Scientists wired up volunteers’ genitals and had them watch animals hump to test a long-held theory

A recent experiment explored whether the simple visual cue of mating movements triggers sexual arousal in heterosexual men and women. The results showed that viewing muted clips of different animal species engaging in copulation failed to increase blood flow to the genitals or elicit self-reported arousal in either sex. These observations were published in the journal Archives of Sexual Behavior.

Lucie Krejčová, a researcher at Charles University in Prague, and her colleagues developed this experiment to test existing theories about human sexual response. They wanted to understand the specific visual triggers that cause the human body to physically prepare for sexual intercourse. Physical arousal often operates independently from a person’s mental or emotional desire.

Historically, studies have suggested a divide between how men and women react to explicit imagery. Men typically experience an increase in genital blood flow only when looking at individuals of their preferred gender. Their bodily responses tend to align closely with their stated sexual orientation.

Heterosexual women often exhibit a different physical pattern in laboratory settings. Past experiments indicated that these women frequently experience increased vaginal blood flow while watching a wide variety of sexual activities. This physical reaction happens even when the actors on the screen do not match the viewer’s preferred gender.

Scholars refer to this phenomenon as a category-nonspecific genital response. Even if a woman does not feel mentally aroused by a video of a non-preferred sexual scenario, her body might still physically react. Researchers have proposed an evolutionary concept called the preparation hypothesis to explain this disconnect.

The preparation hypothesis suggests that the female body automatically increases genital blood flow in response to any sexual cue. This rapid reflex helps protect reproductive organs from potential friction injuries during unexpected or unwanted sexual contact. Because of this defense mechanism, female bodies might respond to a broader array of sexual triggers than male bodies do.

In some older experiments, heterosexual women demonstrated increased genital blood flow while watching primate mating sequences. Some theorists considered that the mechanical act of thrusting itself might act as a universal trigger for female physical arousal. They suspected the rhythmic motion of copulation could be all the body requires to initiate a physical response.

Krejčová and her team wanted to see if these rhythmic movements alone could spark a physical reaction without other contextual clues. They gathered a group of 30 heterosexual men and 28 heterosexual women to participate in a laboratory session. The participants watched a series of 60-second video clips while specialized sensors tracked their physiological changes.

To measure physical arousal in male participants, the researchers used a volumetric penile plethysmography pack. This device relies on an airtight tube that detects fluctuations in air pressure as blood flow increases to the area. For the female participants, the team utilized a vaginal photoplethysmography device.

The female testing tool consists of a small, tampon-shaped acrylic probe equipped with a light sensor. The probe tracks changes in the amplitude of the vaginal pulse, which indicates increased blood circulation to the local tissues. These measurements offer an objective look at how the body reacts to visual stimuli.

The research team curated a specific set of 11 short films to display on a monitor. Two of the videos featured human actors engaging in penetrative sex. One clip depicted a heterosexual couple, and the second clip showed a lesbian couple.

Nine additional videos featured different non-human animals copulating. The team selected the animals based on their evolutionary distance from humans, as well as the visual similarity of their mating movements to human intercourse. The viewers watched chimpanzees, gorillas, lions, zebras, hares, guinea pigs, budgerigars, skinks, and bush crickets.

The researchers removed all audio from the clips so that sexual vocalizations would not act as an extra variable. Each participant sat in a private room to view the randomized video assortment. After each clip concluded, they rated their mental level of sexual arousal on a nine-point scale.

Between the video viewings, the participants completed quick visual puzzle tasks. These simple games required the viewers to find hidden objects in cartoon drawings. The distraction games helped reset the participants’ attention and baseline physical states before the next video played.

The collected data challenged the idea that mating motions universally trigger automatic physical responses in women. When participants watched the animal videos, neither the men nor the women experienced an increase in genital blood flow. The rhythmic movements of the animals did not provoke a genital reflex in the female participants at all.

Instead, both sexes displayed a physical response heavily dependent on the human context of the imagery. The differences in physical arousal across the various animal videos were not statistically significant. Their bodies did not react to the insect, reptile, bird, or non-human mammal mating clips.

The participants’ self-reported feelings of arousal matched their physical measurements perfectly. Neither men nor women felt mentally stimulated by watching the animals mate. They rated the animal videos exceptionally low on the subjective arousal scale.

However, both groups reported high mental arousal while viewing the human couples. This indicates that a person’s conscious evaluation of a visual scene links closely with their physical responses. When participants did not process the visual information as a desirable or relevant sexual scenario, their bodies did not react.

The researchers noted that these outcomes diverge from the older primate studies where women experienced a physical response to bonobos. Those older studies included audio tracks with sexual vocalizations. It is highly possible that the sounds provided an emotional or contextual cue that was missing from the muted videos in the new experiment.

Another explanation relies on cognitive models of sexual response. These psychological models propose that negative emotions, such as aversion or disgust, can interrupt the physiological pathways that lead to arousal. If participants felt mild disgust while watching a skink or a zebra mate, that negative feeling could have suppressed any automatic physical reflexes.

The preparation hypothesis still holds weight for human-oriented interactions. From an evolutionary standpoint, human females would not need to develop a defensive genital reflex for encounters with other species. Such cross-species interactions lack the historical prevalence that would drive natural selection to create a reflex in the first place.

The researchers acknowledged a few limitations in their experimental design. The human video selection only included a heterosexual couple and a lesbian couple. The project lacked a video depicting male homosexual intercourse.

Including a video of two men could offer a better understanding of how gender cues affect physical arousal in both sexes. The team also suggested that future studies could explore how people perceive their evolutionary relationship to different animals. A participant’s personal beliefs about the similarity between humans and primates might influence their physiological reactions.

Finally, the participants in this project were exclusively heterosexual. Future experiments should involve homosexual and bisexual men and women to see if mating movements affect different demographics in similar ways. Examining a wider spectrum of sexual orientations will round out the scientific understanding of what visual cues prompt the human body to prepare for sex.

The study, “Genital and Subjective Sexual Arousal in Androphilic Women and Gynephilic Men in Response to the Copulatory Movements of Different Animal Species,” was authored by Lucie Krejčová, Ondřej Vaníček, Martin Hůla, Kateřina Potyszová, and Klára Bártová.