Moonstruck Primates: Owl Monkeys (Aotus) Need Moonlight for Nocturnal Activity in Their Natural Environment

Primates show activity patterns ranging from nocturnality to diurnality, with a few species showing activity both during day and night. Among anthropoids (monkeys, apes and humans), nocturnality is only present in the Central and South American owl monkey genus Aotus. Unlike other tropical Aotus species, the Azara''s owl monkeys (A. azarai) of the subtropics have switched their activity pattern from strict nocturnality to one that also includes regular diurnal activity. Harsher climate, food availability, and the lack of predators or diurnal competitors, have all been proposed as factors favoring evolutionary switches in primate activity patterns. However, the observational nature of most field studies has limited an understanding of the mechanisms responsible for this switch in activity patterns. The goal of our study was to evaluate the hypothesis that masking, namely the stimulatory and/or inhibitory/disinhibitory effects of environmental factors on synchronized circadian locomotor activity, is a key determinant of the unusual activity pattern of Azara''s owl monkeys. We use continuous long-term (6–18 months) 5-min-binned activity records obtained with actimeter collars fitted to wild owl monkeys (n = 10 individuals) to show that this different pattern results from strong masking of activity by the inhibiting and enhancing effects of ambient luminance and temperature. Conclusive evidence for the direct masking effect of light is provided by data showing that locomotor activity was almost completely inhibited when moonlight was shadowed during three lunar eclipses. Temperature also negatively masked locomotor activity, and this masking was manifested even under optimal light conditions. Our results highlight the importance of the masking of circadian rhythmicity as a determinant of nocturnality in wild owl monkeys and suggest that the stimulatory effects of dim light in nocturnal primates may have been selected as an adaptive response to moonlight. Furthermore, our data indicate that changes in sensitivity to specific environmental stimuli may have been an essential key for evolutionary switches between diurnal and nocturnal habits in primates.     

Seasonal variation of temporal niche in wild owl monkeys (Aotus azarai azarai) of the Argentinean Chaco: a matter of masking?

Among the more than 40 genera of anthropoid primates (monkeys, apes, and humans), only the South American owl monkeys, genus Aotus, are nocturnal. However, the southernmostly distributed species, Aotus azarai azarai, of the Gran Chaco may show considerable amounts of its 24-h activity during bright daylight. Due to seasonal changes in the duration of photophase and climatic parameters in their subtropical habitat, the timing and pattern of their daily activity are expected to show significant seasonal variation. By quantitative long-term activity recordings with Actiwatch AW4 accelerometer data logger devices of 10 wild owl monkeys inhabiting a gallery forest in Formosa, Argentina, the authors analyzed the seasonal variation in the temporal niche and activity pattern resulting from entrainment and masking of the circadian activity rhythm by seasonally and diurnally varying environmental factors. The owl monkeys always displayed a distinct bimodal activity pattern, with prominent activity bouts and peaks during dusk and dawn. Their activity rhythm showed distinct lunar and seasonal variations in the timing and daily pattern. During the summer, the monkeys showed predominantly crepuscular/nocturnal behavior, and a crepuscular/cathemeral activity pattern with similar diurnal and nocturnal activity levels during the cold winter months. The peak times of the evening and morning activity bouts were more closely related to the times of sunset and sunrise, respectively, than activity-onset and -offset. Obviously, they were better circadian markers for the phase position of the entrained activity rhythm than activity-onset and -offset, which were subject to more masking effects of environmental and/or internal factors. Total daily activity was lowest during the two coldest lunar months, and almost twice as high during the warmest months. Nighttime (21:00-06:00 h) and daytime (09:00-18:00 h) activity varied significantly across the year, but in an opposite manner. Highest nighttime activity occurred in summer and maximal daytime activity during the cold winter months. Dusk and dawn activity, which together accounted for 43% of the total daily activity, barely changed. The monkeys tended to terminate their nightly activity period earlier on warm and rainy days, whereas the daily amount of activity showed no significant correlation either with temperature or precipitation. These data are consistent with the dual-oscillator hypothesis of circadian regulation. They suggest the seasonal variations of the timing and pattern of daily activity in wild owl monkeys of the Argentinean Chaco result from a specific interplay of light entrainment of circadian rhythmicity and strong masking effects of various endogenous and environmental factors. Since the phase position of the monkeys' evening and morning activity peaks did not vary considerably over the year, the seasonal change from a crepuscular/nocturnal activity pattern in summer to a more crepuscular/cathemeral one in winter does not depend on a corresponding phase shift of the entrained circadian rhythm, but mainly on masking effects. Thermoregulatory and energetic demands and constraints seem to play a crucial role.     

