Natural History

Monogamy is pretty rare in mammals. Only 3-5% of mammals are monogamous.

Ethologists generally define monogamy as a social organization in which one male and one female live together. Usually, the male helps the female care for any offspring that are born, though he is not always the father. We usually refer to this as “social monogamy.”

Genetic and sexual monogamy is something else. They actually not that common, even in species that thought of as pairing for life. There are countless studies of female songbirds “cheating” on their mates. In colony breeding birds, like budgies, there is often quite a bit of philandering going on. New World quail are often touted for their monogamy, but the truth is that some captive breeders of bobwhites will put compose breeding colonies that consist of one cockbird for every hen.  Some males will mate with multiple hens, and because the custom is to place the eggs in incubators, it really doesn’t matter if the male cares for the young.

I should note that humans are not considered a monogamous species. This may sound somewhat shocking to Westerners, who have lived under a one man and one woman marriage system for centuries, but humans are not truly monogamous.  Not all human cultures have monogamy as the primary structure of the family, and as a construct, it may have never existed until humans developed widespread agriculture and clearly defined property rights–and the rights to inheritance of that property– became important. Even then, we can’t say that monogamy was the default breeding system of human. “True biblical marriage”– a phrase we hear bandied about quite a bit in the US– was often one man and several wives.

So monogamy really shouldn’t be treated as if it were the natural way for humans to reproduce and organize family units.

It’s just that it’s become the main way many societies have decided to operate.

With canids, social monogamy is unusually common.  Virtually every species of canid has some form of social monogamy. In most species, males and females form a pair bond, and the male helps the female raise the pups or kits.

In the social hunting canids, the pair bond is the basis for the pack. There is always a pair-bonded male and female at the center of the pack, and the pups born to those parents are born in the main den and get most of the care from the other members of the pack.

Pair-bonding in canid likely evolved for a very simple reason. Canids are born in a very underdeveloped state, and because the ancestral canids were small and relatively weak carnivores, it was very hard for a female to get enough meat to feed her offspring. Further, she would probably have to leave her den for extended periods of time, which would open her offspring up to predation.

It may have been that the only way that canids could reproduce is if the male stayed and helped the female raise the young.

Now, this is actually a very inefficient way to spread genes. Male animals produce a lot of sex cells. Sex is very cheap for males, and in most mammal species, the male mates with many females to get his genes spread most efficiently.

But from a gene-centered view of evolution, this works only if the females are likely to give birth to young that will survive into maturity.

It may have been that in the ancestral canid, it was virtually impossible for young to survive without the paternal care.

And this may have placed such as strong selection pressure on canids, that this became the most common way they reproduce and organize their social units.

Because males of any species can spread their genes more efficiently through promiscuity, these selection pressures had to have been very strong. Otherwise, we wouldn’t see so much monogamy in canids.

Now, as I pointed out earlier, social monogamy is different from sexual and genetic monogamy. Cheating is actually pretty common in canids– at least in the species that have been studied the most closely. It is fairly common in red foxes. In one study that examined the DNA of red fox kits to ascertain their parentage, 80 percent were fathered by a different dog fox than the one currently pair-bonded to their mother.

Male foxes and other socially monogamous species that do cheat are operating as sort of “half cuckoos.”  Many species of cuckoo are nest parasites. The female cuckoo comes to another bird’s nest and lays an egg in it. The chick hatches after a very short incubation period, and the chick then throws out the eggs that belong to its surrogate parents. The parents raise the cuckoo chick until maturity. They invest so much time and energy into raising it, but the chick they raise does not carry their genetic material. These cheating foxes give half their DNA to fox kits that will be raised by a father that isn’t their own. The kits will have the mother fox’s genetic material, but her pair-bonded mate will be raising offspring that are not his own.

It’s actually not an inefficient way for a male fox to get his genes spread around. It also increases the likelihood that his genes will be carried on into the next generation.

The foxes are not consciously choosing the way they spread their genes. It’s just that their behavior results in this being the way that their genes get carried into the next generation. Strict monogamy places severe controls upon how efficiently and quickly genetic material is spread. It would only exist if there were strong selection pressures to keep monogamy as the main breeding system.

