Age Classes and Sex Differences in the Skull of the Mediterranean Monk Seal, Monachus monachus (Hermann, 1779). A study Based on Bone Shape and Density

Abstract

This study analyzes morphometrically 17 skulls of the Mediterranean monk seal Monachus monachus housed in different Italian Museums and collections. We considered several morphometric variables (31 linear, 1 volumetric and 1 surface area measurements). In addition, we identified, measured and compared two nonmorphometric variables, namely, the bone densities of selected areas obtained using a dual-energy X-ray absorptiometry (DXA) device. The high correlation coefficient of all variables indicated continuous growth with the onset of age. The ranking of the hierarchical cluster analysis identified the presence of three main groups containing individuals of similar sizes: lactating pups and yearlings; subadult individuals and adult females; and adult males. Smaller groups were identified within these clusters, and their respective allocations into two subgroups were argued on the basis of skull development and other factors. The discriminant analysis of the three main groups indicated a discriminant diagnostic key, based on condilobasilar length (CBlr-L); maximum mandibular branch height (MB-H); and surface area of the bulla tympanica. The proposed diagnostic key is useful to classify monk seal skulls of unidentified age and sex. The data reported here suggest that in this species certain adult skull growth features (enhanced tympanic bullae surface area extension, occipital bone density) are sexually dimorphic and possibly related to specific anatomical functions. These functions may include an enhanced auditory capacity; an increased development of the cranial musculature capable of supporting a large skull and guaranteeing the mandibular strength necessary for mastication; and male to male social interactions. Anat Rec, 292:544-556, 2009. V V C 2009 Wiley-Liss, Inc.

Early Evolution of Sexual Dimorphism and Polygyny in Pinnipedia

“…Cranial dimorphism in pinnipeds has been previously discussed only qualitatively (Howell ), or treated quantitatively with a particular focus on otariid (Brunner et al ; de Oliveira et al ; Sanfelice and de Freitas ) or phocid species (Mo et al ), or with a broader taxonomic sampling using smaller sample sizes and/or fluctuating landmark selection (Jones and Goswami , ). This study builds on previous studies to show which traits tend to be most dimorphic, and would be most reliable for identifying dimorphism in fossil taxa.…”

Neurometric evaluations on the brain of the opossum (didelphys marsupialis cancrivora (linnaeus 1758) - a case for cognitive skill -brain development capacity

“…The study utilized a total of eleven (11) animals, consisting of seven (7) females and four (4) males, out of which 3 (three) were the young (2 (two) females and 1 (male) identified by the pelage and dental eruptions (GIULIA, ZOTTI, AGNESSI et al, 2009) collected from peasant hunters. Total of thirteen (13) measurements were taken on the head, brain and skull with some assessed landmarks depicted on Figures 1-4.…”

“…The thawed out frozen heads were weighed on an electronic high precision strain guage sensor, digital weighing scale and (©MY WEIGH i201). Repeated weighing on ice was carried out and the mean recorded (GIULIA, ZOTTI, AGNESSI et al, 2009).…”

Use of Dual-Energy X-Ray Absorptiometry...

“…Additionally, DXA has been applied to wildlife such as the grizzly bear (Felicetti, 2003 as cited in Stevenson & van Tets, 2008). DXA has been used to analyze excised bones of several species of marine mammals including Blainville's beaked whale , Mediterranean monk seals (Mo et al, 2009), and bottlenose dolphins (Lucic et al, 2010). Finally, DXA has been applied to domestic pets including cats (Buelund et al, 2011;Turner et al, 1995b), rabbits (Castaneda et al, 2006;Hanafusa et al, 1995), guinea pigs (Fink et al, 2002), and dogs (Lorinson, 2009;Markel & Bogdanske, 1994a, 1994bMawby et al, 2004;Schneider et al, 2004;Toll et al, 1994;Zotti et al, 2004a).…”