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

Maria Carla De Francesco,Anna Loy

doi:10.1371/journal.pone.0164287

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.

Highlights

Since Darwin [1] sexual dimorphism and ontogenetic allometry have fascinated researches for their various evolutionary and ecological implications, ranging from utilization of different niches by sexes or age classes to avoid intraspecific competition, to the alteration of environmental and predation pressures, or to a combination of these factors [2, 3, 4, 5, 6, 7, 8, 9, 10]
Investigating sexual dimorphism and ontogenetic allometry of dolphin skull and mandible may reveal specific adaptive traits and selective pressures related to reproductive strategies, intraspecific competition, or niche shifting during growth, and on how these factors interact during growth
ANOVA, Procrustes ANOVA, and MANOVA of Procrustes coordinates confirmed a significant difference between sexes in the occipital projection of the North Sea population living in monospecific association (Table 3)

Summary

Introduction

Since Darwin [1] sexual dimorphism and ontogenetic allometry have fascinated researches for their various evolutionary and ecological implications, ranging from utilization of different niches by sexes or age classes to avoid intraspecific competition, to the alteration of environmental and predation pressures, or to a combination of these factors [2, 3, 4, 5, 6, 7, 8, 9, 10]. One of most challenging topic in evolutionary and conservation biology is to investigate if and how sexual dimorphism and ontogenetic allometry interact during growth in response to specific selective pressures [31, 32, 33, 34, 35] This is critically important for understanding if and how resources exploitation varies according to sex or age, and how these intraspecific patterns are eventually affected by reproductive cycles or interspecific interactions [21, 26, 36]. Investigating sexual dimorphism and ontogenetic allometry of dolphin skull and mandible may reveal specific adaptive traits and selective pressures related to reproductive strategies, intraspecific competition, or niche shifting during growth, and on how these factors interact during growth

Methods

Results

Conclusion