The homes we live in and the food we eat might contain chemicals that reduce male fertility - and that goes for both men and their dogs, new research suggests.
Recently, experts have grown ever more worried by what appears to be a 'shocking' drop in human male fertility.
According to the team behind the latest research, some studies show that in the past 80 years alone, there has been a 50 percent global reduction in sperm quality, and no one can figure out why this is happening.
Then, we looked at dogs - and this could be an important clue. In 2016, a team at the University of Nottingham found that sperm quality had also taken a plummet in domestic dogs over the course of several decades.
Unlike the research on humans, this time the scientists could trace back the sperm quality decrease to dangerous chemicals in the dogs' environment and food.
The results had the team curious: did this mean there was something in the shared environment of dogs and people that was to blame?
Their new research certainly suggests this might be the case. Specifically, the team identified two human-made chemicals, commonly found in homes and diets, that had the same adverse effects on both human sperm and dog sperm.
"This new study supports our theory that the domestic dog is indeed a 'sentinel' or mirror for human male reproductive decline," says Richard Lea, a reproductive biologist at the University of Nottingham.
Using sperm samples from 11 men and 9 dogs in the same region, the researchers tested the effects of two human-made chemicals. One was the common plasticiser DEHP, which is commonly found in carpets, flooring, clothes and toys, and which can leach into our food and drink.
The other is polychlorinated biphenyl 153 (PB153), which belongs to a group of industrial chemicals found to be persistent organic pollutants in the 1960s and 70s.
Using the sperm samples and these two chemicals at concentration levels that are commonly found in our current environments, the researchers carried out identical experiments for the men and the dogs. In both subjects and with both chemicals, the effect was reduced sperm motility and increased fragmentation of DNA.
While sample sizes in this study were low, they do build on matching results found in the 2016 study on dogs, as well as past laboratory and other studies that have shown these chemicals are associated with adverse effects on human fertility.
"We know that when human sperm motility is poor, DNA fragmentation is increased and that human male infertility is linked to increased levels of DNA damage in sperm," says lead author Rebecca Sumner, a developmental biologist at the University of Nottingham.
"We now believe this is the same in pet dogs because they live in the same domestic environment and are exposed to the same household contaminants."
As much as we didn't want to involve our dogs in our mess, there is a silver lining for humans. The study suggests we can maybe use dogs to figure out how pollution decreases fertility, and this could be helpful for two reasons.
Firstly, because dog studies on fertility come without all the stigma and social complications of human studies, and secondly, because external influences like diets are easier to control in dogs.
This study has been published in Scientific Reports.