Bioanthro lab activity: Sexual dimorphism

A few weeks ago we examined sexual dimorphism – characteristic differences between males and females – in my Intro to Bioanthro class. Sexual dimorphism roughly correlates with aspects of social behavior in animals, and so we compared dimorphism in our class with what is seen in other primates. For the lab, we collected our body masses, heights, and lengths of our 2nd and 4th fingers, then I plotted the data and we went over it together.

When collecting data on your students, make sure to get permission from your institution and let students know they can opt out of sharing their personal data. I’ve also assigned students randomized ID numbers to help keep their data private and as anonymous as possible.

This activity builds on the first lab we did this year, measuring our head circumferences to estimate brain size and examining how this varies within the classroom. We saw then that our class’s males have  larger brain (well, head) sizes than females. We hypothesized that this was simply due to body size differences – all else being equal, larger people should have larger brains. Now that we collected body mass data, we could test this hypothesis – in fact, when body mass is taken into account, our class’s females have larger brains than males:

Sexual dimorphism in brain size (left), body size (center), and brain/body size.

Sexual dimorphism in brain size (left), body size (center), and brain size relative to body size (right).

These are sex differences based on raw numbers. Another way to look at dimorphism is to se the extent to which sexes deviate from a scaling relationship (“allometry”). Looking to the left plot below, there is a positive linear relationship between body and brain size: as body size increases, so does brain size. As we saw above, male values are elevated above females’ but there is overlap. Importantly, the right plot shows that deviations from this linear trend, quantified as residuals, are not significantly different for the two sexes. So even though females have large brains relative to their body size in absolute terms, this is not exceptional given how brain size scales with body size.

Brain-body allometry in our classroom. Males and females in our classroom do not seem to deviate appreciably from a common pattern of allometry.

Brain-body allometry in our classroom. Males and females in our classroom do not seem to deviate appreciably from a common pattern of allometry.

While lab activities help students to understand patterns in data, this lab also shows students the importance of comparing patterns of variation.  Students learn from readings and lectures that humans show relatively low levels of dimorphism, and this activity helps them see why we say that. Situating our data within the context of primate dimorphism and mating systems, they can ask if there is an adaptive or evolutionary significance behind our level of dimorphism.

Sexual dimorphism in our classroom compared with what is seen in primates with different mating systems and levels male-male competition. Our class values are the stars, and in the right plot blue is males and green is females. Figures from Plavcan (2012) and Nelson & Schultz (2010).

Sexual dimorphism in our classroom compared with what is seen in primates with different mating systems and levels male-male competition. Our class values are the stars, and in the right plot blue is males and green is females. Figures from Plavcan (2012) and Nelson & Schultz (2010).

In this broader comparative context, students tackle what it means for human dimorphism, and ratios of the 2nd digit/4th digit, to be intermediate between what we see in monogamous vs. non-monogamous primates. This can lead some interesting class discussion.

Handout: Lab 3-Sexual dimorphism (Instructions and questions)

ResearchBlogging.orgReferences
 Nelson E, & Shultz S (2010). Finger length ratios (2D:4D) in anthropoids implicate reduced prenatal androgens in social bonding. American Journal of Physical Anthropology, 141 (3), 395-405. PMID: 19862809

Plavcan JM (2012). Sexual size dimorphism, canine dimorphism, and male-male competition in primates: where do humans fit in? Human Nature, 23 (1), 45-67. PMID: 22388772

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Osteology Everywhere: Why we number our premolars 3-4

Portishead* came on the radio the other day, making iTunes display the cover of their album, Third. My inner osteologist rejoiced to see it prominently features a tooth!

Third album cover by Porthishead (2008). Image from Wikipedia

Well not a picture, but rather the name, of a tooth. In each quadrant of your mouth (most likely) are two premolars, commonly referred to as “bicuspids.” In the biz, we usually call these pals,  “P3” and “P4.”

UW 101-1277 mandible, part of the Homo naledi holotype skull. Modified from the Wits media gallery.

UW 101-1277 mandible, part of the Homo naledi holotype skull. Each capital letter stands for the tooth type (incisor, canine, premolar, and molar). Modified from Wits’ image gallery.

You might be wondering why we call them P3 and P4, when there are only two premolars per quadrant — what happened to P1 and P2?  Homology to the rescue!

The ancestral mammalian condition was to have four premolars (and a 3rd incisor) in each side of the jaw. This is a “dental formula” of 3-1-4-3, indicating the numbers of each tooth type from front to back. Over time, different groups of animals have lost some of these teeth. Baleen whales have lost all of them.

P1 and an incisor were lost early in the evolution of Primates. Most Strepsirrhines and New World monkeys retain this primitive”2-1-3-3″ dental formula :

Ring tailed lemur (left) and woolly monkey (right) maxillae, showing the primitive primate dental formula including a P2. For scale, gridlines are 10 mm (left) and 20 mm (right).

Ring tailed lemur (left) and woolly monkey (right) maxillae, showing the primitive primate dental formula including a P2. For scale, gridlines are 10 mm (left) and 20 mm (right). Images from this boss database.

The last common ancestor of catarrhines (living humans, apes and Old World monkeys) lost the P2, and so we have only two premolars left in each side of the jaw. These are homologous with the third and fourth premolars of the earliest mammals. And that’s why we call them P3-4.

*The song was “The Rip.” It’s a very good song with an insanely creepy and trippy video: