Since 2002, our research muse has been owl monkey (Aotus), a native to Neo-tropical region. Due to their evolutionary choice of nocturnal life, owl monkeys do possess many secrets which diurnal living species like humans still cannot fathom. As of now, nothing is known about the circadian variation of melatonin in owl monkeys. Simply stated, two established facts are, (1) melatonin promotes sleep in diurnal primates including humans. (2) Melatonin levels in the cerebrospinal fluid, serum and plasma of diurnal primates like rhesus monkey, chacma baboon, common marmoset and humans peak during night hours. If this is so, how owl monkeys maintain their night alertness and daytime sleep routine and what role melatonin plays in the circadian zeitgebers of owl monkeys still remains a puzzle to be solved.
1. Owl Monkey Aotus
The taxonomy of the owl monkey (Aotus) genus continues to evolve, with advances in chromosome methodology. From a monotypic species A. trivirgatus originally described by von Humboldt in 1812 that remained unchanged until 1973, the number of extant species had multiplied (Groves, 2001; Ruiz-Herreera et al., 2005); currently 11 different species have been recognized (Menezes et al., 2010). Manuel Patarroyo’s group from Colombia had pioneered in developing synthetic malaria vaccine using the susceptibility of Aotus species to malaria, still a scourge of humankind (Baerg and Young, 1969; Pattarroyo et al., 1987; Rodriguez et al. 1990; Nino-Vasquez et al., 2000).
That the owl monkey populations in their native habitat of South American countries such as Colombia and Peru are being depleted due to malarial drug testing (Anon, 1973; Herrera et al., 2002), international trade (Svensson et al., 2016) as well as Amazonian deforestation leading to habitat loss (Brumback, 2001; Shanee et al., 2013; Guzman et al., 2016) is a cause of concern to many primatologists and environmental scientists. Conservation status of Aotus differs according to species/subspecies and countries. While few species are common and not in danger of extinction, few other species such as Aotus miconax and A. brumbecki have been recorded as vulnerable to extinction (Erkert, 1999). It is our fervent hope that this research muse of us will be saved from extinction due to conservation efforts currently being undertaken by researchers and conservationists in the South American countries.
2. Melatonin Puzzle in Owl Monkey
A comparison of 14 parameters in which owl monkeys differ from humans is provided in Table 1. Due to their evolutionary choice of nocturnal life, owl monkeys do possess many secrets which diurnal living species like humans still cannot fathom. After all, in comparison to owl monkeys that chose nocturnal life 12 million years ago, Homo sapiens could enter the night frontier actively, only 140 years ago (just a minuscular fraction in evolutionary time scale), after the invention of electric bulb. Thus, we became curious about the sleep-activity cycle of enigmatic owl monkeys and had attempted to quantitate the behavioral sleep of owl monkey colonies maintained at the Primate Research Institute (Kyoto University, Fig. 1)) and Amami Oshima facility (Tokyo University) as well as the vocalization behavior. As of now, we were able to publish only 6 reports in 17 years! (Sri Kantha and Suzuki, 2006; Suzuki and Sri Kantha, 2006; Sri Kantha et al., 2007, 2009a, 2009b, 2015) Not much to boast, in quantitative terms. But, we are dealing with a research muse which is of an enigmatic variety.
Table 1: Comparison between Owl Monkey and Human
||owl monkey (Aotus)
||human (Homo sapiens)
|1. Origin (yrs.)
||100,000 – 30,000
|2. Current spread
||restricted to Neotropics
|3. Mean wt. (kg)
|4. Activity pattern
|5. Mean skull length (mm)
|6. Mean orbit diameter (mm)
|7. Mean optic foramen area (mm)
|8. Color vision
(blue, green, red)
|9. Photo pigments (max, nm)
||410-430, 530-535, 556-562
|10. Cone: rod ratio
|11. in Chromosome 7
||defective S-cone gene
||S-cone opsin gene present
||single M/L-cone gene
||M+L multiple cone copies
|13. Mating system
||“wide range of grouping”b
|14. Gestation period(days)
Data sources for Parameters
13b: Fuentes (1998); Mating system differs from marriage pattern in humans, which
reflects culturally sanctioned union of economic and sexual relationships to
monogamy enforced by legal restraint in many countries.
Figure 1: Owl Monkey (Aotus azarae) family in a captive family bred at the Primate Research Institute, Kyoto University, Inuyama. New born baby is riding on the back of father, as typical of this monogamous monkey. Mother is seen to the left of the father. To the right of the father, is the elder sibling of the newborn. [photo credit: Sachi Sri Kantha, 2002]
As night work has become increasingly common in advanced and developing economies, humans do suffer from circadian maladaptation to such shift work (Horowitz and Tanigawa, 2002). Majority of the serious accidents in highways, factories and hospitals happen during midnight or early morning hours, when humans are thought to be in the sleep-mode. As such, the role of melatonin hormone in humans have gained much notice (Brzezinski, 1997). Nevertheless, to our knowledge, circadian variation of melatonin in owl monkeys have yet to be explored.
Simply stated, two established facts are, (1) melatonin promotes sleep in diurnal primates including humans (Zhdanova and Wurtman, 1997; Zhdanova et al., 2002). (2) Melatonin levels in the cerebrospinal fluid, serum and plasma of diurnal primates like rhesus monkey (Reppert et al., 1981; Brainard et al., 1981; Wilson et al., 1993, Roth et al., 2001), chacma baboon (Meyer et al., 1981), common marmoset (Guerin and Matthews, 1990) and humans (Brzezinski, 1997) peak during night hours. If this is so, how owl monkeys maintain their night alertness and daytime sleep routine and what role melatonin plays in the circadian zeitgebers of owl monkeys still remains a puzzle to be solved.
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