Territorial behaviour

Anybody who has spent time at the seashore will be familiar with limpets, those hard, flattish shells stuck fast on the rocks. Seeing one of these, motionless on its rock, you may not think it a particularly bright or aggressive sort of an animal. But in nature, when we look carefully, we are often surprised.

Limpets graze on a crust of algae growing on the surface of rocks, rasping it off as they move forward. Individual limpets maintain a specific area of the rock as their territory, and defend it against all comers - not just other limpets, but any of several other species of grazers. If a limpet encounters another grazer, it lowers the forward edge of its shell, and repeatedly strikes and shoves the intruder, until it either falls off, or is pushed outside the territory.

If a predatory snail - one that would attack and eat the limpet - shows up, however, its response is quite different. It raises the forward edge of its shell, and then brings it down sharply on the snail's soft foot; a behaviour called 'stomping'. Usually one such stomp is enough to make the carnivorous snail quickly retract its foot, let go, and fall off the rock. On the other hand, if our limpet encounters an inert object, it simply feeds around it. Remarkably varied and discriminatory behaviour for such an apparently 'simple' animal, don't you think?

Apart from seeing off potential predators, then, defending a territory sequesters a supply of good food from other grazers. The thicker and more luxuriously the algae grow, the smaller is each territory, and the more limpets there are in one place. The biggest limpets with the best territories produce the most offspring. Many smaller ones have to make do with eking out an existence in places where there is little food, or it is of poor quality. But if a good territory falls vacant, one of them will quickly take it over.

There are very many animals, from tiny invertebrates to large mammals, which, like these 'lowly' limpets, maintain a territory and show complex behaviour in defending it. Nearly always this serves to exclude others from a limited source of food. Some will argue, however, that there are many other reasons why animals might claim and defend a territory.

It is often said that a territory is maintained to defend a place to nest. Yet we find that at times when food is very abundant and many more females are breeding, such places are at a premium; there are not enough for all trying to nest. Nevertheless, every female will find somewhere to attempt to raise her young.

For example, one of the famous finches of the Galapagos Islands, the large cactus finch, was thought to nest exclusively in holes in cactus plants (hence its name). But during a time of very high and prolonged rainfall generated by the unusually strong 1982-83 El Niño, their insect prey became super-abundant on the lushly growing vegetation. In response they bred repeatedly, over a much longer period than usual, in places where they had never before been seen to breed, and fledged four times more young than usual. Holes in cactus plants were soon all taken, but this did not stop them. They made their nests just about anywhere; that year more than half their nests were in trees. What had seemed a limiting resource was not.

In Australia there are several species of native ducks that normally nest well above the ground in tree holes. When there is lots of food for them, however, and very many are breeding, there are not enough tree holes for all. Then those that miss out will nest on the ground where their eggs have little more than a screen of grass to protect them.

In the dry interior of Australia when there is lots of fresh green grass after good rains, feral rabbits breed continuously. Then all the subordinate females are driven from the security of warrens by the dominant animals. Nevertheless, with all that good food to eat, they continue to breed, but now have to drop their young in shallow burrows hastily dug in loose sand. With both the ducks and the rabbits their unprotected young are much more likely to be eaten by feral foxes. But the attempt is worth it if only some of this increased production survives.

Interestingly, titmice in the Netherlands illustrate the converse of this, and further emphasise the primacy of food over a place to nest as the key limiting resource. These small forest-dwelling birds normally nest in holes in trees, but in today's carefully managed Dutch plantations such holes are rare. So, routinely, artificial nest boxes are set out for them as it is believed that they protect the trees by eating caterpillars that can defoliate the trees. But often not all of these boxes are occupied. Furthermore, the number that are occupied varies according to the 'richness' of the habitat - more of them are used in mixed broadleaf plantations where there are more caterpillars than there are in pure pine plantations. And within these differing sites, more or less boxes are used from year to year as the number of insects varies with changing weather.

Another, more 'natural' example of the number of nesting sites that are used being dictated by the amount of food available is that of the kangaroo rats which live in the Chihuahuan Desert of Arizona. These animals construct extensive breeding mounds wherein they are protected from weather and predators, and where they store their principal food, seeds. Once built these mounds will last for decades, but in hard times many of them lie empty and neglected for years. After unpredictable good summer rains, however, there is a great increase in the amount of food available for the rats and their numbers rapidly rise. Then old abandoned mounds are quickly rehabilitated and reoccupied.

