We know that dogs respond to human cries of distress (emotional contagion). But is that because we bred them to be more sensitive to humans or is there a universal distress register among animals?

Researchers decided to test that recently by comparing domestic dogs and pet pigs on their reaction to different sounds of the human voice — distressed and non-distressed.

The researchers chose pet pigs because the pig’s history with humans over the last few thousand years is mostly as a food source (and a handy image for verbal abuse), not as a companion animal. Breeding choices doubtless reflected that. The scenario below is very contemporary:

The researchers decided to play recorded sounds of human crying (“a high-arousal, negatively valenced sound”) and humming (“a low-arousal, less emotionally valenced sound”) to both dogs and pet pigs who were the companion animals of citizen science enthusiasts. If the two species reacted differently, the finding would support the view that human breeding choices have shaped dogs’ responses, relative to those of pigs. But if they reacted similarly, the finding would support the view that there is a universally recognized sound quality to distress cries.

Did crying or humming upset the pigs more?

Dogs exhibited higher levels of behaviours associated with increased arousal and negative emotional states and vocalized more in response to crying compared to humming. In contrast, pigs showed more negative and high-arousal behaviours in response to humming than to crying.

Lehoczki, Fanni & Pérez Fraga, Paula & Andics, Attila. (2024). Family pigs’ and dogs’ reactions to human emotional vocalizations: a citizen science study. Animal Behaviour. 214. 10.1016/j.anbehav.2024.05.011.The paper is open access.

The researchers’ interpretation is that the dogs were more upset by crying because they intuited that it is a distress signal in humans. If the pigs were more upset by humming than by crying, the researchers say, their negative response was probably based on the fact that they hadn’t heard much humming before (neophobia). The finding supports the view that the dogs’ contagious (supportive) emotional reactions to human grief are due to sensitivity to human signals, not to the universality of a grief signal.

Of course, that raises a question: If pigs lived with humans for thousands of years, would they become similarly attuned to human grief? It would be selective breeding in the sense that humans would select the nicest, most co-operative pigs to become parents. Of course, we would also have to give up referring to insensitive people as “pigs”…

The researchers discussed their findings with science writer Freda Kreier at Nature:

This could be because pigs don’t interpret crying as a negative emotion, says Natalia Albuquerque, a cognitive ethologist at the University of São Paulo, Brazil. Humming, however, could be “very weird” for pigs, who “don’t know how to process” it.

The findings suggest that, compared with livestock, companion animals might have stronger emotional contagion with humans, she adds. But she cautions that more research is needed. “Pigs are very sensitive,” says Albuquerque. “I was expecting to find that pigs would also show emotional contagion.”

Fraga agrees. “We don’t say that pigs can’t do [emotional contagion],” she says. “The story is really about how good dogs were, not how bad pigs were.”

Freda Kreier, “Dogs might have evolved to read your emotions,” Nature, July 18, 2024

It may not just be selective breeding though…

Dogs are pack animals by nature. They want to please the human who is the top dog. It is easy for them to get used to studying human facial expressions and voices for information about how that is going.

And it is a two-way street. We humans also extensively study and catalogue our dogs’ facial and body expressions for clues:

Pigs, by contrast…

Pigs are herd animals by nature. Herd animals (there are exceptions, like cutting horses) get herded by humans. They may not find it as easy to develop a sensitivity to the specifics of human emotions, even if they are bred to be co-operative companions.

Recent research suggests that pigs simply don’t get as attached to humans as dogs do. As one researcher put it, “the domestication process and intense human socialization alone are not enough to trigger human-analog attachment behavior to the human caregiver in animals”:

But attachment is a two-way street too. How many humans have cared much what pigs are thinking or feeling? People who might cry buckets over the death of Old Yeller are unmoved by the daily routine at the local meat packing plant. Some factors that influence our choices of companion animals may be hard wired, so to speak, on both sides and thus resistant to selection.