The rare socially nesting birds out of the many remind us of three very conspicuous exceptional home-makers among the mammals, two from within the rodents and one from the primate family. In these three, as in both the weavers and the parrots, the home is not just for rearing young. Are they all exceptions? That is, could they have been predicted? If so, then only if we understand the causes. Richard D. Alexander, a zoologist at the University of Michigan with a special interest in the evolution of social behavior, did just that. In the mid-seventies he published and lectured widely on the topic of social behavior, trying to decipher how gregariousness may have evolved to living in a large group with overlapping generations and division of labor, and with only one or two reproducing adults while the rest were all sterile. This suite of four conditions is labeled “eusociality” and it had been found in what were then called “social” bees, wasps, and termites but had not been observed in a vertebrate animal. In trying to illustrate how eusociality (as opposed to mere social behavior) had evolved, Alexander hypothesized a fictitious mammal that, if it were to exist, would be eusocial given certain conditions. His mythical beast was modeled after termites, the eusocial insects that had presumably evolved from ancient solitary cockroaches that had lived in and eaten rotting wood.
Unknown at the time to Alexander, Jennifer Jarvis, a student then working on her PhD at the University of Nairobi in Kenya, was starting to provide the groundwork for his prediction. She was interested in deciphering the biology of a curious type of rodent living in those arid regions in southern Africa where a number of plants have evolved large tubers that are the underground food- and water-storage organs that tide these plants over during long and unpredictable droughts. Her rodent, appropriately called the naked mole rat, family Bathyergidae, relies primarily on these tubers for its food and water and lives “permanently” underground in highly branched tunnel systems that have sleeping and toilet compartments and may extend to over three kilometers in total length. This was precisely what Alexander had predicted for a eusocial mammal, and when that was recognized, these animals’ social biology became the focus of intense study by these and other researchers, working both in the field and with captive animals in the laboratory housed in simulated naked mole rat homes of interconnected cavities. The picture that emerged was that the naked mole rats were indeed eusocial; as many as a hundred individuals of one species, Heterocephalus glaber, live in one tunnel system, but only one female out of the whole colony reproduces, and she is mated by only one to three males. All the rest of the occupants of any one colony help defend it, plug entrances, and work as excavators extending and making new tunnels.
Other species of mole rats are not eusocial, so again the question was, Why did this one species become eusocial? It probably has to do with the same conditions that produced termites from cockroaches.
Termites’ ancestors probably fed on decaying wood, which also enclosed them in a confining space but protected them from predators. Similarly, a naked subterranean mole rat feeding on a huge tuber can make its home there, and the family can remain in relative safety without starving and without venturing outside. A singly excavating mole rat is unlikely to find one of the far-dispersed tubers, and equally unlikely to survive aboveground. But with many excavators in a colony, the odds of one finding a new tuber, after one is eaten, are increased and the costs of sharing small, so social tolerance becomes highly adaptive. Tubers may weigh up to fifty kilograms and supply enough sustenance to last a whole colony of fifty to one hundred individuals for over two months. By their cooperation, the individuals gain by reducing their risk of starvation. The cost is that, in the close quarters, there is opportunity for overpopulation, so neutering becomes the price of food and safety.
Numerous studies after Alexander’s insights, and the empirical work by Jarvis’s and Alexander’s students and others spanning decades, closed the similarities between termites and naked mole rats. The mythical animal proved to be real, validating the idea that the evolution of eusociality probably resulted from confinement of overlapping generations in a safe home.
The adaptations of those mole rats that specialized for underground life include, beyond their eusociality, their almost total loss of hair except for whiskers used as sensory organs, stiff fringes on their feet that aid in digging, and the loss of external ears, loss of vision, and shortening of limbs. All of these apparent “degenerations” are adaptations for underground life. They would be a huge liability aboveground, which then creates ever-greater advantages to staying within the underground colony, creating a spiral that, once started, potentially locked them into eusocial life.
The enclosed space of the home that confines the individuals facilitates the establishment of dominance by the parents over their offspring of succeeding generations, which stay and are physiologically neutered by the dominance hierarchy where reproduction is restricted to the select individuals at the top of the heap. In H. glaber, the only reproducing female and her one to three mates have four or five litters per year, of four to fifteen pups per litter. I think the reason why the sociable weavers and monk parakeets have not become eusocial is simple: they still live in their own “apartments” in the communal house and are free to fly around; they are not under continuous surveillance and control by any other specific individuals. But why did other burrowing rodents living in arid regions in other parts of the world not also share the same home and become eusocial? Here the likely answer is food supply.
