Few animal displays are as alien, yet as touching, as a reunion of two Laysan albatross mates. Having scoured the oceans alone for months and thousands of miles, Laysan albatrosses return to the same stretch of shoreline to mate with the same bird. After clumsily crashing to the ground and testing their shaky legs that have not touched land for months, albatrosses go in search of their mate. Miraculously, they manage to find their partner among the hundreds of other birds packed into their breeding colony. Once they reunite, the albatross couple performs a dance to reform their bond. They stand facing one-another, clacking their beaks, and mimicking each other’s head bobbing. Their twittering and angular wing displays increase in pace and intensity. At the crescendo of their hopping and shrieking, they halt, and break down into a bout of gentle preening and soft embraces in which they copulate. Their choreographed dance may seem foreign to us, but the sentiment behind their tender embraces is universally understood.
Albatrosses may not breed successfully until they are eight or nine years old, but once they settle on a lifelong mate, they will perform these same rituals every time they meet for the rest of their 60-year-life. Bonded together, the couple will tirelessly raise a single chick, fighting against the elements and dwindling fish stocks as they take turns foraging among the endless waves for many days at a time. Do they think fondly about their life partner tending to their chick as they glide effortlessly above the stormy seas? It is tempting to envision their thoughts and see their unyielding devotion to their partner as romantic. One might even go so far to say their behaviour looks akin to love. But what is love for an animal? In comparison to our own complicated relationships, the albatross system may seem refreshingly simple. They find a mate, stick with them for life, and raise offspring together. Is that not what traditional Western ideals tell us we should be doing with our own relationships? So why can’t we love like an albatross? The short answer is because we are not albatrosses. Yet, this relatively straightforward idea brings up the larger issue of why species pair, mate, and raise young differently. More importantly, it begs the question: are we unique in our own capacities for intimacy and love?
The difficulty with studying animal relationships like those between albatross mates is that it is impossible to know how an animal actually feels. We cannot read their thoughts, nor can we ask them whether they think fondly of their partner or long for them while they are apart. Therefore, those who study animal relationships do not refer to non-human relations in terms of “intimacy” and “love” to avoid projecting our inherently biased human experience onto minds we do not fully understand. Instead, what scientists can do more objectively is investigate the behavioural and physiological changes that take place during the breeding process. These observations can reveal the patterns of animal relations and allow us to sometimes extrapolate about the underlying mental states they produce. Enough research has been done through these methods to teach us a considerable amount about animal mating systems and the pair bonds that can unite mates.
Pair bonds result from hormonal changes in the brain. Even in humans, the deep-seated desire we feel for companionship and intimacy has a hormonal basis that developed over evolutionary time. Just as species differ in other biological traits, their hormone levels also differ greatly to dictate how readily and strongly they form pair bonds. This means that even closely related species can deal with sex very differently.
Pair bonds and relationships surrounding reproduction come down to sex. At a basic level, sexual reproduction inherently involves unequal investment by males and females. Eggs are more costly to produce, while sperm is readily abundant and replenishes throughout a male’s lifespan. Hence, females should be choosy about their partners because they are giving up a limited resource to reproduce. At every mating, the female risks squandering her reproductive opportunities with inadequate males or poor genes. The pressure of female choice gives rise to competition between males and drives much of the variation seen between males and females. Pair-bonding, on the other hand, is derived by the need for parental care. In species whose offspring would not survive without the continued input of both partners, bonding between the male and female allows them to better raise their young.
The power of natural selection to produce distinctive traits is no more evident than in its effect on mating systems. Competition for mates produces evolutionary pressures that have shaped some of the most mesmerizing spectacles in the animal kingdom. From haunting male humpback whale song, to the herculean clashes between sparring male rams, sexual selection has produced tremendously beautiful animal displays. But sometimes attracting a partner involves more than just physical demonstrations of fitness. Natural selection is also responsible for producing traits that mirror some of the intimacy we strive for as humans. The cognitive demands of maintaining a strong bond between partners has made it advantageous for certain species to anticipate and expect the desires of their mate. Male Eurasian Jays, for example, can anticipate their partners’ desires and respond, even if her wants directly conflict with their own.[i] They routinely share food with their female partners during the breeding season to reinforce their bonds, and will choose foods to share with her that they believe she desires, even if they themselves do not want that type of food. Being able to attribute a desire to another individual is a cognitive feat rarely seen in the animal kingdom, yet its existence outside of humans gives credence to the idea that we are not so different than other animals, even in our pair bonding. While we do not know whether this type of perspective taking evolved specifically to foster pair bonds in these birds, having the cognitive wherewithal to do so would certainly have its advantages in pleasing a partner.
In other species, however, none of this type of cognitive genius is needed for successful mating. Perhaps one of the most drastic examples in mammals comes from the elephant seal. Elephant seal males do not pair with specific females, but instead guard a large stretch of beach that contains up to a hundred females. As long as they are able to defend the sands from competing males, they can mate with all females that come ashore, and need not invest in their young at all. In fact, as they barrel along the beach with their three tons of blubber and muscle to ward off potential competition, they even can crush their own calves.
