Sexual Dimorphism ~ Another Challenging Variable

This post was prompted by the recent publication of a fascinating article by paleontologist Jordan C. Mallon about our ability or, more specifically, our inability to distinguish between males and females in the available fossils for several prominent dinosaur taxa.  (Recognizing Sexual Dimorphism in the Fossil Record:  Lessons from Nonavian Dinosaurs, Paleobiology, 2017.)

Sexual dimorphism is present when characteristics of the males and the females of a given species differ.  These differences may be reflected in male and female morphology including such features as body shape (e.g., display features such as crests), body size, or coloring, and may also be found in variations in behavior.  Sexual dimorphism is an important attribute that is central to evolutionary biology, particularly as it relates to sexual selection.

With extant animals, some aspects of sexual dimorphism may be obvious as with the African lion species (Panthera leo) whose males sport a distinctive mane and may be somewhat larger than the females.  Pictured below are mounted specimens on display in the Hall of Mammals at the Smithsonian’s National Museum of Natural History.

A word of caution.  Having living animals available for study offers no certainty that we will, in fact, accurately identify sexually dimorphic traits.  The lions in the Hall of Mammal, as posed, promote the widely accepted canard that the female lions do all of the hunting for the pride and the males just lounge around.  It was only in this decade that careful tracking of male and female lions in Kruger National Park (South Africa) using GPS and other technology showed that females hunt in open areas while males do their hunting in denser vegetation shielded from observation.  (Scott R. Loarie, et al., Lion Hunting Behaviour and Vegetation Structure in an African Savanna, Animal Behaviour, Volume 85, 2013.)

Certain patterns of sexual dimorphism have been detected in extant animals.  For example, with respect to size and morphology, biologists Ehab Abouheif and Daphne J. Fairbairn observe, “In most species of animals, females attain larger body sizes than do males (e.g., most spiders, insects, fish, amphibians, reptiles), whereas in most birds and mammals, males are the larger sex.”  (A Comparative Analysis of Allometry for Sexual Size Dimorphism:  Assessing Rensch’s Rule, The American Naturalist, Volume 149, Number. 3, March, 1997, p. 540, references omitted.)

But all bets are off with extinct animals known from the fossil record.  Mallon is not alone in positing that the barriers to identifying the sex of extinct species are particularly high.  Armed only with fossils, we find ourselves without much of what might help to distinguish males from females including gender-specific behavior as well as some morphological attributes such as sex organs and coloring.  Without the information that observation of living animals provides, paleontologists face a daunting challenge.  As evolutionary ecologist Robert J. Knell and his colleagues have written,

Identifying sexual dimorphism in animals known only as fossils is often difficult; specimens of a particular species are sometimes rare, unique, or unavailable, and reliably identifying sex in fossils is often difficult or impossible.”  (Robert J. Knell, et al., Sexual Selection in Prehistoric Animals:  Detection and Implications, Trends in Ecology and Evolution, 2012.)

On the one hand, if the sex-related differences preserved in the fossil record are slight, then, as Knell et al. note, one must work with large samples (IF such sample sizes are actually available).  Apparently, it hasn’t been unprecedented for claims to be made about the sexes of dinosaur fossils based on bones from just two specimens (which is clearly an inadequate sample size).  At times, it may be differences in the sizes of features that distinguish males from females, not the presence or absence of such features in either of the genders.

On the other hand, “when dimorphism is strong, there is a risk that different sexes will be described as different species.”  (Knell, et al.)  That is, relying on morphology to identify species, as is a common and necessary practice in paleontology, poses the risk that significant differences in fossils, possibly related to sexual dimorphism, might be treated as indications of the presence of different species, not the two sexes of the same species.

From his analysis, Mallon concludes that, with the dinosaur fossils at hand, paleontologists, for the most part, cannot tell whether an individual dinosaur was male or female.  In this vein, I would add that one must ignore the nicknames given some of the iconic dinosaur fossil skeletons such as the Field Museum’s Tyrannosaurus rex which is known as “Sue,” the Triceratops horridus cast on display in the Last American Dinosaurs exhibit at the Smithsonian’s National Museum of Natural History which is fondly called “Hatcher” and its companion T. rex cast in the same exhibit carrying the nickname “Stan.”  (The National Museum has Hatcher's fossil bones, while its Stan is just one of many casts that can be found worldwide.  Stan's original fossil bones are in the Black Hills Institute.)  These nicknames honor the collectors who found the original fossils – Sue Hendrickson, John Bell Hatcher and Stan Sacrison.  In all fairness, the Field Museum’s T. rex was nicknamed Sue by Peter Larson because he believes this specimen was, in fact, female.  (See, Jacqueline Ronson, Sue the Tyrannosaurus Has a Sexual Identity Crisis, Inverse Science, December 1, 2016.)

Yes, there are some dinosaur fossils that can be identified as female because they carry eggs or have a medullary bone which, in contemporary birds, supplies the calcium used to fashion egg shells.  Mallon concludes that, for the taxa he studied (nine prominent species including Tyrannosaurus rex, Allosaurus fragilis, a couple of Stegosaurus species, and Protoceratops andrewsi), the available evidence offers “no support for sexual dimorphism.”  Mallon takes pains to make it clear he is not saying that such dinosaurs were not sexually dimorphic, simply that, in his considered view, the fossil record is inadequate to make such a claim.

This is certainly a thought-provoking, revisionist conclusion and it’s not just relevant for dinosaurs.  Mallon lays out the conditions under which sexual dimorphism might be successfully detected in different fossil taxa.

For me personally, his article brought me up short.  I have to admit that for the years that I’ve collected vertebrate fossils (principally shark teeth), I have never pondered the gender of the animals and how that might be reflected in the fossils I have at hand.  So much for my intellectual curiosity and my efforts at taxonomy.

Considering just fossil shark teeth, several of the most important variables that might account for differences in the teeth under study are:

• natural variation among individuals within the same species (hey, this variability is the material that natural selection work with),
• chronological age of the specimens (not only might teeth from younger sharks be smaller, they might also differ in terms of shape),
• position of the teeth in the jaws (most shark families are heterodonts, that is, teeth vary by location in the jaws; depending upon the shark, teeth from the upper jaws can differ from those in the lower jaw, and/or they can vary by location within a specific jaw),
• differential influence of the environment on the health and development of individuals from the same species,
• taphonomic processes (i.e., what happens to an organism in death) which may introduce variability in the fossils that emerge millions of years later, and
• sexual dimorphism.

This list is based primarily on paleontologist Bretton W. Kent’s Fossil Sharks of the Chesapeake Bay Region (1994, p. 2).  Though he does include sexual dimorphism as a factor behind some variation in fossil shark teeth, as far as I can tell, it’s only in a very few species that sexual dimorphism has been identified.  He cites it in just two fossil shark species found in the Chesapeake Bay region – Hexanchus (cow sharks), and Rhizopriondon (sharpnose sharks).

Ultimately, it’s somewhat ironic that, despite his warnings and the unlikelihood of detecting sexual dimorphism in most of my collected fossils, Mallon’s piece has prompted me to be sensitive to that possibility.