Prey Hair and Bone Recovery in Ermine Scats

1. What they did

People who study carnivore diets often pick apart scats and count the hair and bone of prey left inside. To turn those remains into how much prey was eaten, they need to know how much hair and bone survive digestion. Earlier feeding trials with coyotes and bobcats reported that most hair and bone disappeared. Looking at ermine scats, the authors doubted that hair and bone were destroyed to that extent, so they measured it.

• Working out how much hair and bone a vole holds. They killed and skinned 20 meadow voles (15 to 51 g), removed and dried the hair, cleaned the skeletons with dermestid beetles, and dried and weighed both. They related hair weight and skeleton weight to body weight with linear regression.
• Making a vole mash. They ground 28 more voles from the same population into a mash, so that each ermine would eat hair and bone in proportion and could not leave parts uneaten.
• Feeding trial. They fed the mash to 10 caged ermine (5 males and 5 females) for 3 days, more than they needed each day. The ermine were fasted for 8 hours before the trial and overnight at the end, so that all feces were collected. Food eaten, refused food and feces were weighed daily.
• Recovering hair and bone from the feces. They removed fat, boiled and washed the feces through a fine sieve until only hair and skeleton remained, then separated and dried the two. They first tested this sieving method against an earlier method (feces in nylon bags in a washing machine and dryer) using weighed samples of rabbit hair.
• Correcting for the ermine’s own hair. Ermine swallow their own hair while grooming, so under a microscope they counted vole and ermine hairs in 5 random subsamples from each animal and corrected the figures.

They compared males and females, because ermine males and females differ in size.

2. What they found

• The vole mash was 2.5% hair and 5.4% skeleton by fresh weight (mean vole weight 28.3 g). Body weight predicted hair weight and skeleton weight well (r² = 0.84 and 0.89).
• Recovery method: the sieving method lost only 2.8% of the rabbit hair, against 21.6% lost by the washing machine method, and it varied less.
• Ermine hair in the scats: on average 31% of all hair in the scats was the ermine’s own (range 18 to 48%). The highest values came from ermine that were starting to moult.
• Prey hair: essentially all the vole hair that was eaten came out in the feces. Digestibility was -3% in males and -6% in females (combined -5%), not different from zero.
• Bone: about 60% of the skeleton was digested (58% in males, 62% in females), so about 40% was recovered.
• Males and females did not differ for hair or for bone.
• These figures are much lower than earlier reports: bobcats digested 99% of skeleton and 88 to 93% of hair, and coyotes digested 91% of skeleton and 55% of hair.
• In a preliminary trial with ermine fed whole laboratory mice, 19% of the estimated prey hair was left uneaten.

3. What they concluded

• Such large differences between studies must be explained before scat contents can be used to reconstruct carnivore diets.
• Part of the difference may come from selective eating. Many predators avoid eating hair and some skeletal parts (such as skull and feet), especially from large prey. Earlier trials fed whole prey and did not make clear whether uneaten parts were measured. Grinding prey into a mash removes this error, although it does not match natural conditions. The authors do not think grinding changed hair digestion: vole hairs seen under the microscope were intact.
• Predator hair in scats has not been recognised as a problem in earlier digestibility studies, and needs a correction.
• Digestion itself may differ between predator species, possibly because of differences in stomach acidity or how long food stays in the gut. Hair, like feathers, is made of keratin; in some predators digestion may make hair unrecognisable.
• Because essentially all prey hair was recovered, prey hair identification gave a good basis for reconstructing ermine diet. Bone may be less useful, because less of it is recovered.
• Digestibility values should not be carried over from one predator to another, and perhaps not from one prey to another, unless controlled laboratory trials show they are the same. Such trials need standard methods, including measuring the parts of prey that are left uneaten and a carefully controlled way of analysing feces.

6. Other technologies, skills or innovations

• A sieving method for separating hair and bone from feces (fat removed by soxhlet extraction, repeated boiling and washing through a 180 µm sieve, then flotation and filtration), tested against the washing machine and nylon bag method with weighed rabbit hair.
• Grinding whole prey into a mash so that predators eat hair and bone in proportion.
• Counting predator and prey hairs under a microscope (40 times magnification) to correct for grooming hair.
• Dermestid beetles to clean prey skeletons for weighing.