We showed that gentoo penguins produced offshore calls and many individuals formed groups with the calls in the open ocean. These results generally support the hypothesis that the offshore call is related to group association during foraging trips. Thiebault et al.37 revealed that Cape gannets (Morus capensis) used vocal communication among individuals at sea, possibly to avoid collisions. In general, the penguin calls that we analysed were produced by lone individuals, and no vocal interactions with closely neighbouring individuals were recorded in our video-recordings. The absence of vocal interaction among neighbouring individuals may suggest that the function of the offshore call is more likely about group formation, not interaction among members in a pre-formed group37.

After producing offshore calls, the penguins undertook shallower, shorter dives than those before the call, and the video data showed that following their calls, the penguins travelled instead of lingering in a particular area (See Supplementary video S1). Moreover, although it was not possible to analyse detailed vocal exchanges due to the location of the microphone (attached snugly to the penguin’s back), calls made by other individuals (See Supplementary video S2) were also occasionally observed, which implies the occurrence of vocal interactions among multiple individuals. Considering the energetic cost of acoustic signalling, the shallower and shorter dives following a call could be related with production of offshore calls. If there is a trade-off between dive recovery and calling behaviour, it is possible that the calling individuals may incur a slight oxygen debt and accordingly it could affect dive dynamics.

The gentoo penguins did not show a significant difference in the proportion of foraging dives or prey capture rate between before and after producing an offshore call. These results suggest that the call may be a contact call rather than a food or recruitment call; however, an assessment of the abundance of food sources would be needed to draw a definitive conclusion.

We did not find any differences in diving behaviours between calling and non-calling individuals. However, it still remains unclear why some individuals did not produce any offshore calls. Our video-recording could cover only eight hours per individual so we are not able to generalize its foraging and vocal behaviour. We think that multiple recordings on same penguins may show if one individual has different strategies depending on the environmental conditions or food availability.

Then, why did some birds produce offshore calls during the foraging trips? The anti-predation hypothesis does not fully explain our observations of offshore calls, because predation events primarily occurred near the shore. Leopard seals (Hydruga leptonyx), which are one of the primary predators of gentoo penguins at our study site, mainly forage near the shore where the penguins depart for foraging trips21, 41,42,43. Moreover, we did not observe a predator or predatory situations in the videos although it could happen.

Did the offshore calls induce group association? Based on our observation that almost half of the total group associations formed within a minute after a call, the calls are likely to be related to grouping in gentoo penguins. In addition, when an individual joins a group soon after producing a call, the call can be considered as a contact call over short distances, as reported in a previous study on communication in Cape gannets37. We consider it unlikely that the offshore calls are sufficiently loud to act as signals over long distances. Nevertheless, it is not clear how the calls can be efficiently delivered to signal receivers under the windy environment of the open ocean. The offshore calls may not be a sole mechanism to drive group association. Although a half of group associations were observed for one minute, another half group associations did not instantly occur after a call. Also, we did not detect any behavioural changes in diving parameters (maximum dive depth, average dive depth, dive type and prey capture rate) after calling, except diving duration. These imply that unknown mechanisms may also drive group association and theses could not be exclusive with vocal behaviour.

Our data showed that group members forage at the same patch of prey (See Supplementary video S3), but we did not find clear evidence of active cooperative foraging. However, we do not exclude the possibility that sharing a prey patch at the same time could involve passive cooperative foraging33 and increase foraging efficiency through local enhancement44, 45 without intentional awareness or food calls. Information about food can be conveyed by inadvertently producing cues from feeding behaviour44, 45. When seabirds form larger aggregation to forage on inconspicuous prey in the open ocean, individuals may increase detectability on a food patch46, 47.

By deploying GPS and animal-borne cameras, Thiebault et al.47 showed that Cape gannets aggregated at food patches and that the number and frequency of foraging dives increased with the size of the aggregation. Penguins may also gain benefits from group foraging through increased foraging efficiency. Because Antarctic krill, which are the main prey of our studied gentoo penguin population (almost 99% of all prey)34, move in swarms, the penguins may use local enhancement to increase foraging benefits46. To verify the significance of local enhancement on group foraging, the effect of group size at a prey patch on group association should be determined47. By contrast, compared with local enhancement, collaborative foraging requires a higher level of coordination of behaviour among group members in time and place33. Previous research on the foraging behaviour of penguins provided evidence of coordinated behaviour, including synchronous diving, in time 22, 25,26,27, 43. However, because the limited angle of view of the back-borne cameras prevented us from evaluating the presence of other individuals, our results do not provide evidence that the penguins actively coordinated foraging behaviours with group members in place.

In summary, we have described the vocal behaviour of penguins in the open ocean, and we have discussed the possible functions of vocal communication during group behaviour. Based on our observations, we hypothesize that the group foraging behaviour of penguins may be accompanied by their vocal communication in the open ocean. Further investigations of the offshore behaviour of penguins will allow us to better understand their aquatic life. Playback experiments at foraging sites could be used to gain insight into the function of the offshore vocal signal. In addition, the placement of cameras on multiple individuals in the same group would enable a more detailed analysis of interactions among group members. Information on their interactions may enable us to better understand penguin foraging behaviour and test our hypotheses regarding the evolutionary significance of group foraging in penguins.