New behavior: What's my name?

Today 2 of the pups actually turned around while they were digging/chewing at something when I called their name. I wasn't sure if it was the name, or just the sound, so I called each one a couple more times to see if I could get alternate/individual responses. It worked, so I think they know their names (or at least 2 of them do, others will not be long behind). It usually takes about 3-4 days for some newly learned skill to distribute through the litter.

Here's a really interesting paper that explains some pups could more than a week apart in development based on a couple different things:

One finding extremely important to the mystical (mythical?) 49 days time frame was that pups in a single litter can vary in developmental age by a week in each direction though all are born within a few hours. This developmental variation arises from several sources- conception can vary two to three days due to super fetation, delay in implantation of fertilized ova in the uterus may be another two to three days, location in the uterine horn, blood supply to the various embryos, developmental arrests or speedups, differential delay in parturition all can contribute to developmental variability. There is also differential post partum development especially during the first few weeks. This means that by the time the pup reaches 49 days since birth, it can be anywhere between 42 and 56 days old developmentally relative to all other pups in the total population of pups whelped on the same day, even to pups in the same litter. And it is the neural, physiological and physical development, not the exact chronological age, not minutes elapsed since popping into the cold, cruel world that is important in the behavioral stability or lack of it in pups and later when the pups reach adult status.

Note the part I made bold. There are definitely some pups that are ahead in some skills, but I don't know that it's always the same pups -- some get one skill fast, some might pick up the next skill faster. Plus, they are not at training age, just skills like name recognition, affection, recall, etc.