Social dominance hierarchies

We have seen something of the way in which territorial behaviour operates to sequester the limited amount of good protein food in the environment to only a few females and their growing progeny. And how this meant that all others, including previous young, must be expelled from the territory.

Well, the same function is fulfilled by increasingly complex social structures where such expulsion does not happen, or is delayed, or does so only now and then. Here, animals of two or more generations live permanently in a group and 'share' the resources in their habitat. At its most complex this specialised behaviour produces the social groupings we see in the mammals -and ultimately our own societies. But there is no equal sharing in these societies. A strict hierarchy of dominance by a few individuals prevails, and these dominant few get the best food; and only they breed. The concept of democracy and equal sharing and opportunity for all is a recent human cultural overlay - and then all too often honoured more in theory than practice. It certainly plays no part in nature.

But first let me tell you a story about the saving of an endangered species of bird, the Seychelles warbler, reduced to the point where it was found only on a single island, Cousin. These are very strongly territorial birds. They are also what are called 'cooperative breeders'; that is, the young remain in their parents' territory when they are fully grown and help feed the next generation of nestlings. However, they do not themselves breed.

The size of the warblers' territories and the success of their breeding depend on the supply of insect food available in a territory. In a high quality territory with much food young birds stay often for several years. Most of these only leave to breed when a vacancy appears in another good quality territory. Any young bird which waits like this for a good home produces more offspring in its lifetime than any which leave earlier and have to breed in a mediocre territory. Any bird which leaves to attempt to breed in the poorest territories has little chance of ever producing any young.

Destruction of the warblers' habitat had reduced them to only 26 birds on one island. However, careful management to restore the habitat produced a spectacular recovery. In five years the entire island was again covered with territories. Not until that happened, however, were any young birds seen to stay in their parents' territory and 'help'. Prior to that all had immediately left to establish their own territories and breed. Even then, it was only in a few of the richest territories that any young birds were staying.

But after 14 years the population had ceased to grow and stabilised at about 300 birds. By then 'helping' was widely observed in territories all over the island, and from then on the only young birds to leave home were those that could find a vacant territory in which to start breeding.

Once the population was no longer growing, researchers transferred some young birds to two unoccupied islands. These, too, had been renovated to again provide suitable habitats where the birds could establish viable territories. On these vacant islands the warblers bred much sooner, more quickly and more often than those birds left on the original, and now fully populated island. But as had happened there in the early years, none of their young stayed home as 'helpers'; all left immediately to establish new territories. Until, that is, all the high quality territories were occupied. From then on young birds born in these good territories began to stay as helpers. And they did this even although there was still abundant unoccupied space apparently available for new territories. However, investigation revealed that the unoccupied areas were of very low quality, containing little insect food.

There are three points to this story. First, it illustrates that the availability of habitats which contain adequate food dictates the number and size of the territories these birds can establish, and the way their young behaved in these territories. This would never have become apparent had the islands never been denuded, and remained fully occupied by warblers. Second, it revealed that the young birds stayed at home only so long as it was to their advantage to do so, not out of any altruism towards their younger siblings or their parents. Finally, it illustrates the link between strict territorial behaviour on one hand, and hierarchical social groups on the other. In the former, once all good habitat is occupied by territories, all young are driven out - usually to soon die, or at best hang on at the periphery in the hope of a vacancy cropping up. In the latter, young are allowed to stay. This increases their chances of eventually moving into a vacant territory where they can breed successfully. And this is of evolutionary advantage to both them and their parents - all get to pass on more of their genes than they would if the young were kicked out straightaway.

But in all such animal societies the young that remain and reach maturity in the group are themselves prevented from breeding, and are in every way kept subservient to the dominant, breeding adults and their latest young. They get less and poorer quality food, less safe places in the habitat, and are first to die when things get tough. Also they may 'earn their keep', as it were, by making themselves useful to the dominant ones. As I have just discussed, they may help to feed and care for the young. Or they may groom their betters, guard the group against predators and conspecific enemies - and be the first to die from attacks by either. It is better to tolerate such second-class citizenship in the hope of one day getting the chance to pass on your genes by usurping a previously dominant but aging or sick individual, or finding a new place where you can be king pin. The alternative of immediate departure means almost certain death, and no chance to pass on your genes.

