Leaders in agony
THE animal kingdom is inhabited by some species which exist in groups or packs, and these groups are led and dominated upon by individual male or female members. It had been believed for long that animals which are subjugated are the most stressed and suffer from various problems like miscarriages and failure to reproduce. New research, however, shows that it is the other way round - the animal which dominates is the actual sufferer.
Behaviourial biologists Scott and Nancy Creel of Rockefeller University, New York, along with Steven L Monfort, an endocrinologist at the Smithsonian Institution's Conservation Research Centre, Washington, us, report that dominant females in 14 packs of dwarf mongooses had the highest levels of the stress-related hormone, cortisol. Similar studies on female baboons and male and female wild dogs indicate that dominance comes at a high price. In baboons, it leads to higher rates of miscarriages, while in dogs and mongooses persistent stress shortens the lifespan of the alpha/dominant animals (Science, Vol 271, No 5247).
The Creels' study is the first of its kind where hormonal levels have been studied in several free-ranging rather than captive species. The older theory about subordinates with higher chronic stress levels arises from studies conducted on captive colonies of rodents and primates in the '50s and '60s, They found that the subordinates had higher cortisol levels and suffered from ill health and reproductive failure. Higher stress levels in subordinates support the theory of dominance where survival of the fittest was the main feature; the Creels embarked on their study believing this. "We thought that stress could be a factor in suppressing the subordinates' reproduction," says Nancy Creel.
The researchers collected samples of urine and faeces of mongooses and dogs to establish the basal stress hormone levels for each species and to obsme any disparity among individuals. Contrary to expectations, they found higher glucocorticoid (a corticoid that affects glucose metabolism) levels in alpha males and females - probably as a result of their aggressive behaviour.
It is still unclear how this phenomenon affects, the animals' health. According to behaviourial ecologist Sam Wasser of the University of Washington, alpha female baboons do have higher rates of miscarriages, but Wasser has not analysed stress hormone levels. Biologists have not been able to find a similar response in mongooses and dogs. "They must be paying )port e the price in another way, perhaps a shorter lifespan," says Scott Creel.
The methodology adopted in the study has come under fire. According to critics, the hormonal levels derived from urine and faeces can never be the same as that extracted directly from the animals' blood. The accuracy in the time of collection and the level of steroids in the animals' urine have also been questioned. Notwithstanding its criticism, the study has made biologists realise the potential variety of relationships that can exist between animal behaviour and hormones. "We are realising that hormonal levels are more and more situation dependent," says Jeanne Altmann, a behaviourial ecologist at the University of Chicago in US.