Intra- and Interspecific Interactions as Proximate Determinants of Sexual Dimorphism and Allometric Trajectories in the Bottlenose Dolphin Tursiops truncatus (Cetacea, Odontoceti, Delphinidae)

Abstract

Feeding adaptation, social behaviour, and interspecific interactions related to sexual dimorphism and allometric growth are particularly challenging to be investigated in the high sexual monomorphic Delphinidae. We used geometric morphometrics to extensively explore sexual dimorphism and ontogenetic allometry of different projections of the skull and the mandible of the bottlenose dolphin Tursiops truncatus. Two-dimensional landmarks were recorded on the dorsal, ventral, lateral, and occipital views of the skull, and on the lateral view of the left and the right mandible of 104 specimens from the Mediterranean and the North Seas, differing environmental condition and degree of interspecific associations. Landmark configurations were transformed, standardized and superimposed through a Generalized Procrustes Analysis. Size and shape differences between adult males and females were respectively evaluated through ANOVA on centroid size, Procrustes ANOVA on Procrustes distances, and MANOVA on Procrustes coordinates. Ontogenetic allometry was investigated by multivariate regression of shape coordinates on centroid size in the largest homogenous sample from the North Sea. Results evidenced sexual dimorphic asymmetric traits only detected in the adults of the North Sea bottlenose dolphins living in monospecific associations, with females bearing a marked incision of the cavity hosting the left tympanic bulla. These differences were related to a more refined echolocalization system that likely enhances the exploitation of local resources by philopatric females. Distinct shape in immature versus mature stages and asymmetric changes in postnatal allometry of dorsal and occipital traits, suggest that differences between males and females are established early during growth. Allometric growth trajectories differed between males and females for the ventral view of the skull. Allometric trajectories differed among projections of skull and mandible, and were related to dietary shifts experienced by subadults and adults.

Geometric morphometric methods as complementary tools to investigate variability in common dolphins (Delphinus sp.) using museum specimens

“…Pillai trace, Wilks’ lambda, Hotelling's T 2 , and Roy's greatest root) were concordant in returning a highly significant effect of area ( P < 0.0001 for all tests), no significant effects of sex ( P > 0.5 for all tests), and a significant effect of area and sex interaction ( P ranging between 0.014 and 0.020). These outcomes suggest the likely existence of SShD in selected populations (Cheng & Kuntner, 2015), similar to recent findings in T. truncatus (De Francesco & Loy, 2016).…”

“…Although dolphins are characterized by a general absence of sexual dimorphism (Ralls, 1977;Ralls & Mesnick, 2002), the results obtained on common dolphins confirm the existence of SSD, with males larger than females, as observed by and Perrin (1984). Additionally, the results of MANOVA on shape coordinates indicated a significant interaction between marine area and sex, suggesting the possible occurrence of SShD in selected populations (Cheng & Kuntner, 2015), similar to the recent findings in T. truncatus showing SShD in populations living in monospecific associations (De Francesco & Loy, 2016). These results deserve further investigation in terms of the role of size and shape in sexual dimorphism in the common dolphin.…”

“…Moreover, male dolphin skulls continue to grow along the same allometric trajectory as females, reaching a final stage with more marked traits; for example, a more compressed braincase and longer rostral region. Similar ontogenetic scaling and traits were also described in T. truncatus by De Francesco and Loy (2016), suggesting that the pattern of allometric growth is preserved throughout the Delphininae subfamily. Based on Gould (1966) and Weston (2003), this pattern is indicative of change in the duration of growth rather than changes in prenatal development or novel modes of postnatal growth.…”

“…Geometric morphometrics is considered a preferred tool of integrated taxonomy (Cardini et al, 2009; Loy, 1996, 2007), and often shows a strong correlation with genetic differentiation (Heyning & Perrin, 1994; Natoli, Birkun, Aguilar, Lopez, & Hoelzel, 2005; Natoli, Peddemors, & Hoelzel, 2004; Stockin, Amaral, Latimer, Lambert, & Natoli, 2014). Following evidences in Tursiops truncatus (De Francesco & Loy, 2016) and Stenella coeruleoalba (Loy et al, 2010), our analyses were also aimed at assessing any phenotypic differentiation of the rare and endangered Mediterranean stock, that could support the existence of a distinct MU, as suggested by mitochondrial DNA and nuclear markers (Natoli et al, 2008). Following the fate of most Mediterranean biodiversity (Bianchi & Morri, 2000), the common dolphin has experienced a drastic decline over the last 40–50 years in the Mediterranean Sea (Bearzi et al, 2003).…”

Effects of Monetary Incentives in Physician Groups: A Systematic Review of Reviews

“…Aviki et al [ 17 ] focused on oncological care, and their review indicated positive effects but did not provide sufficient evidence. For example, one of the studies [ 34 ] showed an increase in guideline adherence but only for two of the five types of cancer analyzed. Another study [ 35 ] discovered a reduction in hospitalization and radiotherapy, but the cost of chemotherapy drugs increased.…”