The evolution of cathemerality in primates and other mammals: a comparative and chronoecological approach

Non-primate mammalian activity cycles are highly variable across and within taxonomic groups. In contrast, the order Primates has historically been recognized as displaying a diurnal-nocturnal dichotomy that mapped, for the most part, onto the taxonomic division between haplorhines and strepsirhines. However, it has become clear over the past two decades that activity cycles in primates are not quite so clear cut. Some primate species--like many large herbivorous mammals, mustelids, microtine rodents, and shrews--exhibit activity both at night and during the day. This activity pattern is often polyphasic or ultradian (several short activity bouts per 24-hour period), in contrast to the generally monophasic pattern (one long bout of activity per 24-hour period) observed in diurnal and nocturnal mammals. Alternatively, it can vary on a seasonal basis, with nocturnal activity exhibited during one season, and diurnal activity during the other season. The term now generally employed to describe the exploitation of both diurnal and nocturnal phases in primates is 'cathemeral'. Cathemerality has been documented in one haplorhine, the owl monkey, Aotus azarai, in the Paraguayan and Argentinian Chaco and in several Malagasy strepsirhines, including Eulemur spp., Hapalemur sp. and Lemur catta. In this paper, we review patterns of day-night activity in primates and other mammals and investigate the potential ecological and physiological bases underlying such 24-hour activity. Secondly, we will consider the role of cathemerality in primate evolution.     

The Effect of Moonlight on Scopoli's Shearwater Calonectris diomedea Colony Attendance Patterns and Nocturnal Foraging: A Test of the Foraging Efficiency Hypothesis

Moonlight is known to affect the nocturnal behaviour and activity rhythms of many organisms. For instance, predators active at night may take advantage from increased visibility afforded by the moon, while prey might regulate their activity patterns to become less detectable. Many species of pelagic seabirds attend their colony only at night, in complete darkness, avoiding approaching their nest sites under moonlight. This behaviour has been most often interpreted as an antipredator adaptation (‘predation avoidance’ hypothesis). However, it may also reflect a lower foraging efficiency during moonlit nights (‘foraging efficiency’ hypothesis). Indeed, moonlight may reduce prey availability because preferred seabird prey is known to occur at higher depths in moonlit nights. Using high‐accuracy behavioural information from data loggers, we investigated the effect of moonlight on colony attendance and at‐sea nocturnal foraging in breeding Scopoli's shearwaters Calonectris diomedea. We found that birds departing for self‐feeding trips around the full moon performed longer trips than those departing around the new moon. On nights when the moon was present only partly, nest burrow entrances took place largely in the moonless portion of the night. Moreover, contrary to predictions from the ‘foraging efficiency’ hypothesis, nocturnal foraging activity increased according to moonlight intensity, suggesting that birds increased their foraging activity when prey became more detectable. This study strengthens the idea that colony attendance behaviour is strictly controlled by moonlight in shearwaters, which is possibly related to the perception of a predation risk.     

