What Is a Nocturnal Animal? Equipment for a World Without Light

Night vision trades detail for light-gathering and the anatomy shows the trade throughout. Eyes are larger relative to the head, since a bigger aperture collects more light, and pupils open wider, with slit pupils in several species allowing a greater range between fully open and fully closed than a round pupil can. The retina is dominated by rods rather than cones, which sacrifices colour and acuity for sensitivity, and rod signals are pooled so that many detectors feed one output, which further increases sensitivity at the cost of resolution. A reflective layer behind the retina, the tapetum lucidum, bounces unabsorbed light back through the sensors for a second chance, which is what produces eyeshine when a torch is directed at a fox or a cat and which improves sensitivity while blurring the image slightly. Some species have exceptionally light-sensitive vision, and behavioural experiments have shown certain nocturnal insects and vertebrates discriminating colour by starlight, which is well beyond human capability.

Vision is supplemented or replaced and the alternatives are diverse:

• •Echolocation in bats and in some other mammals and birds, emitting sound and reading the returning echoes, which supplies detail in complete darkness that vision could not
• •Enlarged and asymmetrically placed ears in owls, which allow a sound to be located vertically as well as horizontally and permit hunting by hearing alone
• •Whiskers, which map surroundings by touch and are actively swept in several rodents
• •Infrared sensing in pit vipers and some other snakes, detecting the heat of warm-blooded prey through dedicated organs
• •Highly developed olfaction, which many nocturnal mammals rely on for navigation, foraging and communication
• •Electroreception in some aquatic species, which works regardless of light entirely

The benefits explain why the strategy has evolved so many times. Predator avoidance is the strongest, since diurnal hunters cannot follow, and many species have shifted towards nocturnality specifically in response to a predator, which is documented experimentally. Heat and water conservation matter enormously in arid regions, where being active in cool darkness reduces water loss dramatically, and many desert animals are nocturnal for that reason alone. Competition avoidance allows two species with similar diets to coexist by partitioning time rather than space. Prey availability drives it where the food itself is nocturnal. And avoiding humans has become a major driver, with a substantial study finding that mammals worldwide have become measurably more nocturnal in response to human presence, increasing night activity by around a third on average, which is a rapid and widespread behavioural shift with consequences that are not yet clear.

Strictly nocturnal and strictly diurnal are the ends of a range and most animals sit somewhere along it, with named categories for the middle:

• •Crepuscular, active at dawn and dusk, which is where a great many mammals actually fall and which catches the low light that suits eyes adapted for either extreme
• •Matutinal and vespertine, distinguishing morning from evening activity, which matters because the two differ in temperature and in who else is about
• •Cathemeral, active irregularly across the whole cycle, documented in several primates and increasingly recognised as common
• •Seasonally shifting patterns, with species switching between day and night activity as temperature, daylength or predation pressure changes
• •Facultative shifts driven by disturbance, where a normally diurnal species becomes nocturnal near human activity and reverts where it is absent
• •Individual variation within a population, which is substantial and which means describing a species by one category loses real information

Night-adapted animals encounter a problem their evolution did not anticipate. Artificial light disrupts navigation in species that orient by natural light cues, which is why moths circle lamps and why turtle hatchlings move inland instead of towards the sea. It alters predator and prey balance, since illuminated areas advantage sighted predators and disadvantage animals relying on darkness for cover. It shifts activity timing and suppresses hormones governing daily and seasonal cycles. It fragments habitat functionally, since a lit road can be as effective a barrier to a light-averse species as a physical one. Bats show species-specific responses, with some exploiting insects gathered at lights and others avoiding lit areas entirely, which changes which species can use a landscape. The remedies are the same as for light pollution generally, meaning shielding, reduced intensity, warmer colour temperature and switching off, and they are effective immediately because the disturbance stops when the light does.

Larger eyes, wider pupils, rod-dominated retinas with pooled signals and a reflective layer behind the retina all trade detail for sensitivity, which is what produces eyeshine. Echolocation, asymmetric ears, whiskers and infrared sensing supply what vision cannot. Mammals worldwide have become measurably more nocturnal in response to human presence, and artificial light disrupts exactly the species best adapted to darkness.