Masturbating macaques give scientists a hand with semen collection

Dr Ruth Thomsen spent a whopping 400 hours in the field observing masturbating macaques. Image: markfountain52, Flickr

At least sixty species of primates (and that includes us humans) masturbate regularly. Lots of proteins ... all wasted. But in one group of primates at least, the byproducts of the males' favourite hand exercise are now being put to good scientific use, while helping the primates at the same time.

Between 1997 and 1998, Dr Ruth Thomsen, now with University College London, monitored fifteen wild male Yakushima macaques, observing their masturbation habits for more than 400 hours! During the course of her research, Thomsen witnessed macaques, er, taking matters into their own hands hundreds of times.

It sounds like a pretty unusual way to spend your time, but those 400 hours yielded some very useful results. In 2013, Thomsen's hard work laid the foundation for demonstrating a non-invasive method of collecting semen from masturbating macaques. She had solved a longstanding challenge among researchers by acquiring fresh semen samples from wild primates in the field.

Biologists treat semen like a report card of primate health and wildlife diseases. For a start, they can extract information about the fertility of males in wild populations. Fertile males – lots of active sperm – help increase wild primate numbers. "In practice, sperm quality assessment could help programmes to manage populations of endangered species," says Dr Sascha Knauf, a pathologist at the German Primate Center.

Studying sperm also helps scientists study disease. Non-human primates transmit many viruses in their semen, including herpes B, hepatitis B, Ebola and simian immunodeficiency virus (SIV). Some of these kill the non-human primate hosts and can potentially infect humans too.

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 that the scientists lubricate and insert into the rectum of the anaesthetised male, or with metal foils wrapped onto the penis and testicles of the conscious male. But these methods cause more than just a tingling in the nether regions; they can also reduce the quality of the ejaculate, says Knauf.

What's more, electro-stimulation methods have not worked in the field. The logistics are a headache and anaesthetising wild animals risks hurting both the animals and the scientists (and it stresses other members of the primate group). It's these challenges that have prevented researchers from studying semen in wild primate populations.

When Dr Thomsen approached the problem, she reasoned: If it's their semen we want, why not let the masturbating macaques do all the work for us?

Yakushima macaques, named after their native Yakushima Island, are especially busy monkeys. On the island, the males masturbate on average four times per hour, especially when they sense a female in heat. Although not all of these efforts end with ejaculation, a good proportion does.

Not all of male macaques were keen to cooperate. Many licked the ground or their hands clean before Thomsen got close. Image: Dr. Ruth Thomsen

Right after a male Yaku macaque masturbated and ejaculated, Thomsen would approach him – which would usually cause the monkey to back away ... leaving the prized product for Thomsen to collect. Armed with a pipette, she would collect the ejaculate into tubes and analyse the semen within thirty minutes.

Not all of the male macaques were keen to cooperate, however. Many licked the ground or their hands clean before Thomsen got close (semen happens to be highly nutritious). Some males even bared their teeth to scare her off.

Another challenge was maintaining the semen in its natural temperature range of 32°C-35°C. For example, if a male ejaculated onto a very warm surface (such as sun-warmed rock in the afternoon), the heat would destroy all of the sperm. And 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".

Despite these challenges, the method worked. Thomsen collected and analysed semen from half of the 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.

The German Primate Center's Dr Knauf recognizes the potential of Thomsen's method and believes it could be used among the great apes (like gorillas and chimpanzees) in order to examine why some males breed better than others. However, he worries about the quality of the semen samples, as diseases might be overlooked in low-quality specimens.

Since wild male primates are generally not enthusiastic about parting with their semen, especially not to scientists wielding pipettes and syringes, Thomsen's method requires further refining – but she is certainly on to something with this non-invasive technique. The study of semen can teach scientists like Thompson important lessons about some of our closest relatives ... but only if they figure out a way to get their subjects to stop slurping up the samples.

Top header image: markfountain52, Flickr