Cathemeral describes an animal whose activity is spread across both day and night rather than being confined to one or the other. Unlike diurnal species (active by day) or nocturnal species (active by night), a cathemeral animal may forage, travel, and socialize during any hour of the 24-hour cycle. The term was coined after field researchers in Madagascar noticed a lemur species that simply refused to stick to a schedule, and what was once considered a quirky exception has turned out to be surprisingly common across the animal kingdom.
Where the Term Came From
The word “cathemeral” traces back to fieldwork on Mayotte Island in the Comoros archipelago during the mid-1970s. Primatologist Ian Tattersall observed that the Mayotte brown lemur spread its activity almost evenly across day and night. By a rough measure, the daytime activity-to-rest ratio averaged about 0.27, while the nighttime ratio averaged about 0.28, meaning the lemurs were essentially equally likely to be active or resting regardless of whether the sun was up.1PubMed. The concept of cathemerality: history and definition Tattersall built the word from Greek roots meaning “through the day,” and though he came up with it in 1978, it did not appear in a formal publication until 1987-88.
For years, cathemerality was treated as a footnote in behavioral ecology, assumed to be a rare oddity confined to a few lemur species. That perception has shifted dramatically. Researchers now recognize cathemeral activity patterns in arthropods, fish, birds, and a wide range of mammals, and a growing body of work argues the pattern deserves a permanent seat alongside “diurnal” and “nocturnal” as a standard category for classifying how species use time.2PubMed. Cathemerality: a key temporal niche
Why Animals Adopt a Round-the-Clock Schedule
The appeal of cathemerality boils down to flexibility. An animal that can shift its activity into daytime or nighttime hours depending on conditions has options that a strictly diurnal or nocturnal species does not. Three major pressures have been proposed as drivers: temperature, food availability, and predation risk. In practice, all three interact, and studies on different species have found inconsistent results about which one matters most.3PubMed. Why be diurnal? Or, why not be cathemeral?
Diet quality, though, has emerged as a particularly strong predictor in at least some cathemeral primates. Mayotte brown lemurs, for example, shift heavily toward nighttime activity during the dry season, when fruit quality drops. Sugar and protein content of their food decreases while fiber increases, so they appear to compensate by stretching their foraging across more hours.4Folia Primatologica. Cathemerality in the Mayotte Brown Lemur (Eulemur fulvus): Seasonality and Food Quality A comparative study of two brown lemur populations found that diet quality was the only factor with a statistically significant effect on how diurnal or nocturnal the animals were, supporting the idea that metabolic needs tied to nutrition can push an animal into cathemeral scheduling.5Animal Behaviour. Proximate and ultimate determinants of cathemeral activity in brown lemurs
Temperature and the Cost of Staying Warm
For warm-blooded animals, being active during cold periods costs extra energy because the body has to work harder to maintain its internal temperature. The flip side is that resting during warm periods saves energy. This logic, sometimes called the circadian thermoenergetics hypothesis, predicts that cathemeral animals should shift more of their activity into daylight hours during cold seasons, taking advantage of solar warmth while resting through frigid nights.
Azara’s owl monkeys in the Argentine Chaco are a textbook case. These monkeys are active during both day and night, but their daytime behavior changes strikingly with the seasons. From the warmest to the coldest months, their daytime resting dropped by about 31 percent while daytime foraging shot up by roughly 131 percent. In general, daytime activity increased as temperatures fell seasonally.6Animal Behaviour. Thermoenergetic challenges and daytime behavioural patterns of a wild cathemeral mammal So while these monkeys are cathemeral year-round, the balance between their daytime and nighttime activity is not fixed. It tilts with the thermometer. Cold winters push more activity into daylight; mild summers allow them to spread their effort more evenly.
How Moonlight Shapes the Night Shift
Light availability at night is not constant, and cathemeral animals are remarkably sensitive to lunar cycles. The same Azara’s owl monkeys that adjust to temperature also adjust to the moon. During full-moon nights, their nocturnal activity accounted for about 52 percent of total daily activity, compared with only about 26 percent on new-moon nights. After busy full-moon nights, the monkeys took it easy the following morning, while new-moon nights were typically followed by mornings of higher activity. Researchers found a striking relationship between locomotor activity and light levels, with the animals’ preferred range corresponding to the illumination found at dawn, dusk, and during full moons.7PLOS ONE. Moonstruck Primates: Owl Monkeys (Aotus) Need Moonlight for Nocturnal Activity in Their Natural Environment
Even more dramatically, during lunar eclipses, when the moon’s light was completely shadowed, the monkeys’ nighttime movement nearly stopped. Activity was negatively masked by the absence of light at times when the animals would normally have been at peak nocturnal activity. This strongly suggests that moonlight is not just a convenience for cathemeral species but a prerequisite for their nighttime foraging.
