Smooth Brains and Sensitive Whiskers: How Manatees Understand Their World

Manatees (Trichechus) are large, slow-moving animals whose outward appearance may seem unremarkable. Yet their nervous system and sensory organization form one of the most unusual combinations found among mammals. Compared with marine mammals that have been extensively studied—such as dolphins and sea lions—research on manatee cognition and behavior remains relatively limited. Much of this limitation stems from their conservation status, which makes controlled behavioral experiments difficult to conduct. Nevertheless, decades of neurobiological work have gradually revealed how manatees perceive their environment, navigate vast aquatic landscapes, and process sensory information through a brain that differs markedly from that of other marine mammals. To understand how manatees think, one must begin with their sensory systems and brain structure.

West Indian manatee（Trichechus manatus）（Image source：Reid, Jim P, U.S. Fish and Wildlife Service，CC0 1.0 ）

Manatees belong to the order Sirenia, a lineage of aquatic herbivorous mammals that includes three living species of manatees and one species of dugong. Among these five known species, four survive today, while Steller's sea cow disappeared in the eighteenth century due to human hunting. Sirenians represent a distinctive evolutionary branch of mammals, closely related to terrestrial elephants yet adapted to life in water. Their evolutionary path has produced a suite of unusual traits, including a large body supported by an expanded digestive system, a low metabolic rate, paddle-shaped tails, reduced hind limbs, and slow movement patterns. Their brains possess one of the lowest relative brain-to-body ratios among mammals and are characterized by a strikingly smooth cerebral surface known as lissencephaly. In addition, their sensory systems emphasize hearing and touch while visual and chemical senses are comparatively reduced.

Facial sensory hairs of the West Indian manatee（Image source：T.Voekler， CC BY-SA 3.0 ）

One of the most remarkable features of manatees is their tactile sensory system. Unlike most mammals, which possess both insulating fur and specialized whiskers, manatees retain only sensory hairs across their bodies. More than five thousand of these hairs cover the skin, with roughly two thousand located on the head and face. Each hair follicle is encased in dense connective tissue and contains multiple mechanoreceptors that are supplied by abundant nerve fibers. This organization allows the hairs to detect minute disturbances in the surrounding water. Experiments show that manatees can perceive hydrodynamic changes as small as nanometer-scale displacements when water flows toward the snout. Such sensitivity enables them to interpret water movement, locate objects, and potentially detect environmental boundaries through subtle fluid dynamics.

Facial hairs serve several functions beyond hydrodynamic detection. In active tactile exploration, manatees use them to discriminate textures with a level of precision comparable to the human fingertip. The largest hairs around the mouth work together with facial muscles to grasp and manipulate vegetation before guiding it into the mouth—a behavior termed “oripulation.” Correspondingly, the brain devotes extensive neural resources to processing tactile information from these hairs. Large clusters of neurons within the somatosensory cortex appear to represent individual sensory hairs, suggesting a highly detailed neural map of the body's tactile surface.

In addition to touch, hearing plays a major role in the manatee sensory world. Behavioral studies demonstrate that manatees can detect sounds across a wide frequency range from approximately 0.25 to 72 kilohertz. They can also determine sound direction, process rapid temporal changes in acoustic signals, and maintain sensitivity even in noisy environments. Although their auditory abilities do not reach the specialized level of echolocating dolphins, they nevertheless exceed the hearing performance typical of most terrestrial mammals.

Vision, by contrast, is relatively modest. Manatees possess dichromatic color vision, meaning their retinas contain only two types of cone photoreceptors. Their underwater visual acuity is about twenty arc minutes, significantly lower than that of dolphins or sea lions. However, color perception may partially compensate for this limited spatial resolution by helping them distinguish objects and environmental features. Chemical senses appear to play a more limited role. Field observations suggest that manatees may use taste and possibly weak olfactory cues to identify individuals, locate freshwater sources, or detect environmental signals, though rigorous psychophysical experiments are still lacking.

Despite this uneven distribution of sensory strengths, manatees display impressive navigational abilities in the wild. Individuals regularly travel across large open-water regions such as Florida's Big Bend and maneuver through the maze-like channels of mangrove ecosystems. Amazonian manatees (Trichechus inunguis) inhabit river systems that change dramatically with seasonal flooding, requiring them to remember routes and adapt to shifting waterways. These behaviors imply the presence of long-term spatial memory and flexible orientation skills. Some researchers suspect that young manatees learn migration routes and habitat knowledge socially from their mothers or other group members.

