What's In John's Freezer?

Putting my morphologist hat back on today, I had an opportunity to dissect an Elegant-crested tinamou (Eudromia elegans) for the second time in my life. The last time was during my PhD work ~20 years ago. In today’s dissection I was struck by another reminder of how studying anatomy is a lifelong learning experience and sometimes it’s really fun and amazing even when it’s stinky.

Tinamou foot. I did know that tinamous don’t have a hallux; big “perching toe” (1st/”big toe” in us); true of ratites/palaeognaths more generally. Unlike a chicken or many other birds. Just the three main toes (2, 3 and 4) are here.

Stomach-Churning Rating: 7/10; you gotta have guts to learn about intestine-churning stuff.

Tinamous are neat little partridge-like ground birds but they are not close cousins of partridges or guineafowl at all. Their closest cousins are other ratites/palaeognaths such as ostriches and kiwis. And hence they are found in South America, especially Patagonia in Argentina. I’ve seen them there, much to my enjoyment.

Said tinamou.

What struck me today was that, as I delved into the digestive system of this bird, I saw features that were unfamiliar to me even after having dissected many species of birds from many lineages. The intestinal region was very lumpy, with little bud-like pockets full of dense droppings. Furthermore, on separating the tubes of the small and large intestines I realized that most of the intestinal volume itself was caecum (normally a modest side-pocket near the juncture of the small and large intestines). Indeed, that caecum was caeca (plural): it had two massive horns; it was a double-caecum, feeding back into the short rectum and cloaca. Birds have variable caeca and it is typical to see subdivision into two parts, but I’d never seen it to this degree.

Oh why not, here’s the gizzard/stomach showing its grinding pebbles and bits of food, plus the strong outer muscle layers (pink) for driving that grinding. Small intestine heads toward the bottom of the image. Yes, we do need a better dissection light…

I had to question my anatomical knowledge at this point, wondering if I was identifying things incorrectly—did I really screw up somehow and these were other organs, like giant ovaries? But no, they were clearly full of faecal matter; they were digestive organs. I finished the dissection, still puzzled, and hit the literature. Right away, Google-Scholaring for “tinamou caecum” I found the answer, here (free pdf link):

“at least one species (Elegant Crested Tinamou, [Eudromia elegans]), the ceca contain multiple sacculations, resulting in structures that look much like two bunches of fused grapes.”

The caeca in question.

OK buddy, those are the little lumpy buds I saw. Bunches of grapes—exactly.

And later:

“The paired ceca of the Elegant Crested Tinamou are extraordinary and probably unique within Aves (Fig. 3): long and wide (12.5-13.0 X 2.2- 2.5 cm; Wetmore 1926) and internally honeycombed by many small diverticula. These outpocketings gradually diminish in size and organization from the base to the tip of the organ, apically showing a more spiral form of internal ridges like ratite ceca. Externally, the basal diverticula protrude from the ceca as pointed lobes, gradually becoming flatter but still clearly apparent toward the organ’ s tip.”

Whoa! I never knew that! So I happened to be dissecting a bird, fairly common in its homeland, that has a really bizarre and singular form of caeca/ceca! That hit my morphologist sweet-spot so I was very pleased and decided to share with you. It is one of those many examples of times when you quickly go from confusion to illumination as a scientist, emerging with a neat fact about animal biology. And journal articles help you get there!

The bare “brood patch” on the back end of the tinamou’s belly; a nicely hotspot for keeping eggs warm. Perhaps for brooding bad puns, too.