Deposition of growth layer groups in dentine tissue of captive common dolphins <i>Delphinus delphis</i>

Abstract

Knowledge of age structure and longevity (maximum age) are essential for modelling marine mammal population dynamics. Estimation of age in common dolphins (Delphinus spp.) is primarily based on counting Growth Layer Groups (GLGs) in the dentine of thin, decalcified and stained sections of teeth. An annual incremental deposition rate was validated for Delphinus spp. 30-years ago through the use of tetracycline. However, it is not known if the pulp cavity becomes occluded in older individuals or GLGs continue to be deposited in dentine tissue. To investigate the deposition of GLGs in dentine tissue, teeth samples were obtained during the necropsies of two short-beaked common dolphins (Delphinus delphis) that were held in captivity for 31 and 33 years in New Zealand. Individuals were captured together in Hawkes Bay, North Island, New Zealand and classified as juveniles based on physical appearance. Teeth were processed in two ageing laboratories, using four different bone decalcifiers, two sectioning techniques incorporating the use of both a freezing microtome (-20°C) and paraffin microtome, and two different stains. An age was estimated for one of the dolphins, in line with that proposed based on estimated age at capture and period in captivity. However, a hypomineralised area was observed in the dentine tissue close to the pulp cavity of the second individual, preventing estimation of maximum age. The presence and structure of this anomaly is explored further within the study.

Criteria for assessing maturity of skulls in the common dolphin, Delphinus sp., from New Zealand waters

“…The two oldest dolphins, both long‐term captive dolphins, were ≥33 and ≥35 yr in age (see Murphy et al . ).…”

“…Age was estimated based on counting growth layer groups in dentine tissue (see Murphy et al . for an overview of the methodology).…”

“…A Kruskal-Wallis test was run to determine whether median total body length (TBL) differed significantly with maturity status and sex. Age was estimated based on counting growth layer groups in dentine tissue (see Murphy et al 2014 for an overview of the methodology).…”

A review of age estimation methods in marine mammals with special reference to monodontids

“…Lockyer et al (2010) reported that GLGs in the cementum of harbour seal tooth sections are more prominent with Toluidine Blue while haematoxylins such as Mayer's or Ehrlich's were better for dentine. Murphy et al (2014) suggested that Toluidine Blue may have a better affinity for tissues decalcified using organic acids and chelating agents. Lockyer (1993Lockyer ( , 1995 preferred Ehrlich's acid haematoxylin as it is more durable and does not fade so rapidly over time.…”

“…In more recent years, the development of commercially prepared mixtures of acids, such as RDO (Apex Engineering Products Corporation, Illinois, USA), has significantly lowered decalcification times to hours. Murphy et al (2014) investigated the use of 4 different decalcifying agents, and found that Formical-4™ (Decal Chemical Corporation, New York, USA) provided the best sections for age estimates of two captive short-beaked common dolphins known to be over 30 years old. However, decalcification took 4-6 weeks.…”