Part 4. Humans are (Blank) -ogamous: Promiscuity & Physiology

This is the fourth part on the evolution of human mating behavior, comparing evidence for promiscuity and pair-bonding in our species. Please see the introduction here.

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We left off with a list of eight traits in humans suggesting promiscuity in humans. Admittedly, the previous post was a little thick, as it dealt with imprinted genes and population genetics. The current one concerns human reproductive physiological and anatomical traits consistent with a multiple-partner mating structure, building on a couple of points addressed by Ryan and Jethá in their book. If you’re paying attention, that’s three posts concerning promiscuity and one (yet-to-be-written) on pair-bonding. Perhaps it seems I’m stacking the deck, but please reserve judgment. One reason more space is needed to make the case for the evolution of promiscuity is that the biology is less well known, and more effort is needed to bring it into the light. That single post on pair-bonding will be an important one, and quality matters just as much as quantity.

Continuing on with our list of traits hinting at promiscuity…

9. Sexual dimorphism in body size. This point remains somewhat contentious. In the majority of anthropoid species (monkeys, apes, and humans), males are the larger sex, with the degree of dimorphism ranging from slight to extreme (Plavcan 2001). This pattern correlates strongly with mating structure and male-male competition (Plavcan and van Schaik 1997). For monogamous species like gibbons, males and females tend to be roughly the same size. In species where females prefer larger males or where males compete for access to females, bigger males will leave behind more descendants. This is true for polygynous gorillas and dispersed, territorial orangutans, where males are physically about twice as large as females. A good non-primate example is elephant seals. On the other hand are horseshoe crabs, where smaller males cling to the backs of larger females and wait for the release of her eggs. This ‘reverse dimorphism’ is found in a few primates, but is slight and only in some prosimians such as lorises and lemurs.

Former sumo wrestler Konishiki and his wife Chie Iijima, an obviously cherry-picked example of extreme dimorphism. (From smh.com.au).

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Part 3. Humans are (Blank) -ogamous: More on Promiscuity, & Genetics

This is the third part on the evolution of human mating behavior, comparing evidence for promiscuity and pair-bonding in our species. Please see the introduction here.

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Part 2 pertained to human behaviors that suggested a human propensity for promiscuity (primate sexuality, the excessive sexual capacity of humans, infidelity rates, cultural variation in marriage practices, number of lifetime sex partners, etc.). This post and the next are concerned more with clues from our genes, anatomy, and physiology suggesting promiscuity. I realize these things are not clearly demarcated. My advisor at Binghamton, Mike Little, liked to say that “biology is behavior, and behavior is biology.” But I think in general most people would agree that while behavior has a genetic component, it is more plastic than are anatomical structures.

We left off with a list of six traits hinting at promiscuity. I don’t want to simply rehash what Ryan and Jethá address, so this post addresses some additional points on genetics before returning to their book in the next submission. Continuing with that list…

Fetal ultrasound at 4.5 months, profile view

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Student Research on War, Health, and Biology

With grades submitted earlier this week, the Spring 2011 semester is officially in the books. In my ANTH 324 class, “A Biocultural Approach to War,” students were required to write a literature review paper on the ways that war impacts human biology and health.

In my opinion, research papers are valuable in upper level undergraduate classes because they give students the freedom to pick a topic of their choice, within the boundaries of class goals. While it seems self-evident that war’s effects on biology are negative, this relationship is not always straightforward for every possible outcome variable. Thus, it is necessary to explore the evidence thoroughly.   

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The Sex Ratio at Birth Following Periods of Conflict

Note: this paper was written by UMass Boston undergraduate student Johnny Xu for my Spring 2011 class ANTH 324: “A Biocultural Approach to War.” I asked his permission to post it here. 

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Birth Sex Ratio and Infant Mortality: Adaptations or By-products?

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1. Introduction

            The purpose of this paper is to provide manifold reasons attempting to explain why the birth sex ratio following war periods tend to rise in favor of males and what this implies in correlation with infant mortality; and, most of all, to answer the following question: is the combination of these findings proposing that this is an adaptive response of the parent to produce the sex with higher survival prospects in the given environment, or is this simply the by-product of environmental forces?

 

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One Planet. One Species. Homo sapiens.

“One planet, one experiment.”
………………..— Edward O. Wilson. 1992. The Diversity of Life.

Hadzabe men (wikimedia commons)

The BBC has compiled what looks to be an absolutely visually stunning television series, titled ‘Human Planet.’  The footage is said to contain video from 80 different locations, highlighting the relationship of humans to various ecological conditions.1 The description from the website:

Uniquely in the animal kingdom, humans have managed to adapt and thrive in every environment on Earth. Each episode takes you to the extremes of our planet: the arctic, mountains, oceans, jungles, grasslands, deserts, rivers and even the urban jungle. Here you will meet people who survive by building complex, exciting and often mutually beneficial relationships with their animal neighbours and the hostile elements of the natural world.”

Have a look for yourself at the preview:

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