In most cases, males are easily separated from females during the breeding (rutting) season by the presence of antlers and, typically, a mane of longer hair. During the spring, young males can often be separated from females by the presence of developing antlers – these develop from bony structures, called pedicles, which constitute part of the skull. Some authors point to it being possible to sex newborn calves and even foetuses based on the swellings that will eventually form the pedicles. Indeed, in 1973 deer biologist Gerald Lincoln described how he was able to sex foetuses as young as 60 days old (some six months before birth.) by looking at head swellings associated with antler development. There are some (rare) instances where males fail to develop antlers altogether, usually because they fail to develop normal pedicles – these animals are referred to as hummels or notts and all other sexual development seems normal.
Closer inspection of the animal may reveal the penis sheath—the external genitalia is evident in foetal deer by just under two months old—and, especially during the rut, the stag’s underside will often be stained with urine. Males are typically larger and heavier than females and differences may be noticeable in the shape of the head. In a 2003 paper to the journal Acta Theriologica, three Spanish biologists demonstrated some sexual differentiation in the mandible (lower jaw) of Red deer from Spain. The biologists studied 126 mandibles and found that female mandibles are fully grown more than a year before those of males. The researchers also found that several of the measurements they took—including the angle of the jaw and size of some sections at the back of the jaw—were consistently larger in males than females. The biologists also observed significant variation associated with climatic factors, however, which may lead to some ambiguity in assigning sex. In his book Kia: A study of Red deer, Ian Alcock noted that Red deer have very elongated faces and that youngsters have quite short faces that elongate as they get older such that adult hinds have very long noses.
Outside of simply studying a deer’s appearance, it may sometimes be possible to sex them by proxy, using their droppings. One study, published in 1994, demonstrated that it was possible to sex Fallow deer on the basis of their scat pellets, although no similar data were included for Red deer. In 2008, however, a group of Chinese biologists were able to correctly assign the sex of 108 (59%) of the 183 faecal samples collected from Red deer by subjecting them to genetic analysis. Finally, a study published during 2005 by a team of American biologists, led by Douglas Tolleson at the Texas A&M University, demonstrated that it was possible to identify both sex and species of deer by bouncing infrared light off their scat pellets in a process known as Near Infrared Reflective Spectroscopy (or NIRS, for short). However, the team had mixed success with their samples, being able to identify Fallow more consistently than Red and females with greater accuracy than males.
The sexes generally spend most of the year apart (see Behaviour and Social Structure), coming together in the Autumn to mate (see Breeding Biology). Males are called stags, females are hinds and the young are calves.