The Myth of Monogamy: Fidelity and Infidelity in Animals and People by David P. Barash; Judith Eve Lipton
Authors: David P. Barash; Judith Eve Lipton
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history was largely monogamous, with at least some male-female differences in Homo sapiens due to the fact that adultery figured importantly in that history.)
Back to the birds, where an important recent study focused on a small species known as the collared flycatcher, on the Swedish island of Gotland. It provides strong evidence that extra-pair copulations in this supposedly monogamous animal give certain males a distinct reproductive advantage. Collared flycatcher males have a white forehead patch; females don't. This
undermining the myth: males 27
patch is a secondary sexual characteristic. It also appears to be a status signal, whose size varies depending on its bearer's nutritional status as well as social success. Knowing the size of this patch, we can predict which collared flycatcher will win a territorial dispute between males. Moreover, if experimenters artificially increase the size of this patch, the fortunate males are more likely to establish a territory in the first place. Female collared flycatchers mated to males with large white forehead patches even produce a larger proportion of sons. This seemingly odd finding makes sense if males with prominent patches are likely to father sons with prominent patches. There would then be a reproductive payoff if such males father a comparatively large number of sons, which, in turn, will be relatively more successful than daughters ... who lack such distinguishing marks and, as females generally, are less likely to distinguish themselves reproductively.
The researchers found extra-pair paternity among collared flycatchers in 26 of 79 broods, accounting for 71 of 459 nestlings. After carefully analyzing their results, they concluded that "selection via variation in paternity"--- that is, some males having offspring via EPCs with females mated to other males--can be more important than "selection resulting from mate fecundity" (that is, more important than having greater numbers of offspring with their socially defined mates). Among collared flycatchers, in short, the major route to male reproductive distinction is not having more offspring with their "wives" but rather fathering offspring by various "lovers" who are already "married."
There have been many other studies along these lines, looking at species that are socially monogamous, most of them showing not only that males with highly developed secondary sexual traits have higher reproductive success but also that such success comes via EPCs. A typical one, in the prestigious journal Nature, was titled "Extra-Pair Paternity Results from Female Preference for High-Quality Males in the Blue Tit." Other studies have shown that male paternity--the proportion of offspring in his nest that are genetically his, as well as the number of offspring he will father in the nests of other males--is connected not only to the presence or absence of certain secondary sexual traits but also to the degree to which his secondary sexual characteristics are expressed.
Here are some notable examples: "DNA Fingerprinting Reveals Relation Between Tail Ornaments and Cuckoldry in Barn Swallows" showed that male barn swallows sporting more deeply forked tails are more likely to win the hearts of neighboring females. Another article, "Correlation Between Male Song Repertoire, Extra-Pair Paternity and Offspring Survival in the Great Reed Warbler," reported that in this European species--like barn swallows, ostensibly monogamous--males with a large variety of songs are also likely to have a variety of sexual partners. And don't overlook
28 THE MYTH OF MONOGAMY
this gem, potentially encouraging to any readers getting a bit long in the tooth: "Old, Colorful Male Yellowhammers, Emberiza citrinella, Benefit from Extra-Pair Copulations." Among these birds, males grow more colorful as they age. It appears that older, more colorful males therefore give promise of having a desirable set of longevity genes, which in turn are attractive to females.
Back to the birds, where an important recent study focused on a small species known as the collared flycatcher, on the Swedish island of Gotland. It provides strong evidence that extra-pair copulations in this supposedly monogamous animal give certain males a distinct reproductive advantage. Collared flycatcher males have a white forehead patch; females don't. This
undermining the myth: males 27
patch is a secondary sexual characteristic. It also appears to be a status signal, whose size varies depending on its bearer's nutritional status as well as social success. Knowing the size of this patch, we can predict which collared flycatcher will win a territorial dispute between males. Moreover, if experimenters artificially increase the size of this patch, the fortunate males are more likely to establish a territory in the first place. Female collared flycatchers mated to males with large white forehead patches even produce a larger proportion of sons. This seemingly odd finding makes sense if males with prominent patches are likely to father sons with prominent patches. There would then be a reproductive payoff if such males father a comparatively large number of sons, which, in turn, will be relatively more successful than daughters ... who lack such distinguishing marks and, as females generally, are less likely to distinguish themselves reproductively.
The researchers found extra-pair paternity among collared flycatchers in 26 of 79 broods, accounting for 71 of 459 nestlings. After carefully analyzing their results, they concluded that "selection via variation in paternity"--- that is, some males having offspring via EPCs with females mated to other males--can be more important than "selection resulting from mate fecundity" (that is, more important than having greater numbers of offspring with their socially defined mates). Among collared flycatchers, in short, the major route to male reproductive distinction is not having more offspring with their "wives" but rather fathering offspring by various "lovers" who are already "married."
There have been many other studies along these lines, looking at species that are socially monogamous, most of them showing not only that males with highly developed secondary sexual traits have higher reproductive success but also that such success comes via EPCs. A typical one, in the prestigious journal Nature, was titled "Extra-Pair Paternity Results from Female Preference for High-Quality Males in the Blue Tit." Other studies have shown that male paternity--the proportion of offspring in his nest that are genetically his, as well as the number of offspring he will father in the nests of other males--is connected not only to the presence or absence of certain secondary sexual traits but also to the degree to which his secondary sexual characteristics are expressed.
Here are some notable examples: "DNA Fingerprinting Reveals Relation Between Tail Ornaments and Cuckoldry in Barn Swallows" showed that male barn swallows sporting more deeply forked tails are more likely to win the hearts of neighboring females. Another article, "Correlation Between Male Song Repertoire, Extra-Pair Paternity and Offspring Survival in the Great Reed Warbler," reported that in this European species--like barn swallows, ostensibly monogamous--males with a large variety of songs are also likely to have a variety of sexual partners. And don't overlook
28 THE MYTH OF MONOGAMY
this gem, potentially encouraging to any readers getting a bit long in the tooth: "Old, Colorful Male Yellowhammers, Emberiza citrinella, Benefit from Extra-Pair Copulations." Among these birds, males grow more colorful as they age. It appears that older, more colorful males therefore give promise of having a desirable set of longevity genes, which in turn are attractive to females.
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