A large part of the literature on sensory perception and behavior in dolphins is devoted to its well-developed vocal and echolocation abilities. In this review, we aim to augment current knowledge by examining the literature on dolphins' entire "Merkwelt" (which refers to everything a subject perceives, creating a crucial part of the subject's Umwelt). We will show that despite extensive knowledge on audition, aspects such as context relatedness, the social function of vocalizations or socio-sexual recognition, remain poorly understood. Therefore, we propose areas for further lines of investigation. Recent studies have shown that the sensory world of dolphins might well be much more diverse than initially thought. Indeed, although underwater and aerial visual systems differ in dolphins, they have both been shown to be important. Much debated electro-and magnetoreception appear to be functional senses according to recent studies. Finally, another neglected area is chemoreception. We will summarize neuroanatomical and physiological data on olfaction and taste, as well as corresponding behavioral evidence. Taken together, we will identify a number of technical and conceptual reasons for why chemosensory data appear contradictory, which is much debated in the literature. In summary, this article aims to provide both an overview of the current knowledge on dolphin perception, but also offer a basis for further discussion and potential new lines of research.Keywords: cetaceans, Delphinidae, Tursiops truncatus, audition, vision, electroreception, magnetoreception, chemoreception DOLPHIN'S UMWELT Sensory perception is essential for the survival of organisms, be it for the detection of (un)favorable physical conditions, the presence/absence of food or predators, the detection of communication signals or the recognition of social partners. It is crucial for any species to perceive regularities and changes in the properties of their abiotic and biotic environment.

Magnetoreception, meaning the perception of magnetic fields, is supposed to play an important role for orientation/navigation in some terrestrial and aquatic species. Although some spatial observations of free-ranging cetaceans' migration routes and stranding sites led to the assumption that cetaceans may be sensitive to the geomagnetic field, experimental evidence is lacking. Here we tested the spontaneous response of six captive bottlenose dolphins to the presentation of two magnetized and demagnetized controlled devices while they were swimming freely. Dolphins approached the device with shorter latency when it contained a strongly magnetized neodymium block compared to a control demagnetized block that was identical in form and density and therefore undistinguishable with echolocation. We conclude that dolphins are able to discriminate the two stimuli on the basis of their magnetic properties, a prerequisite for magnetoreception-based navigation.

Chemosensory perception in cetaceans remains an intriguing issue as morphological, neuroanatomical and genetic studies draw unclear conclusions, while behavioral data suggest that dolphins may use it for food selection or socio-sexual interactions. Experimental approaches have been scarce due to the practical difficulties of testing chemoreception in wild dolphins. Go/no-go tasks are one elegant way to investigate discrimination abilities; however, they require to train the animals, thus preventing spontaneous responses and hence the expression of preferences. Here, we aimed at testing potential spontaneous responses to chemical stimuli and developed novel procedures. First, we conducted a study to test whether captive dolphins respond to a biologically relevant smell. Therefore, we placed dead fish within an opaque barrel at the border of the pool and counted the number of respirations at proximity as an indicator of investigation. The same dead fishes were presented several times during experiments lasting three consecutive days. From the second day on (i.e., when the odor composition changed), dolphins breathed more often close to the fish-smelling barrel than close to the visually identical but empty control barrel. Second, we conducted a study to test whether dolphins are able to discriminate food flavors. Captive dolphins are commonly provided with ice cubes as a source of enrichment. We took this opportunity to provide ice cubes with different flavors and to compare the reaction to these different flavors as a measure of discrimination. Hence, we used the latency of return to the ice cube begging spot as a measure of discrimination from the previous ice cube flavor. Thus, our method used a non-invasive and easily replicable technique based on the spontaneous begging responses of dolphins toward more or less attractive items bearing biological relevance. The procedures used enabled us to show that dolphins may discriminate odors and flavors respectively.

Among vocal learners, some animal species are known to develop individually distinctive vocalizations, and others clearly learn to produce group signatures. The optimal vocal sharing hypothesis suggests that vocal divergence and convergence are not compulsorily exclusive and both can be found at different levels in a given species. Being individually recognizable is socially important even in species sharing vocal badges. Acoustic divergence is not systematically controlled as it can simply be due to interindividual morphological differences. We tested that hypothesis in a species known to learn their family vocal dialect socially: the orca (Orcinus orca). We identified 13 different call types, including some shared by all group members, some shared only by 2 or 3 individuals, and others particular to 1 individual. Sharing was higher between males than between females. Three of our 4 orcas each produced a unique call type, which was preferably emitted. The call types shared by all orcas still presented individual acoustic distinctiveness that could, to some degree, be explained by morphological differences. We found evidence for strong similarities between some of the call types of our captive orcas and the call types of their ancestors, which are Canadian and Icelandic free-ranging orcas. Our findings suggest that captive orcas use a complex vocal repertoire enabling each individual to produce sounds that are similar to some of their partners', which might be used as social badges to advertise their preferential bonds, as well as individual-specific calls. Our findings open new lines of research concerning the functional value of a balanced "diverging-converging" vocal system.