Researchers have determined that the fertility of male dogs all over Britain has been steadily declining over the past three decades, for a whopping 30 percent across five common breeds. There isn’t any real danger of them losing their ability to reproduce anytime soon, but the findings could have serious implications for their human owners, the team believes.

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Image credits Zach Zupancic/Flickr.

Ok, I don’t know about you but there’s two things that consistently make my day better no matter what’s going down: the dog jumping up and down for joy when he sees me come home, and the fact that my plumbing works like a charm. But, in a kind of depressing quantum link, the two of them seem to be connected — and science says there’ll be a whole lot less up and down going on.

“The dogs who share our homes are exposed to similar contaminants as we are, so the dog is a sentinel for human exposure,” said lead researcher Richard G. Lea, from the University of Nottingham in the UK, for the The New York Times.

Lea and his team have been assessing the fertility of a population of male service dogs at an English center for disabled people. They started in 1988, and since then they’ve analyzed a total of 232 dogs of five different breeds — Labrador retrievers, golden retrievers, curly coat retrievers, border collies and German shepherds. The team chose to work with these dogs because their health and lineage are excellently recorded and they’re all being raised in one location in the same conditions.

Each year, the team would test the fertility of a selection of 42 to 97 dogs via sperm samples. At varying intervals throughout the 26 years of study, the dogs with the poorest sperm quality were removed from the test group. When they measured the percentage of sperm with healthy motility — the ability to swim in a straight line — the researchers found that it dropped by 2.4% each year. Even when not taking data from the dogs who were removed into account, sperm motility declined by an average of 1.2% every year from 2002 to 2014, for an overall decline of 30% over the entire study’ duration.

And the bad news don’t end here.

“Between 1994 and 2014, they also noticed that the mortality rate of the female puppies, although small, showed a threefold increase,” writes Jan Hoffman for the NY Times. “And the incidence of undescended testicles in male puppies, also small, had a 10-fold increase, to 1 percent from 0.1.”

Lea’s team isn’t sure what’s causing this, but they believe that it all comes down to the presence of environmental chemicals called PCBs and phthalates in the dog’s semen and testicles (removed by vets during routine desexing procedures.) Once widely used for paints and plastic masses, PCBs were banned back in the 1970s and ‘80s, and diethylhexyl phthalate (DEHP) has been noted for its potential health risks. But even if they’ve fallen out of use, their long half-life means that they’re virtually everywhere today. Including, as the team found, in trace amounts in the food the dogs eat.

“The scientists cannot determine how the chemicals were introduced into the food supply; these are not additives,” says Hoffman. “But Lea and his colleagues speculate that they could be in the packaging as well as in water that came into contact with any ingredients.”

Ok, so what do dogs’ little swimmers have to do with us? Well, the same chemicals that affect them affect us, too. There are more than 60 studies that report a recent decline in the quality of human semen in the years between 1938 and 1991. Their results are hotly debated, but the evidence that the incidence of undescended testicles in human babies and cases of testicular cancer are on the rise, isn’t. By itself, however, the data isn’t enough to establish a link between these chemicals and the effects the team is seeing — there are just too many other chemicals at play here.

“If you think about it, we are exposed to a cocktail. Who knows how many chemicals are out there and what they are doing?” Lea said.

“What we have been able to do here is just to pull out ones that we know are present, and we have tested those in terms of their effects and it does suggest there is an impact. The next stage – and it is a big next stage – is trying to tease out what else is there and how those chemicals are interacting.”

The paper, titled “Environmental chemicals impact dog semen quality in vitro and may be associated with a temporal decline in sperm motility and increased cryptorchidism” has been published online in Scientific Reports.