Radiographic assessment of pectoral flipper bone maturation in bottlenose dolphins (Tursiops truncatus), as a novel technique to accurately estimate chronological age

Abstract

Accurate age estimation in wildlife conservation is an important diagnostic tool in the interpretation of biological data, necropsy examination, reproductive status and population demographics. The most frequently utilized methods to age bottlenose dolphins (Tursiops truncatus) include tooth extraction; counting dental growth layer groups and dental radiography. These methods are inaccurate in dolphins > 13 years old, due to overlapping of the growth layer groups in dolphins and worn teeth. Establishing a non-invasive method of accurately aging bottlenose dolphins across the entire age range is important to long term conservation efforts to understand health status, lifespan, reproduction and survivability. A database of 126 radiographs from 94 dolphins of known chronological age was utilized to establish the stages of skeletal ossification over time. A numerical score from -1 to 8 was assigned to 16 anatomic locations on the pectoral radiograph, to create a formula to estimate age. The most informative areas to evaluate morphologically were the metaphyseal regions of the radius and ulna, and the proximal and distal epiphysis of metacarpals II and III. Third order polynomial regression calculated separate age predictor formulas for male and female dolphins, with females reaching sexual maturity earlier than males. Completion of epiphyseal closure of the long bones correlated with average sexual maturity. Managed care dolphin ages could be properly estimated with decreasing precision from within 3 months in animals < 5 years old, to within 5 years in animals > 30 years old. This diagnostic tool could also be applied to diagnose atypical ossification patterns consistent with nutritional, developmental or growth abnormalities, and identifying subclinical health issues. In conclusion, knowledge of the lifespan and the onset of sexual maturity for each species will allow this model to be applied to other cetaceans, facilitating age estimation via pectoral radiography in future research.

Alpha- and gammaherpesviruses in stranded striped dolphins (Stenella coeruleoalba) from Spain: first molecular detection of gammaherpesvirus infection in central nervous system of odontocetes

“…Dorsoventral pectoral x-rays were taken in left flipper of all individuals using a conventional veterinary x-ray equipment (Dual Vet X-PL Kit 8Kw 125Kvp 100 mA 250mAs Portable n/s PKL12345) and 70kVp and 6.4mAs exposition setting following Barratclough et al 2019 [ 30 ]. Physiologic maturation was classified based on bone epiphyseal maturation at the distal epiphysis of the radius and ulna [ 31 ] as well as total length and gonadal development stage.…”

Age determination of common bottlenose dolphins (Tursiops truncatus) using dental radiography pulp:tooth area ratio measurements

“…Recently, measurements of skeletal ossification from pectoral flipper radiographs have been reported to successfully age dolphins from managed populations to > 30 years, however, precision decreased in animals > 20 years [ 9 ]. Precision of dental radiographic methods are similar in animals < 10 years, while pectoral flipper radiography does appear to be more precise in older dolphins.…”

“…Thus, researchers have developed a variety of methods in an attempt to estimate age in marine mammals, each giving varying accuracy and precision. These techniques include, but are not limited to, long term photographic-identification (photo-ID) monitoring in live animals [1], morphometric comparisons in live and dead animals [2][3][4], dentinal growth layer groups from extracted teeth (GLGs) [1,3,5,6], bone GLGs in carcasses [7,8], skeletal ossification live dolphins [9], bone density [10][11][12], micro-CT scanning [13], radiometric aging in auditory bullae from carcasses [14], ear plug layers in mysticete carcasses [15], aspartic acid racemization in carcasses [16][17][18], baleen length or radiocarbon dating in carcasses [19,20], bomb radiocarbon dating in extracted teeth for animals alive in the 1960s [18,21], telomere sequence analysis in live animals [22][23][24], DNA methylation analysis in live animals [25], and fatty acid signatures in live or dead animals [26][27][28][29]. Unfortunately, many of the aforementioned techniques are inapplicable for use in studies with living dolphins, are unsuitable to be used in field studies, and/or provide poor precision.…”

“…Measurements of bone density alone, or in conjunction with length, were recently found to poorly correlate to age in bottlenose dolphins [ 10 ], despite earlier studies reporting reasonable correlation between age and bone density in younger dolphins [ 11 , 12 ]. Most recently in dolphins managed under human care, analysis of skeletal ossification using pectoral flipper radiographs was shown to accurately estimate age, with variability increasing with age, using a relatively non-invasive approach [ 9 ]. However, the technique has not yet been validated in wild animals, nor directly compared to other age estimation techniques.…”

Multi-tissue methylation clocks for age estimation in the common bottlenose dolphin

“…As such, other aging methods have been evaluated for use with bottlenose dolphins or other cetaceans including eye lens aspartic acid racemization[8, 9], fatty acid composition[10–12], radiocarbon 14 dating from fallout[13], telomere length[14], radiograph changes in tooth to pulp ratio[15] and pectoral flipper bone ossification[16]. However, all methods vary in inherent accuracy and must be calibrated against some age estimate of the species in question, most often by using GLG counts, and each have limitations for field use and accuracy across different age classes.…”