Who is there? Captive western gorillas distinguish human voices based on familiarity and nature of previous interactions

Catcalls: exotic cats discriminate the voices of familiar caregivers

Background

The ability to differentiate familiar from unfamiliar humans has been considered a product of domestication or early experience. Few studies have focused on voice recognition in Felidae despite the fact that this family presents the rare opportunity to compare domesticated species to their wild counterparts and to examine the role of human rearing.

Methods

We tested whether non-domesticated Felidae species recognized familiar human voices by exposing them to audio playbacks of familiar and unfamiliar humans. In a pilot study, we presented seven cats of five species with playbacks of voices that varied in familiarity and use of the cats' names. In the main study, we presented 24 cats of 10 species with unfamiliar and then familiar voice playbacks using a habituation-dishabituation paradigm. We anticipated that human rearing and use of the cats' names would result in greater attention to the voices, as measured by the latency, intensity, and duration of responses regardless of subject sex and subfamily.

Results

Cats responded more quickly and with greater intensity (e.g., full versus partial head turn, both ears moved versus one ear twitching) to the most familiar voice in both studies. They also responded for longer durations to the familiar voice compared to the unfamiliar voices in the main study. Use of the cats' name and rearing history did not significantly impact responding. These findings suggest that close human contact rather than domestication is associated with the ability to discriminate between human voices and that less social species may have socio-cognitive abilities akin to those of more gregarious species. With cats of all species being commonly housed in human care, it is important to know that they differentiate familiar from unfamiliar human voices.

Grey wolves (Canis lupus) discriminate between familiar and unfamiliar human voices

The ability to discriminate between different individuals based on identity cues, which is important to support the social behaviour of many animal species, has mostly been investigated in conspecific contexts. A rare example of individual heterospecific discrimination is found in domestic dogs, who are capable of recognising their owners' voices. Here, we test whether grey wolves, the nearest wild relative of dogs, also have the ability to distinguish familiar human voices, which would indicate that dogs' ability is not a consequence of domestication. Using the habituation-dishabituation paradigm, we presented captive wolves with playback recordings of their keepers' and strangers' voices producing either familiar or unfamiliar phrases. The duration of their response was significantly longer when presented with keepers' voices than with strangers' voices, demonstrating that wolves discriminated between familiar and unfamiliar speakers. This suggests that dogs' ability to discriminate between human voices was probably present in their common ancestor and may support the idea that this is a general ability of vertebrates to recognise heterospecific individuals. Our study also provides further evidence for familiar voice discrimination in a wild animal in captivity, indicating that this ability may be widespread across vertebrate species.

Wild mockingbirds distinguish among familiar humans

Although individuals of some species appear able to distinguish among individuals of a second species, an alternative explanation is that individuals of the first species may simply be distinguishing between familiar and unfamiliar individuals of the second species. In that case, they would not be learning unique characteristics of any given heterospecific, as commonly assumed. Here we show that female Northern Mockingbirds (Mimus polyglottos) can quickly learn to distinguish among different familiar humans, flushing sooner from their nest when approached by people who pose increasingly greater threats. These results demonstrate that a common small songbird has surprising cognitive abilities, which likely facilitated its widespread success in human-dominated habitats. More generally, urban wildlife may be more perceptive of differences among humans than previously imagined.

Familiarity mediates apes' attentional biases toward human faces

In zoos, primates experience markedly different interactions with familiar humans, such as the zookeepers who care for them, compared with those with unfamiliar humans, such as the large volume of zoo visitors to whom they are regularly exposed. While the behaviour of zoo-housed primates in the presence of unfamiliar, and to a lesser extent familiar, humans has received considerable attention, if and how they spontaneously distinguish familiar from unfamiliar people, and the cognitive mechanisms underlying the relationships they form with familiar and unfamiliar humans, remain poorly understood. Using a dot-probe paradigm, we assessed whether primates (chimpanzees and gorillas) show an attentional bias toward the faces of familiar humans, with whom the apes presumably had a positive relationship. Contrary to our predictions, all subjects showed a significant attentional bias toward unfamiliar people's faces compared with familiar people's faces when the faces showed a neutral expression, both with and without a surgical face mask on, but no significant attentional bias when the faces showed a surprised expression. These results demonstrate that apes can spontaneously categorize humans based on familiarity and we argue that the attentional biases the apes showed for unfamiliar human faces reflect a novelty effect.