Saturday, January 21, 2017

Is Monogamy for the Birds?


If monogamy is the pairing of a single male and a single female to cooperate in the generation and rearing of offspring, this looks like monogamy.  A male and a female penguin have given birth to a chick that they are now rearing.

If this is so, then it might seem that Thomas Aquinas was right in arguing that birds show that monogamy is natural, and thus part of natural law, for those animals who require the cooperative parental care of both mother and father to secure the generation, feeding, and education of their offspring.  Thus, natural law dictates monogamous marriage for human beings insofar as they are like these birds in needing biparental care for children.  I have written previously about Aquinas's biological argument for the natural law of monogamy (here and here).

Across most groups of animals, monogamy is rare.  But over 90% of birds are monogamous, in that pairs of males and females form partnerships in the breeding season to generate and rear offspring.  The male can contribute in many ways--by helping to build and maintain the nest, by helping to incubate the eggs, by feeding the chicks and the mother, by guarding the nest, and by protecting the offspring from predators.  The importance of male parental care has been demonstrated experimentally: if male parental care is reduced or eliminated, reproductive success (measured as the number of fledged young per brood) is reduced (Alcock 2013, 224-25).  This seems to confirm Aquinas's argument.

The emperor penguins show the intense devotion that bird parents can have for their offspring.  They nest on the ice in Antarctica in the middle of the winter, caring for one egg, and then chick, in total darkness with temperatures down to minus 80 degrees Fahrenheit and wind of up to 100 miles an hour.  Here is how biologist Bernd Heinrich describes it:
"The eggs are laid in May and June after the birds have walked far enough onto the ice so that the spring ice breakup will not reach the young until they are old enough to handle life in the water.  After the female has laid her egg, it is carefully transferred onto the male's feet and tucked up into his brood pouch.  he is then stuck with the egg (or vice versa) as she returns to the sea.  During his continuous care of the egg during her over two-month absence, he loses about 40 percent of h is body weight.  It is pitch black most of the time, except possibly for the southern lights shimmering in the skies.  He sleeps a great deal, but during the howling blizzards at the deep subzero temperatures the whole colony of thousands of males of which he is a part contracts and presses against each other to conserve heat.  Other penguins are highly territorial, but the emperors cannot be, or they would not survive the cold.  Without each other for protection against the lements, they would too quickly use up their fat reserves and then starve; they need to huddle during their long fast in the cold to conserve calories.  By July, when the eggs start to hatch, the males may feed the chicks with milklike secretions produced by glands in the esophagus.  But then the females start to return from the sea to bring predigested food that they can regurgitate.  In the crowds of hundreds, they all call for their mates, and somehow the mated pairs recognize each other, are attracted only to each other, and unite, transferring their young chick from the male's feet onto the female's and providing it with food that she has brought.  He, by now emaciated, then heads to the sea to feed himself and also collect food for his chick.  His return journey is made shorter by the receding ice edge, melting in the spring warmth." (Heinrich 2010, 83-84)
This story is now well-known from the hit documentary film of 2005--March of the Penguins, narrated by Morgan Freeman, who spoke of this as "a heartwarming story about family and the power of love."  Like Aquinas, some religious conservatives pointed to this film as showing that these birds could teach human beings about the virtues of monogamous love and parental care.

But some people protested that to speak of these birds as showing "love" was crudely anthropomorphizing these animals.  Similarly, some people have complained about Aquinas's "biologizing" of natural law as failing to see that nonhuman animals cannot provide any moral standards for human beings, because human morality has nothing to do with animal biology.

Heinrich was asked by the New York Times to write an article about this controversy as to whether it was proper for Freeman to speak of these penguins as showing "love."  Heinrich wrote:
"The unspoken rule is that this four-letter word is to be applied only to one creature on earth, Homo sapiens.  But why?  A look at the larger picture shows this presumption of exclusivity is utterly unproved.  In a broad physiological sense, we are practically identical not only with other mammals but also with birds--muscle for muscle, eye for eye, nerve for nerve, lung for lung, brain for brain, hormone for hormone--except for difference of detail of particular design specifications.  Functionally, I suspect love is an often temporary chemical imbalance of the brain induced by sensory stimuli that causes us to maintain focus on something that carries an adaptive agenda. Love is an adaptive feeling or emotion--like hate, jealousy, hunger, thirst--necessary where rationality alone would not suffice to carry the day.  Could rationality alone induce a penguin to trek 70 miles over the ice in order to mate and then balance an egg on his toes while fasting for four months in total darkness and enduring temperatures of minus 80 degrees Fahrenheit and gusts of wind of up to 100 miles an hour?  And bear in mind that this 5-year-old penguin has just returned to the place of its birth from the open sea, and thus has never seen an egg in his life, and could not possibly have any idea what it is or why it must be kept warm.  Any rational penguin would eventually say, 'To heck with this thing, I'm going back for a swim and eat my fill of fish.'" (Heinrich 2010, 80-81)
Although Heinrich was expecting criticism from his fellow biologists, those who wrote to him praised him for recognizing the evolutionary biology of emotions.  The criticism came from everyone else, who said that it was degrading to human dignity to speak of love as a chemically induced state of mind serving the same reproductive function as it does for other animals.

