Horses Cross‑modally Recognize Women And Men Part 1

Several studies have shown that horses can cross-modally recognize humans by associating their voices with their physical appearance. However, it remains unclear whether horses can differentiate humans according to different criteria, such as the fact that they are women or men. Horses might recognize some human characteristics, such as sex, and use these characteristics to classify them into different categories. 

Gender is a biological characteristic, and memory is one of the most important functions of our brains. There have been various opinions and ideas about the relationship between gender and memory. Research shows that there are some differences in memory between men and women, but that doesn't mean one is weaker or stronger than the other.

Generally speaking, women have better memory than men. Research shows that women have sharper, longer-lasting, and more refined memories. Women's brains manage memory information better and can move from short-term to long-term memory more quickly. Additionally, women are better at remembering verbal and emotional content.

Men's memory, on the other hand, pays more attention to spatial memory and material information, because men tend to be better at processing visual-spatial information and operating objects. This can be seen in the presence of men in certain high-risk fields, such as the military, aerospace industry, and engineering.

However, it is important to stress that these gender differences do not mean that one gender has better or worse memory than the other. Behavioral scientists say that memory is the same for both sexes, although there are some gender differences. This is because the human brain's structural and functional principles are the same.

The impact of gender differences on memory is less obvious, and each of us can improve memory by maintaining a healthy lifestyle and getting enough sleep. For example, eating healthy, exercising, studying, relieving stress, and developing good sleep habits can help our brains work better and process memory information.

In summary, there are some differences in the effects of gender on our memory, but that doesn't mean one gender is worse than the other. We can improve our memory through correct lifestyle and good habits, which will have a positive impact on memory for both men and women. It can be seen that we need to improve memory, and Cistanche deserticola can significantly improve memory because Cistanche deserticola is a traditional Chinese medicinal material that has many unique effects, one of which is to improve memory. The efficacy of minced meat comes from the various active ingredients it contains, including acid, polysaccharides, flavonoids, etc. These ingredients can promote brain health in various ways.

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This study aimed to explore whether domesticated horses can cross-modally recognize women and men according to visual and auditory cues, using a preferential-looking paradigm. We simultaneously presented two videos of women's and men's faces, while playing a recording of a human voice belonging to one of these two categories through a loudspeaker. 

The results showed that the horses looked significantly more toward the congruent video than toward the incongruent video, suggesting that they can associate women's voices with women's faces and men's voices with men's faces. Further investigation is necessary to determine the mechanism underlying this recognition, as it might be interesting to determine which characteristics horses use to categorize humans. These results suggest a novel perspective that could allow us to better understand how horses perceive humans.

Horses (Equus caballus) have advanced social cognition abilities, especially in their interaction with humans [e.g.,1–8 ]. Individual recognition is important in social interactions, and horses can discriminate between humans based on visual or vocal cues, similar to many other domesticated species, such as cats, cows, dogs, horses, pigs, and sheep9. 

Previous studies have demonstrated that horses spontaneously recognize their handler, last seen six months prior, from a picture10. It seems that horses do not solely rely on an easy cue, such as hairstyle, for recognition, suggesting that face recognition is a holistic process. Indeed, horses are also able to link faces from photographs to people in real life, indicating that horses do not process photographs of human faces as simple abstract shapes11. In a study investigating the cross-modal categorization of human emotions, horses matched visual and vocal cues for the same emotion (joy or anger)12. Horses also use multimodal cues to recognize familiar humans. 

In one study, horses were visited by a familiar person who then passed out of the horse's field of vision; the horses were then more surprised to hear a playback of another human's voice (of the same sex) than that of the familiar visitor. This suggests that horses can make the connection between the person they just saw and the voice that they heard13. In another study, horses were demonstrated to be capable of cross-modal recognition of familiar individuals. When placed in front of two people (of the same sex), they looked preferentially towards the person corresponding to the voice being broadcast14. However, horses might use different human characteristics to rely on for recognition. 

