Sexual dimorphism and taxonomy

The final of my posts on characters that vary in populations that can cause problems for taxonomists. I’ve already covered ontogeny (growth) and intraspecific variation and now for perhaps the most tricky aspect of them all, sexual dimorphism. For those who have not come across the term before this basically refers to differences between genders as exhibited (typically) by adult organisms. It should be easy to see how this complicates things by taking humans as an example. In general men are taller than women, with proportionally broader shoulders and a narrower pelvis but of course the range of intraspecific variation covers most if not all of this, and ontogeny can cloud the issue further (young teenage girls are often taller than their male counterparts as they hit their growth spurt earlier). If you only have part of a skeleton (like an arm) to work from it could be very hard to say if you have a tall woman or short man before you.

Even if we go outside of mankind to animals with more obvious sexual dimorphism like a peacock, it’s not entirely clear how much of the obvious male and female differences would remain if you stripped off the feathers. Some for sure (like the males’ fighting spurs) but it would not be as clear. Even with obvious bony differences (such as the antlers and horns of deer and antelope) these can be shed regularly or lost in some individuals. In some species male and females both have horns and even very similar horns to each others and in some (like reindeer) females retain their antlers at times when males lose theirs. Even if as a palaeontologist you had a fairly complete skeleton of a male and female antelope, it might be clear that they are similar enough to be the same species and different enough to be sexual morphs, but it may not be clear which was which.

As such palaeontologists (or at least the ones working on archosaurs) do not often deal with sexual dimorphism. Some archosaurs (like crocs for example) really don’t have much difference to detect. Others might be present, but to sort it out first one needs a good sample size of individuals to work with (even if you have ten specimens, they might all be males, or a variety of ages, or if the pieces are non-overlapping it will be hard to spot patterns of differences). Even then a pattern can be hard to interpret – you might find that specimens fall into two fairly distinct and separate size categories but is this male and female, or two different species (one big and one small)? If it *is* male and female, which is which? It is true that in general male animals are ornamented so one measured group may have horns and another lack them or have smaller horns but again this may be a species split (look at just how similar many antelope and gazelle are in the African savannah that live side by side).

In some cases there are clues available to the lucky few such as the structure of the bones, (recently used to diagnose a Tyrannosaurus specimen as female), eggs or embryos being found inside a female, an enlarged pelvis for egg-laying and so on that can be more fixed but one still has to be careful when identifying putative males. There are few dinosaurs or pterosaurs with enough of a group of specimens to be able to work on effectively, but a number have been suggested as being males and females.

The problem here for a taxonomist is of course that it is so hard to tell these things apart with the limited information we have. It is easy to think that two animals are rather different in size and form and name them as different species when in fact they are two different genders (especially if one has a crest and the other does not for example). Without the large numbers of specimens required to even being to piece together possible differences, it can be futile to try to separate them out. There are probably therefore at least a few males and females languishing as separate species in the records of taxonomists – of the few putative male and female dimorphs that have been suggested, most are hotly contested. Of course in many cases this is largely irrelevant – if you have only a few bones for your specimen, its gender is not much of an issue, but when you have things like the ceratopsians where much of the taxonomic difference lie in the frills and horns that you might also think could well denote male and female differences (as with many modern ungulates) then you can see how things rapidly get much trickier. This can get still more complex as these kinds of characters often only show up in adults and thus juveniles which lack ornaments or have smaller ones can also be thrown into the mix to mess up the efforts of taxonomists.

As a vague conclusion to the three pieces here, taxonomy is about more than just looking for similarities and differences and erecting new species based on differences, or synonymising others based on similarities. One must assess these characteristics themselves to ensure that what you are naming is a genuinely different organism and not just a large and robust adult, a juvenile or a female. There is an awful lot of variation out there in biology and when working with only half a 200 million year old skeleton it can be tricky to keep on top of things. However these aspects (variation, ontogeny and dimorphism) are important and should not be ignored or underestimated.

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4 Responses to “Sexual dimorphism and taxonomy”

  1. 1 Tor Bertin 02/09/2009 at 11:53 am

    So, I just realized that a Tyrannosaurus femur section I was given by some friends about a decade ago has a ring of medullary tissue. That amuses me.

  2. 2 Tor Bertin 02/09/2009 at 11:55 am

    Not that I could have done much but give it away since I was about 11 at the time… still, it’s the principle that counts. 😉

  3. 3 Andy 02/09/2009 at 9:03 pm

    Another problem facing the study of sexual dimorphism is the difficulty in demonstrating sympatry for alleged males and females. . .proper stratigraphic data are so rarely collected! The case of sexual dimorphism in Lambeosaurus and Corythosaurus is a great example – turns out the “males” and “females” of each genus never co-existed!

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