Scientists Think Warm Balls Could Be the Next Male Birth Control

Warming male animals' balls using nanoparticles and electric coils could be effective contraception, researchers say after experimenting on mice.

In this advanced scientific age, there are two questions that remain outside of the science community’s reach: Whether casually getting one’s nut sack too warm for too long causes infertility, and how to make male contraception that doesn’t suck. Hot laptops may affect sperm quality if you’re sitting with one in your lap for hours at a time, and male birth control options are limited mainly to vasectomies and condoms. What if warmed-up nuts has been the answer to male contraception all along?

A research team from the Institute of Reproductive Medicine at Nantong University in China published the results of their experiments on mice testicles in the July 2021 issue of Nano Letters, and found that wrapping their balls with an electric coil and heating up their nanoparticle-filled nuts with the resulting magnetic field reduced fertility temporarily and reversibly. 


Getting there was a gnarly process, though. The researchers previously experimented in a 2013 study with injecting mice balls with modified gold nanorods with near-infrared irradiation—warming them up from 86 degrees Fahrenheit to between 98 and 113 degrees. That study posed the issue as being about stray animal control: a “more convenient, efficient, and cheaper than surgical castration, which can be chosen to treat the overpopulation of pets and stray animals,” they wrote. 

The 2021 study gets more to the point: male contraception that’s non-invasive and reversible. In this study, they tried to solve some of the problems with their earlier work: that injecting tiny bits of precious metals into one’s balls might be severely painful, that heating them up causes “severe dermal damage,” and that shooting one’s balls up with non-degradable nanoparticles could be toxic. 

Illustrations from the study

From the study: "(a) Schematic illustration of the noninvasive contraceptive therapy of a mouse model via intravenous administration. (b) Photographs showing the magnetic capture of IONPs at 0, 3, 10, and 15 min, respectively. (c) Thermal infrared images of treated mice at different time intervals under an AMF (403 kHz) with an output current of 30 A."

This time around, they tried using magnetic iron oxide nanoparticles (which the body can eventually eliminate), injected them into the mice’s veins and then placed a magnet next to their balls for four hours to attract the particles there. After one to four days of injections and magnets, they wrapped an electric coil around the testicles, which warmed up the iron oxide now stored in the balls with the magnetic field generated from the current, again to temperatures between 98 and 113 degrees. 

The mice’s sperm counts went down after several days of treatment, and for those that underwent warming at lower temperatures, their fertility recovered after 60 days. “The number of pups born to pregnant mice was almost the same and no visible morphological defects were observed in all the pups,” the researchers wrote. 

In the hotter tests of the experiment, however, the balls atrophied, and in some—depending on how much iron oxide was injected, and how hot the nuts got—showed “distinct black discolorations” and the testes were “severely damaged at 7 days.” If they were cooked above 113 degrees, the balls didn’t recover. 

As Jeffrey Mo noted in The Conversation, getting human men to voluntarily inject their balls with a magnetic substance then wrap a heating coil around their sack to shrink it and kill their sperm for a couple of months might be a hard sell—it makes vasectomies sound like a cake walk. But in the zoo and animal husbandry market, it might be an appealing option for controlling breeding. 

“Zoos are a very small market, and so drug companies don’t have a lot of motivation to make animal contraceptives,” David Powell, director of the Reproductive Management Center of the Association of Zoos and Aquariums, told Mo.