Animals come together and stay together in a common home for food and safety. If either resource is missing, dispersal is necessary, and domination becomes unlikely. In the American Southwest, for example, there are no plants in the desert that have huge tubers such as those of African plants, so desert rodents such as kangaroo rats, pocket mice, and ground squirrels, although they have underground homes in burrows and plug up the entrances of their homes with earth during the day, must venture out at night to collect their food—seeds on the ground surface. They cannot stay safely sheltered almost permanently in a home as the ancient cockroaches did in excavating wood and using it as their food and their home at the same time, or as the naked mole rats do while confined in their underground tunnel system. But although they cannot avoid being on the ground surface, they can reduce their vulnerable time there. And, unlike the naked mole rats, they have evolved cheek pouches to carry home many seeds to then eat in the safety of their underground homes.
Primates are, with one exception, one of the least creative animals in home building. Baboons sometimes use caves for overnighting. Chimps pull a few branches together in a treetop to create a crude platform for a night’s sleep. Surely monkeys and apes were not limited by intelligence in evolving to build nests or homes, if they needed them. But among all the primates, only humans build homes. Our extreme exception is so obvious that we are almost forced to consider what it is about us that tipped the balance to make us so very different from our close remaining relatives. As before, we must begin with basic biology.
Primates, for the most part, have only one young at a time, and they carry their baby with them rather than leaving it in a shelter. Those species of crustaceans, insects, spiders, and mammals (most famously marsupials) that do carry their young also lack any kind of home-making. Given the convergence of such unrelated groups, one can assume independent origins for lack of home construction, arising from a common denominator. So, then, how come we diverged from the mold to evolve from tree-climbing apelike hominids with little or no need for a nest to build, to living in skyscrapers? How do we differ from other primates, to end up with a strong predisposition both to become highly social and to build shelters? For that we need to search for fundamental differences from the other primates.
In contrast to all other primates, our young are at birth altricial in the extreme, and they continue to depend on constant social support for at least five or six years, and on training for still more years. There is debate about why we ended up with babies who are bor
n naked and helpless except to accept food and express wastes like those of many birds. What that biology means, though, is that, like birds and other mammals with helpless (and especially multiple) young that could not easily be carried around, we probably originated from a unique predecessor that required a “nest,” or we built a safe “nest” and adapted to safe homes, which permitted our young to be left in a safe place so that they could become altricial. As in birds and as in other mammals, the home and the altricial became linked and now are part of the same package. Furthermore, with us it was not just the babies who were bound to that nest. The mother was bound to stay in its vicinity as well—at least more so than in any extant ape where the group can and does travel at will.
Presently existing ape females can be more easily mobile with a baby than naked pre-humans would have been, because ape mothers have fur and a relatively horizontal back for a baby to ride on when they are moving around, and also because ape babies are born with both an insulating covering and the dexterity to hang on to their mothers. In contrast, bipedality created a huge constraint in free movement while carrying offspring—especially because the mother may have had more than one offspring at a time. Furthermore, meat-eating hunter-gathering pre-human hominids needed more, not less, mobility than apes, while having less means to carry even just one young and keep it comfortable at the same time. As a consequence, a pre-Homo equivalent mother (before the advent of tools used to carry babies) who was tied down to a home meant that, as in species of monogamous birds, she needed a helper on whom she could depend to be at least an occasional provider. True, Bushmen carried their babies in slings, and Native Americans used a rack on the back where the babies were as secure as a quadripedal baboon’s are riding on their mother’s back and clinging to her fur, but that was unlikely to have been the case until we were human.
The upshot of all this is that with the male pre-human leaving mom and a helpless infant or two in order to go find food, it was imperative that the offspring reside in a safe place with a guardian, or two, or three, or more. Likely more: we may not have resided in hollow logs or been subterranean, and it is a cliché to say so, but a truth nevertheless that some of our ancestors, and already those more than a million years ago when our predecessor was likely a different species, found a home in caves.
Caves may have been ideal homes, because attack there could come only from one side, but the rarity of caves, and the necessity that they be accessible to good hunting territory, must have generated competition. That is, as with termites and naked mole rats, a good home is a strong inducement to stay at home. The offspring, as those in beaver lodges when the parents produce more young, may have been reluctant to leave and “face the world.” They may have to have been evicted instead. One thing is sure: if they were allowed to stay long enough that there was overlap of generations and crowding, they would have had to compromise some authority and priority, and/or they would have had to provide something of value in order to be tolerated as co-inhabitants. They could have been helpful in defense, in the hunt for food, and in child rearing. Social tolerance, division of labor, cooperation, and subservience to rules or individuals, of a dominance hierarchy in the group, then became necessary and hence valued survival traits.
Human home-making could then have evolved in a manner not much different from that in weaverbirds, and the making of homes was a huge step that allowed us to expand into all sorts of previously unavailable habitat. First it was in Africa itself, and eventually it reached far beyond. Portable skin houses allowed the Native Americans to live on the plains and hunt the bison there, and similar family houses brought other peoples out onto the wide-open steppes of Asia. Sod houses enabled humans to survive the cold winters in northern Europe and Asia, and the invention of homes made from the snow and ice permitted the invasion of the Arctic, with access to a huge bounty of prey along the shores of the Arctic seas. But it was the innovation of multiple-occupancy houses—apartment complexes—in conjunction with agriculture that made the extreme human density of our cities possible. With those, we have also expanded vertically.
