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.