If that title doesn't get hits, nothing will.

In celebration of President's Day (which I have off work), Valentine's Day (which I do not have off work), and Charles Darwin's birthday (which isn't even a holiday), Olivia Judson has written a brilliant article on the mating habits and physiology of Tyrannosaurus rex at her New York Times blog, The Wild Side.

Now if we stipulate that:

1. All little boys love dinosaurs.
   • T. rex is the coolest dinosaur.

   • Some little girls love dinosaurs too.
2. All big boys love sex.
   • Again, some big girls too (probably a great many more than loved dinosaurs when they were little).

   • Okay, I think we can agree that everybody loves sex.

We clearly see that Judson's choice of topic was well-calculated genius. Distilled by this genius is a terribly interesting look at both the evolutionary biology and natural history of dinosaurs and their closest living relatives...

So what can we say about dinosaurs? My guess is that the males had members — but it’s an educated guess. It’s based on an analysis of dinosaur relations.

Two living groups are most closely related to dinosaurs. One is the crocodiles. Male crocodiles have a penis — just one — which, most of the time, they keep tucked inside their cloacae. (In most species of crocodile, it’s hard to determine the sex of living animals without an intimate exam, never mind dead ones.) Compared with the mammalian penis, the crocodile’s has an oddity: sperm is transported along an external groove, rather than through an internal tube.

The other group related to dinosaurs is the birds. Indeed, to be strict about it, birds are dinosaurs. If you look at a family tree of dinosaurs, birds, and other reptiles, you see that the lineage that evolved into dinosaurs split off from the lineage that evolved into crocodiles. Birds, in contrast, evolved directly from a dinosaur lineage. Birds are more closely related to T. rex than they are to any living form.

Birds themselves divide into two main groups, formally known as the palaeognathous and the neognathous. The palaeos comprises the big flightless birds such as ostriches, emus, rheas, and cassowaries, as well as kiwis and an obscure (but flying) group of south American birds, the tinamous; the neos covers everything else. The palaeos have penises; like crocodiles, they keep them tucked into their cloacae. Again like crocodiles, the organ has an external groove for sperm. What’s more, the lineage leading to the other endowed birds, the ducks, geese, and swans, appears to have split off from that of the other neos relatively early.

This strongly suggests that the ancestor of all birds had a penis, and that at some point early in the evolution of the neognathous birds, the penis got lost. Since crocodiles have one, and ancestral birds almost certainly did, and since the two groups have such similar genital morphology, I think it’s a safe bet that the lineages between crocodiles and birds — that is, dinosaurs — had one, too.

Now, the next question — what did it look like? Was it large or small? Fancy or plain? I wouldn’t like to guess. The blue-billed duck (Oxyura australis) is just a little fellow — he weighs less than one kilogram (two pounds) — but his penis measures 28.5 centimeters (11 inches), and it’s covered with knobs. In contrast, the mighty ostrich (Struthio camelus), which can weigh as much as 160 kilograms (350 pounds), has a penis that’s a mere 20 centimeters (8 inches) long. But at least it’s bright red.

Happy Valentine's Day.