River Otter Latrine Site Selection in Arid Habitats of Western Colorado, USA

1. What they did

River otters were wiped out from much of southwestern North America by the early 1900s and have since been reintroduced in Colorado, Arizona and Utah. River otters leave urine, feces and anal gland secretions at latrine sites on land, which they use to communicate with each other, and the presence or absence of latrines is used to track where otters live and whether reintroduced populations persist. Latrine site selection had been described in temperate regions but not in arid and semiarid ones. The authors set out to build a model that predicts where otters place latrines in western Colorado, to make sign surveys more efficient and accurate.

• Finding latrines. In the summers of 2004 and 2005 they surveyed 247 km along six reaches of five rivers by raft or kayak and on foot, searching both banks up to 30 m from the water’s edge. A site counted as a latrine if it had at least 3 feces.
• Choosing random sites. For comparison, they generated 30 random points along the shore of each reach (180 in total) on a map and found them in the field with a handheld Global Positioning System (GPS) unit.
• Measuring habitat. At 42 latrines and 119 random sites they described a 10 m circle: overstory and understory cover (trees, shrubs, rock cliff), shade, substrate (sand, grass, mud, rocks or a large prominent rock), water depth near the bank, and whether fresh beaver sign was within 40 m.
• Modelling. They compared eight models with logistic regression and tested how well the best ones predicted latrines, building each model on 80% of the sites and testing it on the other 20%, five times over.

2. What they found

• Latrine density ranged from 0.03 to 0.42 latrines per km. Otters used rivers from very gentle to steep gradients, including stretches of class IV whitewater.
• DNA from feces and hair at the latrines (from a companion study) showed that at least 21 different otters used them.
• The authors chose the second-ranked model because it was simpler and its variables appeared again and again in the five top models. Latrines were more likely where there was:
• beaver sign within 40 m: the odds of a latrine were about 51 times higher;
• a large prominent rock: about 24 times higher (found at 41% of latrines and only 5% of random sites);
• deeper water: about 3 times higher for every 20 cm of depth (water 60 cm or deeper at 61% of latrines);
• cliff or rock overhang overhead: about 2 times higher for every 20% more cover;
• shade: a small increase for every extra percent of shade.
• The model predicted well: the area under the curve was 0.95, it classified 87% of test sites correctly on average, and it picked out 72% of the latrines.
• Trees were sparse along these rivers and did not predict latrines; otters chose sites under cliffs and rocks instead.

3. What they concluded

• Otters in this dry country choose latrine features much like those chosen in temperate areas, but with cliffs and rocks taking the place of trees as cover. The authors suggest overhead cover gives protection from aerial predators and keeps scent marks from drying out, and that high rocks give a good view and spread scent further.
• Deep pools probably help otters escape predators and matter during low flows and drought, when fish gather in pools. They could not study latrine use at low water because the canyons could not be floated then.
• Beaver activity was the strongest predictor. Beavers probably help otters in dry regions by keeping pools that hold fish during drought and by creating den sites. The authors call for studies of otter and beaver interactions in these rivers.
• The model gives researchers and managers a robust tool for finding latrines and monitoring otters in arid environments.

4. How tracking was or was not used

The study is built on a sign survey: researchers searched riverbanks for otter latrines, identified them from otter feces (a site needed at least 3 feces), and found the point with the most otter activity at each one. They also recorded fresh beaver sign near each site, though the paper does not say what kinds of beaver sign were used. Feces and hair from the latrines were collected for DNA in a companion study. No trailing was reported.

5. Tracker or observer certification

Not reported. The paper does not say who identified the latrines and beaver sign or what experience they had. Four people are thanked for assistance in the field.

6. Other technologies, skills or innovations

• Random sites generated in a Geographic Information System (GIS), ArcView 3.2, from national hydrography data, and located in the field with a handheld GPS unit.
• A spherical densiometer to measure overstory density, checked against visual estimates of shade.
• Water depth ranked by body height (below knee, below waist, above waist) in the first year and measured with a graduated rod in the second. The two methods were closely correlated.
• DNA fingerprinting of feces and hair collected at latrines (reported from a companion study).

7. Implications for conservation (from the paper)

• The model variables are easy to spot in the field with little training and could improve monitoring of river otter reintroductions in southwestern North America.
• Surveys for latrines in arid areas should concentrate where beavers are present.
• Remote mapping cannot replace field surveys: prominent rocks at the bottom of deep canyons are hidden from above, and beaver activity changes over time. Planning reintroductions or checking for otters will still need surveys for latrines on the ground.