Diel activity of resident and immigrant waterbirds at Lake Turkana, Kenya

Of the 42 dominant species of waterbirds at Lake Turkana, Kenya, 14 foraged uniformly throughout the day and night, five foraged mostly during the night, five foraged during both the night and day but with diurnal peaks, 17 were exclusively diurnal and only one was exclusively nocturnal. Species with uniform feeding activity usually captured small prey, using tactile or visual plus tactile cues; most diurnal species captured large prey, using visual cues. However, some species which fed mostly at night, or uniformly, relied exclusively on visual cues. We found support from only one species that moonlight influenced foraging activities. Palaearctic immigrants spent significantly more time foraging than partial migrants and residents; they were also smaller and mainly microphagous. Only gulls and terns were restricted to diurnal feeding, presumably by their need to see and capture prey while flying. The other groups were formed by species which foraged uniformly over 24 h or partially by day or night. These patterns indicate that in most waterbirds feeding activities are not basically tied to any phase of the diel cycle. Since most waterbirds display some degree of nocturnal activity, time budget studies based only on diurnal observations are likely to be misleading.     

Non-additivity among multiple cues of predation risk: a behaviorally-driven trophic cascade between owls and songbirds

Non-lethal effects of predators on prey are initiated in the form of responses to direct and indirect cues of predation risk. Like their lethal equivalents, non-lethal effects may affect species further down the food web initiating a behaviorally-driven trophic cascade. I presented a direct cue of predation risk, owl vocalizations, to white-footed mice ( Peromyscus leucopus ) during either a new or full moon (indirect cue). Mice reduced their activity in space by nearly two-thirds in response to playbacks of owl vocalizations during a full moon. However, neither moonlight (full vs new) nor the presence/absence of owl calls had an effect on space use when each cue varied singly. Previous studies have demonstrated a tight correlation between spatial activity in mice as used in the current experiment and nest predation rates on ground-nesting birds. Because moonlight is a ubiquitous deterrent of activity in nocturnal rodents I used of long-term nesting records the veery ( Catharus fuscescens ) to test whether nest predation rates were correlated negatively with moonlight. For half the lunar cycle (∼full moon to new moon) predation rates decreased with moonlight as predicted. During the second half of the lunar cycle predation and moonlight did not correlate as expected, but this was likely due to the depletion of vulnerable nests after a period of in which predation rates were at their maximum near the full moon. These studies suggest that the non-lethal effects of predatory risk on mice (i.e. changes in space use) cascade to affect their prey. Through the mechanism of reduced space use by rodents, perceived predation risk has the potential to significantly and indirectly affect songbird nest predation rates.     

Nocturnal ranging by a diurnal primate: are ring-tailed lemurs (<Emphasis Type="Italic">Lemur catta</Emphasis>) cathemeral?

Cathemerality, an activity pattern comprised of distinct periods of diurnal and nocturnal activity, is a trait found among several of the Malagasy strepsirhines and one species of Aotus. Because occasional anecdotal reports suggest that some diurnal primates can be active at night, I investigated the possibility of nocturnal ranging behavior in the ring-tailed lemur (Lemur catta) by using global positioning system (GPS) collars programmed to collect data across a 24-h period. Five individuals in a provisioned, free-ranging L. catta colony on St. Catherines Island, Georgia, USA, wore GPS collars across 1 week of the mating season. Results revealed that night ranging behavior occurred between the h of 1900 and 0530. An evaluation of the effect of moonlight on nocturnal activity showed that a greater rate of travel occurred during moonlit periods as opposed to periods when the moon had not yet risen. Distance travelled at night decreased across the deployment period, likely because of a decrease in available moonlight over time, as the lemurs were collared during a waning moon. Fewer mating opportunities over time may have also been responsible for the decrease in night ranging, because the number of females in estrus declined across the deployment period. Future research is needed to separate the effects of moonlight and mating activity on night ranging in this species, as well as to evaluate whether L. catta in Madagascar show night ranging similar to L. catta on SCI. These data raise the possibility that L. catta may be cathemeral, with an activity pattern fluctuating between diurnality and cathemerality in accordance with shifts in environmental conditions.     

Seasonal Variation of Temporal Niche in Wild Owl Monkeys (Aotus azarai azarai) of the Argentinean Chaco: A Matter of Masking?

