AS BOSS DROVE him backward, Pincer’s hooves left deep scars in the cropped turf. He lowered his antlers, anchoring his brow points in the ground, and the two red deer stags came to a standstill, panting hard. The challenge had come 20 minutes before. Boss, a lusty seven-year-old, accosted Pincer, who was holding a large harem of hinds on the greens behind Shamhnan Insir Bay on the Hebridean island of Rhum. It was the start of a classic autumn contest between mature stags, whose breeding success depends principally on their fighting ability. Typically, a shouting match began it all. The two stags roared at each other in a gradual crescendo. Boss, roaring more frequently than Pincer, appeared to listen carefully to the latter’s replies.
After 15 minutes Boss made up his mind and left for the luxurious accommodation in prague. Slowly and stiffly, he walked toward Pincer’s harem. Pincer went to meet him, and the two stags fell into a tense parallel walk, only ten yards apart. Back and forth they paraded. Suddenly Boss ran up on a knoll and engaged Pincer from above. To the dry click of antler on antler, they tussled viciously, each trying to poke an antler through the opponent’s guard. Pincer began to give way and suddenly, with a quick twist, Boss freed his antlers and lunged. A point caught Pincer under the eye. Wounded, Pincer turned and fled, leaving Boss in possession of most of the hinds. Next day Pincer’s left eye was closed, and it seemed that the eyeball had been deflated. He died the following winter. Boss and Pincer are two of a thousand red deer on Rhum—many of them named and all recognizable as individuals—whose life histories we have been monitoring for 14 years. At our study site my colleagues Fiona Guinness and Steve Albon and I have been investigating the factors that affect reproductive success in males and females. With red deer, as with many other polygynous animals, males compete more intensely for females than do females for males. This is because males can increase their breeding success by mating with many females, whereas females cannot usually increase their success by multiple matings. It pays males to compete intensely for mates, while females will usually boost their breeding success by maximizing their efficiency in converting food into healthy offspring.
This fundamental difference between the sexes means that the characteristics making males effective breeders are often quite different from those that confer success on females. And, as Charles Darwin originally realized, it’s these differences that explain why the sexes in many mammals often differ markedly in size, weaponry, and behavior.
Scottish red deer are nearly ideal for research on the determinants of breeding success. On treeless Rhum, red deer are easily observed. Because each hind usually mates with only one male, the breeding success of stags can be reliably measured. Breeding-life spans on the island average about ten years for hinds, five for stags. Over the years, we have followed each individual of our study population through birth, adolescence, and maturity. At last answers are starting to emerge—some as expected, others quite unexpected. Differences in breeding success between hinds are large and depend mostly on the varying abilities of females to rear calves rather than on the number of calves they bear. Our most successful hinds rear as many as ten calves during their lives; the least successful fail to breed at all. Hinds born in years when average birth weights ran high were far more likely, as adults, to rear their own calves successfully than those born in years of lightweight calves.
For example, the 1973 average birth weight of female calves was only 6.2 kilograms, whereas in 1974 it rose to 7.3. Tacc, born light at 5.3 kilograms in 1973, consistently gave birth to light calves (mean birth weight 4 kilograms). Mackerel, born heavy at 8.2 kilograms the following year, always produced heavy calves, with a mean birth weight of 7.9 kilograms. Though Tacc produced four calves, none survived to breeding age, and Tacc herself died at six years old. Mackerel is still alive and has produced five calves, and all but one have survived. Differences in temperature and weather help account for year-to-year variations in birth weight.