Orang-utans' sex habits seem to come under pressure when conditions are tough, leading young males to use violence in order to have sex with older females, writes William Reville.
We have all seen animals copulating, particularly dogs. Among canines, I have often seen a bitch not interested in a dog's advances, but I have never seen the dog use violence in pursuit of his goal. I assumed this to be the case with all animals. However, Anne Maggioncalda and Robert M. Sapolsky describe an exception to this general rule among orang-utans in Scientific American June 2001.
The orang-utan is one of the four great apes - the others are gorillas, chimpanzees and bonobos. They are found in Southeast Asia, the only great ape found outside of Africa. Hundreds of thousands of orang-utans lived there 10,000 years ago, but today they are limited to parts of Borneo and Sumatra.
The mature adult male orang-utan is on average twice the size of the female. The mature male carries flamboyant secondary sexual signals - wide cheek pads, well-developed throat sac for emitting loud calls, and brightly coloured hair - all advertising fitness and fertility to the female. During "normal" development males reach puberty around eight years of age and spend a few years in a sub-adult phase, being the size of an adult female. They reach full size and attain secondary sexual characteristics by age 12 to 14. At least, that was the traditional understanding of researchers until the 1980s.
It is now known that some adolescent male orang-utans can remain in a sub-adult state of arrested development until their late teens, i.e. for 10 or more years, half their potential reproductive life.
Many things can cause arrested development in animals. Shortage of food can delay maturation. The evolutionary logic that applies is that, if it is reasonably likely that an animal will starve to death, it makes no sense investing valuable nutrient resources developing secondary sexual characteristics. Various genetic disorders that cause biochemical abnormalities can also delay maturation.
The social environment causes arrested development in some male orang-utans. The precipitating factor is the presence of a dominant male in the vicinity. It was initially believed that this was a stress-induced pathology, i.e. the bullying and intimidation of the dominant male frightened the adolescent males, causing biochemical changes in their bodies that delayed maturation.
Maggioncalda and Sapolsky now report that this condition is not pathological but is an adaptive evolutionary strategy. The developmentally retarded adolescents are capable of impregnating females, but by staying small and looking immature they minimise food requirements and diminish the risk of conflict with adult males. However, the disturbing aspect of this strategy is that these retarded adolescent males copulate forcibly with the female - they rape. Social regulation of sexual maturation rate has also been observed in other species. In all cases it has been interpreted as being caused by stress. The biochemical indicators of stress are elevated levels of certain hormones, e.g. epinephrine, glucocorticoids and prolactin, in the blood and urine.
However, when researchers measured these hormone levels in juveniles, arrested adolescents, adolescents developing normally and mature adult male orang-utans, it was found that levels were the same in juveniles, arrested adolescents and adults, but were double this figure in adolescents undergoing the sexual developmental spurt. In other words it is the developing adolescents, not the arrested adolescents, who are under stress.
Levels of reproductive hormones (e.g. testosterone and lutenising hormone) were similar in arrested adolescents and mature males and arrested adolescents have mature functional sperm and good-sized testes.
According to modern evolutionary theory, animal behaviour has evolved to maximise the chances of an individual passing on his/her genes. Different species have evolved several different strategies. Among the orang-utans, the conventionally understood male strategy is to grow very big, develop expensive (metabolically) secondary sexual characteristics to impress the female and intimidate other male competitors.
There is a price to be paid for this strategy. For example, food gathering to maintain this size is very important and during famine the larger animals are less likely to survive. Also, of course, the physical aggression that goes along with being a dominant male is not good for the health.
There is an advantage for some, therefore, in being developmentally arrested adolescents. They don't have to gather so much food and, because they look like children, they do not attract much aggression from the mature adult males. Sexually developing adolescents, on the other hand, do attract aggressive attention which explains their high levels of stress hormones.
Arrested adolescents are quite successful in impregnating females. Studies have indicated that they father about half the orang-utan babies. The problem for these arrested adolescents, however, is that the mature females are usually not receptive to them and fiercely resist their efforts to copulate. The arrested adolescents successfully copulate by forcing themselves on the females.
Forcible copulation as a routine means of siring offspring has not been observed in the other great apes, or, to my knowledge, in other species. It is disappointing to find it among orang-utans, but facts cannot be denied. The authors strongly point out that, because rape occurs among orang-utans and something similar is observed in humans, it would be wrong to conclude that rape has a natural basis and is therefore an unavoidable part of human society. There are very distinct differences between human rape and orang-utan rape. For example, nobody has ever seen an orang-utan deliberately injure a female during copulation, a feature not uncommon in human rape. Orang-utans have evolved a unique set of behaviours to enable them to survive in their environment and it would be naive to draw parallels between their behaviour and human behaviour.
William Reville is associate professor of biochemistry and director of microscopy at UCC