Among the more than 40 genera of anthropoid primates (monkeys, apes, and humans), only the South American owl monkeys, genus Aotus, are nocturnal. However, the southernmostly distributed species, Aotus azarai azarai, of the Gran Chaco may show considerable amounts of its 24-h activity during bright daylight. Due to seasonal changes in the duration of photophase and climatic parameters in their subtropical habitat, the timing and pattern of their daily activity are expected to show significant seasonal variation. By quantitative long-term activity recordings with Actiwatch AW4 accelerometer data logger devices of 10 wild owl monkeys inhabiting a gallery forest in Formosa, Argentina, the authors analyzed the seasonal variation in the temporal niche and activity pattern resulting from entrainment and masking of the circadian activity rhythm by seasonally and diurnally varying environmental factors. The owl monkeys always displayed a distinct bimodal activity pattern, with prominent activity bouts and peaks during dusk and dawn. Their activity rhythm showed distinct lunar and seasonal variations in the timing and daily pattern. During the summer, the monkeys showed predominantly crepuscular/nocturnal behavior, and a crepuscular/cathemeral activity pattern with similar diurnal and nocturnal activity levels during the cold winter months. The peak times of the evening and morning activity bouts were more closely related to the times of sunset and sunrise, respectively, than activity-onset and -offset. Obviously, they were better circadian markers for the phase position of the entrained activity rhythm than activity-onset and -offset, which were subject to more masking effects of environmental and/or internal factors. Total daily activity was lowest during the two coldest lunar months, and almost twice as high during the warmest months. Nighttime (21:00–06:00?h) and daytime (09:00–18:00?h) activity varied significantly across the year, but in an opposite manner. Highest nighttime activity occurred in summer and maximal daytime activity during the cold winter months. Dusk and dawn activity, which together accounted for 43% of the total daily activity, barely changed. The monkeys tended to terminate their nightly activity period earlier on warm and rainy days, whereas the daily amount of activity showed no significant correlation either with temperature or precipitation. These data are consistent with the dual-oscillator hypothesis of circadian regulation. They suggest the seasonal variations of the timing and pattern of daily activity in wild owl monkeys of the Argentinean Chaco result from a specific interplay of light entrainment of circadian rhythmicity and strong masking effects of various endogenous and environmental factors. Since the phase position of the monkeys' evening and morning activity peaks did not vary considerably over the year, the seasonal change from a crepuscular/nocturnal activity pattern in summer to a more crepuscular/cathemeral one in winter does not depend on a corresponding phase shift of the entrained circadian rhythm, but mainly on masking effects. Thermoregulatory and energetic demands and constraints seem to play a crucial role. (Author correspondence: hans.erkert@t-online.de)     

Temporal activity patterns of North China leopards and their prey in response to moonlight and habitat factors

The nocturnal activities of predators and prey are influenced by several factors, including physiological adaptations, habitat quality and, we suspect, corresponds to changes in brightness of moonlight according to moon phase. In this study, we used a dataset from 102 camera traps to explore which factors are related to the activity pattern of North China leopards (Panthera pardus japonensis) in Shanxi Tieqiaoshan Provincial Nature Reserve (TPNR), China. We found that nocturnal activities of leopards were irregular during four different lunar phases, and while not strictly lunar philic or lunar phobic, their temporal activity was highest during the brighter moon phases (especially the last quarter) and lower during the new moon phase. On the contrary, roe deer (Capreolus pygargus) exhibited lunar philic activity, while wild boar (Sus scrofa) and tolai hare (Lepus tolai) were evidently lunar phobic, with high and low temporal activity during the full moon, respectively. In terms of temporal overlap, there was positive overlap between leopards and their prey species, including roe deer and tolai hare, while leopard activity did not dip to the same low level of wild boar during the full moon phase. Human activities also more influenced the temporal activity of leopards and wild boar than other species investigated. Generally, our results suggested that besides moonlight risk index (MRI), cloud cover and season have diverse effects on leopard and prey nocturnal activity. Finally, distinct daytime and nighttime habitats were identified, with leopards, wild boar, and tolai hare all using lower elevations at night and higher elevations during the day, while leopards and roe deer were closer to secondary roads during the day than at night.     