The influence of the moon extends well beyond primates. A study of terrestrial mammals on tropical forest floors, some of the darkest habitats on Earth, found that half of all species studied responded to lunar phases. Some changed how nocturnal they were, others changed their overall activity levels, and some did both. Avoidance of full-moon periods was more common than attraction to them, with about 30 percent of species avoiding the full moon compared to 20 percent that were drawn to it. Nocturnal rodents tended to hide during bright nights, while hoofed mammals were more likely to increase activity.8PubMed Central. The moon’s influence on the activity of tropical forest mammals For species already cathemeral, this lunar sensitivity adds yet another layer of scheduling flexibility.
Eyes Built for Both Worlds
One of the more fascinating questions about cathemeral animals is how their eyes cope with the vastly different light conditions between midday sun and a moonlit forest floor. Strictly nocturnal mammals tend to have large corneas to capture as much light as possible, while diurnal species have smaller ones with features suited to bright environments. Cathemeral lemurs of the genus Eulemur land right in the middle: their relative cornea sizes are intermediate between diurnal and nocturnal primates, and they match well with other cathemeral mammals outside the primate order.9PubMed. Eye morphology in cathemeral lemurids and other mammals This suggests that cathemeral species have evolved eyes that work reasonably well at a broad range of light levels rather than being optimized for one extreme.
At the retinal level, the pattern holds. Nocturnal lemurs have lower densities of cone photoreceptors (the cells responsible for color and detail vision in bright light) and higher densities of rods (the cells that detect dim light). Cathemeral and diurnal lemurs, by contrast, have higher cone densities.10Journal of Comparative Neurology. Diversity of photoreceptor arrangements in nocturnal, cathemeral and diurnal Malagasy lemurs However, the picture is not a perfectly clean three-way split. When researchers look at overall eye shape across mammals more broadly, cathemeral species overlap completely with both nocturnal and diurnal ones. Most diurnal and cathemeral mammals have eye shapes that actually resemble those of nocturnal birds and lizards, a legacy of the evolutionary history of mammals as a group.11PubMed Central. Eye shape and the nocturnal bottleneck of mammals
The practical upshot is that cathemeral animals are not simply nocturnal animals that sometimes wake up during the day, nor diurnal animals pulling occasional all-nighters. Their visual systems reflect a genuine adaptation to functioning across the full light spectrum that a 24-hour cycle delivers.
Not Just Lemurs
The early focus on Malagasy lemurs gave cathemerality a primate-centric reputation, but the pattern is far broader. The same 2023 review that argued for cathemerality’s inclusion as a standard temporal niche documented its presence across arthropods, fish, birds, and mammals well beyond the primate order.12PubMed. Cathemerality: a key temporal niche Among mammals, large cats provide a vivid example. Pumas in Brazil are naturally cathemeral, and research tracking their behavior in protected versus unprotected areas found that pumas in protected areas displayed the cathemeral pattern expected under natural conditions, with activity distributed across the 24-hour cycle.13Global Ecology and Conservation. Effects of anthropogenic disturbance and land cover protection on the behavioural patterns and abundance of Brazilian mammals
In Madagascar, even among lemurs, the degree of cathemerality varies species by species. A study of three lemur species found that both Eulemur species displayed distinct cathemeral activity patterns, with longer travel distances at night than during the day. Meanwhile, Varecia variegata editorum was significantly less active at night, with shorter nocturnal travel distances and a smaller nighttime range.14PubMed Central. Living in the Dark: Exploring the Factors Driving Nocturnal Activity in Three Lemur Species The factors that influenced nocturnal activity, including light levels, temperature, and rainfall, varied across the three species, reinforcing the idea that cathemerality is not one fixed behavior but a flexible strategy tuned by local conditions.
When Human Activity Pushes Animals off Schedule
If cathemerality is about flexibility, then human disturbance is one of the pressures that can bend that flexibility in unfortunate directions. The same Brazilian study that documented natural cathemeral behavior in pumas found that protection status significantly affected the activity patterns of five out of six mammal species studied. Four of those species shifted toward greater nocturnality in unprotected areas. For pumas, the pattern was clear: they showed significantly more nocturnal activity in non-protected areas, suggesting that human presence was pushing a naturally cathemeral animal into a more exclusively nighttime lifestyle.15Global Ecology and Conservation. Effects of anthropogenic disturbance and land cover protection on the behavioural patterns and abundance of Brazilian mammals
Domestic dogs played a measurable role. For giant anteaters, dog abundance was the single most important factor in activity pattern shifts, with nocturnality increasing as dog numbers rose. For agoutis and pumas, loss of native vegetation was the key driver, with nocturnality increasing as natural habitat decreased. The implication is that human encroachment does not simply reduce wildlife populations; it warps their temporal behavior, potentially cutting off their access to daytime resources and forcing them into suboptimal scheduling.
