The larger a dog is, the colder its body will probably be, according to a new study that found a strong inverse correlation between canine body size and temperature.
The explanation for this phenomenon remains a controversial mystery, but the study's authors suspect it is tied to a dog's metabolism, since smaller animals tend to have higher metabolic rates.
An earlier study on people came to a similar conclusion, with larger body mass predicting a colder person. In both dogs and humans, health and the individual's maintenance of temperature under different conditions appear to be mostly unaffected, perhaps due to the laws of physics.
Co-author Roberto Refinetti explained to Discovery News that large dogs should be better at maintaining temperature in the cold, but worse in the heat or, "more accurately, identical animals with larger body sizes lose body heat more slowly than small animals because of the physics of heat transfer, thus they should be better at avoiding hypothermia in the cold, but should also be worse at avoiding hyperthermia in the heat."
Zefinetti, a physiological psychologist at the University of South Carolina, and colleagues Giuseppe Piccione, Francesco Fazio and Elisabetta Giudice began their investigation hoping to better understand the daily rhythm of body temperature in dogs.
"Body core temperature in dogs has a temporal pattern similar to that of humans," explained Refinetti. Dogs tend to be colder in the morning and hotter in the evening.
The researchers studied newborn puppies, as well as the puppies' mothers, from three variously sized breeds: Neopolitan Mastiff, boxer and basset hound. Raised under controlled conditions, the pups all developed a daily rhythm of body temperature at six weeks of age. That timing suggests "there is a biological clock in the brain, which is believed to be the source of rhythmicity," Refinetti said.
In addition to regulating body temperature, this biological clock in mammals seems to help control daily patterns of activity, heart rate, blood pressure, hormone release, and more — even potty breaks.
During the study, however, the scientists noticed that the basset hounds tended to be hotter, in general, than the other dogs, with the mastiffs coldest of all, so they devised another study that recorded the rectal temperature of 115 adult dogs from 19 different breeds. Temperatures were documented at around noon each day, the heat midpoint for most mammals.
The researchers again noted that smaller dog breeds, such as the Yorkshire terrier, tended to be warmer than dogs from larger breeds, like the Great Dane.
The findings have been accepted for publication in the Journal of Thermal Biology.
Jacopo Mortola, a professor of psychology at McGill University, told Discovery News that the conclusions about the daily pattern of body temperature are supported by other research on people and rats, but he is puzzled by the finding that small dogs tend to be warmer than larger ones.
Mortola wondered if the way the researchers took the dogs' temperature might have affected the results since "rectal temperature in these dogs may not fully reflect their core body temperature, slightly underestimating it in the larger dogs because, in large animals, rectal temperature is more peripheral to the body core."
Mortola said that other body heat studies in animals have instead measured abdominal temperature with "chronically implanted temperature transmitters."