Correlation and Causation (Social Monogamy)

Lack of consensus and continued scientific debate shows once again that data is not as objective as scientists want it to be. While evidence being used to directly counter each other’s claims between “Male infanticide leads to social monogamy in primates” and “The Evolution of Social Monogamy in Mammals,” the two publications agree on one point: Paternal care is a consequence of social monogamy rather than a cause. How will biology curricula across the nation adapt?

The conclusion that scientists make will affect all lessons involving mammal behavior. Even AP Biology’s leading textbook explains that monogamy arose from a need for parental care (1, p. 1124). According to this hypothesis, mammal parents found it advantageous to invest more energy in carrying for offspring, so that they could survive and go on to reproduce. Despite the dichotomy of both articles, both refute this nurturing hypothesis as a cause for social monogamy. Education generally mentions only one of the three hypotheses for social monogamy; we do not take into account male infanticide or geographic range. The two articles diverge at this point.

Opie et al. determined male infanticide to be the cause of social monogamy in mammals. Because female primates undergo a shorter lactation period when aided by their partners, Opie et al. believe this to be a significant environmental pressure for social monogamy. It also reduces infanticide risk. Ensuring survival for offspring means greater perpetuation of genes and reproductive success. Opie et al. conclude that paternal care and females’ discrete ranges follow social monogamy. Based on their report, the two hypotheses may contribute to maintenance of social monogamy, but Opie et al. emphasize that those developments do not precede social monogamy. Process of elimination points to the high risk infanticide hypothesis.

Lukas and Clutton-Brock’s publication supplements their hypothesis with explanations as to why other hypotheses do not work. They maintain paternal care as a consequence and point to a female’s distance from other females as the main factor. In fact, they write that “detailed field studies have found no evidence of any form of male contribution to care in 94 of 229 (41%) socially monogamous species,” discrediting paternal care as not even fully correlational. For example, Dik-dik are explained to be monogamous yet provide no paternal care.

They find the infanticide hypothesis unlikely because “Male infanticide is typically found in species where the duration of lactation exceeds the duration of gestation (6, 28). This is the case in few socially monogamous species (20 of 75 species, 27%) compared with species where females are solitary [148 of 335 species, 44%”.  Simply put, male infanticide is not as relevant as solitary living. Lukas and Clutton-Brock believe discrete ranges cause social monogamy because males have a harder time finding other female partners. Furthermore, they have pointed to primate ancestry as supporting evidence.

I find Lukas and Clutton-Brock’s argument compelling because of how they present data. Their statistics on the aforementioned male infanticide and paternal care seem more significant than data from Opie et al. Their hypothesis also has more grounding in evolution. It makes sense that socially monogamous species evolved from solitary ancestors

I do not think these articles have changed my understanding of monogamy in primates. Opie et al.’s statistic of 34% to 64% of infant deaths caused by infanticide in some species surprised me. I definitely know more than I did before, but if two articles make opposing conclusions as to the cause of monogamy, then it may very well be a combination of causes rather than only one.

1.    Campbell, N. A., & Reece, J. B. (2005). Biology. San Francisco: Pearson, Benjamin Cummings.

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