The diversity of mating systems and pair bond relations that result from selection pressures is dizzying, yet there are many parallels between animal systems and our own. Some have such a strong desire for a bond with another individual that they seek relationships that transcend reproduction. Homosexual pair bonds are quite normal in many species.[ii] In other cases, the dynamics of non-human relationships could easily be mistaken for the drama of a reality TV show. Fighting between pairs, and cheating on partners is commonplace in many mating systems. In humans “cheating” carries the assumption that a partner is being morally dubious. In animals, however, “extra pair copulations” are a natural phenomenon that is easily explained in terms of maximizing one’s reproductive success. Being socially monogamous—i.e. bonding with a partner to help raise offspring, but still mating with other partners—allows one to share the burden of parental care but still obtain genes from a potentially better partner. While human cultures vary in their tolerance of homosexuality and cheating, they are both universal features of human relations found throughout the world. Does this mean animals are more human than we often give them credit, or that humans are no more than animalistic tendencies shrouded in conceptions of culture?
Charles Darwin’s oft-quoted theory that “[t]he difference in mind between man and the higher animals…certainly is one of degree and not of kind”[iii] applies to mating systems as accurately as any other cognitively motivated trait. In questioning our inherent superiority over the forces of natural selection, we are compelled to accept that we are no different than any other species in owing our feelings towards a partner to our evolutionary history. Scrutinizing non-human mating systems may, therefore, help us better understand ourselves. This idea has motivated a huge body of literature into the reproductive behaviours of many species.
In trying to understand our own mating system, it is tempting to look at our closest relatives. However, primates do not have a consistent mating strategy and comparison to them leaves us little certainty about our own system. There are species such as the hamadryas baboon, in which dominant males defend and mate with a harem of females, traveling in large groups as they graze the African savannahs. In contrast, female tamarins, a species of small tropical forest monkey, frequently have two male partners who help her raise their offspring together. Sex in our closest relative, the chimpanzee, for instance, is different again. Wild chimpanzees live in male-dominated social groups and violence between neighboring groups is frequent. When females reach reproductive age, they emigrate from their natal group, but can only leave and join another while they are reproductively receptive; a state which is advertised through a large pink swelling that inflates during estrus, and functions as a “pink passport” to allow them to travel safely to neighboring groups.[iv] In contrast, bonobos, the cousin of the chimpanzee, are purported to have a radically different system in which sex is open, frequent and happens between all members of a group, including members of the same sex, juveniles and elders. Bonobos are known for their promiscuity because they use sex as a way to diffuse tension. The juxtaposition of these two opposing approaches to sex in our closest relatives has fostered an existential crisis over whether humans are designed to be violent war-mongers or free-loving hippies.[v] In reality, however, idealizing the lifestyle of the “make love, not war” primate does not get us any closer to understanding our own systems of love.
The crux of the issue lies in whether we can ascribe morality to animal sexual systems. By doing so we are inherently projecting our own human-centric views onto other species systems that we may not understand. By demonizing chimpanzee violence, we are forgetting that sex is part of chimpanzee life history that has allowed them to survive in their tropical forest habitat. In any mating system, sex is the key to survival. Without it, no sexually reproducing species would pass on their genes. Therefore, traits that favor successful reproduction are more likely to persist in the population.
The expression of mating systems, including our own, is not, however, solely dependent on genes. The environment can play a role too in influencing patterns of reproduction. In some species, individuals will employ different mating strategies depending upon their physiological state. For instance, horseshoe crabs generally secure a mate by clasping onto her back as she heads to shore so they may copulate on the sand together. Males that experienced more difficult environmental conditions, such as being unable to obtain adequate nutrition, will adopt a strategy of sneaking in on a couple and stealing a copulation with another crab’s partner. While various types of environmental variability can provoke different mating strategies, the influence of the environment is most exaggerated in humans as our cultures dictate much of how we mate.
As humans, our cultural norms greatly influence our mating choices and what we expect to gain in terms of intimacy with our partners. We are similar to other animals, however, in being unable to escape some of the biological heritage that predisposes us to seek out bonds with other people. Hence, when studying animal pair bonds, it is endlessly tempting to compare our own situation to animal models. Wouldn’t life be easier if we could count on our partner like an albatross can count on theirs? Equally, there are sure to be many men who wouldn’t mind having relations like a hamadryas baboon, or many females that wouldn’t mind trying out the tamarin monkey’s system. However, exalting any animal system out of context risks glossing over the challenges animals face in finding mates and raising offspring—challenges that shaped the very creation of their system. We cannot mate like an albatross, or love like a bonobo, any more than we can swim like a fish or fly like a bird. We experience different forces of attraction because we have each evolved along a unique evolutionary trajectory. We forget that our experience of love is just as much part of being human as having two legs. Yet at least we can commiserate in the fact that we are not alone in desiring the company of others, even if only for a short while, on a distant shoreline after a long journey home.
[i] Ostojić, L., Shaw, R. C., Cheke, L. G., & Clayton, N. S. (2013). Evidence suggesting that desire-state attribution may govern food sharing in Eurasian jays. PNAS, 110(10), 4123–8. doi:10.1073/pnas.1209926110
[ii] Homosexual acts are widespread throughout the animal kingdom, but do not always involve pair bonding behaviour. Only animals with heterosexual pair bonds demonstrate homosexual pair bonds. However, it is common enough that even in the aviaries used by my research group, a pair of female rooks engage in bonding behaviours.
[iii] Darwin, C. (1871, 1896). The decent of man and selection in relation to sex. New York: D. Appleton and Company.
[iv] Chimpanzees are not alone in exhibiting estrus swellings, as many other primate species do too.
[v] This question gained much sensational press, much of which may have been exaggerated. A summary of the glamorization of bonobos can be found here: http://www.newyorker.com/magazine/2007/07/30/swingers-2