What’s on the Menu at Fort St. Vrain?

So what’s on the menu at Fort St. Vrain? While we were up in the nest, I decided to collect prey remains. We don’t have the necessary permits to take feathers (of which I found only two, both belonging to prey), but there were plenty of skulls and a few turtle shells. I got them home, laid them out on a table, and started ID’ing them. Some Moms bring home t-shirts and postcards. I bring home skulls and photos of prey.

The photo below shows all of the remains I recovered from the nest, along with the book that Elizabeth Ries and I used to identify them. In all, we found 35 fish remains (skulls, skull fragments, opercular bones, jawbones, and an otolith), seven prairie dog skulls and one foot, one desert cottontail skull, one common muskrat skull, three western painted turtle shells, and some unknown vertebrae. Even in a relatively dry environment, the eagles show a clear preference for fish. As you can see from the opercular bones and skulls, some of the fish were pretty large!

From bottom to top: six right opercular bones, six fish skulls and skull fragments, 11 left opercular bones and one otolith, seven fish jawbones, three western painted turtle shells, vertebrae, four unknown skulls, seven prairie dog skulls, one muskrat skull, one desert cottontail skull

This blog will deal with mammal remains! To make the job easier, I started by assuming that most of the mammalian skulls were probably prairie dogs given the close proximity of a prairie dog colony to the nest. We divided them into groups of skulls that looked alike (which of these things is not like the other?) and got the skull book out!

Top to bottom: desert cottontail, muskrat, black-footed prairie dog

Take a look at the skulls above. The top skull is quite a bit different than the other two, with a high, rounded top and a slightly different eye socket and muzzle (although much of the muzzle was gone). The bottom two skulls are flatter, ‘boxier’, and have different suture patterns. I decided to start with the bottom skull, since we had seven of them and just one each of the other two. Sure enough, Animal Skulls, A Guide to North American Species verified that we had a black footed prairie dog! Our skull had well developed post-orbital processes (the bone spurs jutting out almost perpendicular from the skull over the eye sockets), v-shaped temporal ridges (the ‘V’ on the skull), well-developed occipital crests (the ridge at the back of the skull), and (not real visible in this photo) broad nasals with squared posterior edges. It was a prairie dog! But what were the other two?

I started by thinking about the animals I had seen around the nest. How about a rabbit? Elizabeth and I began with looking at what kind of rabbits lived in the area. After a lot of comparison, we settled on a desert cottontail. The skull has a relatively round, large braincase, the eye sockets are similar, with a posterior extension that is “long, broad, and often fused to the braincase” (this helped us rule out brush rabbit), and a broad and high intraorbital region, or top of the skull.

Elizabeth suggested we take a look at the ventral, or underside, of the skull. Good idea!

Desert cottontail, ventral view

This turned out to be very helpful! She observed that the teeth lined up exactly with the desert cottontail’s teeth, the heart-shaped structure at the top of its mouth formed by the palatal bridge begins near the anterior of the second cheek tooth and terminates just into the fourth tooth, the incisive foramina are large, and the skull has a medium forum magnum that resembles the one pictured in Skulls. Unfortunately, we didn’t have a mandible to compare, but I feel pretty comfortable calling this a desert cottontail.

Since the ventral view was so helpful in identifying mystery skull #2, we decided to start with the underside in skull #3. Wow – look at those distinctive teeth! This should be easy!

Ventral view

The teeth were distinctive, but not as much as we hoped. We eventually ruled out northern pocket gophers. Like this skull, they have smooth incisors, but the skull shape was all wrong. We went on to rule out other types of pocket gophers and eventually just ended up looking for teeth that matched the skull as I racked my brain to think about what else could be out there. And then we found the muskrat skull. Voilà!

It was a muskrat, not a pocket gopher!

Muskrats are semi-aquatic rodents found all over the United States, including Colorado. The skull has orange, smooth mandibles, the anterior edge of the squamosal has a strong angular projection (look at the squared off ridges protruding over the eyesockets), giving the braincase a ‘squared’ appearance, and its temporal ridges are developed (look at the back of the skull). The teeth descend in size to the posterior, the first lower molar has five closed triangles or loops, and the incisive foramina (aka the long slits visible in the roof of its mouth in the ventral view) are long and thin, with the posterior edges aligned with the anterior edges of the toothrows.

Elizabeth and I found the skulls quite fascinating. We discussed the distinctive teeth of the muskrat, which is the only mammal of the three to eat underwater. Its short tooth ‘plate’ allows it to grind tough fibers behind closed lips, which keeps water from running down its throat. We noted the high, rounded skull and facial tilt of the rabbit skull, which may help it to leap and bound – something neither prairie dogs nor muskrats do – by keeping its nose down and out from its line of sight. And we talked about the high post-orbital processes of the prairie dog, which also occur in some squirrels and marmots. These bony barbs help anchor ligaments, pulleys, and other connective tissue, but why do prairie dogs have them when muskrats don’t? Were they inherited from a distant ancestor, or do they reflect a more recent evolutionary development. Even though the animals were long gone, their skulls told us a lot about the way they had lived.

We’ll look at fish and turtles in the next blog!

Why so many prairie dogs versus other mammals? There is an active colony very close to the nest, just below some large power poles with great cross braces. The eagles can perch right above the colony and wait for an unwary prairie dog to get a little too far from its hole. Prairie dogs are also relatively large and meaty – a nice size dinner for the amount of energy expended catching it!

Interested in ID’ing the skulls of North American animals? Check out Animal Skulls, a Guide to North American Species, by Mark Elbroch. We would not have been able to make ID’s without it! 

Did you know that prairie dogs are believed to have a rudimentary language? Given Ma and Pa’s presence, I assume they have a call for ‘eagle’: http://www.npr.org/2011/01/20/132650631/new-language-discovered-prairiedogese