Width of cementum growth layer groups and its possible use in monitoring reproduction in female ringed seal : an exploratory pilot study

Sammanfattning

The Baltic ringed seal (Pusa hispida botnica) population has faced historical declines due to both hunting and environmental pollution. It is slowly recovering but there is a need for new methods in order to monitor and assess pregnancy rates. The count of growth layer groups (GLGs) in teeth cementum have been used to age ringed seals for decades and the method is well established. This pilot study explored the possibility to measure the widths of the GLGs for subsequent use in monitoring reproduction events in female ringed seals. Canine teeth from eleven ringed seals were used (eight females, three males). The annular GLG widths were compared using a proportional width index (PWI) that corrects for body size difference between individuals and age trends. The PWI values were classified into juvenile years (<6) or adult years (>5). The variance in PWI during adult years was significantly higher in females (0.005062) than in males (0.002065, p<0.001). When comparing the distributions of the PWI values in females (Hartigan’s dip test) the females showed a significant multimodality (p=0.049), forming two significantly different subgroups. This pattern may suggest the influence of pregnancy and lactation on one of the subgroups. During this pilot study, technical difficulties made comparing GLGs of known reproductive status impossible. Addressing these difficulties could be a way forward for future research.