In some red fox populations, the half cuckoo strategy gets the both of best worlds. Male foxes can spread their genetic material rapidly and efficiently, and the offspring get cared for by a pair-bonded male.

This is sort of middle ground, and it’s probably what most wild dogs actually do. They are only socially monogamous, and there is at least some cheating going on.

But there are exceptions.

Coyotes are very strictly monogamous. A recent study of urban coyotes found that without exception, females gave birth to pups that were fathered by the male with whom she had a pair bonded. Similar rates of monogamy have been suggested for golden and black-backed jackals, though they have not necessarily been confirmed through genetic testing.

Coyotes and golden jackals are very similar to the primitive ancestor of the wolf. Indeed, what may be the oldest extant subspecies of wolf has long been classified with as a subspecies of golden jackal. So it is very likely that the ancestral Canis species was a very strictly monogamous species.

Contrary to popular belief, wolves are not strictly monogamous. As David Mech wrote in The Wolves of Minnesota (2003):  “Wolves have long been considered monogamous. However, in reality, wolves are as monogamous–or non-monogamous–as human beings” (pg. 75). Mech then goes on to describe male wolves mating with multiple females. He points out that no one has seen a female wolf mate with multiple males, but the exact parentage could be determined through DNA testing.

Mech doesn’t point out that wolves actually have two reproductive strategies. The first of these is the most common, and the one that every one knows.  A wolf pack is actually an extended family. It consists of a pair-bonded male and female and their grown offspring. Occasionally, siblings of the pair are included in the pack, but most of the pack is a male and female and their grown offspring.

Now, these grown offspring almost never remain in their natal packs their entire lives. At some point, they’ll want to mate, and in wolves, mating is very strictly controlled within packs. The mated pair get free license to mate with each other, but because wolves have both inbreeding avoidance behavior and social suppression of estrus in the subordinate females, the chances of more than one female getting pregnant from other pack members is pretty low. Female wolves won’t mate with their brothers, and they will only mate with their father if their mother dies. If they try to mate with their father, they are likely to be attacked. If the males try to mate with their mother, she usually won’t let them, and if they try, their father will attack them.

However, virtually all female wolves get pregnant at some point in their lives. This includes subordinate females that are in their natal packs.

How does this happen?

Well, there are wolves that use another strategy for reproduction. These are the so-called “Casanova wolves” or “lone rangers.”  These are male wolves that leave their parents’ territory and then hang out on the margins of the territories of established packs. When the estrus time comes, it is not unusual for a subordinate female to become pregnant by a “ramblin’ man.”

This strategy is more risky for two reasons.  The first is that the chances of Casanova wolf being killed when he shows up on a pack’s territory are pretty high. Wolves believe in “No trespassing: Violators will be eaten.”  Quite a few dogs have figured this out the hard way!

The other is wolf society almost guarantees that his pups won’t be born.

Now, in a wolf pack, only that female that has the pair bond with the male gets preferential access to dens and food. Both of these resources are typically quite scarce, so the female with the pair bond gets these resources first. If they run out, the pups belonging to the subordinates don’t survive. In some cases, it is possible that the main breeding female kills off the offspring of the subordinates– and this has certainly been observed in captive packs. That this one female gets to have preferential access to the dens and to the food is the definition of dominance.

There is some evolutionary advantage here. If all these female wolves have pups and the subordinate females lose them, then they will have a milk supply and strong maternal behavior for the main breeding female’s litter.  These two features would have been present in most female wolves whether they became pregnant or not. The false pregnancy and the subsequent lactation and maternal behavior observed in many domestic dogs is also evident in wolves, but if the females have had puppies, these behaviors will be that much stronger.

However, there have been wolf populations where multiple females have produced pups and raised them to maturity. The main breeding female’s pups are sired by her pair-bonded mate, but those of the subordinates are all sired by Casanovas. Some wolf packs– particularly those in Yellowstone– seem to be more tolerant of multiple breeders. This may be because Yellowstone wolves have access to rather extensive prey resources.

