Sensory Perception in Cetaceans: Part II—Promising Experimental Approaches to Study Chemoreception in Dolphins

Abstract

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.

A sense of scale: Foraging cetaceans' use of scale‐dependent multimodal sensory systems

“…A recent study demonstrated that bottlenose dolphins are capable of perceiving the odor of a dead fish in air at a distance of 2.5 m, and discriminate between fish and nonfish flavors (Kremers et al . ). Small fungiform papillae that may be associated with taste buds were recently documented on the dorsal surface of a gray whale calf tongue (Kienle et al .…”

“…These indications of chemoreception ability suggest that individual cetaceans may use olfaction and gustation cues to localize and discriminate prey (Kremers et al . ), or potentially sense salinity levels to evaluate local oceanographic conditions (see Oceanographic stimuli). In the dolphin scale‐of‐senses schematic (Fig.…”

“…Again, although genetic and some anatomical studies indicate that odontocetes have also lost olfactory capability (Pihlstr€ om et al 2008, Kishida et al 2015, other evidence indicates well-developed olfactory tubercle in odontocetes (Oelschl€ ager and Oelschl€ ager 2009) and chemoreception in the nasal cavity of harbor porpoise (Behrmann 1989). A recent study demonstrated that bottlenose dolphins are capable of perceiving the odor of a dead fish in air at a distance of 2.5 m, and discriminate between fish and nonfish flavors (Kremers et al 2016b). Small fungiform papillae that may be associated with taste buds were recently documented on the dorsal surface of a gray whale calf tongue (Kienle et al 2015), demonstrating evidence of gustation in baleen whales.…”

Sensory Perception in Cetaceans: Part I—Current Knowledge about Dolphin Senses As a Representative Species

“…Kuznetzov (1990) proposed the term "quasi-olfaction" to describe the chemical sense in dolphins that combines characteristics of both gustation and olfaction. Recent behavioral studies suggest that dolphins are able to detect airborne odors and discriminate between different flavors (Kremers et al, 2016).…”

“…For instance, in contrast to captive dolphins, wild dolphins cannot be trained; therefore, other methods may have to be developed for the same research tasks. While chemoreception may be tested in captive dolphins by using odor sources close to the pool or ice cubes (see Kremers et al, 2016), floating dispensers (such as the ones used for chlorine tablets in swimming pools) may be adopted for wild dolphins. For olfactory studies, the substance to be tested is simply placed inside the device.…”