The same effect can be observed in animals and one theory is that it's a stress reaction to densely populated environments. https://academic.oup.com/endo/artic

The same effect can be observed in animals and one theory is that it's a stress reaction to densely populated environments. https://academic.oup.com/endo/article/162/11/bqab154/6354390 https://academic.oup.com/endo/article/162/11/bqab154/6354390