Scientists have assessed the fertility of male dogs in Britain over the past three decades to find that it’s declined by a whopping 30 percent across five common breeds.
While the researchers aren’t concerned that dogs will lose their ability to reproduce any time soon, they do say the find could have serious implications for human fertility, pointing to the possibility that industrial chemicals in our food packaging could be to blame.
"The dogs who share our homes are exposed to similar contaminants as we are, so the dog is a sentinel for human exposure," lead researcher Richard G. Lea, from the University of Nottingham in the UK, told The New York Times.
Back in 1988, Lea and his team decided to monitor changes in dog fertility by analysing a population of service animals at a centre for disabled people in England.
A total of 232 dogs from five different breeds - Labrador retrievers, golden retrievers, curly coat retrievers, border collies and German shepherds - were included in the study, and the fertility has been tested every year up to 2014.
As Jan Hoffman explains for The Times, the benefit of working with these dogs in particular is that, not only do they come from an environment where systematic record-keeping is kept for their health and lineage, but they’re also being raised in one location with uniform conditions.
Each year, a selection of 42 to 97 dogs within the group had their fertility tested via sperm samples, and at different intervals throughout the 26 years, dogs with the poorest sperm quality were removed from the test group.
When the researchers looked at the percentage of sperm with healthy motility - the ability to swim in a straight line - they found that it dropped by 2.4 percent every year between 1988 and 1998.
Once the dogs with the worst sperm were removed from the group, the team found that the sperm motility continued to decline by 1.2 percent every year from 2002 to 2014, with an overall decline of 30 percent across the entire study period.
And there were other problems, too.
"Between 1994 and 2014, they also noticed that the mortality rate of the female puppies, although small, showed a threefold increase," says Hoffman. "And the incidence of undescended testicles [where testes fail to correctly descend into the scrotum] in male puppies, also small, had a 10-fold increase, to 1 percent from 0.1."
Lea and his team are yet make any definitive conclusions about the cause, but they did confirm the presence of environmental chemicals called PCBs and phthalates in both the dogs’ semen and in testicles removed by vets during routine desexing procedures.
They also found traces of the chemicals in the food given to the dogs.
Once used in the manufacture of plastics and paints, PCBs were widely banned back in the 1970s and ‘80s, and diethylhexyl phthalate (DEHP) has been noted for its potential health risks. But they have a long half-life, and are virtually impossible for us to avoid completely.
"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."
The most worrying thing here is that more than 60 studies - though controversial - have reported a recent decline in human semen quality, in the 53 years between 1938 and 1991, and, as Tim Radford reports for The Guardian, PCBs and phthalates "are ubiquitous, and have been linked to both fertility issues and birth defects".
And while the decline in human semen quality is still hotly debated, the recent increases testicular cancer and undescended testicles in human babies are not.
But whether or not harmful environmental chemicals are to blame has yet to be confirmed, and Lea and his colleagues have so far only made a correlation between the two things in dogs - not a causative link.
"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 told The Guardian.
"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 results have been published in Scientific Reports.