A re-evaluation of the role of vision in the activity and communication of nocturnal primates

This paper examines the importance of vision in the lives of nocturnal primates in comparison to diurnal and cathemeral species. Vision is the major sense in all primates and there is evidence that the eyesight of nocturnal species is more acute and variable than has previously been recognized. Case studies of the behaviour of a galago and a loris in open woodland habitats in relation to ambient light show that Galago moholi males are more likely to travel between clumps of vegetation along the ground when the moon is up, and during periods of twilight, whereas they retreat to more continuous vegetation and travel less when the moon sets. This is interpreted as a strategy for avoiding predators that hunt on the ground when it is dark. The travel distances of Loris lydekkerianus are not affected by moonlight but this species reduces its choice of food items from more mobile prey to mainly ants when the moon sets, indicating the importance of light when searching for high-energy supplements to its staple diet. Evidence is presented for the first time to indicate key aspects of nocturnal vision that would benefit from further research. It is suggested that the light and dark facial markings of many species convey information about species and individual identity when animals approach each other at night. Differences in the colour of the reflective eye-shine, and behavioural responses displayed when exposed to white torchlight, point to different kinds of nocturnal vision that are suited to each niche, including the possibility of some degree of colour discrimination. The ability of even specialist nocturnal species to see well in broad daylight demonstrates an inherent flexibility that would enable movement into diurnal niches. The major differences in the sensitivity and perceptual anatomy of diurnal lemurs compared to diurnal anthropoids, and the emergence of cathemerality in lemurs, is interpreted as a reflection of evolution from different ancestral stocks in very different ecosystems, and not a recent shift towards diurnality due to human disturbance.     

Infant Development and Parental Care in Free-Ranging <Emphasis Type="Italic">Aotus azarai azarai</Emphasis> in Argentina

Studies of infant development and parental behavior in free-ranging owl monkeys have been constrained by their nocturnal habits. Taking advantage of the cathemeral activity pattern of Aotus azarai azarai in the Argentinean Chaco, we describe the development of a cohort of free-ranging infants born in an owl monkey population in Formosa Province. We observed 7 infants, whose birth dates are known to the nearest week, to record details of their development and care between October 1999 and March 2000. We collected 92 h of behavioral data in 76 sessions. The infants were almost never off the parents during the first 4 wk of life, and we observed no infant being transported by a nonadult. The parent carrying the infant traveled most frequently in the middle of the group, sometimes first, but rarely last. The mean duration of 33 nursing episodes is 69 sec. After nursing, the infant was more likely to return to the nonnursing adult than to remain with the mother suggesting that in owl monkeys the infant may be primarily attached to the adult male in the group. Infants began to explore, to manipulate and to consume solid foods during the second month. Our observations are comparable to ones on captive breeding groups of Colombian owl monkeys (Aotus lemurinus) and Bolivian owl monkeys (A. azarai boliviensis) under controlled conditions of temperature, illumination and food availability.     

Aerial insectivorous bat activity in relation to moonlight intensity

It is commonly assumed that aerial insectivorous bats in the tropics respond to moonlight intensity by decreasing their foraging activity during bright nights due either to an increase in predation risk, or to a reduction in insect availability.The effect of moonlight on bat activity can be measured both between nights and within a single night. However, few studies have simultaneously used both approaches, and most authors generally compare bat activity with lunar phases. Our main aim was to evaluate how moonlight influences aerial insectivorous bat activity at different time scales: between nights and within the same night. Activity of five bat species was measured using autonomous ultrasound recording stations and moonlight intensity percentages retrieved from the Moontool program nightly throughout a 53-day sampling period. Only one species (Myotis riparius) responded negatively to moonlight, while two species (Pteronotus parnellii and Saccopteryx leptura) increased their foraging activity in moonlight. For Cormura brevirostris and S. bilineata, moonlight intensity did not affect activity level. Bat activity was greater for all species at the beginning of the night, independent of the presence of the moon, indicating that foraging just after the sunset is adaptive. Thus, bat response to the effect of moonlight intensity is more apparent between nights than within a single night and may depend on species-specific traits, such as flight speed, flexibility in habitat use and body size.     

