The use of camera traps to estimate occupancy and describe activity patterns for the pygmy hippopotamus (Choeropsis liberiensis) on and around Tiwai Island, Sierra Leone

Background/Question/Methods

Techniques for monitoring rare and cryptic species have developed rapidly since the introduction of remote monitoring devices. The pygmy hippopotamus (Choeropsis liberiensis) is an endangered species endemic to the Upper Guinea Forests of West Africa. Immediate threats to this species include deforestation and hunting; however, the ecology of wild populations is poorly known. Camera trap technology has permitted increasing understanding into the ecology and behavior of elusive wildlife. However, occupancy estimates have never been attempted on the pygmy hippo. We applied a presence/absence sampling technique to estimate occupancy and describe activity patterns for the pygmy hippopotamus. Tiwai Island Wildlife Sanctuary, a 12 km² island on the Moa River in Sierra Leone, was previously documented for the presence of pygmy hippos. During 2008-2011, we designed a grid of fifteen 1 km² blocks on the Upper and Lower Tiwai Islands and employed camera traps at random locations within each block. Cameras were set out for 14 or 21 days for a total 12 survey periods.

Results/Conclusions

Hippos were detected on a total of 28 occasions over 3,211 trap nights and overall had very low detection probabilities (p < 0.05) with an overall site occupancy estimation of 0.48. Seasonality produced differences in detectability and occupancy. In 3 sampling periods, no pygmy hippos were observed.  Pygmy hippopotamus detections mainly occurred in the late dry season. When combined with camera trapping conducted for assessment of high use areas, pygmy hippos were detected an additional 44 occasions. Unlike previous studies, our photographs revealed that pygmy hippos are active throughout the night with no peak hours of activity (35 events from 00:01-08:00 and 37 events from 16:00-00:00). Camera traps are a remote monitoring technology that can give greater insight into a little known species like the pygmy hippopotamus. Results from this study will aid in designing future camera trap studies, which in turn can lead to improved conservation assessments.