Puppies are an Altricial Species
Puppies are an altricial species, meaning that, when they are born, they are born in a pretty much immature state. This means that unlike precocial species such as foals, calves and fawns, puppies are unable to walk when they are born and they depend to a great extent on their mothers during their first weeks of life.
When puppies are born, and during the first two weeks of life, they are unable to eliminate on their own (mother dog must lick their bottoms to stimulate them to eliminate) and they are not fully mobile, meaning that they must sort of crawl if they need to move from place-to-place. On top of this, newborn puppies are unable to regulate their core body temperatures.
In Need of Warmth
Unable to regulate their body temperatures on their own, puppies must strongly rely on warmth for their survival.
Young puppies lack the shivering reflex, that important reflex that allows muscles to start automatically shaking in small movements for the purpose of generating warmth.
Additionally, young puppies don't have insulating fat, that layer of fat known for preventing substantial body-heat loss. Young puppies therefore depend on the warmth of their mother and litter mates in order to stay warm.
When in the wild mother dog had to occasionally step out of the den, sleeping in piles allowed the pups to keep warm as a heat-conservation mechanism, explains Michael Fox in the book" 'Understanding Your Dog."
When puppies are born, their temperature is around 97°F, but then as the puppies develop, they reach the “normal” 101°F at around three weeks of age.
Prior to this time, breeders usually rely on some type of supplemental heating source (heat lamp, heating pad) to ensure the puppies do not get chilled. During the neonatal stage, it is therefore natural for puppies to pile up on each other if they are feeling cold. This is an instinctive behavior.
Did you know? According to research conducted by Yngve Zotterman, of the Swedish Research Council, puppies come equipped with special "heat sensors" in their noses which helps them find their way towards their mom and siblings in case they get separated and risk being chilled.
Risks for Illness
Warmth is so crucial in newborn puppies that, if a puppy becomes chilled, a cascading chain of events takes place.
The puppy will lose his suckling reflex, his intestinal system begins to shuts down, and his stomach will fail to push food forward into the small intestine for digestion, explains Myra Savant-Harris in the book: "Puppy Intensive Care, A Breeder's Guide to Care of Newborn Puppies." For this reason, it is imperative to never feed a chilled puppy.
As mentioned, in general, the puppies' ability to regulate their temperature improves once the puppies mature. In general, things get much better once puppies reach the age of three weeks to about three weeks and a half when, warmth-wise, they need just minimal support from the breeder, caretaker or mom.
During this time, puppies may no longer sleep in piles as often as they did when younger. You may therefore catch them sleeping more side-by-side in row rather than in a heap, unless the area is excessively cold. Later on, as the pups develop further, you'll notice them sleep more spread out.
The Power of "Sticking Together"
Just because puppies develop and start regulating their body temperature better, doesn't mean they'll give up sleeping in piles totally.
You can still catch them sleeping in heaps, or at least, closely together. The reason for this is that puppies instinctively seek each others' company as social beings.
It ultimately feels reassuring to puppies to stick together. According to Scott and Fuller (1965), by 7 weeks, although competitiveness among siblings prevails, bonding remains strong.
This can be easily noticed upon witnessing the distress vocalizations in 7-week old puppies when isolated even briefly from their litter mates at this time.
This attachment is expected to decline by 10 weeks of age. However, puppies may still sleep in contact with each other if this helps them feel warm and reassured.
"Each pup serves as a social presence for its litter mates and obtains social comfort for itself by huddling close enough to the other young pups in the brood so that it can feel their touch. If all the puppies in the group try to crowd together to feel the comforting presence of one another, the result is a pile of puppies." Stanley Coren, Do Dogs Dream?: Nearly Everything Your Dog Wants You to Know
Now That You Know...
As seen newborn puppies have their own good reasons for sleeping in piles. It keeps them warm, as puppies radiate body heat, and provides them with a sense of togetherness during a time where they are the most vulnerable.
If you are a breeder or you are fostering a litter of puppies, it's important to monitor their behaviors to ensure they are warm.
Just as with raising baby chicks, you may therefore want to take steps in making the area less chilly if you notice young puppies crowding over one another in piles and crying just like baby chicks huddling together when cold.
At the same time though, you must be careful that the area doesn't get too warm which can also be problematic, especially for mother dog who may easily overheat.