Think about people you know, and how you could tell they were around even if you couldn't see them: their voice, perhaps, or even a favored deodorant.
For bottlenose dolphins, it's the taste of urine and signature whistles that allow them to recognize their friends at a distance, according to a study published Wednesday in Science Advances.
"The use of taste is highly beneficial in the open ocean because urine plumes will persist for a while after an animal has left," wrote the team, led by Jason Bruck at the University of St Andrews.
"By recognizing who caused a plume, dolphins would be alerted to the recent presence of that individual even if it had not signaled its presence vocally."
The question of whether animals can attach "labels" to their friends in their minds has been difficult to answer. Prior research has focused on lab-based experiments, leaving it unclear whether animals use labeling when communicating naturally.
Bottlenose dolphins, which use "signature whistles" to selectively address specific individuals, and can remember these for over 20 years, were thus an interesting test case to study.
To investigate, the team presented eight dolphins with urine samples from familiar and unfamiliar individuals, finding they spent around three times as long sampling urine from those they knew.
Genital inspection, in which a dolphin uses its jaw to touch the genitals of another individual, is common in their social interactions, providing a good opportunity to learn the taste of others' urine.
For the purposes of this study, the dolphins were trained to provide urine samples on demand in exchange for food.
Dolphins do not have olfactory bulbs, and the corresponding nerve is underdeveloped, leaving the team certain it was taste and not smell at play.
Next, the team paired urine samples with recordings of signature whistles played via underwater speakers, corresponding to either the same dolphin that provided the urine sample, or a mismatched sample.
Dolphins remained close to the speaker longer when the vocalizations matched the urine samples -- indicating that the two lines of evidence together evoked more interest.
The team suggested that major urinary proteins, as well as lipids, were likely responsible for individual chemical signatures.
"Given the recognition skills revealed in our study, we think that it is likely that dolphins can also extract other information from urine, such as reproductive state, or use pheromones to influence each other's behavior," they said.