This article contains links to several peer-reviewed research studies on the matter.

[e]xamined samples collectively consisting of over 20,000 animals from 24 populations of animals representing eight species living with or around humans in industrialized societies. In all populations, the estimated coefficient for the trend of body weight over time was positive (i.e. increasing). Surprisingly, we find that over the past several decades, average mid-life body weights have risen among primates and rodents living in research colonies, as well as among feral rodents and domestic dogs and cats.