Very commonly territories are established by male animals. They do this, however, not to defend a supply of food, but to sequester one or more females from being mated by other males. First - or exclusive - access to the best females to raise his offspring ahead of those of his rivals is what matters to a male, thus passing his genes, not theirs, to the next generation.

For a female, on the other hand, there is always a surplus of males from which she can pick and choose. What matters much more to her, if she is to be successful in passing on her genes, is access to the best protein food in the environment. This she needs in her own diet to maximise her ability to nurture her foetus or egg, and for the nourishment of her young through their early exponential growth. And this is what most territorial behaviour is about.

Various hunting birds like eagles, hawks and owls provide clear and well-studied examples of this function of territoriality. The more prey there is for a pair to catch, the smaller their territory will be, and the more nestlings they will fledge each year. In poor, larger territories, or in years when there is little in the way of prey, they may not raise any young - in extreme cases not even establish a territory. As we saw briefly in the last chapter, in some species there is a further mechanism for ensuring that fluctuations in the amount of food available is used to maximise the number of young that can be raised to maturity; they hatch two or three eggs over several weeks. In good years they can feed all the nestlings, and all of them survive. But in tough times if they continued to try and feed all their young, none would get enough, and all would die. In this situation the oldest - and largest - nestling will kill and eat, or eject from the nest, its weaker siblings. The young of the Australian (laughing) kookaburra are notorious for this ruthless behaviour. But this way at least ensures that one 'favoured' chick may survive - better than none doing so.

A good example of the way in which territorial birds can respond to weather-driven changes in the supply of their food is that of the Andean condors of Peru. These large, long-lived birds feed, like African vultures, on the dead bodies of mammals. In the high Andes there is a fairly constant and adequate supply of this carrion, and the birds breed continuously. In the deserts of the coast and foothills, however, food is much more limited. On the coastal desert there are enough dead marine animals washed ashore for condors holding territories there to maintain a low level of breeding. In the foothills, on the other hand, where the average annual rainfall is only 9 cm, food is scarce. So scarce that in drought years the condors do not breed at all, even though they continue to defend their nesting sites and maintain their pair bonds.

But in El Niño years there are significant changes in the deserts. Rainfall increases dramatically (during the 1982-83 El Niño more than 4 metres fell in the foothills in nine months). This causes a great increase in the number of deaths among the large herds of free-ranging livestock that are run in the foothills. Pairs of condors which have not bred for years quickly initiate breeding in response to the resulting bonanza of carrion. On the coast, however, it is a different story. Changes in ocean currents generated by El Niño drastically reduce the number of marine carcases washed ashore and the condors holding territories there stop breeding.

There is a final twist to this story. Because of the vagaries of the El Niño cycle, desert-dwelling condors can seldom produce enough young to maintain their numbers over the long term. They can only maintain their populations because they are replenished by immigration of some of the 'surplus' offspring produced by the ones that breed continuously in the more consistent climate of the highlands.

You are probably familiar with many examples of birds forming territories. If you live in Australia you may well have a pair of magpies that raise their young in a territory near your home. And you might have noticed the annual aerial dogfights that precede the carving up of territories before the breeding season starts. But have you also noticed the roaming band of birds that have missed out on a territory? Males and females both, they do not breed but remain ever alert and ready to seize any vacancy should an occupant die. And next season they will be fighting along with the rest to try and gain both a territory and a mate. Usually, however, there are a few dominant birds which hold on to the same territories year after year, driving out their fully grown young before they start breeding again. I have a female magpie that comes to my back door and feeds from my hand. Each year she brings her mate and, in due course her young, to learn to share in the handouts. For 16 years now she has nested in one or other of the tall trees near our house. In that time she has lost at least three mates but has still managed to raise at least one young each year. Among all the magpies that come to my door, she feeds first; all others defer to her. Last spring, after 17 years, she failed to return. But one of her daughters - with a handsome new mate - is now the matriarch coming to my backdoor.

On the other hand you may not be familiar with some of the less apparent animals which defend territories, including many which live entirely on plant food. Let me tell you about some of them.

In America there are tiny aphids, only 0.6 mm long, which form galls on poplar leaves when these are expanding in the spring. Two aphids will fight long and hard with each other to decide which one will settle closest to the base of the leaf. They push and shove, sparring like boxers, sometimes for days, until one (usually the smaller) either falls off or moves away. Aphids which manage to settle at the base of the biggest leaves claim the best territories - areas no more than 3 mm long - where they develop the biggest galls within which they grow to maturity and produce many more offspring than their less fortunate sisters. This is because this position on the leaf is where they can insert their sucking mouthparts into plant sap of the best quality -sap richest in soluble amino acids, the essential ingredients for successful production and growth of their young -as it is imported into the growing leaf.