In America there are small ground-dwelling herbivorous rodents, called marmots, which live in systems of burrows. Their story illustrates a further step along this spectrum, or continuum of complexity of social behaviour. And it illustrates once more that a lack of good food in the environment is the pivotal factor driving these associations.

Their burrows are aggregated on better quality sites: those with most food. Each set of burrows is home to a varying number of marmots, which defend it against other groups of marmots. The better the quality of the site the more marmots in a set. All the animals living in a set are females, and in each there is a single dominant one. She alone breeds. All the other marmots in her set are either her sisters or her grown daughters. Such a group is called a matriline. Young males leave home before they are sexually mature, and adult males maintain a separate hierarchical social structure among themselves. Each dominant male defends one or more matrilines against other males, and breeds with the dominant female in each.

But even in the best of worlds, continued increase in numbers means that sooner or later somebody has to go. Then it is the newly maturing young and those of lowest rank in the group (the two are usually synonymous) which are driven out. Most will soon die; they are, again, the so-called 'doomed surplus'.

This is well seen in populations of feral rabbits in Australia. Because of the immense environmental and economic damage they do their ecology and behaviour have been extensively and intensively studied. They live in warrens - complexes of burrows dug deep beneath rocky outcrops or tree roots, where they are safe from predators and the vicissitudes of the weather. Around each warren the occupants mark and defend a territory. Each warren contains a group of three to five females, one of which dominates the others and claims the best place in the warren to nest and the best food in the territory. The warren also houses two to three males, one of which is dominant. He mates with all the females. The others rarely get a look in, apart from a few lucky couplings - and then only with the lowest ranking females. So both sexes maintain a strict social hierarchy wherein the dominant animals gain first access to the best food and do 90 per cent of the breeding.

All this can change dramatically, however. From time to time widespread rains turn this hot dry land into a sea of green growth. Then all rabbits - even those lowest in the hierarchy - breed continuously and prolifically. Warrens are renovated and enlarged and new ones are established, but still there are more and more females breeding, and no room left for all of them to nest in a warren. This, however, does not stop the breeding. Young females of low status that are kicked out of the warrens dig quite shallow burrows away from the warrens in which to give birth. As we saw before, these are easily dug up by foxes, and few of their young survive. Yet, in spite of this, so long as the green grass lasts, very many more young survive and mature and the number of rabbits continues to increase.

If we look still further along the continuum of social structure we find increasing complexity of that structure.

Within the group the actual behavioural interactions and their complexity will differ. In some, like a herd of deer, it is relatively simple. A single male, having driven off all competing males, will sequester a varying number of females and mate with all of them. At its most complex are the social groupings of primates where a group consists of several related families.

A socially structured group may live in a geographically defined and defended territory, or it may range more or less widely over an area which is frequented by other groups of the same species. Each group will defend itself against conspecifics from all other groups while maintaining a strict and ruthless hierarchy of dominance and unequal distribution of resources within its own group. In every case the same basic function is fulfilled. Dominant males mate with all, or all but the lowest ranking females. Dominant females get first pick of the best males to mate with and commandeer the best food. The available resources in the environment are efficiently concentrated for the wellbeing of a favoured few. While this may seem cruel in human terms, in evolutionary terms it is good, for it means that those individuals that are best at appropriating and using the limited food pass on their genes at the expense of those which are not so good at it. It is also good ecologically, for it sees to it that as many individuals as possible - more than would have been possible in the absence of a social structure - are produced in each generation.

Some experiments done at the University of Wisconsin more than 50 years ago, and long since forgotten by most, clearly showed that the amount of food available is what drives territorial and social behaviours - and limits the growth of populations. Small populations of wild-caught house mice were established in a series of large rooms in an old warehouse. The rooms were lined with sheets of galvanised iron 60 mm up the walls, higher than any mouse can jump, and turned in along the floor to prevent them gnawing out beneath it. Each population was provided with many large cardboard boxes as nesting sites along with an abundance of cotton waste and shredded paper as nesting material. Each room had a permanent and unlimited supply of drinking water. All populations were fed on a diet known to support vigorous growth and breeding of mice; a rich mix of grains, powdered protein rat pellets and powdered dried meat.