Curiously, this navigational competence appears despite relatively modest hippocampal structures in the manatee brain. The hippocampus and parahippocampal gyrus—regions typically associated with spatial memory—are not particularly large compared with those of other mammals. This has led to the hypothesis that manatees may rely on alternative mechanisms for orientation, perhaps integrating tactile information from their sensory hairs with environmental cues such as water flow, boundaries, or subtle physical gradients in their surroundings.

The brain itself presents one of the most intriguing aspects of manatee biology. Adult manatee brains weigh roughly 360 grams—about the size of a grapefruit. Although this is large in absolute terms, it is small relative to the animal's massive body. For many years researchers assumed that relative brain size predicted cognitive ability across species. However, more recent studies indicate that such comparisons are oversimplified. Neural density, connectivity, and internal organization may offer more meaningful insights into cognition than simple brain-to-body ratios.

Perhaps the most striking anatomical feature is the extraordinary smoothness of the manatee brain. The cerebral hemispheres lack the complex folds and grooves—gyri and sulci—commonly found in large mammalian brains. This condition, known as lissencephaly, is extremely unusual for an animal with such a large brain. Among mammals exceeding 100 grams in brain weight, manatees represent one of the most extreme examples of a smooth cortex. Fossil evidence suggests that this feature has been present throughout the evolutionary history of sirenians.

Comparison of the brains of 27 mammalian species; the manatee brain (Manatee) is located toward the upper left（Image source：Cook PF et al. (2025)， CC BY 4.0 ）

Why would such a large brain remain smooth rather than folded? One explanation relates to the thickness of the cerebral cortex. In many mammals, cortical folding results from lateral expansion during development. Manatees possess a comparatively thick cortex, which may reduce the tendency for the surface to fold. Another possibility involves developmental processes associated with basal radial glial cells, which regulate cortical expansion during embryonic growth. If these cells occur at lower densities in sirenians, the cortex may thicken without expanding laterally enough to produce folds.

Another factor may involve the tension created by long-range neural connections. When cortical regions are strongly interconnected, axonal growth can generate mechanical forces that pull related areas together and form folds. If long-distance connections develop later or remain relatively sparse, the forces required to generate gyri may never arise. These possibilities have led researchers to investigate patterns of brain connectivity in manatees.

Recent studies using diffusion tensor imaging on post-mortem manatee brains have begun to reveal how neural pathways are organized. Compared with the brains of California sea lions (Zalophus californianu), manatee brains appear to show simpler patterns of corticocortical connectivity. Neural pathways often form concentric arrangements rather than the more complex network modules seen in sea lions. Connections between the two hemispheres also appear more limited, while pathways within each hemisphere tend to remain local rather than spanning long distances.

These structural patterns do not necessarily indicate inferior cognitive capacity. Instead, they may reflect the ecological demands of manatee life. Manatees have relatively simple social structures, predictable feeding behavior, and sensory systems centered primarily on touch and hearing. Their brains may therefore require less extensive long-range integration across multiple sensory modalities than the brains of more socially complex marine mammals.

Even with this seemingly simplified architecture, manatees still demonstrate clear learning abilities and impressive navigation in natural environments. Many aspects of their cognition—including problem solving, imitation, sequential learning, and symbolic understanding—have yet to be formally studied. Future research may reveal how their distinctive neural organization supports memory, decision-making, and sensory integration in aquatic habitats.

Corticocortical connectivity in the brains of the manatee (top) and California sea lion (bottom)（Image source：Cook PF et al. (2025)， CC BY 4.0 ）

Manatees remind neuroscientists that intelligence and perception can arise through diverse neural designs. Their smooth brains, extensive tactile systems, and unusual patterns of neural connectivity illustrate how evolution can shape cognition in ways that differ greatly from familiar models such as primates or dolphins. Understanding how these gentle herbivores interpret their watery world may ultimately broaden our understanding of how brains evolve to meet the demands of different ecological niches.

Author: Shui-Ye You

Reference:

Cook PF et al. (2025). Manatee cognition and behavior: a neurobiological perspective on an unusual constellation of senses and a unique brain. Frontiers in Behavioral Neuroscience.