But what of the losers? Some find smaller leaves, or a position further out along the leaf from the prime site on the best leaves. There they may produce a few young. Most, however, exist for a while as 'floaters', searching for sites to settle, or challenging holders of territories, but soon dying. They are the 'doomed surplus' that miss out on the chance to survive and breed.

Another form of territorial behaviour by insects is perhaps more nearly akin to the siblicide of nestling raptors. Codling moths, or at least their caterpillars, are all too familiar to anybody trying to grow apples. The moths lay their eggs in the just-formed new fruit and the tiny caterpillars feed on the developing seeds in the ovary. But unless the apple has many large seeds they provide enough food for only one caterpillar; more than one sharing and all Figure 5.1 A mere 0.6 mm long, these would die. Then, the oldest - and largest newly hatched aphids fight each otheroften . , , to the death,for possession of the prime site

- one eats its siblings.This to°,has been at the base of a poplar leaf to form their mentioned in the previous chapter. galls. Photo courtesy of TG Whitham.

Still in the domestic garden, there is another, but more sophisticated example of insect territoriality. There is a very tiny wasp that lays its eggs in the eggs of the green vegetable bug - that large foul-smelling beast that attacks your tomatoes and beans. Again more than one wasp grub per egg and none would survive, so the first and biggest one eats any others. But this rarely happens because each female wasp, after she has laid one egg in the egg of the vegetable bug, wipes her ovipositor back and forth across the top of that egg, depositing a pheromone on its surface. This signals to any other female that the egg is already taken and they respond by not attempting to lay their own eggs in it.

Another fascinating invertebrate example is that of a species of ground-dwelling spider which lives in desert grasslands in Arizona and New Mexico. The females are very territorial. Each builds a funnel in which to shelter, and a sheet of non-sticky webbing in front of the funnel. Any insects moving onto this sheet are instantly detected by the spider in her funnel and she quickly jumps out and grabs it. But beyond this web each spider maintains an area which she fiercely defends against all other females of her species. This includes not just holders of neighbouring territories, but any of the many 'floaters' - individuals that have not been able to gain a territory for themselves. These animals hide wherever they can in cracks and crevices, and subsist by trying to steal prey from the webs of holders of territories. If a territory becomes vacant a floater will quickly move in and take it over, but most do not survive for long, let alone produce offspring.

In places where insects are sparse there are many fewer spiders and they defend territories that are much bigger than those of spiders in the best places. What is more, spiders holding the smallest territories where there is most prey produce 13 times more offspring than their sisters on the poorest land.

An experimenter put some of these spiders in enclosures and either fed them daily with insects or deprived them of food for four weeks. The starved ones increased the size of their web. Well-fed ones, on the other hand, not only reduced the size of their webs, most let them disintegrate altogether. They had quickly learned that an abundant supply of good food arrived every day, so just came to the mouth of their funnel at feeding time to take the insects offered to them! (Who said welfare dependence is confined to humans?)

In contrast to this sort of deliberate experimentation, every so often human manipulation of the environment unintentionally reveals what is happening in nature. One such case involved a study over the past 24 years of serow in northern Japan. Serow are goat-like ungulates that live in mountainous areas of mixed broadleaf forest and plantations of conifers where they browse on the leaves and twigs of a variety of shrubs and bushes. In this

Figure 5.2 The size and number of territories of the Japanese serow expand and contract markedly in response to changing availability of their food plants generated by the clear felling and regrowth of the conifer plantation where they live. Photo courtesy of Keiji Ochiai.

stable habitat males and females maintain separate solitary territories, and vigorously drive off any other individuals of their own sex. They have no predators, and hunting was banned long ago. Over the years of this study, some adults died or vanished, and all young kids left their mother's territory as soon as they were mature. Yet the number and size of occupied territories remained nearly constant as, consequently, did the density of the serow population. One very severe winter slightly reduced their numbers, but they quickly recovered. Then, in one part of the study area, some plantations of mature conifers were clear-felled. The response was dramatic. Many shrubs grew on the cleared land so that there was as much as 150 per cent more food for serow there than in undisturbed parts of the study site. And very soon there were up to six-fold as many serow, occupying territories that were three times smaller, on the cleared sites. However, as the newly planted conifers grew and started to suppress the shrubs on these sites, the number of serow began to decline. But it is estimated that it will be 20 years before numbers fall to those present before the clear-felling.

