Masturbating macaques give scientists a hand with semen collection

Another version of this article first appeared on Earth Touch News Network.


At least sixty species of primates—humans included—masturbate regularly. Lots of proteins, all wasted. Now, someone has finally put the primate males’ favourite hand exercise to good scientific use and help the primates too.

On Yakushima Island, Japan, Dr Ruth Thomsen, now with University College London, has been following a male Yaku macaque for hours. The male stopped and sat on the forest ground. Thomsen trained her eyes on the macaque’s hands.

Then, it happened.

The male macaque parted his legs, reached down and rubbed his penis. More than five seconds, Thomsen counted. Masturbation, checked—the male’s third in the past hour. By then, Thomsen had already witnessed hundreds of masturbation by Yaku macaques.

Yaku macaque adult male masturbating. Credit: R. Thomsen.

Yaku macaque adult male masturbating. Credit: R. Thomsen.

In more than four hundred hours between 1997 and 1998, Thomsen monitored fifteen wild male macaques and observed their masturbation. Kinky science?—Yes, and useful too.

Thomsen’s hard work laid the foundation for demonstrating in 2013 a non-invasive method of collecting semen from masturbating macaques. She solved a longstanding challenge of acquiring fresh semen samples from wild primates in the field.

Biologists treat semen like a report card of primate health and wildlife diseases. Non-human primate transmits many viruses in their semen, for example herpes B, hepatitis B, Ebola and simian immunodeficiency virus (SIV)—these kill the non-human primate hosts and can potentially infect humans too.

Sperm of Yakushima macaque. Credit: R. Thomsen.

Sperm of Yakushima macaque. Credit: R. Thomsen.

Scientists also extract information about fertility of males in wild primate populations from their semen. Fertile males—lots of active sperm—help increase wild primate numbers. “In practice,” says Dr. Sascha Knauf, pathologist at the German Primate Center “sperm quality assessment could help programs to manage populations of endangered species.”

Collecting semen from primates however, is a hassle. For primates in captivity, common methods involve stimulating the primate to ejaculate with weak jolts of electricity. The current is delivered with either an electrode which the scientists lubricate and insert into the rectum of the anaesthetized male, or with metal foils wrapped onto the penis and testicles of the conscious male. Electro-stimulation gives more than just a tingling in the nether regions; electro-stimulation might also reduce quality of the ejaculate, says Knauf.

Electro-stimulation methods have not worked in the field. Logistics is a headache; anaesthetizing wild animals risks hurting both the animals and the scientists, and stresses other nearby primates. These challenges have prevented scientists from studying semen in wild primate populations.

Subtropical rainforest on Yakushima Island.  Thomsen monitored Yaku macaques here. Credit: R. Thomsen

Subtropical rainforest on Yakushima Island. Thomsen monitored Yaku macaques here. Credit: R. Thomsen

Back on Yakushima Island, Yaku macaques male masturbate on average four times per hour, especially when they sense a female in heat. Although not all of Yaku macaque’s masturbation end with ejaculation, a good proportion does.

If it’s their semen we want, Thomsen reasoned, why not have the masturbating Yaku macaques give it to us?

Right after a male Yaku macaque has masturbated and ejaculated, Thomsen approached him; the male would usually move away. Then, armed with a pipette—a tool for transferring tool—Thomsen collected the ejaculate into tubes and analyzed the semen within thirty minutes.

Males however, often refuse to hand over their semen. Many males lick the ground or their hands clean of the nutritious semen before Thomsen got close. Some males even bared their teeth.

Thomsen must also maintain the semen at its natural temperature of 32°C-35°C. If the male ejaculated onto a very warm substrate, e.g., a bare rock in the afternoon, the male has killed all of its sperm. When Thomsen collected at night or early mornings, she kept the semen tubes in “a pocket close to the body to keep the specimen warm”.

Thomsen’s method worked. She collected and analysed semen from half of ejaculations observed, and showed that semen quality—sperm concentration and semen volume—varied greatly among males. These new data counter the prevailing assumption that wild male primates in a population share similar semen quality.

Semen of Yakushima macaque ejaculated onto forest floor.  Credit: R. Thomsen.

Semen of Yakushima macaque ejaculated onto forest floor. Credit: R. Thomsen.

Knauf recognizes the potentials of Thomsen’s method but worries about the quality of the semen samples, as diseases might be overlooked in low quality samples. He adds that Thomsen’s method could be used among great apes, e.g., gorillas and chimpanzees, to examine how some males breed better than others.

Wild male primates refuse to part with their semen, especially not to scientists wielding pipettes and syringes. Although Thomsen’s method requires further refining—for example how to collect the semen before the males eat it off their own hands—it’s obvious who benefits most from Thomsen’s study. This is one for the wild primate males.

Click here for a Youtube video that compiled more non-human primate masturbation than you wish to see. Proceed with care!


  1. Thomsen, R. (2014). Non-invasive collection and analysis of semen in wild macaques. Primates 55, 231–237.
  2. Thomsen, R., and Soltis, J. (2004). Male masturbation in free-ranging Japanese macaques. International Journal of Primatology 25, 1033–1041.
  3. Schaffer, N., Cranfield, M., Meehan, T., and Kempske, S. (1989). Semen collection and analysis in the conservation of endangered nonhuman primates. Zoo Biology 8, 47–60.

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