Mutational changes in S-cone opsin genes common to both nocturnal and cathemeral Aotus monkeys

Aotus is a platyrrhine primate that has been classically considered to be nocturnal. Earlier research revealed that this animal lacks a color vision capacity because, unlike all other platyrrhine monkeys, Aotus has a defect in the opsin gene that is required to produce short-wavelength sensitive (S) cone photopigment. Consequently, Aotus retains only a single type of cone photopigment. Other mammals have since been found to show similar losses and it has often been speculated that such change is in some fashion tied to nocturnality. Although most species of Aotus are indeed nocturnal, recent observations show that Aotus azarai, an owl monkey species native to portions of Argentina and Paraguay, displays a cathemeral activity pattern being active during daylight hours as frequently as during nighttime hours. We have sequenced portions of the S-cone opsin gene in A. azarai and Aotus nancymaae, the latter a typically nocturnal species. The S-cone opsin genes in both species contain the same fatal defects earlier detected for Aotus trivirgatus. On the basis of the phylogenetic relationships of these three species these results imply that Aotus must have lost a capacity for color vision early in its history and they also suggest that the absence of color vision is not compulsively linked to a nocturnal lifestyle.     

Testing the weekend effect hypothesis: Time of day and lunar phase better predict the timing of births in laboratory‐housed primates than day of week

The weekend effect hypothesis proposes that captive primates are more likely to give birth during times of low disturbance and reduced staff activity. The hypothesis specifically predicts that laboratory‐housed primates will be more likely to give birth during the weekend than weekdays when staff activity is reduced. To date, support for the weekend effect hypothesis has been mixed and based on studies with relatively few subjects. To further examine the hypothesis, we analyzed the birthing patterns of three genera of laboratory‐housed primates: squirrel monkeys (Saimiri species, N = 2,090 births), owl monkeys (Aotus species, N = 479 births), and rhesus macaques (Macaca mulatta, N = 2,047 births). Contrary to predictions derived from the weekend effect hypothesis, the frequencies of births during weekends for all taxa were not significantly different from rates that would be expected by chance. However, while there was no variance across days of the week, all three taxa gave birth at nighttime, when staff was absent. This parallels reports of births in wild and captive monkeys, both diurnal and nocturnal, which are more likely to give birth during the night; plausibly a time when the environmental and social disturbance is lowest and the mother is safest to bond with her newborn infant. As all births occurred at night, we also explored the relationship between the lunar cycle and the timing of births timing. While the diurnal primates (i.e., Saimiri and Macaca) were no more likely to give birth on “bright” nights than “dark” nights, owl monkeys (Aotus) had a much higher frequency of births on bright nights than darker ones, and at rates that deviated from chance. Our data provide a more detailed understanding on how the environment may influence captive monkey births but do not support the oft‐cited weekend effect hypothesis.     

Influence of extrinsic variables on activity and habitat selection of lowland tapirs (Tapirus terrestris) in the coastal sand plain shrub,southern Brazil

The objectives of this research were to: 1. evaluate the circadian activity patterns of lowland tapirs (Tapirus terrestris) throughout the seasons and 2. study the influence of moonlight, temperature and rainfall on the activity patterns and habitat selection of this species, in the coastal sand shrub in southern Brazil. From June 2005 to June 2006, eight tapirs were monitored in a large enclosure containing open and vegetation-covered areas, using four camera traps. Differences in activity patterns within seasons were found. Tapir predominately presented nocturnal-crepuscular activity; however, they differed in the winter, with cathemeral activity patterns. Covered areas were mostly used during periods of extreme temperatures, with less diurnal and more nocturnal activities within these areas, on hotter days. Activity in open areas mainly occurred during periods of intermediate temperatures, both during the day and in the night. Moonlight intensity did not influence nocturnal activities. On days of precipitation of 34 mm or more, there was no record of open-area activities, despite constant activity in covered-area.     

