Have you had a good look at the foxes that visit your garden recently? Over the years, do you think they’ve been getting bigger or smaller? The trouble is, there are good reasons to expect both trends.
To explain why, let me first of all introduce you to Bergmann’s rule, arguably the oldest rule in ecology, in fact it predates the coining of the word “ecology” by nearly 20 years.
One of the few ecological rules that really is true more or less all the time, Bergmann’s rule says that among groups of related warm-blooded animals, body size increases as you get further from the equator.
The reason is surface-to-volume ratio, which means that larger animals find it easier to keep warm in cold climates, while small animals lose heat and thus stay cool more easily in hot ones.