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I apologize for the weird question, but based on a Google search it looks like this topic hasn't been discussed before. I'm hoping that enough information about population biology is stored in the fossil record to answer this question.

Humans have wiped out a lot of animals in a very short time, but I realized that mammals might be doing very well.

First off, humans are mammals, and there are 7.6 billion of us. That's quite a lot. I don't believe any other animal species of our body size has such a large population.

Second, humans have pets. A lot of them. And they're almost all mammals. Everywhere we go we take our cats and dogs with us. Many of them become feral and multiply prolifically.

Third, our livestock. With the growth of human civilization came the growth of meat consumption, which, naturally, meant a lot more cattle. I think I read somewhere that cattle are contributing more to global warming than cars, through burping.

Lastly, and probably most importantly: rodents. Cats chased us everywhere because rats chased us everywhere. They thrived on the grain we grew and their population exploded to the point that they became a global pest, and possibly humanity's greatest enemy.

So, my hypothesis is that there are more mammals now than ever before in the history of life. Not in terms of species diversity, but in terms of headcount. If this is true, it would be remarkable, since the rest of the Animal Kingdom is currently suffering through a mass extinction.

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  • $\begingroup$ Interesting question. At first, my knee-jerk answer was "yes". But then I saw you were talking about individuals, and not species. Humans have certainly skewed the numbers, favoring domesticated species, and eliminating others. $\endgroup$ – Karl Kjer Dec 18 '17 at 13:11
  • $\begingroup$ During the ice age, when there were many great plains, there must have been a staggering number of prairie dogs, lemmings, gophers etc. I don't have time for an in-depth literature search but it appears that studies focus on biomass and less on numbers. $\endgroup$ – Roland Dec 18 '17 at 13:26
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There can be perhaps 10 trillion rodents and bats on the planet, so the humans and livestock probably are small compared to a rainforest rodents and bats. The biggest bat colony is 40 million, they decline without unmanaged forests, can reach 4-10 bats per hectare, and the world population might normally be in 10s of billions, save for the use of pesticides and deforestation.

Here's a bit of maths to say that we have erased 1 trillion rodent habitats through monocultures and ploughing. Every cow and sheep field would have previously supported 10-20-50 times more rodents, and they aren't adapted for pesticides and ploughs.

37% of the world's land is farmed Extent of total cultivated land in 2006: 1.5 billion hectares.

If there were 14 mammals per km2 of farmland, then we have displaced 20 billion mammals.

If the pristine habitats previously supported 1 rodent/mammal every 5m2, we would have destroyed the homes of 2000 mice,voles,shrews,squirrels and other mammals by hectare, that's 3000 Billion mammals (worst case scenario) due to monocultures and farming, and 1000 Billion in a best case scenario. Bangladesh rice farms and UK fields both are hostile to mammals.

A 20g mouse for 5m2 comes to the same as one human for 15 square kilometers. A mouse only needs a 3g of seeds/roots/beetles/snails per day to survive, that's about a kilo per year. "blue oak, interior live oak, and black oak respectively produce 2955, 1074, and 616 kg/ha of tree canopy" invertebrates provide 100-300kg/ha.

Perhaps we have erased 1 trillion mammals in the amazon alone.

https://www.worldatlas.com/articles/list-of-primates-by-population.html

Also we have depleted the fish stocks. It is reported that in medieval times, the seas heaved with incredible amounts of dolphins, whales and fish stocks, pristine seal and dolphin numbers could have been about 1 billion. Perhaps today it's about 1/4 of that.

If i had to bet all I own on your theory, I'd say it happened at another time in the previous 60mn years, probably not today.

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  • $\begingroup$ It looks like this comes down to rodent population. I'm surprised the density can go as high as 10mice/m^2 in any large area. Source? $\endgroup$ – StarlightDown Dec 19 '17 at 21:17
  • $\begingroup$ I had a hard time finding any sources that compare mouse density in woods to their density in farmland. But my intuition was always that mice benefitted from human expansion. We wiped out their big predators and left wide open row crops for them to munch on. This back-of-the-napkin calculation gives a ridiculously high mouse count for one agricultural region. Also, consider their introduction as invasives in places like NZ. $\endgroup$ – StarlightDown Dec 19 '17 at 21:18
  • $\begingroup$ I was researching it a bit fast, I did mice m2 in and the first page i had was 10/m2 so i added that, I have edited it. sciencenordic.com/rodent-population-swings-remain-mystery Sep 4, 2015 70 to 100 rodents for each 1000 m2 of forest floor in southern Norway. a cold country with slow growing forests and smaller tree seed harvests. primeval forests have heaps of dead wood everywhere and masses of seedfall. $\endgroup$ – com.prehensible Dec 19 '17 at 22:18
  • $\begingroup$ I did a bit more research. I wouldn't say it's a certainty that more farmland means fewer rodents. This study found a maximum density of 379 rodents/ha in an Ethiopian wheat field. However, this other study found a maximum density of just 35 rodents/ha in an Ethiopian forest. $\endgroup$ – StarlightDown Dec 20 '17 at 6:03
  • $\begingroup$ Of course, these are just two papers, and maybe another two papers will give the complete opposite result. In any case, do you have a source for the 2000 rodents/ha figure in your first post? I looked around, but never found a density that high. $\endgroup$ – StarlightDown Dec 20 '17 at 6:04
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I think that in order to find out whether there as many mammals previously as there are right now, we would need to use procedures like DNA-seq and mTDNA sequencing to identify clusters of mammal extinction or the now extinct ancestors of current mammals.

Here are some relevant articles:

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0003602

http://www.pnas.org/content/91/25/12336.short

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