contemplativeNone of the other languages I know has this specific feature.. in Italian for example animals are all simply "in calore", in Romanian they are "în călduri" etc.
I wonder if a)there are such specific terms maybe in local dialects in particularly agricultural regions and b)this means that in German for some reason communication of the fertile status of specific animals used for protection, nutrition and transport were more important than elsewhere? These terms are also rarely, if ever, used in an out-of-context fashion, as sometimes happens in other languages regarding the sexuality of animals (in Italian for example, "in calore" is quite often used for humans with intense sexual appetites, even if originally it refers only to animals).