What's the Most Monogamous Animal?

What's the Most Monogamous Animal?

Animals, we know, typically lack the hang-ups that make human mating so difficult. You won’t find a bonobo moping around, stewing in jealousy. Nor will you find a bonobo contentedly fucking his or her bonobo-spouse to the exclusion of all other viable bonobos for months or decades at a time. And though that particular species may take it to an extreme—mother-on-son action is not uncommon—their non-monogamous nature inheres in most of the rest of the animal kingdom. Only a minority of species operate on the one-partner model, and of these even fewer practice it on something like a human level. For this week’s Giz Asks, we reached out to a number of experts for their take on the latter group’s most monogamous member.

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Karen Bales

Professor, Psychology, UC Davis, who studies the physiology, neurobiology and development of social bonding, particularly in monogamous species

When I think about monogamy, I don’t think about genetic or sexual monogamy but about what we call social monogamy. Social monogamy sometimes goes along with genetic monogamy, and sometimes doesn’t. Animals that form pair-bonds to each other form really strong, selective emotional attachments. That includes things like a clear preference for their mate—they get upset if their mate isn’t there. The mate can also buffer them against stress, and try to exclude rivals from them. Of course, we see a lot of the same behaviors in human adult relationships, but these pair-bonds are super-strong.

The monkeys I’ve studied exhibit all of these behaviors, and some really cute additional ones that go along with them. For instance, the adults twine their tails together—they’ll sit with their tails wrapped around each other. And the fathers carry the babies most of the time, which is another thing you see a lot in monogamous mammals—the fathers become very involved in infant care. There are many hypothesis about why that is, but the one I believe is right is that the male has a very strong attachment to the female, and he doesn’t have the time to mate with a bunch of other females, so it makes sense for him to maximize the survival and success of the offspring of the female he’s with. So they tend to be very good dads.

“Social monogamy sometimes goes along with genetic monogamy, and sometimes doesn’t.”

David Barash

Emeritus Professor, Psychology, University of Washington, and the author of Out of Eden: The Surprising Consequences of Polygamy

There are remarkably few genuinely and persistently monogamous nonhuman animals. Thus, thanks to DNA fingerprinting in particular, we now know that even such species as swans and eagles, previously thought to be paragons of monogamy, will “cheat” on occasion. My favorite guaranteed monogamous creature is a flatworm that parasitizes certain fresh-water fish. In this species, whose technical designation is Diplozoon paradoxum, male and female meet when they are adolescents, whereupon their bodies literally fuse together, and they remain sexually faithful until death-do-they-not-part!

Some other mammals that appear to be monogamous are the California mouse, a few species of foxes, one or two marmosets, and—get ready for it—the Malagasy giant rat and the fat-tailed lemur. It’s worth emphasizing that whenever it is proclaimed that a given species practices lifelong monogamy, it turns out that once those animals are studied long enough, we find that, in fact, their “monogamy” isn’t completely reliable. This shouldn’t be surprising, because biological systems inevitably show variability, and both sexes nearly always can benefit from multiple sexual partners, especially if they can keep their “official” mate from finding out!

“My favorite guaranteed monogamous creature is a flatworm that parasitizes certain fresh-water fish.”

Daniel J. Kruger

Professor, Evolutionary Psychology, University of Michigan

Many animal species are more monogamous (or less polygynous) than humans. Quite a few likely qualify as “truly monogamous.” The greater the degree of polygyny, the greater the sexual dimorphism in physiology and behavior. Sexual dimorphism is a function of mating competition. The greater the level of polygyny, the greater the skew in male reproductive success, and the more intense male mating competition will be. The exaggerated size, elaborate ornamentation, and armaments seen in the males of highly dimorphic species are great investments of energy and other somatic resources that facilitate success in male mating competition. Because each individual has limited energy and resources, there is a trade-off between investment in mating competition and investment in paternal care of offspring and reproductive partners. Polygynous species tend to have less, if any, paternal investment.

So, the more monogamous a species is, the more similar you would expect females and males to be in physiology and behavior. One of my favorite examples is Emperor penguins, in which females need to return to the sea to feed for two months after laying a large energetically expensive egg. The males care for the egg over the harsh winter with no food, huddling together for warmth and losing considerable weight. Both sexes have an enormous physiological investment for offspring, and are very similar in appearance (they have low sexual dimorphism). You probably cannot tell which are females and which are males, and in fact the penguins themselves cannot identify individuals by appearance. The have a unique little dance that they use to identify themselves and recognize their partners. Of course Emperor penguins are serially monogamous, they are faithful to their mate through the course of the year, but usually have a different mate in the next breeding season.

“One of my favorite examples is Emperor penguins, in which females need to return to the sea to feed for two months after laying a large energetically expensive egg.”

Mark Erno Hauber

Professor, Evolution, Ecology, and Behavior, University of Illinois, Urbana-Champaign

One of the most monogamous birds is the Australasian gannet, which together with several of its relatives (e.g. Masked and Nazca Boobies) seem to show for faithfulness between the parenting birds. The Australasian gannet lays just one egg and it requires prolonged biparental care, so any risk of extra-pair parentage would be immediately predicted to face retaliation by the cuckolded parent.

Zoe Donaldson

Assistant Professor, Behavioral Neuroscience, University of Colorado Boulder

It depends on how you define monogamy. What are the most important features of monogamy? Fidelity? Resource sharing? Whether you ever break up or take a new partner? Scientists don’t usually think in terms of “most” or “least”, but we do distinguish between sexual and social monogamy. The first refers to whether you are sexually faithful while the second focuses on whether there is a particular mating partner with whom you spend most of your time, share resources, and help take care of the offspring. Only a handful of socially monogamous animals are also sexually monogamous. Based on recent studies, this category includes urban coyotes (which was something of a surprise) and a few other species. Either version of monogamy is incredibly rare among mammals; only ~9% of species are thought to be socially monogamous. 

Fun fact: DNA testing for paternity and criminal investigations actually had its origins in scientists simply wanting to know if monogamous birds ever had illegitimate chicks. They examined DNA from chicks in nests to see if the father was the male who helped build the nest and feed the baby birds. It appears that most socially monogamous birds are not sexually monogamous; about 11% of offspring are sired by a male bird who is not the male at the nest most of the time, although this varies from species to species. 

“It depends on how you define monogamy. What are the most important features of monogamy? Fidelity? Resource sharing? Whether you ever break up or take a new partner?”

Rebecca Young

Research Associate and Lecturer, Integrative Biology, University of Texas at Austin

Some duck and goose species form a pair-bond after some kind of courtship and will then be mated for the rest of their lives. They will actually show what’s termed “bereavement” or “mourning” behavior when they are separate from their monogamous mate, which is really an increase in stress. There are other species that have long-term pair-bonds, so they’re not alone here. Birds in general have the highest-known rates of monogamy—only 3-5% of mammals display monogamous mating systems, whereas that number is closer to 75% with birds. That said, many of these monogamous systems are ones in which they’re only monogamous for one breeding season, after which they form a separate pair-bond with a new individual.

MMW