I just came across this month-old question from Kathie Hodge on Facebook, and after 20+ comments, she seems no closer to getting an answer:
Dear cat people, For years I’ve been wanting to know the answer to this one question: what is the organ that cats universally reject when lunching on mice?? It is shaped like a bean and is unpleasant to find on one’s carpet. But it’s better than finding more parts of a mouse. What is the bean??
The cat with whom I am currently sharing a house always leaves mice perfectly intact, often on display in the exact center of a rug. But as it happens, six years ago I spent a winter following bobcats around in northern Vermont, and when they hunted voles they consistently did what Kathie is describing. Here’s a typical kill site:
And a closeup of the “bean”:
I believe this thing is the cecum, which is the beginning of the large intestine. Here are some mouse ceca to compare. Why a cat would so carefully avoid consuming or even puncturing the cecum, I’m not sure. This tidbit from Wikipedia may have something to do with it:
Most mammalian herbivores have a relatively large cecum, hosting a large number of bacteria, which aid in the enzymatic breakdown of plant materials such as cellulose; in many species, it is considerably wider than the colon. In contrast, obligatory carnivores, whose diets contain little or no plant material, have a reduced cecum, which is often partially or wholly replaced by the vermiform appendix. Over 99% of the bacteria in the gut flora are anaerobes, but in the cecum, aerobic bacteria reach high densities.
So, the cecum is full of aerobic bacteria that are useful to a rodent but not to a cat. Are they not only useless, but somehow harmful? Anyone have any insights to add here?
[Added a few hours later] Sharlene Santana left this comment on the Northern Naturalists Facebook Page:
This is very interesting, and it does look like a caecum. Many insectivorous bats discard (squeeze out) the guts of insects and caterpillars when they eat them, and this behavior is thought to prevent the bats from consuming the toxic chemicals found in the plants the insects fed on. Maybe cats are doing something similar to avoid consuming compounds that are toxic or unpalatable in the caecum?
[Added 9/28/2013] Beatriz Moisset just called my attention to this paper that describes Chinese mantids gutting monarch caterpillars before eating them, presumably for similar reasons.
BugTracks 
I was thinking just yesterday of how I became interested in small things because, for me, they embody mystery. When I was a kid it was almost impossible to find answers to questions like this one, so my small world was full of mystery. There was no internet. There were no field guides to bug tracks. These small landscapes had a magical light. Now that I’m mostly grown, I am still most delighted by questions I can’t solve, and even more delighted when their mysteries are revealed. Thanks Charley.
Several years ago I was trying to domesticate and foster a stray cat. He was a big rangy orange hunting machine. One night I heard him in the living room crunching on something he’d caught. I went in to find that all that was left was what looked like a little stomach. In fact, it looked like a tiny textbook drawing of a human stomach, so I assumed that’s what it was. I didn’t have anything I could carefully cut it open with, so I squeezed out the contents – undigested birdseed. For some reason I was convinced at the time that he’d eaten a sparrow, and now I’m not sure if it was because of the birdseed or because I *think*, if I remember correctly, there was also a little birdie foot left over.
I hadn’t thought about that in years, and I assumed at the time it was a fluke. Most references to feline predation claim that prey gut contents supply the cats with whatever vegetable-derived nutrients they need. But that must be an oversimplification.
I have a friend who might know, or will know someone who knows. I’ll do my best to find out & report back!
Interesting… I wouldn’t expect a rodent to swallow seeds whole, so you’re probably right about it being a bird. And a mass of unprocessed seeds in a bird would be in the crop, on the way to getting ground up in the gizzard. I don’t think I’ve ever come across a crop left behind by a cat or other predator.
Blech. Yep, I’ve received small offerings on my carpet that look just like that. Very interesting stuff you turned up on this cat-oddity!
I really need some relief here. My girlfriend is all freaked out with the recent capture and killing of two voles by our feline killing machine. Not because it was particularly gory, but rather, because a fairly large (compared to the vole) brown, football -shaped item was secreted from the creature’s anus. Most likely from being inadvertently compressed. At first I thought it might be a pupa but when a second vole was presented, the same thing happened. Additionally, the first item was actually moving like it was in peristalsis or catastalsis. I did not witness the second one but am assured that it was similar if not identical to the first. Would someone please provide concrete enlightenment so that I might have a bit of relief from Sherlock’s unending inquisitiveness?
Did it look like the organ in my photo above? I assume it was the same thing, which seems to be a cecum, but I’m surprised to hear that it was secreted through the vole’s anus and continued to move on its own.
Surprised? I’m horrified!
Pingback: Four Years of Bug Tracking | BugTracks
I still can’t believe no one knows the answer to this yet. I’ve had three of my cats do the same thing when eating mice, or birds. Some organ is left in the middle of where everyone can’t possibly miss it, and it’s stand alone. Nothing else but my wits to tell me this is from the inside of an animal. What is it??? And why do they leave it out?
Well, as stated in the blog post I’m pretty sure it’s the cecum. Still not totally clear why cats don’t eat it.
I still can’t believe no one knows the answer to this yet. I’ve had three of my cats do the same thing when eating mice, or birds. Some organ is left in the middle of where everyone can’t possibly miss it, and it’s stand alone. Nothing else but my wits to tell me this is from the inside of an animal. What is it??? And why do they leave it out?