Diurnality and nocturnality in nonhuman primates: comparative chronobiological studies in laboratory and nature

Looking for differences in circadian clock characteristics of diurnal and nocturnal nonhuman primates, this article summarizes results of chronobiological studies carried out in various nocturnal, diurnal, and cathemeral prosimian and anthropoid primate species under controlled laboratory conditions, under seminatural conditions, and in the wild. In almost all circadian parameters investigated, no differences were discernible between the two main chrono-ecotypes, either in circadian period length and the influence upon it of after-effects, of light intensity, and ambient temperature, or in the PRC, re-entrainment behavior, rhythm splitting, and internal desynchronization. Diurnal and nocturnal or cathemeral species differed only in the phase of artificial or natural LDs to which their circadian activity phase was adjusted as well as in the characteristics of masking activity upon the rhythms produced by the direct inhibiting or enhancing effects of light. Pronounced lunar periodicity—observed in the activity rhythm of nocturnal neotropical owl monkeys, genus Aotus, in seminatural and natural environments as well as in wild cathemeral Malagasy lemurs, genus Eulemur—is shown to result from masking effects of moonlight. In captive Eulemur fulvus albifrons, a change from dark-active over cathemeral to light-active behavior, without concurrently changing the circadian phase-setting of activity to D, was produced by direct masking effects of a stepwise reduction of darktime luminosity on an LD 12:12 cycle. Long-term activity recordings carried out in wild diurnal Malagasy sifakas (Propithecus verreauxi) and cathemeral redfronted lemurs (Eulemur fulvus rufus), as well as in wild nocturnal owl monkeys (Aotus a. azarai) of the North Argentinean Chaco, yielded in all species distinct bimodal long- and short-day activity patterns with pronounced peaks during dusk and dawn. Applying Pittendrigh's two-oscillator concept to these results, it is hypothesized that the differences in chrono-ecotype behavior may result from variations in internal coupling and external phase-setting of morning and evening oscillators (m, e) to dawn and dusk, interacting with direct masking effects of light.     

Behavior,Ecology, and Demography of <Emphasis Type="Italic">Aotus vociferans</Emphasis> in Yasuní National Park,Ecuador

Given its broad geographical distribution, Aotus is a productive genus for comparative studies that evaluate how different ecological factors influence the morphology, behavior, ecology, and demography of closely related species. During 18 mo we collected demographic, ranging, and activity data from owl monkeys (Aotus vociferans) in Yasuní National Park in eastern Ecuador. To collect demographic data, we monitored the trail system several times per week searching for groups. To characterize patterns of activity, we recorded the time when the subjects began and ended their nocturnal activity, and we collected data on range use and daily path length during 12 full-moon and 12 new-moon night follows of 1 radiocollared group. They ranged in size between 3 and 5 individuals (n = 4 groups). All groups were strictly nocturnal, beginning their activity between 1800 and 1900 h and finishing it between 0500 and 0600 h. The territory size of the radiocollared group was 6.3 ha. On average, the subjects traveled 645 m per night (±286 m) and ranged farther during full-moon than new-moon nights. The owl monkeys used a small number of preferred daytime sleeping trees. Our data conform well with previous studies of other tropical owl monkeys from Colombia and Perú. A comparison of tropical owl monkeys with more temperate Aotus azarai from the Argentinean Gran Chaco reveals that grouping patterns, day range length, and territory size are relatively conserved across the genus despite dramatic differences in body size and activity pattern.     

The effects of feedback lighting on the circadian drinking rhythm in the diurnal new world primate Saimiri sciureus

