Effect of diet on mass loss of bobcat scat after exposure to field conditions

1. What they did

Most of what we know about carnivore diets comes from scats. Researchers decide which species left a scat from its shape, size and smell, then identify the prey from hair, teeth, bones, toenails or scales inside it. Earlier studies looked at how well prey survives digestion, but nobody had asked whether a carnivore’s diet changes how fast its scat breaks down once it is lying in the field. If some scats fall apart faster, they may become unrecognizable sooner, and the prey in them could be undercounted.

To test this, the authors:

• Fed captive bobcats a single prey type. Twelve bobcats at the Mississippi State University bobcat research facility were each randomly given one diet for 5 days in March 2002: mice and rats, rabbit (alternating head and tail halves so each animal got the same amount of hair and bone), or deer scrap meat with hide and bone, obtained from hunters. Before the trial each bobcat was fed a whole chicken and then fasted for 24 hours. Scats were collected daily. Two juveniles were housed together, fed the same diet and their scats pooled.
• Put the scats out in the field. Without freezing them (to keep them as natural as possible), they broke each scat into two sections along natural breaks and weighed them. They laid the pieces in random order in a grid on a fallow food plot in southwestern Georgia: groups about 5 m apart, individual scats 30 cm apart.
• Weighed them again. Half of each group was collected after 3 weeks and the other half after 6 weeks, and they compared the percentage of the starting mass that remained.

2. What they found

• At the start, the scat pieces averaged 17.0 g. Deer scats averaged 16.4 g (range 9.8 to 20.8 g), rabbit scats 17.4 g (14.0 to 21.6 g), and mouse and rat scats 17.3 g (15.3 to 20.3 g).
• Diet mattered. Scats from the mouse and rat diet kept 65% of their mass and rabbit scats 72%, which was not a meaningful difference. Deer scats kept only 44%, clearly less than both.
• Time mattered. Scats collected after 3 weeks kept 72% of their mass; those collected after 6 weeks kept 49%.
• Diet and time did not interact: deer scats lost more mass than the others at both 3 and 6 weeks.
• None of the scats disappeared completely.

3. What they concluded

• The authors suggest that the more mass a scat loses, the harder it becomes to tell which species left it.
• A possible bias in diet studies: if, for example, a scat could no longer be recognized after losing half its mass, a 6-week-old bobcat scat containing deer would never be collected, while one containing mice, rats or rabbit would. Deer would then be underrepresented in the bobcat’s diet purely because of how their scats break down.
• Biomass estimates: correction factors for working out how much prey was eaten are built from fresh scats. Because scats lose mass over time, biomass of prey is likely underestimated, and if loss differs by prey, prey could be ranked in the wrong order.
• Why deer scats break down faster: the authors suggest prey size and digestibility. Large prey contain proportionately less hard material (bones, teeth, hair) than small prey, so deer scats hold more soft residue that rain, drying and decomposing organisms break down more readily.
• Search intervals: with only 2 intervals tested, they could not set an ideal search interval, but searching as often as possible should help keep scats recognizable and reduce prey-specific bias.
• Next steps: the study covered spring only. They expect the relative differences between prey to hold in other seasons, but the absolute rate may vary with season and climate. Studies through the year and on other predators are needed.

4. How tracking was or was not used

No tracking was done in the field: the scats came from known captive bobcats and were placed out by the researchers. The study is about scat as a sign. The authors did not test whether people could still identify the aged scats; they used mass loss as a stand-in for how recognizable a scat stays.

6. Other technologies, skills or innovations

• A controlled captive feeding trial, with each animal on a single prey type, so each scat’s contents were known.
• Splitting scats along natural breaks to standardize size, and weighing each scat before and after exposure.
• Daily weather records from the Georgia Automated Environmental Monitoring Network.

7. Implications for conservation (from the paper)

• Diet studies based on scat may under-represent prey whose scats break down faster, such as deer in bobcat diets.
• Searching for scats as often as possible should help reduce this prey-specific bias.
• Research is needed in other seasons and on other predators to see whether diet-specific scat mass loss is a common bias.