So that's the first story (cones and rods in the eyes) about the similarity between humans and dogs. The second relationship between people and our canine friends is another physiological similarity that can be considered maybe to be a little disturbing.
Researchers have noted over the last few decades that the sperm count of males—both dog and human—have been decreasing. Specifically, sperm motility—the ability of sperm to swim in a straight line—has diminished. If sperm can't swim energetically and directly toward the female egg, fertility suffers. In addition, traces of PCBs and phthalates are being found in both dog and human semen. These artificial chemicals have been linked to birth defects. Whether or not they contribute to decreased sperm activity has yet to be determined.
A comprehensive evaluation of human sperm viability has so far not been carefully and consistently measured, due to the fact that the issue is so complex for humans. Dogs are simpler, so recent studies spanning a 26-year period at Nottingham University in the UK have meticulously measured the decrease in dog fertility. It is significant.
So what is the cause of the carefully documented canine reduction in fertility? No smoking sperm gun has as yet been discovered, but the likely contributors—in light of the measured PCBs and phthalates—points to the presence of toxins and chemicals in dogs' environments as the likely causes.
Dogs are our best friends. We share much of the same living space, and we are exposed to the same chemicals they are. Thus our canine buddies may be in the same sinking fertility boat that we are. Human male fertility has been measured and is definitely on the decline, although it has yet to be adequately quantified. So it raises the question: Are we—humans and dogs—headed together towards a fertility problem? Stay tuned.
In summarizing this pair of dog duet posts: The mammalian eyes of dogs and humans—although both have evolved from those tiny mammals scurrying in the dark to avoid dinosaurs—still retain the rods and cones of those long ago times—though in different proportions. We also share a declining sperm quality. In interesting ways “man's best friend” seems to be headed down a similar evolutionary path with us. Maybe we're closer than we think; maybe more than just good buddies.