Social females rarely live in groups with their adult sons, presumably, according to evolutionary theory, to avoid inbreeding. If a son does stay with his natal group when he reaches maturity, he might mate with his mother, as has been seen rarely for chimpanzees (Goodall, 1986). For example, in a small family of gibbons in Thailand (Hylobates lar), the adult male disappeared leaving his presumed son, almost an adult at eight years of age, with his mother and an empty territory (Brockelman et al., 1998). The son duetted with his mother and could have set up a new life with her, but instead chose to leave both her and their domain.

When mothers and adult sons do live in the same group, however, and if she is high-ranking in a species with a dominance hierarchy, she can be of great help to her son. It is to her advantage to help increase the chances that he will become a breeding male who can pass along their DNA to the next generation. Here we will consider the behaviors of bonobos, chimpanzees, woolly spider monkeys, coyotes, leopards, and lastly, the pros and cons of researchers providing food such as bananas and sugar cane to “their” animals. This activity has both positive (it speeds up individual identification and habituation of the animals) and negative (it interferes with the natural lives of the animals and fosters unnatural behavior to an unknown extent) results.

Male animals are far less likely to be bonded in male–male friendships than are female dyads because, at heart, male priority is to mate with females. Males are therefore usually rivals. However, most females only mate when they are in estrus, which never lasts very long, so male friendships may blow hot and cold. For example, adult male blue monkeys (Cercopithecus mitis) sometimes groom each other and hang out with particular friends, but this easy companionship exists only when there are no females around. The same is true of patas monkeys (Erythrocebus patas). In species in which males and estrus females are present together in a group, males are typically antagonistic or, at best, grudgingly tolerant of each other (Kappeler, 2000). In such groups the biggest, most aggressive males tend to have the most success with females, so males have evolved to be larger than females (Neuhaus and Ruckstuhl, 2002). Male primates may form coalitions to attain power for some end result (for example, combat against another group), but these are ephemeral in nature and do not usually last. Human brothers and male relatives are often close friends but so, too, can they become enemies. About 1800, ruling sultans had a tradition of fratricide, where brothers, nephews, and any other threats to ascendancy were generally imprisoned and killed (Nagel, 2004).

This chapter considers friendships among primates (chimpanzees, gorillas, Tibetan macaques, and woolly spider monkeys), horses, lions, cheetahs, wolves, bears, dolphins, and birds (geese, rheas, black swans, Humboldt penguins, and long-tailed manakins).

Hundreds of social species mingle freely with other social species if they are not predators (which, by contrast, except for lions are usually solitary rather than gregarious). Zebras forage alongside wildebeest, waterfowl and shore birds of many species mix together, colonial species of marine birds crowd on cliff faces, and a variety of birds gather at bird feeding stations. Such sociality may stress feeding resources, but it improves safety for the many animals. Here, however, we are interested in special friendships between two or a few adults of different species.

Newspapers often run fetching pictures of two such adult animals cozying up together (and many more involving a cute infant, which do not concern us here). In my “Interspecies Friends” file I have best buddy photographs of a Siamese cat and a parrot, a red setter and a hamster, two swans and a goose, a deer and a horse, a poodle and a budgie, dogs and rabbits, a dog and a fawn, and a cat and a squirrel, all taken by or for the proud human companions of these odd couples. Often one of these animals was an orphaned youngster when it met its pal; if animals become familiar with each other from an early age, they are often fast friends for life. Early experiments have shown that when a kitten is raised with a rat, the cat will not kill its friend, even though it may kill other rats (Kuo, 1930). Animal Sanctuaries are where such curious friendships flourish, genial homes as they are to needy animals of various species (Hatkoff, 2009).

We think of ourselves as a monogamous species, so it seems strange that among our closest relatives, chimpanzees, bonobos, and gorillas, males and females do not form pairs but rather consort with a variety of mating partners. Few primates know who their father is. The most devoted couples among non-human species, instead, are birds of whom over 90 percent are monogamous (Adkins-Regan and Tomaszycki, 2007) – socially monogamous, that is; recent genetic research into the DNA of nestlings proves that many species have males and females who commonly mate with individuals other than their avowed partner. The necessity for social monogamy in birds is related to reproduction, with the laying of eggs by females rather than the production of live young who nurse from their mother. The eggs have to be protected during incubation and the insatiable nestlings fed until they are able to fend for themselves. This requires a huge amount of parental effort, especially for migratory species that have a limited season in which to rear young able to join in the fall southern migration. In long-lived birds especially, this effort can be best organized if the parents work together efficiently. If pairs of a migratory species are bonded, they can breed earlier in the season because they do not have to hunt for and court a partner; their young have more time to grow strong before they must fly south. A seasoned pair has learned how to raise their young effectively; for example, who will take which shift in incubating the eggs and then in foraging for food for the nestlings: “Each pair must work out its own rhythm, which takes some time to perfect and gives the advantage to established couples” (Burger, 2001). Bonded couples are far more likely to raise young successfully than are non-bonded pairs.

In social mammals when male and female young are born into the same litter, they tend to bond together as they grow up, constantly in each other’s presence. When they reach puberty, however, members of one sex or the other usually disperse from their natal group; this breaking of the brother–sister bond prevents inbreeding. In human beings, there is no such dispersal. An inborn incest taboo prevents a brother and sister from mating, but often the two remain close friends throughout their lives, which are enriched by this relationship.