Two separate experiments were run, and each lasted for a year. In the first one the mice were given a fixed and limited amount of food each day, in the second they were provided with a constant surplus of food.

In the first experiment the numbers of mice increased rapidly, but the population abruptly stopped growing when the 'food crisis', as the experimenters called it, was reached; when the daily amount of food was no longer sufficient to allow all mice to eat as much as they needed. From then on their numbers started a slow, steady decline. The survival of young had been very high until the food crisis. At this point, however, all but one of the few young born soon after died, and no female showed any evidence of pregnancy from then on; there was a complete cessation of breeding. All adults continued to obtain enough food to survive and to maintain their body weight, but all became physiologically and behaviourally sexually inactive.

Space was not limiting. More than half the animals in the room were crowded into just one 'house' while many of the other boxes remained empty and with unused nesting material. But the young could not disperse out of the room, which is what happened in unconfined populations that were given the same daily ration of the same food. And the females in these unconfined populations continued to produce young. So the key was the amount of food per animal.

Experiments with laboratory rats and mice have shown that quite moderate fasting will stop their oestrus cycle without them losing any condition. An experiment with American white-footed mice demonstrated the same thing -but with one important difference. Individual females were fed equal ad lib amounts of food, identical except that some received diets containing less protein. Just a 10 per cent drop from an intake of protein on which 100 per cent of females bred, stopped nearly all breeding within three weeks. Yet none of the mice - even those with 30 per cent less protein in their diet - had lost any weight, nor was the level of their carcase fat reduced. So an individual mouse's capacity to breed is immediately and drastically reduced when not just the amount of food, but the amount of protein she eats, is reduced by so small an amount, or for so short a time, that it has no detectable influence on her physical condition.

In the second Wisconsin experiment there was a very different outcome. Numbers increased steadily and at times exponentially, but the population again stopped growing quite suddenly. This time, however, it was not because of any decline in the number of litters being produced, but because practically none of the young survived. By the time this happened crowding had reached the stage where there was general social instability. There was much intense aggressive activity with fighting very prevalent. Fighting was mostly, but not entirely, between males, 75 per cent of which were mite-infested, scabby and with open wounds. Females had ceased to defend their nests which were constantly being encroached and damaged by other adults. Worse, they were eating their pups. This cannibalism was the major cause of the death of the young - and of the population ceasing to grow.

But matters didn't stop there. A gradual decline in numbers followed. And this was due to continued social discord among the crowded animals. All semblance of the normal male hierarchy was gone so that mating became disrupted and chaotic. Many males pursued and fought over any female in oestrus, and few managed to copulate properly. At and around feeding sites fighting and harassment became so bad that the amount of food being eaten per individual fell below the level known to be necessary for females to keep breeding. This, and the diminished rate of conception as many failed to be impregnated in the first place, led to a decline in the numbers pregnant and the rate of births. Throughout all this the rate of mortality of mature mice remained unchanged, but that of newly weaned sub-adults increased markedly as they missed out in the scrabble with their elders for the food.

So, whether a population does not have enough food or enough space, if the young cannot disperse it will cease to grow.

In the first situation, left to itself the population would have continued to be stable as long as the food supply did not change. Over time there would be minor changes in the number of animals, as once sufficient adults had died to allow the survivors to get enough to eat, some breeding would replace them. But, of course, in nature there is never a steady and unchanging supply of good food. Quite the reverse. Both the amount and quality of food available for natural populations is constantly changing, often by a great deal.

The second situation is an aberration because it does not arise in nature. There is never a permanent excess of high quality food available, and young will always disperse, even if to their death. Forcing this doubly impossible situation on the animals resulted in their behaviour and physiology becoming totally inappropriate. Evolved to maximise the use of what food there is in the habitat to produce as many young as possible, and to see to it that they then dispersed to find new sources of food, they were now pathological. Presumably a population left in this situation would have eventually adjusted to such conditions, but I imagine it would be a long process and even crueller than the experiment proved to be.

Was this article helpful?

0 0

Post a comment