Canine fertility has plummeted over the course of three decades, according to a study from researchers at the University of Nottingham. Published in Scientific Report, the study found a sharp decline in sperm quality in stud dogs.

The 26-year-long study took samples from five breeds of stud dogs (Labrador retriever, golden retriever, curly coat retriever, Border collie and German shepherd) at an assistance dogs breeding center. An analysis counted sperm in the collected semen.

"The strength of the study is that all samples were processed and analysed by the same laboratory using the same protocols during that time and consequently the data generated is robust,” said Gary England, who administered the semen collection process and is the foundation dean of the School of Veterinary Medicine and Science.

The researchers found a 2.5 percent decline per year between 1988 and 1998 and a 1.2 percent annual downfall between 2002 and 2014. They also discovered that the sperm collected contained environmental contaminants at levels high enough to hinder sperm motility

The researchers attribute the decline in dog fertility to environmental contaminants, as they found the chemicals that alter sperm quality are also found in an array of commercial dog foods.

"This is the first time that such a decline in male fertility has been reported in the dog and we believe this is due to environmental contaminants, some of which we have detected in dog food and in the sperm and testes of the animals themselves,” said Richard Lea, who led the researcher and is affiliated with the University's School of Veterinary Medicine and Science, in a statement. "We looked at other factors which may also play a part, for example, some genetic conditions do have an impact on fertility. However, we discounted that because 26 years is simply too rapid a decline to be associated with a genetic problem."

The findings are preliminary but they hold a greater significance: they can offer a potential explanation for the supposed decline in human semen quality that some studies have found. The deteriorating quality of human semen has been debated amongst experts for decades, as critics note many of these studies

"While further research is needed to conclusively demonstrate a link, the dog may indeed be a sentinel for humans—it shares the same environment, exhibits the same range of diseases, many with the same frequency and responds in a similar way to therapies,” said Lea. "The Nottingham study presents a unique set of reliable data from a controlled population which is free from these factors. This raises the tantalizing prospect that the decline in canine semen quality has an environmental cause and begs the question whether a similar effect could also be observed in human male fertility."