In all mammals, the bond between mother and daughter is tight at birth, with the infant staying close to her parent for support and comfort as well as for milk. As the youngster grows, however, she becomes increasingly independent. This shift has been documented for Soay sheep living on the Scottish island of St. Kilda (Clutton-Brock et al., 2004). They are free-ranging and thought to be intermediate in type between wild sheep and domestic sheep; the latter have had their social instincts bred out of them to some extent so that they will perceive a shepherd as their leader rather than an elderly ewe (Darling, 1969). Although they seldom interact with their mother, Soay juvenile and yearling daughters usually graze within 20 meters (22 yd) of her as their mutual bond weakens (Clutton-Brock et al., 2004). (The bond between mother and son dissolves much faster, with the lamb on his own by six months of age.) By the time the daughter is about two years old, this friendship is gone, too. She is no more likely to associate with her mother than with any other female in their group. From a human point of view, it is nice to know that the mother likely will, by that time, have another young offspring at heel.

In other species such as those described as follows (chimpanzees, gorillas, baboons, Japanese monkeys, and ring-tailed lemurs), some daughters remain close to their mothers when they reach maturity, continuing to interact with her in a variety of ways. These species are all primates. Presumably other species have close mother–adult daughter ties, too, but research has not focused on this possibility.

“Animals are such agreeable friends – they ask no questions, they pass no criticisms,” George Eliot wrote long ago. Often we believe, because we love an animal very much, that we are loved in return. This usually applies to companion dogs and cats, who are limited to one section in this chapter because such human–pet relationships are so common and universal. But that our animal friends love us can be a dangerous assumption. Dale Lott’s (2002) mother, who thought that she and her horse Smoky were great buddies, was killed when he suddenly threw himself backward so that the saddle horn was driven into her heart. Lott also describes a rancher in Idaho who raised a bull from a calf. Even when the animal was full-grown, he would let the man pet him and climb onto his back. Then one day the bull killed the man, mutilating his body and refusing to allow it to be removed from the corral. Lott writes that bison, for example, are “immune to our charm, sincerity, personal integrity and peaceful intentions,” no matter what we may think. One man, who kept a few pet bison, was stunned one day when a young bull attacked him. The upward thrust of his horn into the man’s belly and ribs was so strong that it chucked him over the fence where a veterinarian was able to save his life.

We cannot ever know if people and animals, even reptiles, can be best buddies. The people may think they are, but their buddy may “think” otherwise, as the above examples indicate. The first four relationships described in this chapter about Elsa, the lion, mountain gorillas, olive baboons, and sociable whales do seem to depict true friendship. The animals are free to stay or leave the people they relate to, so there is equality between them. The second type of example includes horses and elephants who work for individual people. These animals and their human exploiters may have a good relationship, but it is not equitable. The animals must do what the humans expect and want. The same is true for the wolf Brenin and the hyrax Tsavo (and dogs), examples of the third type, but these animals, at least, were not expected to work for their keep. They did not try to escape from their close human buddies but were still subject to their humans’ whims. The fourth and final type includes individuals who were subject to close human supervision by researchers devoted to teaching them to learn information that presumably exhibited their intelligence. These animals had no life of their own, although they were very close to their teachers who spent thousands of hours with them, drilling them on subjects important to people. But they were locked up at night when their work was done.

Every day for many years I watched our three female cats, Silver, Tiger, and Gomer. Silver and Tiger were best buddies but Gomer was a loner. Actually, Silver and Tiger were half-sisters, although how would they know this? Silver, named by my young daughter Mary (although the cat was black with a white throat patch) came first, as a kitten given to our family of five by friends who had a sexually too-active female. A year later they gave us Tiger, with gray tabby markings, also as a kitten. Silver cuddled and licked her new young friend as they snuggled together. But when full-grown, both ignored the calico cat Gomer who arrived at our house as an older adult. Gomer retaliated by hissing and striking out at them with her paws. Did Silver and Tiger know somehow that they were related, which cemented their lifelong bond? Was it because Tiger came as a kitten whom Silver could mother, while Gomer (also known fittingly as Crosspatch) did not? Is it impossible for cats to become friends as adults, even though they live for years together in the same house?

Because these cats daily reminded me that animal friendships have been under-studied and under-reported, I decided to make this my next book’s topic: not alliances involving youngsters that are highly instinctual – mothers and their nursing young or siblings growing up together; not matings between males and females that are short-term; but friendships between adult individuals in the wild, perhaps for evolutionary reasons or just because a duo has bonded and wants to spend much time together relaxing, feeding side by side, or grooming each other – and not all animals either, as at present we know little about the sensibilities of cold-blooded species. However, there is recent proof that three-spined stickleback fish recognize by olfaction and like to hang out with individuals from their own neighborhood (Ward et al., 2007), that lizards kept as pets can distinguish between different human family members (Lee Harding, personal communication, 2010), and that dominant male hissing cockroaches from Madagascar defend a territory against other males within which they court several females who are free to come and go as they wish (Bullington, no date). Perhaps in a few decades, after far more research has been done, we will have to acknowledge that, like birds and mammals, individuals of cold-blooded vertebrates and invertebrates can also have sophisticated social systems and special friendships.