But how do we know they was doin' it?

There have been a bunch of articles recently about extinct creatures and at which point in their lives they began to mature sexually. The titles of the articles initially turned me off (no pun intended), like Big Dinosaurs Had “Teen Sex”. I don’t like it when media uses racy headlines to get my attention and sometimes I rebel and wait a long time before checking out articles that have said racy headlines.

I was curious about those “dinosaurs and reptiles had sex at really young ages” articles. I didn’t know a lot (still don’t) about how research is done on the ages/growth rates of dinosaurs and I had a lot of questions, like 1) How can you tell the age of a fossilized animal? 2) How can you tell when a fossilized animal started having sex, roughly*? and 3) How do the rates of growth vs. sexual maturation compare to extant animals?

After my initial rebellion, I took a peek at some of the dino/reptile sex articles and found some answers:

1) The ages of the subjects of the articles, dinosaurs and avian reptiles, were determined by studying growth rings. The bones of both dinosaurs and avian reptiles feature tree-ring-like rings that indicate periods of growth. The rings, according to Gregory Erickson, associate professor at Florida State University, are likely formed annually due to the similarities to the growth rings of organisms with common ancestry and give a good approximation of the age of the animal.

2) Birds, when they ovulate, develop “medullary bone” in the marrow spaces of their bones. The medullary bone acts as a reserve for the calcium required to build eggshells. Medullary bone was found in the bones of Tyrannosaurus rex and compared to the medullary bone of extant ratites (an ostrich and an emu). So…there’s female dinosaur with what looks like medullary bone and her bones also yield growth rings. Her age can be determined and the presence of medullary bone reveals that at that age, the dinosaur was reproducing.

Medullary bone of D) Tyrannosaurus rex E) Emu F) Ostrich

3) After some reading, I don’t feel very confident about the initial answer I got, but here it is, anyway: birds reach full size very quickly (within a year) but don’t begin to reproduce until years later. Crocs, alligators, lizards and snakes, on the other hand, grow throughout their lives and are capable of reproducing before full grown. Dinosaurs seem to fall in the middle somewhere, growing more slowly than birds and more quickly than reptiles.

*Okay, this post is impossible to write without innuendo…sorry.

References:

Erickson, G. M. 2005. Assessing dinosaur growth patterns: A microscopic revolution. Trends in Ecology and Evolution: 20: 677-684.

Peake, T. (2005, June 2). Dinosaur fossil bone leads to gender, age determinations. Dinosaur Fossil Bone Leads to Gender, Age Determinations. NC State University press release. Accessed February 16, 2008 at http://www.ncsu.edu/news/press_releases/05_06/133.htm

Roach, J. (2008, March 12). Ancient flying reptiles likely had sex as youths. National Geographic News. Retrieved March 16, 2008, from http://news.nationalgeographic.com/news/pf/75336744.html

Roach, J. (2008, January 14). Big dinosaurs had “teen sex.” National Geographic News. Retrieved March 16, 2008, from http://news.nationalgeographic.com/news/pf/57625340.html

Schweitzer, M.H., Wittmeyer, J.L. & Horner, J.R. 2005 Gender-specific reproductive tissue in ratites and Tyrannosaurus rex. Science 308, 1456–1460, (doi: 10.1126/science.1112158).