While Aquinas insisted on the uniqueness of human beings as the only animals capable of conceptual thought and speech and of morality in the strict sense, he would have agreed with Henrich that nonhuman animals do have emotions, that these emotions are what motivate animal movement, and that they do have "a certain likeness of moral good [similitudo boni moralis] in them, in regard to the soul's passions" (ST, I-II, q. 24, a. 4, ad 3).  We can know that they have such emotions, because "the internal passions of animals can be gathered from their outward movements: from which it is clear that hope is in dumb animals" (I-II, q. 40, a. 3).

It is not clear, however, that the modern evolutionary understanding of mating among penguins and other birds fully supports Aquinas's claim that birds show a monogamous bonding of male and female in the generation and care of offspring that manifests the natural law of monogamy for human beings.

For Aquinas, natural monogamy requires a pair-bonding of male and female over their entire life that excludes any sexual mating with anyone else outside the pair.  What scientists now know about birds suggests that they depart from this standard in three ways.  First, the monogamous pair-bonding of birds almost never lasts longer than one or two breeding seasons.  Second, even while the pair-bond lasts, there is often copulation with others outside the pair.  Third, the pair-bonding is sometimes between birds of the same sex.

While many people found the March of the Penguins to be an endearing display of "family values," some people pointed out that the bonding between the mated pair of penguin parents and the bonding of the parents with their offspring almost never last beyond one breeding season.  Emperor penguins can live up to 50 years.  And once a mated pair has reared one chick, the male and the female must forage independently for several years; and when they return to the colony for another breeding season, they find different mates.  So once the kid is out of the nest, the parents are free to divorce and find new mates. 

This is true for most birds, although a few, such as ravens, seem to pair for life.  Ravens form a lifelong attachment to their mates.  This has various benefits for them.  Raven young require a longer period of rearing than is the case for other birds.  If a raven pair holds a territory throughout the year, they don't have to look for a new nesting site every spring.  A raven pair also hunts and forages together, so that their strong and enduring bond makes them a coordinated team.

But even with raven couples, it is unclear as to whether they might occasionally engage in sexual mating with birds outside the pair.  In the 1980s, new techniques in molecular genetics allowed scientists for the first time to determine the genetic parentage of birds, and they were surprised to discover that many of the offspring being reared by pair-living individuals were genetically related to the female but not the male, and thus they were products of what came to be called extra-pair copulations (EPCs) (Reichard 2003; Alcock 2013, 224-41).  Ornithologists had always assumed that monogamous male birds were advancing their reproductive success by helping their female partners in caring for their genetic offspring.  But now it seemed that some monogamous males were being cuckolded, because they were caring for the offspring of another male.

So it seems that social monogamy is not necessarily the same as genetic monogamy or sexual monogamy.  Pair-living birds who cooperate in caring for offspring are in a monogamous social partnership.  But if some of the offspring are genetically related to the female but not the male, then we could say that the mating system here is not genetic monogamy, but either genetic polyandry (a female mating with multiple males) or genetic polygyny (a male mating with multiple females), although the system for caring for offspring is socially monogamous. 

This creates two puzzling questions for the Darwinian scientist.  What do males gain from being socially monogamous but genetically polygynous, when this means that some males will be cuckolded?  And what do females gain from being socially monogamous but genetically polyandrous, when this means that they have to trick their social partner into caring for another male's offspring?  Evolutionary biologists have offered various hypotheses as to how the benefits of these mating strategies outweigh the costs, with costs and benefits measured in terms of reproductive fitness.  But there is no general agreement on which hypothesis is correct.

This is a similar puzzle in explaining human monogamy.  Through much of human history and in some societies today, polygyny (men with multiple wives) has been permitted or even preferred.  This makes sense from an evolutionary perspective, because, as Robert Trivers (1972) argued, polygynous mating manifests the difference in mating effort and parental investment between males and females.  The most reproductively successful men have many more children than the most fertile women, because while a man can potentially impregnate thousands of women, a woman can give birth to no more than about 20 to 25 children at the outer limit.  So it seems that a man's reproductive fitness is promoted by investing his time and energy in mating with as many women as possible and investing very little in the parental care of his many offspring, while a woman's reproductive fitness is promoted by investing a little in mating and investing a lot in the parental care of her few offspring. 

