It's one thing for a dog to resemble his owner, but quite another for his sperm to follow suit. A recent study by researchers at the University of Nottingham showed a decline in the semen quality of dogs over the past 26 years, which could help explain a decline in human male fertility that has been documented over the past 70 years. While some have questioned the validity of some of the human studies, other experts have blamed this human fertility decline on environmental contaminants.
"Since dogs share our homes, they are exposed to similar environmental conditions including chemical pollutants. It is possible therefore that the dog is a sentinel for human exposure to chemicals in the environment," says lead researcher Dr. Richard Lea in an email interview. "If this is the case, then whatever factors that are affecting dog semen may also affect the human."
The study spanned the years 1988-2014, during which the semen of stud dogs bred to help the disabled were monitored. The sample included pups of five breeds – German shepherds, border collies, curly coat retrievers, Labrador retrievers and golden retrievers.
"The advantage of using this population of dogs is that we know everything about them in terms of where they live, what they eat, their general health etc.," says Lea, an associate professor of reproductive biology at the University of Nottingham's School of Veterinary Medicine and Science. "In a wider population of dogs that are not monitored in this way, the study would be much more difficult to carry out."
The results were certainly significant – the pooches experienced around a 30 percent decline over the time period in sperm motility (how actively the sperm moves in a forward direction or in a large circle). Generally speaking, males (both humans and dogs) are more likely to sire offspring if their sperm are active. Male puppies produced from the same population of stud dogs also showed a higher rate of cryptorchidism, where the testes do not descend into the scrotum when they should. "We do not know if this is related to the decline in semen quality or if there is a link with environmental contaminants," Lea says.
The Nottingham researchers found the same chemicals present in commercial dog food, the dogs' sperm and the testes of pups undergoing routine veterinary castration. Two of these chemicals, diethylhexyl phthalate (DEHP) and polychlorinated bisphenol 153 (PCB153) were at concentrations that have disturbed reproductive functions in other species. These species include farm animals like bulls, rams and goats — and human males, too.
"However, we do not know if these chemicals are responsible for the 26-year decline in motility – this is our hypothesis but we have no evidence that it is the case," Lea notes. "It is very difficult to conclusively show that a chemical in a food (for example) is responsible. The reason for this is that we are exposed to mixtures of chemicals, many of which interact."
A comparatively small number of these environmental chemicals have been shown to affect sperm movement or other parameters of reproductive health, according to Lea. "Some examples are chemicals derived from plastics (phthalates), flame retardants (polybrominated diphenyl ethers) or the banned polychlorinated biphenyls (PCBs) which had an industrial use but persist in the environment."
Don't be too distraught about the study results yet, though. Although canine semen quality has declined, Lea says the dogs involved still produced sperm within the normal range, and are still plenty fertile enough to get the job done.
"We don't know yet if the decline in motility that we have observed will continue," says Lea, adding, "The possible impact of chemical contaminants on the canine population is hypothetical and therefore requires further work."