Day Monkey Night Monkey Story
Queila Neubecker
But Day Monkey doesn't recognise the creatures of the night, confusing fireflies for winking eyes, bats for flying mice and the moon for a banana. Night monkey is baffled by the brightness of the sun and confuses moths for butterflies and giants for chimpanzees.
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Night monkeys are the only truly nocturnal monkeys with the exception of some cathemeral populations of Azara's night monkey, who have irregular bursts of activity during day and night. They have a varied repertoire of vocalisations and live in small family groups of a mated pair and their immature offspring. Night monkeys have monochromatic vision which improves their ability to detect visual cues at night.
Night monkeys are threatened by habitat loss, the pet trade, hunting for bushmeat, and by biomedical research. They constitute one of the few monkey species that are affected by the often deadly human malaria protozoan Plasmodium falciparum, making them useful as non-human primate experimental subjects in malaria research. The Peruvian night monkey is classified by the International Union for Conservation of Nature (IUCN) as an Endangered species, while four are Vulnerable species, four are Least-concern species, and two are data deficient.
Night monkeys have large brown eyes; the size improves their nocturnal vision increasing their ability to be active at night. They are sometimes said to lack a tapetum lucidum, the reflective layer behind the retina possessed by many nocturnal animals.[8] Other sources say they have a tapetum lucidum composed of collagen fibrils.[9] At any rate, night monkeys lack the tapetum lucidum composed of riboflavin crystals possessed by lemurs and other strepsirrhines,[9] which is an indication that their nocturnality is a secondary adaption evolved from ancestrally diurnal primates.
Night monkeys can be found in Panama, Colombia, Ecuador, Peru, Brazil, Paraguay, Argentina, Bolivia, and Venezuela. The species that live at higher elevations tend to have thicker fur than the monkeys at sea level. Night monkeys can live in forests undisturbed by humans (primary forest) as well as in forests that are recovering from human logging efforts (secondary forest).[10]
During the daylight hours, night monkeys rest in shaded tree areas. These species have been observed exploiting four different types of tree nests, monkeys will rest in; holes formed in the trunks of trees, in concave sections of branches surrounded by creepers and epiphytes, in dense areas of epiphyte, climber and vine growth and in areas of dense foliage.[18] These sleeping sites provide protection from environmental stressors such as heavy rain, sunlight and heat. Sleeping sites are therefore carefully chosen based upon tree age, density of trees, availability of space for the group, ability of site to provide protection, ease of access to the site and availability of site with respect to daily routines.[18] While night monkeys are an arboreal species, nests have not been observed in higher strata of the rainforest ecosystem, rather a higher density of nests were recorded at low-mid vegetation levels.[18] Night monkeys represent a territorial species, territories are defended by conspecifics through the use of threatening and agonistic behaviours.[19] Ranges between night monkey species often do overlap and result in interspecific aggressions such as vocalizing and chasing which may last up to an hour.[11]
Night monkeys are primarily frugivorous (fruit eating species) as fruits are easily distinguished through the use of olfactory cues,[20] but leaf and insect consumption has also been observed in the cathemeral night monkey species A. azare.[11] A study conducted by Wolovich et al., indicated that juveniles and females were much better at catching both crawling and flying insects than adult males.[21] In general, the technique used by night monkeys in insect capturing is to use the palm of the hand to flatten a prey insect against a tree branch and then proceed to consume the carcass.[21] During the winter months or when food sources are reduced, night monkeys have also been observed foraging on flowers such as Tabebuia heptaphylla, however this does not represent a primary food source.[11]
The night monkey is socially monogamous, and all night monkeys form pair bonds. Only one infant is born each year. The male is the primary caregiver, and the mother carries the infant for only the first week or so of its life. This is believed to have developed because it increases the survival of the infant and reduces the metabolic costs on the female. Adults will occasionally be evicted from the group by same-sex individuals, either kin or outsiders.[25]
The family Aotidae is the only family of nocturnal species within the suborder Anthropoidea. While the order primates is divided into prosimians; many of which are nocturnal, the anthropoids possess very few nocturnal species and therefore it is highly likely that the ancestors of the family Aotidae did not exhibit nocturnality and were rather diurnal species.[26] The presence of nocturnal behavior in Aotidae therefore exemplifies a derived trait; an evolutionary adaptation that conferred greater fitness advantages onto the night monkey.[26] Night monkey share some similarities with nocturnal prosimians including low basal metabolic rate, small body size and good ability to detect visual cues at low light levels.[27] Their responses to olfactory stimulus are intermediate between those of the prosimians and diurnal primate species, however the ability to use auditory cues remains more similar to diurnal primate species than to nocturnal primate species.[27] This provides further evidence to support the hypothesis that nocturnality is a derived trait in the family Aotidae.
