Biorhythm tracks enamel thickness in humans and great apes.

Although evidence of a biorhythm retained in tooth enamel as Retzius periodicity (RP) was identified in the 19th century its significance for mammalian growth and life history has only recently been discovered. This study builds upon our recent work where we hypothesised the biorhythm may have a role in enamel growth, and that its periodicity may change from deciduous to permanent teeth. Here we test this hypothesis. We compare RP between deciduous second and permanent first molars within the maxillae of four human children. We report the first RP’s for deciduous teeth from modern great apes (n=4), and compare these to new data for permanent teeth (n=18) from these species, as well as to previously published values. Results show RP changed within the maxilla of each child, from thinner to thicker enameled molars. RP of 5 days for great ape deciduous teeth lay below the lowermost range of those from permanent teeth from modern orangutan and gorilla, and within the lowermost range of RP’s from chimpanzee permanent teeth. When considered alongside our earlier reported correlation between RP and formation time, these observations provide further evidence that RP is associated with enamel growth processes in humans, and can change from deciduous to permanent teeth within an individual. Our data suggest these associations might extend to great apes. We conclude that enamel growth should be considered alongside other physiological systems when developing predictions around RP as a measure of an underlying biorhythm.