Pet dogs keep their feet from freezing
Dogs can keep warm on frozen ground, thanks to a specialised circulation system in their paws, say scientists.
Researchers in Japan used electron microscopes to study the internal structure of domestic dogs' paws.
They found that heat was transferred from the artery to the network of veins, meaning that cooled blood could not return to the body.
The system has been recognised in many other animals' extremities, including penguins' beaks and dolphins' fins.
The findings are published in the journal Veterinary Dermatology.
Arctic foxes and wolves are well known for their adaptations that help them to regulate a constant body temperature in cold conditions.
- Arctic foxes have dense and rigid fur on their feet which keeps the pads away from the freezing ground
- The pads are connective tissue and fat which is more resistant to freezing
- They have a "rete mirabile" or "wonderful network" of veins in their feet and legs that works as a heat exchanger where cooled blood is warmed up to maintain a constant temperature
(Additional research by Dr Arnaud Tarroux from the University of Quebec, Canada)
Previous studies showed that the canines can keep the tissue in their feet from freezing even in temperatures of -35C.
Dr Hiroyoshi Ninomiya and his team at the Yamazaki Gakuen University in Tokyo, Japan, set out to discover if this ability was also common to domestic dogs.
Using electron microscopes, the researchers were able to examine the internal structure of dogs' paws.
They found that the very close proximity of the arteries to the veins in the footpad meant that heat was conducted from one blood vessel to another.
So when blood in the paw's veins cooled on contact with the air or ground, warm blood pumping from the heart - through the neighbouring artery - transferred its heat.
The blood was therefore "warmed up" before it returned to the body - preventing the dog's body from cooling down, whilst also keeping the paws at a constant temperature.Cold specialists
"It is well known that penguins in the Antarctic have a counter current heat exchange system in their wings and legs to prevent heat dissipation and keep the body warm," said Dr Hiroyoshi Ninomiya.
"When we found that dogs also have such an excellent system in their paws, we were excited."
Anatomist Dr Sarah Williams from the Royal Veterinary College says the evidence could be a revelation for dog experts.
"Up until now, it was not considered necessary for domestic species to posses such a specialisation.
"This discovery has interesting evolutionary implications, and may suggest that the ancestors of the domestic dog lived in cold climates [in order] to bring about such an adaptation."