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prompted a great deal of work on the hearing of aquatic and semi-aquatic mammalian species (Au et al., 2000;Wartzok and Ketten, 1999;Richardson et al., 1995;Tyack et al., 2006;Nachtigall et al., 2005). The polar bear Ursus maritimus is the only bear species classified as a marine mammal (Rice, 1998), but there has been no audiometric examination of their hearing. According to the most comprehensive review of animal hearing studies (Fay, 1988), and a search of the literature published since, in fact no measurements have been completed on the hearing of any bear.One way to estimate the hearing of a species is to examine the calls of its prey and its response to those calls. Ringed seals Phoca hispida and bearded seals Erignathus barbatus are prominent in the diet of polar bears (Stirling, 2002). In some areas, the predator-prey relationship between ringed seals and polar bears is so interrelated that a count of the population of one of them can indicate the population level of the other (Stirling and Øritsland, 1995). Polar bears' preferred prey items are the newborn pups and subadults (Stirling and McEwan, 1975), and they primarily hunt seals in areas of moving pack ice, which include known important locations of seal birth lairs (Smith, 1980). Four types of vocalizations made by ringed seals can be heard at all times of day in the Arctic spring: (1) low-pitched barks, (2) high pitched yelps, (3) low and high pitched growls and (4) short descending chirps (Stirling, 1973). Sonograms of the recorded sounds indicated that most of the energy was relatively low frequency below 2·kHz, with some harmonics up to 8·kHz.The behavioral responses of polar bears to the calls of ringed seals recorded under water and then presented to the bears in air were measured (Cushing et al., 1988), and elicited similar responses from two recently captured bears. The bears erected their ears, lifted their heads, visually scanned the room and then began sniffing. As the ringed seal calls continued to be played the bears became active, paced their cage, groaned and chuffed, then pawed and chewed at their cage bars. All of these behaviors were observed only rarely in the baseline behavioral examinations prior to the presentation of ringed seal sounds, indicating that the bears responded to their primary prey's underwater vocalizations, presented in air, in a manner that indicated some importance of in-air hearing in detecting and locating their under-ice prey. Cushing's observations suggest that if polar bears could hear the underwater vocalizations of the ringed seals they might use seal vocalizations as a method to locate their favorite prey. It has also been noted (Stirling and Thomas, 2003) that the distinct trills of bearded seals might also provide a prominent cue for polar bear localization of these animals. A While there has been recent concern about the effects of sound on marine mammals, including polar bears, there are no data available measuring the hearing of any bear. The in-air hearing of three polar bears was measured using ...

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The use of passive acoustic monitoring (PAM) is increasingly used as a monitoring tool in e.g environmental impact assessments. However, very few studies have focused on validating PAM data against independent observations which is critical in order to compare data from different studies and areas, and over time. In August 2007 we tested two types of odontocete PAM equipment: version 5 T-PODs (Chelonia, U.K.) and PCL's (Aquatec, Lochborough, UK) by comparing acoustic detections to visual theodolite tracings of wild harbour porpoises. Eight T-PODs with known detection thresholds and four PCLs were mounted in four clusters. Detection thresholds of the T-PODs varied from 117 to 125dB re 1μPapp. Based on visual tracks of 28 porpoises it is shown that detection ranges correlated inversely with detection threshold. However, PAM-detection also strongly depended on the angle between the swim track of porpoises and the line to the data logger. Some porpoises were not detected despite swimming within 20m of the nearest data logger. It was not possible to distinguish between recordings from one and more than one animal on the PAM-recordings, despite that the number of animals was known from the corresponding tracks. All in all, the results showed good correspondence between PAM recordings and visual tracings, as well as among PAM recordings.