For instance, horses might categorize humans according to their age, sex, or size. In a recent study, horses associated children's voices with children's faces and adults' voices with adults' faces15. This present study now focuses on the recognition of men and women.

In humans, there are differences between women and men in both physical appearance and voice16, although there are some exceptions to this general rule. These differences emerge by sexual maturity, during puberty and adolescence17; thus, women and men are sexually dimorphic18. A study conducted by O'Toole and collaborators in 199819 reported that the faces of women and men in photographs were correctly categorized 95% of the time in their database. In addition, the voices of adult women and men differ. On average the fundamental frequency is 120 Hz for women's voices and 200 Hz for men's voices20. Women retain higher frequencies than men21. 

This disparity is due to sexual dimorphism in the vocal tract anatomy of adults, as men have a larger larynx22. After adolescence, men have also more resonant voices due to their longer vocal tract23. As well as individual behavior, the vocal expression of masculinity or femininity can be controlled even before puberty, before the anatomical difference in the vocal apparatus emerges 24.

Different species seem to be able to use these characteristics to discriminate between and modify their behavior towards women and men. African elephants (Loxodonta africana) categorize humans based on visual, auditory, and olfactory cues25. For example, they categorize humans according to the level of threat they represent: as they are hunted only by men, elephants discriminate against humans based on sex because this characteristic can dramatically affect predation risk26. 

Thus, since certain humans are predators of these elephants, they must develop the ability to discriminate between categories of humans to recognize the most dangerous ones. Dogs also seem to be able to discriminate between women and men. They discriminate between two individuals more easily if those two persons are of different sexes27. Moreover, several studies have reported that dogs differ in their interactions with women and men: they bark less at women and look more towards men28, display more stress-related behaviors when interacting with men29, and show more relaxed behaviors and lower cortisol levels when petted by women than by men30. Moreover, male dogs were less likely to approach and make body contact with an unfamiliar man31 and urinated more when walked by unfamiliar women than when walked by unfamiliar men32. 

In addition, dogs cross-modally recognize people according to sex: when hearing a voice, dogs expect to see an appropriate sex-matched visual cue33. To date, it remains unknown whether horses that are regularly in contact with humans can cross-modally recognize women and men. Thus, we sought to evaluate this ability in the current study.

This study aimed to investigate whether horses can associate a woman's voice with a woman's face and a man's voice with a man's face, using a preferential-looking paradigm. We based our protocol on one already used successfully with horses to explore cross-modal recognition of human emotions12 and cross-modal recognition of children and adults15. Two mute videos of people talking (one of a woman's face, one of a man's face) were simultaneously presented to the horse while a woman's voice or a man's voice was broadcast, this was repeated in 6 trials with different stimuli. 

We hypothesized that horses would look preferentially at one of the videos depending on the vocal stimulus. We analyzed the gaze duration, the latency to the first look, and the number of looks toward the videos. We did not predict the gaze direction as, according to the literature, the gaze can either be directed towards the congruent stimulus14 or the incongruent stimulus12,15. To determine if hearing women's voices or men's voices induced different emotional reactions in the horses, we compared their variation in heart rate and the behavioral signs of emotion (e.g., defecating, shaking their head, pawing at the ground, rearing, and vocalizing) while hearing the two types of voices. 

Indeed previous studies, using a similar protocol, noticed that horses' heart rate increased more while hearing children's voices than adults' voices15 and anger vocalizations than joy vocalizations12. As animals can sometimes display different kinds of reactions towards women and men26,28–32, the type of stimuli might affect horses' reactions differently.

Materials and methods

Subjects. This study included forty Welsh mares aged 4–13  years old (mean age±SD=8.94±2.45). The mares were born and bred at the experimental unit of INRAE (PAO, INRAE, Nouzilly, France, Animal Physiology Experimental Facility, https://doi.org/10.15454/1.5573896321728955E12). Those horses were only used for research purposes. They were kept in herds in a pasture in spring and summer or large stables with straw bedding and daily access to an outside paddock during autumn and winter. The experiment took place in winter. Hay and water were available ad libitum. These horses were fully habituated to humans. All the horses were introduced to the experimental setup during familiarization sessions. Nine horses were excluded from the experiment since they did not reach the fixed criterion of the familiarization sessions (see the Familiarization with the Experimental Setup section). Therefore, thirty-one horses were included in the final testing.

