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 herbivoresHerbivore. The study of their natural history and adaptations provides insights into the range of possibilities in mammalian evolutionEvolution, including their ethology and behavioral ecology. Their large body sizeBody size accommodates an expanded digestive system necessary to process the large amounts of foodFood ingested, and it also confers thermalThermal advantages and protection from predationPredation. Other thermoregulatory adaptations include a divergent blubberBlubber arrangement, dense heavy skin, and a series of counter-current heat exchangers to balance both heat lossLoss, 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 benthicBenthic substrate, where their foodFood is often located, to the surface where air is inhaled. Their hydrostasisHydrostasis allows for precise control over buoyancyBuoyancy, which in turn reduces energetic costs of movement. Their mode of foodFood acquisition involves both the sensory and motor functions of facial vibrissaeVibrissae. This muscular-vibrissal complexMuscular-vibrissal complex is capable of numerous varied and detailed movements due to the hypertrophy of muscles into a muscular hydrostatMuscular hydrostat or shortened elephantine trunk-like muzzle. Their pachyosteosclerotic bonesBone pachyosteosclerotic, that function so well as part of their buoyancyBuoyancy control and hydrostasisHydrostasis system, are also brittle, like a ceramic material, and prone to fracture. Sirenian perceptionPerception of the aquatic environment is largely through somatosensation (touch and hydrodynamic receptionHydrodynamic reception) and hearing, although vision and tasteTaste (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 receptionHydrodynamic reception that is analogous to the function of the lateral line system in fishFish and amphibians. This mode of reception likely playsPlay a role in sirenian spatial orientation and navigationNavigation. Novel structures in the brainBrain (RindenkerneRindenkerne) are likely responsible for information processing of touch and hydrodynamic reception and provide a substrate for multi-modal sensory perceptionPerception capabilities. RindenkerneRindenkerne may also playPlay a key role in mediating interesting behaviors such as synchronous breathingBreathing synchronous 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.

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