If the successful rearing of human offspring typically requires parental care from both parents, then we might expect that women would want to mate with men who seem likely to be caregiving fathers.  A polygynous father will have to divide his paternal caregiving among his many offspring, and we can expect that the wives will have to compete with one another for the father's parental investment in their children.  But if the father controls great social resources (high status and great wealth), he can possibly provide those resources to his wives for the successful rearing of the children.

Aquinas says that polygyny is partly natural and partly unnatural (ST, suppl., q. 65, a. 1).  It is natural insofar as it is possible for one man with great resources to mate with multiple wives and provide for the successful generation, feeding, and education of their children.  But it is unnatural insofar as the jealousy of the multiple wives will drive them to fight amongst themselves in competing for the husband's attention and resources, and thus the conjugal bonding of husband and wife will not achieve the sort of marital friendship that is possible in a monogamous marriage.  In fact, the wives in polygynous marriages are often treated as almost the slaves of the husband.  The conflict among the co-wives can be softened somewhat by sororal polygyny--one man marrying sisters--because then each woman will be related to the children of the other women as t heir nieces or nephews.

By contrast, polyandry (one wife with multiple husbands), Aquinas says, is totally contrary to nature, because the wife can be impregnated by only one man at a time, and the men will be reluctant to invest parental care in children if they are uncertain as to whether the children are theirs (ST, suppl., q. 65, a. 1, ad 8).  The sexual jealousy of men is stronger than that of women, so that it is much harder for men to share a wife than it is for women to share a husband.  And yet the conflict among the co-husbands can  be softened somewhat by fraternal polyandry--one woman marrying brothers--because then each man will be related to the children of the other men as their nieces or nephews. 

Aquinas thought human polyandry was impossible, but actually there have been a few rare cases of polyandry, particularly in Tibet, Nepal, and adjacent areas.  And often these have been cases of brothers marrying one woman.  This seems to have been an adaptation to a harsh mountainous environment where farming is difficult, and arable land is scarce, and so brothers marry a single wife so that their land is held together rather than divided between the brothers, and this also maximizes the number of males for working the land (Durham 1991; Levine 1988; Sanderson 2014, 175-80).  The brothers often fight over this arrangement.  Clearly, they are making the best of a bad situation.  In showing the instability of polyandrous marriages, they confirm Aquinas's observation that polyandry is unnatural (Levine and Silk 1997).

So should we agree with Aquinas that we are left with monogamous marriage as the most natural form of marriage?  By the time Aquinas was born in the 13th century, the Catholic Church had abolished polygyny and made monogamy the only legal form of marriage across Europe.  Aquinas saw this as the fulfillment of natural law, because monogamous marriage best satisfies the two natural ends of marriage--the parental care of children and the marital friendship of men and women.

Notably, however, Aquinas admitted that there were exceptions to this norm of monogamy.  A wealthy woman might provide for the successful rearing of her children without need for the father's help (SCG, III, chap. 122, sec. 7).  And a wealthy man might generate a child through fornication and then help the unmarried mother in the rearing of their child (ST, II-II, q. 154, a. 2) .  But Aquinas claimed that marriage law must conform to what is generally best and not what works only in exceptional cases.

Even if we agree with Aquinas about monogamous marriage as naturally normative, however, we might wonder whether his ignorance of how to trace genetic lineage blinded him to the distinction between social monogamy and genetic monogamy, or between marriage systems and mating systems.  Beginning in medieval Europe, monogamous marriage has spread around the world as the legal norm; and thus it seems that as Aquinas argued, human beings have followed the example of the monogamous birds.  But now that we see that even most of those monogamous birds are engaging in extra-pair copulations, we have to doubt that social monogamy coincides with genetic monogamy.  And isn't this often as true for human beings as for birds--that human beings who are socially monogamous are often genetically or sexually polygamous?  Or, in other words, monogamous marriage systems are not necessarily monogamous mating systems?  (See Low 2003.)  Doesn't this suggest that human beings are by nature a mildly polygamous species, and consequently that genetic monogamy is almost never fully attained across a society, although socially-imposed monogamous marriage systems are common?

If this is true, then perhaps Aquinas was unrealistic in upholding life-long heterosexual monogamy with extensive biparental care and absolutely no extra-pair copulation as the natural standard.  He might have been right that by nature this satisfies the fullest range of natural human desires--including sexual mating, parental care, conjugal bonding, familial bonding, and friendship.  But predictably many human beings will fail to achieve this.  It might, therefore, be prudent for marriage law to tolerate these human imperfections--by, for example, legalizing no-fault divorce and remarriage and by not punishing adultery and fornication as crimes.

