Hello, everyone! This week, I'm going to talk about a specific mechanism of evolution that can shape species in many different ways - sexual selection. One thing to keep in mind while reading this is that it is incredibly difficult, if not impossible to tell the sex of a fossil organism in the vast majority of cases, so even if there are two distinct morphs of a species it is not necessarily possible to tell whether it is due to sexual dimorphism, geographical variation, or some other factor. However, there are some features in many groups of fossil organisms that seem to have played some role in mating behaviors and displays, and sexual selection may have helped to shape their development.
One of the key requirements for sexual selection is anisogamy - this means that one sex produces small gametes, and the other sex produces large gametes. Biologically, this is how we can assign male or female - it is not the presence of a certain chromosome (not all creatures share the same sex chromosome system, and some species have temperature-dependent sex determination), but rather the size of the gametes they produce that defines their sex. Males produce the smaller gametes, and females produce the larger gametes. Since these two gametes are very distinct, selection can favor different traits in males and females.
One of the very important things about the size difference is that the larger gametes (eggs) are MUCH more expensive to produce than smaller gametes (sperm), and as such they are a limiting resource. For this reason, females are often much "choosier" than males - they stand to lose far more fitness than males by making bad mate choices (a male can easily produce more sperm if he makes a bad choice, it costs far more for the female to make more eggs).
As you may have noticed as you've read this, sexual selection is a logical equivalent to natural selection. Heritable traits in males that increase mating success should increase in frequency, whereas heritable traits that decrease mating success should decrease. These traits may be elegant displays, horns or spikes to fight off other males, or simply a high sperm count.
There are two subtypes of sexual selection. These two types are called intrasexual selection and intersexual selection:
Intrasexual selection is the result of interactions between individuals of the same sex. One of the most easily observed examples are those that happen before copulation. In these scenarios, there is competition between individuals of the same sex for mating opportunities (often males). Examples of traits that may be favored by this type of sexual selection are visual displays, traits that make the animal look larger, horns/tusks [or other features] used for physical confrontation, and features that help to establish dominance such as coloration or vocalizations.
Intersexual selection is the result of interactions between individuals of the opposite sex. In these scenarios, one sex preferentially mates with individuals of some specific phenotype, and consequentially those individuals displaying said phenotype produce more offspring. Examples of features that may be favored by intersexual selection include vivid color patterns/ornamentation, vocalizations, and display behaviors.
As I stated above, it is unfortunately very difficult to distinguish sexes of fossil animals, and as such we can not always determine whether or not there was sexual selection occurring. For example, it was originally hypothesized that the crests on species such as Dilophosaurus were used to attract mates, but due to the fact that there is no evidence of sexual dimorphism in the species (and even if there was, it would be very difficult to be sure), it is considered more likely that they were used for species recognition.
However, there are some species that show evidence of specialized features that may have been at least partially shaped by sexual selection. One such example are the frills and horns of Triceratops. There is considerable evidence that Triceratops engaged in non-fatal intraspecific combat (Petersen et al., 2013; Reid, 1997; Horner and Goodwin, 2009; Horner and Lamm, 2011; Farlow and Dodson, 1975), though we do not know if this was based on competition for mates - if one were to identify all individuals showing cranial pathologies linked to such combat as the same sex, that might reinforce the idea that sexual selection played a role, but the behavior could also be unrelated to mating and simply be a way of settling territorial disputes. Triceratops frills apparently began to develop at a young age (before the onset of sexual maturity), and were likely also used for display and species recognition (Goodwin et al., 2006). Whether the display was simply shape or if there were vivid colors is unknown, though if the idea of sexual selection playing a role in the evolution of frills and horns is correct, there may have been distinct colors patterning that area to help attract mates.
Additionally, as I talked about in my Creature Feature about Pachycephalosaurus, there is similar evidence for headbutting behavior in that species. Like with Triceratops, it is not known whether or not this behavior was based on competition for mates, or if it was simply a way of settling territorial disputes (or both), but the idea is the same.
To sum it up, sexual selection is a very important driver in the development of many features as observed in modern taxa, and I personally believe it is safe to assume that it played some role in the evolution of prehistoric creatures as well. However, the difficulty of sexing fossil animals and the lack of preservation of soft tissues such as non-bony crests/flaps or vivid colors means that even if it did play a role, it would be very difficult if not impossible to say so with any great degree of certainty. Dinosaur Battlegrounds could help to investigate the possibility of sexual selection playing a role in the evolution of certain animals by running simulations with different AI behaviors specifically regarding mating preferences/behaviors in one [or both] sexes, and see whether or not the results match existing data. This is yet another example of how Dinosaur Battlegrounds' nature as a full paleoecosystem restoration can help us answer questions that we cannot uncover from the rocks.
I hope you enjoyed this week's Sci-Day! I'd like to thank my Evolutionary Biology professors for providing great lecture materials that I could use to help organize this post.
Acknowledgements:
Peterson, J. E.; Dischler, C.; Longrich, N. R. 2013. Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae). PLoS ONE 8 (7): e86820.
Reid, R. E. H. 1997. Histology of bones and teeth. In: Currie, P. J. and Padian, K, editors. Encyclopedia of Dinosaurs. Academic Press, San Diego, CA. 329-339.
Horner, J. R.; Goodwin, M. B. 2009. Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus. PLoS ONE 4 (10): e7626.
Horner, J. R.; Lamm, E. 2011. Ontogeny of the parietal frill of Triceratops: a preliminary histological analysis. Comptes Rendus Palevol 10: 439-452.
Farlow, J. O.; Dodson, P. 1975. The behavioral significance of frill and horn morphology in ceratopsian dinosaurs. Evolution 29: 353-361.
Goodwin, M. B.; Clemens, W. A.; Horner, J. R.; Padian, K. 2006. The smallest known Triceratops skull: new observations on ceratopsid cranial anatomy and ontogeny. Journal of Vertebrate Paleontology 26 (1): 103.
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