It should be noted that domestic dogs generally use a variant on the Cassanova strategy. Pair-bonding is virtually unknown in domestic dogs, though it should not be considered nonexistent.

In a book that gets trashed mainly because of its somewhat extreme anthropomorphism, Elizabeth Marshall Thomas describes a pair bond that existed between her two Siberian huskies. She unfortunately called their pair bond a marriage, and well-known scholars trashed it. However, the behaviors that existed between those two huskies were not that different than those between two wolves. The male actually vomited up food for his mate, and the female would not allow other males to mate her. The male may have had some women in the side, but Thomas never mentioned this dog mating with any other dogs on his travels. (She also rather infamously let him wander the streets of Cambridge, Massachusetts, and for several years, she followed him on bicycle to see what he was doing on his ramblings.)

But she also describes the other way in which dogs reproduce– the main way. When the male husky is given away because of his excessive roaming, the female reverts to the normal dog mode of reproduction, even trying to mate with one her adult male sons.  Thomas describes what female dogs normally do as “a businesslike sexual encounter, wherein a female who cares little or nothing for the male seeks only to get herself bred.” Thomas goes on to describe street dogs copulating on the streets of San Jose, Costa Rica.  She describes their mating behavior is simply a fleeting affair, though she couldn’t see the entire episode because she was afraid of what sort of attention she might have been drawing to herself by watching such a spectacle.

Dogs generally reproduce this way. Because they have become polygynous, selective breeding is much easier. If a male has  desired trait, he can be bred to multiple females, and if a female produces a trait when mated with one male, one can easily breed her to another.

Wolves are not nearly so willing to have their mates chosen for them. Many wolves want to form pair bonds with their mates, just as some dogs do. And if you’re forming a pair bond, you’re going to be more selective about your mate.

The question of why dogs use this Casanova strategy has not been fully answered.

As I noted earlier, one of the reasons that is suggested as to why Yellowstone wolf packs tolerate multiple breeding females is that Yellowstone wolves have much greater access to prey. Resources are less scarce, and the subordinate females can raise their puppies to maturity. The Casanova strategy works when this is the case.

It is possible that the wolves that became dogs found themselves in a very resource rich environment when they began scavenging off the massive surplus kill sites that ancient humans created when they drove ungulates like horses, red deer, and reindeer off cliffs or into box canyons, where they could be entrapped and easily dispatched with boulders or well aimed spears. Some wolves may have adapted their entire culture to living near humans and the riches of surplus killing. The wolves may have even helped participate in the hunt, which would mean that humans would have left them some booty on purpose.

In that sort of environment, the resources would not be that scarce, and the Casanova strategy would work very well. And because Casanova wolves can sire more puppies over their lifetimes, it wouldn’t take very long before wolves with a lowered tendency to pair bond would swamp the population.

And then, as time went on, humans were much more willing to help the wolves raise their puppies, and the selection pressures that ensured very strong paternal parenting would have been relaxed significantly. Although most male dogs love puppies, only a small percentage will vomit up food for them.

Now, we don’t have direct evidence that increased food supply results in more polygyny in wolves. However, on Round Island in the Bering Sea, red foxes typically are polygynous. They live on an island with vast numbers of breeding seabirds, so the vixens can raise their kits on their own. However, when the Bering Sea experiences El Niño, the seabirds can’t raise offspring. Seabirds often rely upon small fish, which are more easily caught in cooler water. When El Niño hits, the fish can swim faster, and the birds can’t catch them. And some species of fish don’t like the warmer water at all and don’t hang out near the island, which means it is very difficult for the parent birds to raise any chicks. The foxes primarily eat the chicks and the eggs, and if there aren’t a lot of chicks produced every year, then the resources get tighter, and the foxes have to hunt voles and eat walrus carcasses that wash up on the beach. Under those conditions, the vixens that have pair bonded are more likely to raise kits to maturity than those that have had them on their own.

And this has generally been the model for understanding when canids will adopt monogamy or polygyny.

But there is an exception to the rule.