We have no way of knowing what our original homes looked like because they were almost certainly perishable and would not have fossilized. The structures of extant human homes in many environments likely are close to what might have been. Given our imagination, we would have been aware of what other animals did. Perhaps our homes at first converged to resemble (and possibly mimic?) those of the resident birds’. The Pygmies of the Ituri Forest in central Africa build round huts next to each other. Woven from branches and covered with leaves, these huts are reminiscent of weaverbird nests. The Maasai, and the Pueblo Indians, built and still build ovenlike adobe huts with a side entrance that in form resemble the homes of swallows, which are often colonial. Maasai huts are arranged in a small colony serving as a semipermanent home that is surrounded by a wall of thornbushes as a defense against lions. The Anasazi of the American Southwest built cliff dwellings with clay and straw, building materials much like those used by swallows, phoebes, and some other birds. At the “Cliff House” at Mesa Verde National Park, some 150 rooms are packed adjacent to each other. In Europe, earlier people probably lived in natural cavities in rocky cliffs, much like the ancestors of the monk parakeets. These people, like the monk parakeet ancestors, presumably learned to plug the holes of the cliff openings with wooden material. This would have been a first step to building a shelter with a wall or two against a cliff, and ultimately of attaching other homes to it, to create more walls, until more and more rooms were created, so that a communal structure with numerous entrances to separate identical family rooms allowed for efficient climate control, a settled life, nurseries for the young, and division of labor. However, all traces of similar homes of predecessors in Europe, Africa, Asia, America, or Australia would have vanished much as any bird nests of grass, leaves, and sticks would have vanished. What we see in caves represents a sample that is likely hugely biased by differences in preservation.
What then were the implications of many pre-humans or early human individuals sharing the same home? First, several individuals living together permits specialization, a phenomenon that is almost always linked with eusociality but can exist independently of it. Specialization concerning division of labor can result in a massive increase in efficiency, which then can lead to tapping into new resources, and therefore result in growth and permit the existence of even more individuals.
Eusocial bees are an excellent example of this. There are worldwide hundreds of species of bees that are solitary. A female makes her modest home, such as a burrow, and raises her own offspring there. Some species, as the bee specialist Charles Michener described, may have nests with side burrows and/or cells where several young are reared at once or overlapping with each other. Most solitary bees, under selective pressure to provide for their offspring, specialize in foraging from the flowers of specific species of plants (or from a group of species with similar flowers). They thus must restrict their seasonal activity to those often-narrow seasonal times when their food sources are in bloom.
Eusocial bees, on the other hand, tend to be generalists that learn skills of flower handling, so different individuals from the same home often specialize on different flowers. Having multiple specialists in the same home where there is sharing widens the bees’ niche. Bumblebees are a good illustration of this. Bumblebees are fairly general flower foragers; they are not hard-wired specialists for any one flower kind. By following any one individual bee from the time she emerges from her cocoon in the nest until she dies, one sees the following: the young bee stays in the nest the first few days and does any number of various house chores, but when she leaves the nest to become a forager, she flies almost indiscriminately to sample various kinds of flowers. Eventually she settles on one main one. She will soon know where to find these, and she will fly without hesitation to them and learn to handle them faster, to extract their nectar and/or their pollen. She may have, aside from her “m
ajor,” a “minor” flower, which gives her flexibility in case her major stops blooming. Thus, one Bombus fervidus worker from any given home may major on jewelweed and minor on New England aster, a second one may major on aster and minor on jewelweed, a third major on mint and minor on a yellow composite, a fourth major on a lily and minor on wild carrot, and a fifth perhaps collect honeydew from aphids. Together they have all the food-producing sources covered, both at any one time as well as through their oft-shifting availability from early May until late fall. Not only is this constant availability of food resources possible because of their social organization, it also makes their social organization possible in a self-reinforcing cycle. As in the eusocial naked mole rats, the risks of running out of food are greatly reduced.
Multitasking vastly expands the bees’ niche, but then an inevitable consequence of that capacity of hugely enhanced efficiency in tapping resources sets the stage, close on its heels, for another consequence, namely, following the ability to tap vastly more resources, maybe even seemingly inexhaustible ones, they will with certainty become limiting. This is due to the fact of the biological “law” of reproduction proceeding until it encounters limits. If there were no limits placed on reproduction and resources were inexhaustible, any planet where this (impossibility) occurred would eventually explode. That is, given some reprieve from epidemic disease and predators, there would come a time when new selective pressures would emerge as a means of regulating reproduction. There is no animal society on this or on any other planet in which this would not be true. Since we do not see anywhere anything other than a stabilization of the populations, it can be inferred that there has been and is very strong selective pressure against catastrophic reproduction.
The Homing Instinct Page 20