Determining the age of marine mammals through examination of growth layer groups (GLG’s) in teeth was developed in 1950 by Scheffer and is now a routine practice used to determine the age of dolphins. GLG accumulates over time in a constant cycle, and each GLG usually represents a year of age. Myrick examined seven Hawaiian Spinner dolphins held in captivity at Sea Life Park, Hawaii.
The teeth of four deceased dolphins that were administered tetracycline (TCL) in their lifetime at Sea Life Park were examined for TCL labels in the dental tissues, including one whose age was known. Three live dolphins in captivity at Sea Life Park were clinically treated with TCL through intramuscular injections at three month intervals. In order to monitor the accumulation of GLG’s, a tooth was pulled from each dolphin once every six months. The Spinner Dolphin teeth were prepared by taking untreated thin sections of the teeth – grinding, polishing and decalcifying them. Additionally they cut the teeth along a longitudinal plane and stained with Mayer’s haematoxylin-stained (D/S) thin sections. Teeth were examined under UV light to highlight the fluorescent TCL labels, then the label and inter-label thicknesses of the layers were measured.
Results showed a repeating sequence of annual dentinal layering and the GLG contained a series of patterns containing a repetition of a thin, light GLG-boundary layer, a thicker dark layer, another thin light mid-GLG layer and then a thick dark layer. The layers were arranged in 13 pairs of fine layers, and each pair represented one lunar month. This type of cyclical dental layering is found in pinnipeds, dugongs and beaked whales, and from this experiment is now known to occur in Hawaiian Spinner Dolphins. This supports the theory that there is an internal clock within the mammal that starts at birth and cycles every lunar month. Using this information, they determined that the GLG-boundary layers and mid-GLG layers were formed at six month intervals. These layers were uniform and could be counted in reverse to determine the age, month and year of birth for each dolphin. Surprisingly, each dolphin was not found to be born during the same season; spinner dolphins have a two-cycle reproductive pattern, one in the spring and one in the fall. Prior to this experiment, reproductive seasonality in Hawaiian Spinner Dolphins was also suggested by Norris and Dohl.
Myrick, A. C. et al. 1984. Calibration of Dental Layers in Seven Captive Hawaiian Spinner Dolphins, Stenella Longirostris, Based On Tetracycline Labeling. Fishery Bulletin. Vol 81, 1 pp. 207-225.