Morphological and Sensory Innovations for an Aquatic Lifestyle

Abstract

Sirenians have evolved novel innovations relative to other terrestrial and marine mammals for life as aquatic herbivores. The study of their natural history and adaptations provides insights into the range of possibilities in mammalian evolution, including their ethology and behavioral ecology. Their large body size accommodates an expanded digestive system necessary to process the large amounts of food  ingested, and it also confers thermal advantages and protection from predation. Other thermoregulatory adaptations include a divergent blubber arrangement, dense heavy skin, and a series of counter-current heat exchangers to balance both heat loss and heat gain. Due to their herbivorous niche in relatively shallow environments, sirenians have evolved an unusual skeletal system and arrangement of their lungs, diaphragm, and digestive system, and a re-arrangement of the thoracic and abdominal cavities, that enable easy transitions from the benthic substrate, where their food is often located, to the surface where air is inhaled. Their hydrostasis allows for precise control over buoyancy, which in turn reduces energetic costs of movement. Their mode of food acquisition involves both the sensory and motor functions of facial vibrissae. This muscular-vibrissal complex is capable of numerous varied and detailed movements due to the hypertrophy of muscles into a muscular hydrostat or shortened elephantine trunk-like muzzle. Their pachyosteosclerotic bones , that function so well as part of their buoyancy control and hydrostasis system, are also brittle, like a ceramic material, and prone to fracture. Sirenian perception of the aquatic environment is largely through somatosensation (touch and hydrodynamic reception) and hearing, although vision and taste (chemoreception) are also important to some degree. Sirenians are one of a few mammalian groups in which all hairs on the body are sensory hairs that mediate exquisitely sensitive hydrodynamic reception that is analogous to the function of the lateral line system in fish and amphibians. This mode of reception likely plays a role in sirenian spatial orientation and navigation. Novel structures in the brain (Rindenkerne) are likely responsible for information processing of touch and hydrodynamic reception and provide a substrate for multi-modal sensory perception capabilities. Rindenkerne may also play a key role in mediating interesting behaviors such as synchronous breathing of groups when sleeping. These innovations are important in terms of sirenian conservation and form the basis for much of the sirenian ethology and behavioral ecology observed. Sirenians are a special group of mammals with unusual and interesting morphological and sensory innovations for aquatic life that we are just beginning to understand and explore.