Head adaptation for sound production and feeding strategy in dolphins (Odontoceti: Delphinida)

Abstract

Head morphology in toothed whales evolved under selective pressures on feeding strategy and sound production. The postnatal development of the skull (n = 207) and mandible (n = 219) of six Delphinida species which differ in feeding strategy but exhibit similar sound emission patterns, including two narrow-band high-frequency species, were investigated through 3D morphometrics. Morphological changes throughout ontogeny were demonstrated based on the main source of variation (i.e., prediction lines) and the common allometric component. Multivariate trajectory analysis with pairwise comparisons between all species was performed to evaluate specific differences on the postnatal development of skulls and mandibles. Changes in the rostrum formation contributed to the variation (skull: 49%; mandible: 90%) of the entire data set and might not only reflect the feeding strategy adopted by each lineage but also represents an adaptation for sound production and reception. As an important structure for directionality of sound emissions, this may increase directionality in raptorial feeders. Phylogenetic generalized least squares analyses indicated that shape of the anterior portion of the skull is strongly dependent on phylogeny and might not only reflect feeding mode, but also morphological adaptations for sound production, particularly in raptorial species. Thus, postnatal development seems to represent a crucial stage for biosonar maturation in some raptorial species such as Pontoporia blainvillei and Sousa plumbea. The ontogeny of their main tool for navigation and hunting might reflect their natural history peculiarities and thus potentially define their main vulnerabilities to anthropogenic changes in the environment.

The ontogeny of asymmetry in echolocating whales

“…Using NBHF sounds might be advantageous for navigating the complex environment that surrounds them [39], and it also allows these taxa to avoid predation by large predators such as killer whales, which do not hear sounds at such high frequencies [4,39]. Nevertheless, they employ a variety of feeding strategies, from the specialized suction feeder Kogia to the raptorial feeder Pontoporia [4,39,40]. Morphologically, while they convergently evolved similar inner ear shapes in order to use this specialized type of echolocation [4], they do not consistently display similar skull traits.…”

“…They prevalently inhabit coastal and riverine environments, with the exception of Kogiidae [4]. Using NBHF sounds might be advantageous for navigating the complex environment that surrounds them [39], and it also allows these taxa to avoid predation by large predators such as killer whales, which do not hear sounds at such high frequencies [4,39]. Nevertheless, they employ a variety of feeding strategies, from the specialized suction feeder Kogia to the raptorial feeder Pontoporia [4,39,40].…”

Rostrum abnormalities in the endangered Indian Ocean humpback dolphin (Sousa plumbea) in South Africa

“…These species present distinct patterns of rostrum morphology, but they both depend on postnatal ontogeny for its maturation (Frainer et al. 2015, 2021). Humpback dolphins present a peculiar anteriorization (i.e.…”

“…2016; Frainer et al. 2021) and for prey capture (Werth 2006). Consequently, wounds and/or congenital anomalies of this structure could directly affect the feeding strategies and abilities of individuals.…”

Evolutionary drivers of morphological differentiation among three bottlenose dolphin lineages,Tursiopsspp. (Delphinidae), in the northwest Indian Ocean utilizing linear and geometric morphometric techniques

“…Head shape has also been proposed to be related to adaptation for efficient echolocation (eg. Frainer et al 2021). Changes to the orbit of the AS-Ta lineage might also be a result of living in a more turbid environment, again, the extreme being found in river dolphins, which have small eyes (Herald et al 1969).…”