The large hippopotamus Hippopotamus amphibius is categorized as Vulnerable on the IUCN Red List. In West Africa, threats to the species are exacerbated by high human population densities and significant habitat degradation and loss. In Nigeria, Africa's most populous nation, many hippopotamus populations occur outside protected areas, in lakes, inland rivers and reservoirs; the largest populations yet recorded occur in large reservoirs in the country's northern region (Baker et al., 2020, Aquatic Conservation, 30, 1996–2003).

In October 2021, we conducted a census of hippopotamuses at the 300 km² Dadin Kowa Dam Reservoir, on the Gongola River in northeastern Nigeria. Most of the reservoir lies in Gombe State, with smaller areas in Yobe and Borno States. As part of the survey, funded by The A.G. Leventis Foundation, we investigated reports from local farmers of several sightings of a ‘mom and two babies’. On two occasions, we observed a single adult female with two calves in flooded farmland along the western side of the reservoir; we also captured the trio in a drone video. On the second visit, one calf was initially resting under a tree c. 150 m from the female and second calf, both of which were mostly submerged in aquatic grasses. At the time of our study, the area around the reservoir was widely covered by farmland, notably guinea corn Sorghum bicolor. Local farmers reported observing the trio leave the water to forage in farmland during the day. We did not observe other hippopotamuses in proximity to the female and two calves.

Twins are rare in hippopotamuses. According to data from the Association of Zoos and Aquariums (AZA), of the 561 recorded births among hippopotamuses in AZA institutions from 1880 to 2021, only 6 (1%) were twins. In European zoos from 1850 to 2020, twins were reported < 1% of the time: 11 twins in 1,562 births (data from the European Association of Zoos and Aquaria). Given the precarious status of hippopotamuses in Nigeria, our observation of twins is encouraging. The most recent reports received from local people, in mid December 2021, indicated that both calves had survived, but that the trio may have moved elsewhere because of receding water levels.

We thank the Gombe Goes Green Project and the Gombe State Ministry of Environment for their support.