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Age determination of sperm whales (Physeter macrocephalus) from the west coast of Jutland, Denmark : The 13th Danish Marine Mammal Symposium

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Abstract

Age determination of sperm whales (Physeter macrocephalus) by counting growth layer groups (GLG´s) in the teeth is to some extent considered to be subjective and only relative, due to: 1) Limited validation of GLG counts to “known age” of the individual; 2) Variation in methods for preparation of teeth e.g. acid (pH) and duration of etching; 3) Difference in interpretations of GLG´s between readers1,2. Bearing in mind these challenges, the age of three sperm whales stranded in Denmark in 2012 and 2014 were determined by counts of GLG´s in the erupted teeth from the lower jaw and comparing these with the number of GLG´s obtained from rudimentary teeth in the upper jaw. Teeth were obtained from 3 adult male sperm whales; MCE 1642, stranded at Nr. Lyngby Strand, Denmark in 2012; and MCE 1644 and MCE 1645, who stranded at Henne Strand, Denmark in 2014. From each whale one non-erupted tooth from the maxilla and one erupted mandibular tooth was cut longitudinally in two half´s with a diamond blade saw and grained with sandpaper gain 800. One half of each tooth was etched in 10% acetic acid for 7 hours and the other half was etched in 15% acetic acid for 3.5 hours. The GLG´s were counted several times by two readers. Based on counting of GLG´s the average estimated ages of the three sperm whales were between 29 and 39 years. However, some deviation due to intra- and inter reader differences was observed. The number of GLG´s in the rudimentary teeth did not differ significantly from the GLG´s of the mandibular teeth. Pulp stones were seen in both erupted and non-erupted teeth from all three whales. Further studies are needed to develop the current preparation techniques to make clear and more easily readable GLG´s to obtain more accurate age determination of sperm whales.