Remember that urban coyote study I mentioned earlier?

Urban coyotes have access to great resources. They can scavenge out of dumps. They raid trash cans. They can hunt cats. They can steal pet food.

But they don’t develop polygyny. They remain monogamous.

I don’t know of any studies in which the paternity of coyote pups living in other enviroments was examined, but it is a fair bet to say that if coyotes in urban environments are highly monogamous, then those living in rural areas probably are, too.

It may be that selection pressures that produced pair-bonding in coyotes is so strong that it is very hard for them to change it.

There may also be something hormonal in what is keeping them so tightly bonded.

In recent years, there has been a lot of research on prairie voles. Prairie voles are unusual in that they are monogamous. All other voles in North America are polygynous.

So this set up an interesting research question:

Why were prairie voles so willing to form pair bonds while the others were not?

It turns out that prairie voles have more receptors in their brains for particular hormone called oxytocin than other voles. In particular, prairie voles have a large numbers of these receptor in a part of their brain called the nucleus accumbens, which plays a vital role in the pleasure and reward system. When these voles get together, oxytocin, which is associated with sexual attraction, is released, and this oxytocin actually stimulates the reward and pleasure function of this part of the brain, which means that the being together becomes an even more pleasurable experience than it would be for a polygynous vole.

I wonder if something like this is going on with coyotes. I wonder if there is a difference in the number of oxytocin receptors in the brains of coyotes, wolves, red foxes, and dogs.  It could explain why monogamy is so rare in domestic dogs and why it is almost the rule in coyotes.

It is likely that monogamy in coyotes is one factor keeping this species distinct. Although there is plenty of evidence that many populations of coyote are mixed with dogs, they all still remain primarily coyote in ancestry. The distinctness of the coyote species may be that they have such a strong tendency toward monogamy. Wolf and dog genes may enter the population at times, but they may happen only before a coyote bitch forms a pair bond with a member of her own species. Or maybe male wolves and large male dogs are powerful enough to be able  to drive her mate off and to force her into copulation. (All wolf and coyote hybrids that have been detected have been male wolf and female coyote crossed. The same is generally assumed for coyotes and domestic dogs, but there is evidence of dog mtDNA sequences in some coyote populations, which means there must have been some way that female dogs could have worked their way into the coyote gene pool.  I think the intentional release of coydogs as game animals is a possibility. It seems unlikely that a domestic bitch would be able raise her hybrid pups in the wild.)

It may have been that the wolf’s flexible social and mating systems made them much more easy to domesticate than other related species. It also may have been that the wolf is naturally a very social species, but it is not as intensely social as African wild dogs or dholes are.

At the base of canid mating and social systems is the mated pair. All living canids have some version of a monogamous social system, but only a handful have been found to be genetically and sexually monogamous. Most of them will cheat, and most of them will adjust their mating and social system to fit the relative abundance of food resources.

But monogamy is rare in mammals.

For it to have ever been used by creatures as successful as those in the dog family, there had to have been intense selection pressures for that behavior.

And it had to have worked very well.

We really don’t know when canids started being monogamous, but it was probably very early on. Otherwise, it wouldn’t be such a common feature for this family.

Monogamy in canids is one reason why we like them.

They are like us– with husbands and wives and little kids.

That’s how we anthropomorphize.

Farley Mowat once extolled the wolf for its supposed monogamy, and he then half jokingly excoriated the dog for picking up too many of many of man’s promiscuous happens.

That’s not really an accurate picture, but it is certain true that when some species of wild dog form a pair bond, it is very tight.

It is something like modern Western marriage.

But it has a much stronger evolutionary reason for its existence than our institution.

The truth is dog species probably would not have been able to survive without forming pair bonds.

It always took two parents to raise the kits or pups to maturity.

Domestic dogs don’t need pair bonds anymore. They have humans to take care of their offspring.

In a weird way, we’ve become like the host bird to the parasitic cuckoo. The dogs whelp their puppies and nurse them for eight weeks.

Then we take over the raising of them.

They don’t have any of our genes, but they are our kin.

And I don’t think we’d have it any other way.