What, then, is happening in all these examples? Is territorial behaviour, as many ecologists would have us believe, a form of population control which reduces the numbers in a population so as to conserve limited resources for the use of future generations? Or is it making sure that what resources are available are shared out equally among those seeking to use it? Neither explanation is correct. Altruism is a purely human concept. Territoriality has nothing to do with conserving resources for the next generation nor of sharing them among the present one. To do either of these things would, in any case, be evolutionary suicide. Others intent on using every bit of food they can get would quickly outstrip those altruistically 'trying' to restrain themselves so as to ensure others got a fair share, or, equally improbably, conserving some for future generations. And in doing so those 'selfish' individuals, by using all the food they can, would leave more offspring than the altruists, ensuring that their genes would quickly dominate the gene pool.

In a world where there is rarely enough food - and more particularly, enough protein food - for all those trying to eat it, one way to make sure that what food there is gets used as effectively as possible, is to channel the limited supply to only some of the many individuals seeking to use it. All but a 'favoured few' are denied the use of a scarce resource for the production of the next generation. And only these few breed successfully. Some of the rest may find an inferior site and produce some offspring, but most - the 'doomed surplus' - may manage to subsist for a time, but eventually die without reproducing. But, of course, they are not a surplus at all. They are a reserve. Whenever the amount of food available increases, some of them will establish new (and usually smaller) territories to make use of it. And should a vacancy arise in an established territory one of them will quickly take it up. In either case the available food is put to good use - producing more of their own kind.

In all the examples I have told you about, and in hundreds more, this is what territorial behaviour achieves. And, though it may seem paradoxical to some, in so doing it maximises, not reduces, the number of individuals that the environment can support in each generation.

Finally, interesting illustrations of the prime place of protein food in the maintenance of territoriality are some responses to 'unexpected' concentrations of good food - both man-made and natural - in the habitat. Black bears in North America are solitary, mostly herbivorous, and strictly territorial. They will not entertain another bear near them. But where they can gain access to humans' rubbish dumps - a rich source of protein food - they suspend this intolerance of their own kind while all gather to partake of the feast. A similar thing happens with other, equally solitary bears. Grizzly bears gather to feed on the annual runs of salmon up the rivers, and even entirely carnivorous polar bears come together and 'socialise' at places where there are a great many seals to catch. In all cases otherwise aggressively individualistic animals 'happily' tolerate others nearby while all are busy harvesting a bonanza of food in a world usually distressingly short of food. But, in each case, all is not entirely sweetness and light. Within a group they quickly establish a hierarchy where physical dominance decides who gets the lion's share of the pickings.

Nor is this behaviour confined to bears. In northern Norway stoats - also fierce holders of individual territories - congregate at rubbish dumps of tourist lodges. What is more, they persist at these places when in the surrounding countryside their numbers have crashed following the collapse of populations of voles - their natural food. A similar story emerged of wolves congregating and feeding at night at rubbish dumps in several European countries. Also in Europe red and roe deer are frequently fed throughout the winter to help maintain their number for hunting. Where these animals congregate at the feeding stations established for them, wild lynx soon learn to congregate for an easy meal. Lynx, wolves and wolverines behave in the same way, and inflict heavy losses on reindeer which are herded together and fed through the winter in northern Scandinavia.

There is another story that I must tell here. It is a further example of how wild animals subsisting on a less than adequate diet quickly learn to congregate to take advantage of food concentrated in their environment by human activity. But it is also an illustration of the unintended flow-on consequences that this may generate. It is not the sort of story that would ever get into the scientific literature, but it was told to me by a respected colleague who assures me that it is true.

Wild feral goats have become so abundant in the Flinders Ranges of South Australia that they constitute a serious threat to native plants and marsupials. One means of reducing their numbers, and at the same time making some money from them, has been to round them up, halal-slaughter them, and ship them frozen to Middle East countries. There it eventuated that some very wealthy families cook and present a whole goat at each meal. Any of the carcase not eaten at that meal is not considered worthy of being kept and is thrown onto a dump in the desert. There troops of local baboons gather and feast upon this unexpected and persisting supplement to their normally uncertain scavenged diet. This large extra input of animal protein into the baboons' diet has resulted in a huge increase in their numbers; so much so that they have become a serious pest. An example of an attempt at environmental conservation in one place causing an environmental problem in another!

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  • HAYDEN ROBERTSON
    Why animals claim and defend territories?
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