Maybe rodents regulate a ratio of protein to dietary carbohydrates in order to achieve metabolic benefits (reduced insulin levels, improved blood glucose control, and, in the long term, reduced weight & fat gain)

What does self‐selection of dietary proteins in rats tell us about protein requirements and body weight control? Patrick C. Even  Joséphine Gehring  Daniel Tomé. Obesity Reviews, January 5 2021. https://doi.org/10.1111/obr.13194

Rolf Degen's take: https://twitter.com/DegenRolf/status/1346836414521081859

Summary: Omnivores are able to correctly select adequate amounts of macronutrients from natural foods as well as purified macronutrients. In the rat model, the selected protein levels are often well above the requirements estimated from the nitrogen balance. These high intake levels were initially interpreted as reflecting poor control of protein intake, but the selected levels were later found to be precisely controlled for changes in dietary protein quality and adjusted for cold, exercise, pregnancy, lactation, age, etc. and therefore met physiological requirements. Several authors have also suggested that instead of a given level of protein intake, rodents regulate a ratio of protein to dietary carbohydrates in order to achieve metabolic benefits such as reduced insulin levels, improved blood glucose control, and, in the long term, reduced weight and fat gain. The objective of this review was to analyze the most significant results of studies carried out on rats and mice since the beginning of the 20th century, to consider what these results can bring us to interpret the current causes of the obesity pandemic and to anticipate the possible consequences of policies aimed at reducing the contribution of animal proteins in the human diet.