Many living systems send auditory signals to communicate with others, including to attract, announce, or warn. These sounds must be audible to the intended recipient in a variety of conditions, such as in wind, water, and solids. As a result, each living system has specific sounds best adapted to its environment. For example, some birds live in loud habitats, like those alongside noisy streams or in windy areas. These birds use sound frequencies that can be heard by other birds over competing environmental noises.
For living systems, sensing sound and other vibrations is important for communicating and detecting conditions within their environment. Living systems must locate a signal’s source so that they can move toward it (such as when it is food or a potential mate) or away from it (such as when it is a predator). To prompt an appropriate response, living systems must sense these signals, recognize their amplitude or volume (which is sometimes very low), and determine their direction. Living systems must be attuned to signals relevant to them and able to distinguish these from irrelevant sounds to avoid expending unnecessary energy. For example, owls’ ears are asymmetrically placed. This enables them to detect sounds more accurately, which helps them locate small prey at night and avoid wasting energy chasing down irrelevant sounds.
Perceiving motion is important for a living system to sense where it is in relation to a moving environment, which is critical in locating resources or wayfinding. This applies whether the environment itself is in motion (such as water movement coming from a nearby fish) or the living system is moving within a stationary environment (such as a bird flying through the air). Because motion dampens over distance and the cost of missing those motion signals is high, living systems must be quite sensitive to these signals. For example, fast‑flying big brown bats have microscopic, stiff, domed hairs on their wing membranes that act as a sensor array to monitor flight speed and airflow conditions.
Living systems must identify other living systems and objects to navigate, feed, escape predators, find resources, and more. The ability of living systems to “see” varies widely, with “seeing” including not only sight, but other means of sensing shapes or patterns, such as smell or echolocation. For example, a hawk can see much more detail than humans while other organisms can see far less detail. Nevertheless, each living system is capable of detecting shapes and patterns to the extent it needs to for survival. In addition to perceiving physical shapes or patterns, living systems can also sense landscape patterns at various scales. The purple sea urchin, for example, does not have eyes, yet can pick out fine details in its environment by using its entire surface as a compound eye. The urchin’s spines shield light coming from wide angles, further refining its focus.
Although free from many of the physical obstructions found on land and in water, organisms that move through the air must still avoid hitting each other and objects in their flight path, such as trees and mountains. They also need to navigate from one place to another, which presents a different challenge. They use one strategy to navigate around obstructions and another to move towards their destination. For example, bats use echolocation to detect both obstacles and prey. To navigate during migration, bats also use vision, sun orientation, and likely other strategies.
Class Mammalia (“breast”): Bats, cats, whales, horses, humans
Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.
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