Do domesticated horses still respond to the sound of traditional predators, even though they and their forebears haven’t encountered them for generations?
Researchers in Poland, in a paper just published in the journal BMC Veterinary Research, explored the question in an experiment involving 19 Polish Halfbred horses.
Iwona Janczarek and her colleagues said that the horses did, indeed, react to recorded vocalizations of the gray wolf and the Arabian leopard, but their responses were weak.
“Despite the fact that predator vocalizations are usually unfamiliar to present-day domestic horses, the horses responded with signs of anxiety,” they reported.
The response was elicited both by the wolf — which horses co-evolved with but have had no recent exposure to — and by the Arabian leopard, from which the horses have been mostly isolated.
The behavioral responses were less distinct than the physiological changes, they reported.
There was also a more pronounced response to the growl of the leopard than the howl of the wolf, the study team reported.
This, they said, may indicate that the horses were more frightened of the sound from an unknown threatening predator than by one known to their ancestors.
“The differing response can be also due to differences in the characteristic of the predators’ vocalizations.”
The study team noted that predatory attacks on horses can be a problem in some parts of the world, particularly when considering the recovering gray wolf population.
“Across millions of years of evolution, the survival of equids has depended on their capacity to adapt to their environment. In open grasslands, equids could typically detect the presence of predators by sight, smell, and/or hearing. The anti-predator defense response in horses is to flee from a fear-inducing cue.”
In Europe and northern Asia, packs of gray wolves were the main predators of large ungulates until recent ages, and horses of the breed used in the study co-evolved with wolves but have had no recent exposure.
In turn, leopards are distributed across southern Asia and sub-Saharan Africa and are now critically endangered. They avoid preying on large animals, such as plains zebras.
Polish Halfbred horses have been evolutionarily isolated from leopards, and only the Thoroughbred ancestors in the bloodlines of the Polish Halfbreds, having originated in part from Oriental horses, could be familiar with leopard vocalizations.
However, these Oriental ancestors were imported only in the 18th century.
“Hence, for the horses living in Europe a few centuries ago, the vocalizations of wolves were familiar, whereas those of leopards were alien.
“Currently, wolves are usually unfamiliar, and leopards are entirely strange to horses in Europe.”
The six-strong study team described an experiment involving 19 healthy leisure horses, aged 6 to 10, kept by the university. They were all Polish Halfbreds that originated from matings of German horses with Wielkopolski Warmblood breeds. All the study horses had various percentages of Thoroughbred ancestry in their pedigrees.
The horses were divided into three groups according to the percentage of Thoroughbred ancestry in their pedigree within the parental and grandparental generations. Six horses were 75% Thoroughbred, seven were half-Thoroughbred, and six were 25% Thoroughbred.
The horses were separately exposed to the recorded sound of a gray wolf howling and an Arabian leopard growling for five minutes, with the animals monitored for 10 minutes before exposure, during exposure, and for 10 minutes after. Heart activity was monitored continuously during the test, and salivary levels of the stress hormone cortisol were measured before and after exposure to the animal cries.
Genetic coding and sensitivity
The study team had hypothesized that a higher proportion of Thoroughbred horse ancestry in the pedigree would result in higher emotional excitability in response to predator vocalizations, given that this breed is known for its sensitivity. This proved to be the case.
The tendency towards a stronger internal reaction to predators in horses with a higher proportion of Thoroughbred genes suggested that the response intensity was partly genetically coded.
Discussing their findings, Janczarek and her colleagues said their work was the first attempt to test whether the sounds of predators are frightening to horses bred by humans, and to assess their responses.
Changes to the heart-rate levels recorded in the study suggested that the horses were more interested in the sound rather than frightened.
“The predator vocalizations seemed to be too weak of a stimulus to induce intense fear and consequently a flight response, instead eliciting only a short-term increase in locomotor behavior and vigilance.
“Ceasing the stimulus resulted in a less intense but similar short-term response to that elicited by the playback, which may be interpreted as the sudden silence disquieting the horses.”
Nevertheless, the observations and changes in heart rate and cortisol levels recorded implied some level of stress.
The finding that the external response was less distinct than the internal physiological changes may imply that the horses handled and trained by humans are able to manage their emotions, they said.
“Domestication aimed to tame horses and to prompt them to develop the ability to detect human signals. Horses became more dependent on humans and are now presumably less able to react appropriately to problems by themselves.”
They concluded: “Our findings suggest that the present-day horses’ abilities to coexist with predators are weak, presumably due to domestication, which tamed the horses and was followed by long-term selection for utility traits.”
They warned that the growth in wolf populations in Europe after centuries of decline might be dangerous for horses in the future.
“Humans should protect horses against predation, especially when introducing them into seminatural locations.”
The full study team comprised Janczarek, Anna Stachurska, Witold Kędzierski, Anna Wiśniewska and Agata Kozioł, all with the University of Life Sciences in Lublin; and Magdalena Ryżak, with the Polish Academy of Sciences, also in Lublin.
Janczarek, I., Stachurska, A., Kędzierski, W. et al. The intensity of physiological and behavioral responses of horses to predator vocalizations. BMC Vet Res 16, 431 (2020). https://doi.org/10.1186/s12917-020-02643-6