Pregnancy and lactation easily fit within a 365-day year for sheep with time for a dry period. However, for cattle predicted lengths of pregnancy and lactation sum to more than 365 days. So to calve every year a cow must get pregnant while lactating and be both pregnant and lactating for some months. Cows are capable of this but are under more pressure than sheep to perform to this level every year.
Added to this is the extra load on some animals through selection for higher productivity. Modern dairy cows are an extreme example where they produce more milk than a calf needs. This puts more pressure on getting cows pregnant at the same time every year.
Beef cows run in less favourable environments may be under equivalent pressure at breeding time. Twinning may be “a bridge too far” for an annual calving beef cow herd.
Some traits show less variation within species than expected from body size and GSS. The most notable example is pregnancy length. Dog breeds differing in size 25 times – for example a Great Dane, 75kg, versus chihuahua, 3kg – have pregnancy lengths that vary little, typically 60-65 days. GSS theory predicts such different sized animals would have very different pregnancy lengths – 164 days for a 75kg adult, 70 days for 3kg. Why pregnancy length does not differ much within a species is not clear.
Dogs are an extreme example. From a wolf ancestor extreme variations in size, shape, colour and temperament have been bred. No other species has been changed so much by humans guiding breeding for utility (terrier versus wolfhound for hunting) or for fashion whims in pet breeds. In theory all dog breeds can interbreed – one definition of a species – even if physically improbable for extremes in size.