This seems to follow from Aquinas's prudent observation that it does not rightly belong to human law to punish all vices:
"Human law is established for the collectivity of human beings, most of whom have imperfect virtue.  And so human law does not prohibit every kind of vice, from which the virtuous abstain.  Rather, human law prohibits only the more serious kinds of vice, from which most people can abstain, and especially those vices that inflict harm on others, without the prohibition of which human society could not be preserved.  For example, human laws prohibit murders, thefts, and the like" (ST, I-II, q. 96, a. 2).
Therefore, Aquinas argues, human law "permits some things because it is unable to direct them, not because it approves them" (ST,  I-II, q. 93, a. 3, ad 3; q. 96, a. 2).  If human law permits without approving what we know by nature to be vices, we can rely on social pressure in civil society to disapprove of those vices, but without the coercive punishment of law.  This is what some scholars have identified as the "permissive natural law" that supports the modern idea of natural rights (Tierney 1997, 2014).

Moreover, as I have argued (here), there is some empirical evidence that Aquinas was correct in claiming that by nature monogamous marriage supporting sexual partnership and parental care promotes human happiness

"Permissive natural law" might also permit without approving homosexuality.  Aquinas condemns homosexuality as clearly "contrary to nature" for two reasons.  First, nonhuman animals do not engage in homosexual conduct.  Second, homosexuality does not lead to procreation and parental care of children (Summa Theologica, I-II, q. 30, a. 3; q. 31, a. 7; q. 94, a. 3, ad 2, q. 94, a. 6; II-II, q. 154, aa. 11-12).. We now know, however, that Thomas was mistaken about both of these points (as I have argued in a previous post here).

Scientists have observed homosexual behavior in 471 animal species--167 species of mammals, 132 species of birds, 32 species of reptiles and amphibians, 15 species of fishes, and 125 species of insects and other invertebrates.  Scientists have also observed that same-sex pairs have successfully reared young in at least 20 species.  In some cases, one or both partners are the biological parent(s) of the young they raise together.  In other cases, the partners adopt and care for young without being the biological parents.  Moreover, in some cases, the same-sex couples seem to be more successful in their parenting than opposite-sex parents.

We also now know that homosexuality is biologically natural in that it arises through the interaction of many biological factors in the early development of fetuses and children--genes and sex hormones shape the body and the brain in early life so that people are naturally predisposed to become heterosexual, bisexual, or homosexual (LeVay 2011; Sanderson 2014, 144-52). 

Penguins are one of the birds that show homosexuality.  Penguins can mate with a same-sex partner, incubate a fertile egg, and then raise their chick for three months.  If Aquinas is right about birds providing us with models of the natural law of monogamy and parental care, does this show that homosexual monogamy and parental care is natural?  If so, does this suggest a Thomistic natural law argument for the Supreme Court's decision in Obergefell v. Hodges (2015) upholding the constitutionality of same-sex marriage (as I have argued here, and here,).


Alcock, John. 2013. Animal Behavior, 10th ed. Sunderland, MA: Sinauer Associates.

Durham, William H. 1991. Coevolution: Genes, Culture, and Human Diversity. Stanford, CA: Stanford University Press.

Heinrich, Bernd. 2010. The Nesting Season: Cuckoos, Cuckolds, and the Invention of Monogamy. Cambridge, MA: Harvard University Press.

LeVay, Simon. 2011. Gay, Straight, and the Reason Why: The Science of Sexual Orientation. New York: Oxford University Press.

Levine, Nancy. 1988. The Dynamics of Polyandry: Kinship, Domesticity, and Population on the Tibetan Border.  Chicago: University of Chicago Press.

Levine, Nancy, and Joan Silk. 1997. "Why Polyandry Fails: Sources of Instability in Polyandrous Marriages." Current Anthropology 38: 375-98.

Low, Bobbi S. 2003. "Ecological and Social Complexities in Human Monogamy." In Ulrich H. Reichard and Christophe Boesch, eds., Monogamy: Mating Strategies and Partnerships in Birds, Humans, and Other Mammals, 161-76. Cambridge: Cambridge University Press.

Reichard, Ulrich H. 2003. "Monogamy: Past and Present." In Reichard and Boesch, Monogamy, 3-25.

Sanderson, Stephen K. 2014. Human Nature and the Evolution of Society. Boulder, CO: Westview Press.

Tierney, Brian. 1997. The Idea of Natural Rights. Grand Rapids, MI: Eerdmans.

Tierney, Brian. 2014. Liberty & Law: The Idea of Permissive Natural Law, 1100-1800. Washington, DC: Catholic University of America Press.

Trivers, Robert. 1972. "Parental Investment and Sexual Selection." In Bernard Campbell, ed., Sexual Selection and the Descent of Man, 1871-1971. Chicago: Aldine.


Anonymous said...

You're still insisting on "natural homosexuality" despite never addressing the arguments presented here:
and here:

As always, you're a Darwinian of convenience.

Narciso said...

I would also like to see Prof. Arnhart discuss the arguments presented by Anon above.