As the ancestor of Aotidae was likely diurnal, selective and environmental pressures must have been exerted on the members of this family which subsequently resulted in the alteration of their circadian rhythm to adapt to fill empty niches.[26] Being active in the night rather than during the day time, gave Aotus access to better food sources, provided protection from predators, reduced interspecific competition and provided an escape from the harsh environmental conditions of their habitat.[20] To begin, resting during the day allows for decreased interaction with diurnal predators. Members of the family Aotidae, apply the predation avoidance theory, choosing very strategic covered nests sites in trees.[28] These primates carefully choose areas with sufficient foliage and vines to provide cover from the sun and camouflage from predators, but which simultaneously allow for visibility of ground predators and permit effective routes of escape should a predator approach too quickly.[20][18] Activity at night also permits night monkeys to avoid aggressive interactions with other species such as competing for food and territorial disputes; as they are active when most other species are inactive and resting.[20]
Night monkeys also benefit from a nocturnal life style as activity in the night provides a degree of protection from the heat of the day and the thermoregulation difficulties associated.[28] Although night monkey, like all primates are endothermic, meaning they are able to produce their own heat, night monkeys undergo behavioural thermoregulation in order to minimize energy expenditure.[28] During the hottest points of the day, night monkeys are resting and therefore expending less energy in the form of heat. As they carefully construct their nests, night monkeys also benefit from the shade provided by the forest canopy which enables them to cool their bodies through the act of displacing themselves into a shady area.[28] Additionally, finding food is energetically costly and completing this process during the day time usually involves the usage of energy in the form of calories and lipid reserves to cool the body down. Foraging during the night when it is cooler, and when there is less competition, supports the optimal foraging theory; maximize energy input while minimizing energy output.[28]
While protection from predators, interspecific interactions, and the harsh environment propose ultimate causes for nocturnal behavior as they increase the species fitness, the proximate causes of nocturnality are linked to the environmental effects on circadian rhythm.[29] While diurnal species are stimulated by the appearance of the sun, in nocturnal species, activity is highly impacted by the degree of moon light available. The presence of a new moon has correlated with inhibition of activity in night monkeys who exhibit lower levels of activity with decreasing levels of moon light.[29] Therefore, the lunar cycle has a significant influence on the foraging and a nocturnal behaviors of night monkey species.[29]
However, while this does explain social monogamy, it does not explain the high degree of paternal care which is exhibited by these primates. After the birth of an infant, males are the primary carrier of the infant, carrying offspring up to 90% of the time.[30] In addition to aiding in child care, males will support females during lactation through sharing their foraged food with lactating females.[34] Generally, food sharing is not observed in nature as the search for food requires a great degree of energy expenditure, but in the case of night monkey males, food sharing confers offspring survival advantages. As lactating females may be too weak to forage themselves, they may lose the ability to nurse their child, food sharing therefore ensures that offspring will be well feed.[34] The act of food sharing is only observed among species where there is a high degree of fidelity in paternity. Giving up valuable food sources would not confer an evolutionary advance unless it increased an individual's fitness; in this case, paternal care ensures success of offspring and therefore increases the father's fitness.[34]
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