Age estimation in the harbour seal (Phoca vitulina) based on the closure of skull sutures and synchondroses

Abstract

Age estimation of individuals is an important tool for the management of marine mammals and is mostly done by microscopic analysis of growth layer groups (GLGs) in teeth (cementum and dentin) and bone. The present study evaluated the use of sutural and synchondrosal closure in the skull of the harbour seal (Phoca vitulina) as a potential non-destructive alternative for age estimation. For that, we scored the ectocranial closure of 12 selected sutures and synchondroses in a sample of 303 Eastern Atlantic harbour seals (P. v. vitulina; 138 males, 165 females), for which data on age at death were available, based on determined gross-morphological criteria in individuals younger than 1 year and cement layer analysis in canines in older individuals.A strong positive relationship between the sum of closure level scores (SCS) and age (males: r = 0.8797, females: r = 0.8825) was recorded, which was stronger than that for the relationship between age and condylobasal length (CBL, males: r = 0.7085, females: 0.7086) (all p-values <0.0001). In adult individuals (≥5 years), CBL was higher in males than females (p < 0.0001), while SCS did not significantly differ between the two sexes (p = 0.148). Our findings show that the analysis of sutural/synchondrosal closure is a valid alternative for age estimation in harbour seals, thereby confirming results of previous studies on other pinniped species.

The tympanic region of the skull in extant pinnipeds: A pilot study of auditory morphological disparity using linear morphometric principal component analysis

“…Within museum collections it can be difficult to always know the exact age of an individual, so for mammalian cranial material, looking at the stasis of suture closures, especially the basioccipital suture, can be a good indicator of at least sexual maturity (Jones & Goswami, 2010; Kahle et al, 2023; Kardong, 2009). The average age of sexual maturity for pinniped males and females ranges from 3 to 8 years of age, but they do not reach full adult size until closer to 8–10 years old, depending on the species (Kahle et al, 2023; King, 1983; Ridgeway & Harrison, 1981; Rosas et al, 1993). Even if these pinnipeds are not full‐sized adults, the sexually mature individuals present similar cranial proportions to adults, therefore distinguishing them from juvenile morphology.…”

“…Even if these pinnipeds are not full‐sized adults, the sexually mature individuals present similar cranial proportions to adults, therefore distinguishing them from juvenile morphology. In this study, adults are individuals who have completely fused sutures (Table S1), subadults are estimated to be sexually mature individuals that may have varying degrees of suture closure (stages 2 and 3; Kahle et al, 2023), and juveniles are sexually immature individuals with open sutures (stage 1; Kahle et al, 2023) and different cranial proportions than their adult counterparts. Adults and subadults were deemed appropriate to include in the PCA.…”