coming close to those two. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [December 9, 2011, 12:16am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/14 "2011-12-09T00:16:25Z")
Incidentally, there is an [earlier thread on this topic](http://boards.straightdope.com/sdmb/showthread.php?t=617915), and no one then could find a mammal species more numerous than humans (7 billion) or a bird species more numerous than chickens (25 billion). It’s possible that a species like Mus musculus might outnumber humans, but I’d like to see an estimate of its numbers. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [December 9, 2011, 12:18am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/15 "2011-12-09T00:18:07Z")
> [@Fubaya](#): > > There’s rodents, bats, then everything else. Rodents are said to be around 40% of all mammals and considered to be the most numerous, with bats in second place at 20%. I found [this chart](http://upload.wikimedia.org/wikipedia/commons/c/c7/Mammal_species_pie_chart.png) on wikipedia which doesn’t show anything coming close to those two. That chart is interesting, but it shows numbers of species, not numbers of living specimens within each species. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [December 9, 2011, 12:23am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/16 "2011-12-09T00:23:51Z")
> [@Giles](#): > > Note that the wolf species Canis lupus is quite numerous, because it includes domesticated dogs – there are about 400 million of those, vastly outnumbering their wild siblings. I would have thought there were more. That’s only ~1 for every 20 hoomans. No one is buying my thoughts that it’s Peter Cottontail? If we rule out rodents, it has to be some sort of a rabbit or a bat. > [@t-b](#): > > The correct answer (if knowable) is more likely to be toward the bottom of a hierarchy. Prey species survive by reproducing in very large numbers, compared to the predator species that eat them. You’re assume the prey is also a mammal. And in those cases, you’re right. But when your prey species is insects, it’s a different ball game. Hence, the possibility of a species of bat being our candidate. ---
**Author:** ![Rachellelogram](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rachellelogram/32/8689_2.png) [@Rachellelogram](https://boards.straightdope.com/u/Rachellelogram)\ **Post date:** [December 9, 2011, 12:26am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/17 "2011-12-09T00:26:22Z")
> [@Giles](#): > > Incidentally, there is an [earlier thread on this topic](http://boards.straightdope.com/sdmb/showthread.php?t=617915), and no one then could find a mammal species more numerous than humans (7 billion) or a bird species more numerous than chickens (25 billion). It’s possible that a species like Mus musculus might outnumber humans, but I’d like to see an estimate of its numbers. Makes you wonder how many chickens would exist if they weren’t being bred by humans. Thanks for the numbers! ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [December 9, 2011, 12:50am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/18 "2011-12-09T00:50:09Z")
> [@Stoid](#): > > The ocean is different, since almost everything in it qualifies as a predator of some kind or another; the ocean has nowhere near enough plant life to support very many vegetarians. Unless pelagic animals have learnt how to photosynthesize, this cannot possibly be true. > [@John\_Mace](#): > > If we rule out rodents, it has to be some sort of a rabbit or a bat. I do not think the OP intends us to rule rule out rodents, she just wants a specific species named rather than the whole order _rodentia_. Note also that although rabbits are widespread, your Eastern Cottontail rabbit is not teh same species as rabbits found outside eastern North America. ---
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [December 9, 2011, 1:50am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/19 "2011-12-09T01:50:10Z")
> [@njtt](#): > > I do not think the OP intends us to rule rule out rodents, she just wants a specific species named rather than the whole order _rodentia_. Correct. So the question is probably which rodent? Which bat? ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [December 9, 2011, 1:59am UTC](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555/20 "2011-12-09T01:59:27Z")
Doing some Wikipedia research on potential contenders, it appears the most likely correct answer is the Norway rat \*(Rattus norvegicus{/i], brown rat, Hanover rat, sewer rat, etc.). The other two potential rodents, the house mouse _Mus musculus_ and th common rat _Rattus rattus_, are nowhere as widely distributed as the Norway rat. While bats are _very_ widely distributed, they speciate very easily, and it appears that no one species is anywhere near as widespread as _R. norvegicus_ or even _M. musculus_. The four most common genera of bats are little brown bats (Myotis), big brown bats (Eptesicus), pipistrelles (Pipistrellus), and Old World horseshoe bats (Rhinolophus). No single species, even taking a “lumper” viewpoint, is anywhere near as widespread as the rodents. And so far as i can tell, nothing else even comes close. [Next page](https://boards.straightdope.com/t/what-is-the-most-numerous-free-living-species-of-mammal-outside-of-humans-and-rats/605555.md?page=2)