For per cent weight change, 24 out of 24 time trends were positive (i.e. increasing). The probability of all out of 24 independent trend estimates being in the same direction by chance is 1.2 × 10^{−7}. For the odds of obesity, 23 out of 24 cases were positive (*p* = 3.0 × 10^{−6}; and ). When we combine males and females of each species into a single analysis, we find that in all 12 populations, per cent weight change and odds of obesity time trends were positive (*p* = 4.9 × 10^{−5}, for 12 out of 12 in the same direction). Given these overwhelmingly significant results at the ensemble or meta-analytical level, we describe the results below for samples from each individual population focusing on the magnitude of the coefficients. Standard errors, confidence intervals and *p*-values are shown in and .

| **Table 1.**Body weight changes per decade. (Per cent weight (PCT_WT) = 100 × (weight/norm), where norm is the sex-specific median of ‘baseline’ distribution and baseline distribution is distribution in first time interval if time intervals (more ...) |

We first examined primates living in highly controlled environments with nearly constant living conditions and diets. Across all three macaque populations, meta-analytically averaging the estimates weighted by the inverse of their variances yielded values of 7.7 per cent for the increase in body weight and a 86 per cent increase in the odds of obesity for males, and 7.9 per cent for the increase in body weight and a 144 per cent increase in the odds of obesity for females, on a per-decade basis. In the combined sex analysis, we find a 7.7 per cent increase in body weight and a 114 per cent increase in the odds of obesity. Among colonized chimpanzees, males and females, respectively, experienced a 33.2 and 37.2 per cent weight gain per decade, and a nearly 18-fold and 11-fold increase in the odds of obesity. In vervets, for females and males, respectively, there were 9.4 and 2.9 per cent increases in body weight per decade associated with 83 and 834 per cent increases in the odds of obesity. Among marmosets, females experienced a 9.7 per cent increase in body weight per decade, and a 1.73-fold increase in the odds of obesity. Among males, there was a 9.2 per cent increase in body weight per decade, and a 64 per cent increase in the odds of obesity.

Among mice in control groups in the National Toxicology Programme (NTP), there was a 11.8 per cent increase in body weight per decade from 1982 to 2003 in females coupled with a nearly twofold increase in the odds of obesity. In males there was a 10.5 per cent increase per decade. Among female rats in the NTP, there was a 0.2 per cent increase in body weight per decade, coupled with a 45 per cent increase in the odds of obesity, while among males there was a 6 per cent increase in body weight per decade coupled with a 1.25-fold increase in the odds of obesity.

Among animal species living in less-controlled environments, female cats experienced a 13.6 per cent increase in body weight per decade and an 84 per cent increase in the odds of obesity. Among male cats, there was a 5.7 per cent increase in body weight per decade, however a slight (not statistically significant) reduction in the odds of obesity. Male dogs experienced a 2.2 per cent increase in body weight per decade coupled with a 7 per cent increase in the odds of obesity per decade. Among female dogs, there was a 3 per cent increase in body weight per decade and a 13 per cent increase in obesity.

Finally, we examined a population of animals living close to people but not under their direct control. For the 1948–2006 time period, male rats trapped in urban Baltimore experienced a 5.7 per cent increase in body weight per decade from 1948 to 2006 and a nearly 20 per cent increase in the odds of obesity. Similarly, female rats trapped in urban Baltimore experienced a 7.22 per cent per decade increase in body weight, along with a 26 per cent increase in the odds of obesity. From 1948 to 1986, male rats trapped in the rural area gained 4.5 per cent in body weight, while females gained 5.2 per cent, and the increases in the odds of obesity were, respectively, 19 and 26 per cent. We did not find any evidence of nonlinear changes in weight increase and change in obesity prevalence that was statistically significant in any of the animals considered.

We next examined whether these body weight increases were different for male versus female animals. Recognizing that these analyses should be treated with some caution given that we are pooling across species, we compared the meta-analytically derived (i.e. averaged across all species and weighted by inverse of variances) point estimates, for males and females, and find that female animals experienced greater per cent weight gain and increase in the odds of obesity, but results are statistically significant only for the sex difference in the increase in odds of obesity (*Z* = 432, *p* < 0.0001).

Similarly, we examined whether body weight increases were greater for laboratory versus non-laboratory animals. The non-laboratory animals included urban rats, rural rats, and domestic cats and dogs. Again, we compared the meta-analytically derived estimates for each of these groups, and find that the laboratory animals show a greater increase in per cent weight gain and odds of obesity than non-laboratory animals (*Z* = 5.37, *p* = 0.003 and *Z* = 111, *p* < 0.0001, respectively).