This matters because a cathemeral animal forced into strict nocturnality loses the very flexibility that cathemerality provides. If the evolutionary advantage of being cathemeral is the ability to shift activity toward the most favorable conditions at any given time, then an animal locked into nighttime-only activity by human disturbance is functionally losing an adaptive tool.
Cathemerality in Captivity
Zoo and sanctuary populations offer a revealing window into whether cathemerality is hardwired or environmentally triggered. A study at the Duke Lemur Center tracked 88 lemurs across seven species using wrist-mounted motion loggers. As expected, the strictly diurnal sifaka showed the least nocturnal activity. The three cathemeral Eulemur species showed the most evidence of round-the-clock activity, and their nighttime activity was significantly higher than the sifaka’s. Interestingly, ring-tailed lemurs and ruffed lemurs, species not traditionally classified as cathemeral, also showed some evidence of nighttime activity in captivity, raising questions about whether the boundary between cathemeral and non-cathemeral species is sharper in the wild than in managed environments.16PubMed Central. Activity patterns in seven captive lemur species: Evidence of cathemerality in Varecia and Lemur catta?
The captive setting strips away many of the environmental pressures thought to drive cathemerality: there are no predators, food is provided on a schedule, and temperatures are relatively controlled. The fact that cathemeral species still distribute their activity across day and night under these conditions hints that the behavior has a genuine circadian component, not just a situational one. But the appearance of cathemeral-like patterns in species not known for it in the wild suggests that many animals may have latent capacity for round-the-clock activity that ecological pressures normally suppress.
How Researchers Measure It
Classifying an animal as cathemeral is trickier than it sounds. You cannot just see an animal awake at night once and declare it cathemeral. Researchers use accelerometer-based motion loggers (actigraphy) that record movement minute by minute, then calculate metrics from the data. One approach generates a day-to-night activity ratio: a value of 1.0 means equal activity across day and night (consistent with cathemerality), values above 1 indicate a diurnal bias, and values below 1 indicate a nocturnal bias. Because this ratio can be skewed by extreme values, some researchers also use a bounded cathemerality index scaled from 0 to 1, which places species on a standardized continuum for easier comparison.17Journal of Human Evolution. Are humans cathemeral? The diurnal activity model of Homo sapiens challenged
These measurement tools matter because they allow direct cross-species comparisons. Without a standardized metric, one field team’s “cathemeral” might mean something different from another’s. The day-night ratio has the advantage of being straightforwardly interpretable, while the cathemerality index provides a way to place species along a gradient rather than forcing them into hard categories. Both approaches rely on defining when “day” and “night” begin and end, which is itself nontrivial since twilight periods are biologically active times for many species. Most studies peg the cutoffs to astronomical twilight rather than simple sunrise and sunset.
Could Humans Be Cathemeral?
The question might sound absurd at first. Humans are textbook diurnal animals: we sleep at night, work and forage during the day, and have color vision adapted to sunlight. But the same actigraphy tools used to study lemurs and owl monkeys have recently been turned on humans, and the results are provocative enough that at least one paper has directly challenged the diurnal model of Homo sapiens.18Journal of Human Evolution. Are humans cathemeral? The diurnal activity model of Homo sapiens challenged While the full findings are still being digested by the field, the basic argument is that human activity, measured objectively by minute-by-minute motion data rather than by cultural norms, is more spread across the 24-hour cycle than our self-image as a strictly daytime species would suggest.
This is not entirely surprising in a modern context: shift workers, night-owl chronotypes, and societies where nightlife is central to the culture all routinely show significant nighttime activity. What makes the question scientifically interesting is whether the human capacity for flexible scheduling reflects deeper evolutionary roots, perhaps tied to the same mammalian toolkit that produces cathemerality in other primates. The eye-shape data adds an intriguing layer here: as noted earlier, most diurnal and cathemeral mammals have eye morphologies consistent with a nocturnal evolutionary past, and humans are no exception.
Cathemeral Animals and Ecosystem Connections
Beyond individual survival, cathemeral behavior can have ripple effects through ecosystems. In seed-dispersal networks, for instance, diurnal and nocturnal frugivores form somewhat separate communities centered around the fruiting trees they visit. Cathemeral frugivores serve as connectors between these daytime and nighttime networks because they interact with fruit sources during both periods.19Trends in Ecology & Evolution. Ecosystem functioning across the diel cycle in the Anthropocene This bridging role means that cathemeral species may contribute disproportionately to the connectivity and resilience of plant-animal interaction networks. If cathemeral seed dispersers decline or shift their activity patterns in response to human disturbance, the effects could cascade through the plant community by weakening the links between day and night dispersal events.
Pollination networks show a similar structure, with day-active and night-active pollinators forming partially distinct communities. The principle is the same: any species active across both periods acts as a bridge. In heavily fragmented or disturbed habitats where some species have already been lost, the remaining cathemeral species may become even more critical for maintaining these ecological connections. This is an area where the science is still developing, but it points to a broader significance of cathemerality that goes well beyond the animal’s own fitness.