This is a zoomed-in image of a three-year old northern fur seal tooth viewed under polarized light (photo by Jason Baker).  The "PN" indicates pre-natal dentin, which is tooth material deposited before the seal pup is born.  The "N" indicates birth, or natality, and each subsequent number corresponds to individual nursing events separated by periods of fasting while the mother is away on foraging trips.  The dark line at the top of the image was deposited when the pup was weaned, at four months of age.