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

Abstract

Knowledge about the maturity status of specimens included in evolutionary, taxonomic or life history investigations is fundamentally important. This study investigated the use of the degree of cranial suture fusion, the developmental status of cranial bones, and the degree of tooth wear as indicators for cranial maturity status in Delphinus sp. from New Zealand waters. In total, 15 sutures, one joint and three nonmetric characters were assessed on 66 skulls obtained from stranded and bycaught individuals sampled between 1932 and 2011. A suture index (SI) was computed based on 10 sutures, in which degree of fusion was correlated with age and the three misclassification indices (MI), calculated for a given suture, were <50%. In addition to these, five premaxilla‐maxilla fusion and seven tooth wear categories were assessed. Results suggest that New Zealand Delphinus sp. skulls should be regarded as cranially mature if at least two of the following criteria are met: (1) individuals assessed as sexually mature, (2) aged ≥ 11 yr, (3) SI ≥ 8, and (4) premaxilla‐maxilla fusion ≥ 75% of the length of the dorsal side of the rostrum. Presence of any number of rostral teeth worn to the gum line provided further evidence for cranial maturity.

Morphological differences between coastal bottlenose dolphin (Tursiops aduncus) populations identified using non-invasive stereo-laser photogrammetry

“…Such parameters are linked to morphological and age-specific processes, often regulated by physiological, ecological, evolutionary, and anthropogenic factors 3 . Morphometric data of cetaceans contribute to the assessment of individual- and population-based reproduction 4,5 , health 6,7 and demography 8–10 and in conjunction with genetic information, taxonomic status 11,12 . Despite its many applications, obtaining accurate morphometric data on free-ranging cetaceans is challenging.…”

Accuracy and reliability of cetacean cranial measurements using computed tomography three dimensional volume rendered images

“…Osteological specimens, particularly crania, represent ideal structures for the study of evolutionary and geographic variation in vertebrate morphology. In cetaceans, the morphometric assessment of cranial measurements parameters may provide additional information to identify sex, age [ 1 – 2 ], evolutionary history, geographical variation [ 3 – 6 ] and sexual dimorphism [ 3 , 5 – 6 ]. Cetacean cranial morphometrics often rely on manual measurement [ 7 ], requiring extensive time and manpower for skeletal excarnation [ 8 – 10 ].…”

Use of mass spectrometry to measure aspartic acid racemization for ageing beluga whales

“…Other means of age estimation that have provided a relative measure of age include pelage or skin color (Herzing ), fusion of cranial sutures (Jordan et al . ), and fatty acid composition (Marcoux et al . ), which allowed individuals to be placed in age groups corresponding to life stages ( e.g ., dependent young, juveniles, adults).…”

“…Mammal ages have been estimated using different methods; however, the need for more precise and accurate estimates resulted in the key development of ageing techniques that examine the layered structure of deposited material in several persistent tissues, primarily teeth, less often bone, and in some cases, from other layered structures or from chemical signals (Stewart et al 1996, Garde et al 2012, Marcoux et al 2015. Other means of age estimation that have provided a relative measure of age include pelage or skin color (Herzing 1997), fusion of cranial sutures (Jordan et al 2015), and fatty acid composition (Marcoux et al 2015), which allowed individuals to be placed in age groups corresponding to life stages (e.g., dependent young, 1 juveniles, adults). Marine mammalogists pioneered age estimation from counting growth layers in teeth (Laws 1952); recent research has been directed toward verifying the amount of time represented by a growth layer (i.e., calibration or validation) to ensure that growth layer counts are consistent among studies (Pinedo and Hohn 2000, Stewart et al 2006, Lockyer et al 2007, Frie et al 2011.…”