Experimental design.

Horses were tested individually in a stall (3.5×4.5 m, Fig. 1). They were placed in front of two white projection screens (1×2 m) located in the right and left corners of the stall. A loudspeaker was located between the projection screens, in front of the horse.

During testing, the horses were attached with two leading reins, one on each side of the head. For safety reasons, an assistant stayed next to the horse during testing. The assistant kept his back to the projection screens and looked at the ground, but was ready to intervene in case the horse reacted extremely or dangerously. The side of the horse (right or left) that he stood on was counterbalanced between the horses. The assistant was blinded to which videos were played on the two projection screens so as not to influence the horse via unintentional cues.

One camera was placed under each projection screen to record the horse's behaviors. A third camera was placed up high behind the horse to provide an overview of testing; this camera allowed the experimenter to watch and supervise the test sessions from outside the stall. Additionally, the horses were equipped with a heart monitoring system (Polar Equine RS800CX Science, Polar Oy, Finland) to measure their heart rate.

Familiarization with the experimental setup.

Before the experiment, the horses were familiarized with the experimental setup. The familiarization sessions took place in the mornings. During the familiarization sessions, videos of landscapes were presented on the two projection screens, while the loudspeaker played recordings of birdsong. Horses underwent at least 5 min of familiarization. Then, the assistant checked the heart rate of the horse. 

The familiarization session stopped when the horse reached the determined criterion: the horse exhibited a heart rate lower than 80 bpm for at least one minute and stood in front of the projection screen for one minute without requiring any assistance intervention. The familiarization session lasted for a maximum of 10 min. If the horses reached the criterion in the morning, they were tested in the afternoon. If the horses did not reach the criterion, they were familiarized again the next morning. If the horses did not reach the criterion during the two familiarization sessions, they were excluded from the experiment. In total, nine horses were excluded. With more familiarization sessions, these horses would have probably reached the criterion. 

However, we decided to limit the number of sessions to homogenize the testing conditions between the horses; and also for ethical reasons (if the horses were not comfortable in front of the screen, we preferred to not insist).

Test procedure.

The test sessions took place in the afternoons. The test sessions consisted of 6 trials. During each trial, two mute videos were played simultaneously, one on each projection screen, while a voice was played from the loudspeaker placed between the projection screens. Specifically, the two mute videos consisted of one video woman's video and one man's video, while the voice consisted of a repeated sentence said by a woman or man. The stimuli (videos and voices) presented to the horses were different in the 6 trials. 

In addition, the voice played was not the original voice of one of the two people in the two videos to avoid any bias due to the horses potentially matching the mouth movements with the voice they were listening to.

All the stimuli were recorded under the same conditions in the same room at INRAE. The videos were recordings of the faces (Fig. 2) of 12 people, we recorded 6 men and 6 women aged 23–34 years old. During the recording of the video stimuli, the people remained motionless and faced the camera, placed 2 m away, with their backs against a white wall. People had to say a neutral sentence. Te sentence was simple: "I like pistachio ice cream; I like chocolate ice cream" said in french ("J'aime la glace à la pistache; j'aime la glace au chocolat"). The audio part was recorded at the same time. 6 recorded voices were kept (3 women's voices and 3 men's voices) to have one voice for each trial.

For each trial, a couple of videos and the voice were repeated 4 times for a total duration of 16 s. Between each trial was a 5-s break in which the screens were black and no sound played. Therefore, the test had a total duration of 121 seconds. Horses underwent 6 trials in total: 3 trials with a woman's voice and 3 trials with a man's voice. The order and sides of the women's and men's videos were counterbalanced between horses. 

Additionally, the order of the voice (man's or woman's voice) and the side of the congruent video was semi-randomly distributed between trials, such that videos that were congruent with the voice were played an equal number of times on the right side and the left side for each horse.

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