Feedback lighting provides illumination primarily during the subjective night (i.e., the photosensitive portion of the circadian cycle) in response to a given behavior. This technique has previously been used to test the nonparametric model of entrainment in nocturnal rodents. In three species (Rattus norvegicus, Mesocricetus auratus, and Mus musculus), the free-running period of the locomotor activity rhythm was similar whether the animals were exposed to continuous light or discrete light pulses occurring essentially only during the subjective night (i.e., feedback lighting). In the current experiments, feedback lighting was presented to squirrel monkeys so that light fell predominantly during the subjective night. Feedback lighting was linked to the drinking behavior in this diurnal primate so that when the animal drank, the lights went out. Despite the seemingly adverse predicament, the monkeys maintained regular circadian drinking rhythms. Furthermore, just as the period of the free-running activity rhythms of nocturnal rodents exposed to continuous light or feedback lighting were similar, the period of the drinking rhythms of the squirrel monkeys in continuous light and feedback lighting were comparable (25.6 +/- 0.1 and 25.9 +/- 0.1 hours, respectively), despite a substantial decrease in the total amount of light exposure associated with feedback lighting. The free-running period of monkeys exposed to continuous dark (24.5 +/- 0.1 hours) was significantly shorter than either of the two lighting conditions (P < 0.001). The results presented for the drinking rhythm were confirmed by examination of the temperature and activity rhythms. Therefore, discrete light pulses given predominately during the subjective night are capable of simulating the effects of continuous light on the free-running period of the circadian rhythms of a diurnal primate. The response of squirrel monkeys to feedback lighting thus lends further support for the model and suggests that the major entrainment mechanisms are similar in nocturnal rodents and diurnal primates.     

Relationship between moonlight and nightly activity patterns of the ocelot (Leopardus pardalis) and some of its prey species in Formosa,Northern Argentina

The moon can profoundly influence the activity patterns of animals. If predators are more successful under bright moonlight, prey species are likely to respond by shifting their own activity patterns (predator-avoidance hypothesis). However, the assumption that prey will necessarily avoid full-moon nights does not take into account that moonlight also allows prey to more easily detect predators, and to forage more efficiently. Thus, nightly activity patterns could depend on night vision capabilities (visual-acuity hypothesis). To consider the possible influences of moonlight and to distinguish between these hypotheses, we used camera-trapping records of a predator, the ocelot (Leopardus pardalis), and several of its night-active prey to compare activity patterns under different moonlight conditions. The ocelots' activity patterns were not strongly related to moonlight, but showed a slight tendency for higher activity during brighter nights. Tapeti rabbits (Sylvilagus brasiliensis) and brocket deer (Mazama americana) showed a clear preference for brighter nights. White-eared opossums (Didelphis albiventris) also showed a trend to be less active in new moon light. In contrast, smaller grey four-eyed opossums (Philander opossum) and the poor eye-sight nine-banded armadillo (Dasypus novemcinctus) showed similar activity patterns across all moon phases. Since activity patterns of most prey species were not shifted away from the activity of the ocelot, the differences between species are probably linked to their night vision capabilities, and emphasise the need for more information on the visual system of these taxa. Their activity patterns seem to be less strongly linked to avoidance of predation than previously thought, suggesting that foraging and predator detection benefits may play a more important role than usually acknowledged.     

Seasonal variation in the activity patterns of the Orkney vole Microtus arvalis orcadensis

In captivity, Orkney voles Microtus uruulis orcudensis show an activity pattern that consists of regularly alternating periods of rest and activity with a periodicity of about 2.8 hours. Captive voles kept under ambient conditions are equally active day and night at all times of the year. Free-living voles also show a regular short-term rhythm of activity but in addition the partition of activity between day and night varies seasonally. At two sites on Orkney Mainland cyclic variation in diurnality with a period of 12 months were recorded. Although surface activity occurred during both the day and night throughout the year, day activity predominated, particularly during the winter months. The cycle appeared to be synchronous at the two study sites.
Seasonal variation in diurnality appeared to be associated with population density and due largely to the proportion of juveniles in the population. Given the 12-month cycle of diurnality found in the Orkney vole, phase relationships with photoperiod are likely to be consistent between years. This may explain why populations of this subspecies do not appear to undergo multi-annual population cycles.
Despite intense levels of predation by raptors, including the short-eared owl which switches between nocturnal and diurnal hunting, Orkney voles exhibit a relatively simple and highly predictable cycle of diurnality. This cycle is unlikely therefore to have evolved as a means of preventing predator specialization with respect to time of day, season or year.