We studied the spontaneous vocal behaviour of captive wolves at the International Wolf Center (IWC) in Minnesota (spring 2019 and winter 2020), and the Centro del Lobo Ibérico Félix Rodríguez de la Fuente (CLIFRF) in Spain (winter 2020). We used AudioMoth recording devices to record wolf howling 24 hr/day. We identified 412 solo howl series and 403 chorus howls and found differences between wolves at the two centres. Vocal rates for North American wolves at the IWC (7.8 chorus howls/day in spring and 4.8 chorus howls/day in winter) were higher than rates obtained for Iberian wolves from CLIFRF (3.8 chorus howls/day in winter). Howling rates obtained in our study were similar to those obtained for captive Mexican wolves and greater than those reported for wild wolves. Hourly distribution of howling was also different between centres. The greatest howling activity identified at IWC was at pre-sunrise, while at CLIFRF the peak occurred at sunset. Weather conditions had little influence on the vocal behaviour of the captive wolves we studied. We show the potential of passive recorders to study topics of animal acoustic communication, such as vocal rates and temporal patterns, that have not been deeply addressed due to technological constraints.