With the invention of agriculture, humans domesticated three of those ruminants to help them with all sorts of tasks.
But rumination has a downside: it's not very efficient. In the process to turn plant matter into food, the animals lose a lot of energy.
One of those energy losses is methane gas. It forms when the bacteria break down the plants and is then burped into the air.
The amount of methane that an animal burps differs, from 2kg per year for a gazelle to 50kg per year for a giraffe. Our cows score even higher, between 50 - 100kg per year.
For thousands of years, huge herds of ruminants have walked the earth and burped methane.
So what's different now?
See for yourself...