Documenting orang-utan sleep architecture: sleeping platform complexity increases sleep quality in captive Pongo

Of the extant primates, only 20 non-human species have been studied by sleep scientists. Notable sampling gaps exist, including large-bodied hominoids such as gorillas (Gorilla gorilla), orang-utans (Pongo spp.) and bonobos (Pan paniscus), for which data have been characterized as high priority. Here, we report the sleep architecture of three female and two male orang-utans housed at the Indianapolis Zoo. Sleep states were identified by scoring correlated behavioural signatures (e.g., respiration, gross body movement, muscle atonia, random eye movement, etc.). The captive orang-utans were focal subjects for a total of 70 nights (1013 h) recorded. We found that orang-utans slept an average of 9.11 h (range 5.85-11.2 h) nightly and were characterized by an average NREM of 8.03 h (range 5.47-10.2 h) and REM of 1.11 (range: 0.38-2.2 h) per night. In addition, using a sleeping platform complexity index (SPCI) we found that individuals that manufactured and slept in more complex beds were characterized by higher quality sleep. Sleep fragmentation (the number of brief awakenings greater than 2 min per hour), arousability (number of motor activity bouts per hour), and total time awake per night were reduced by greater quality sleep environments. Therefore, comfortable sleeping environments reduced arousability and improve sleep quality in captive orang-utans. These results support Fruth & Hohmann's (1996) hypothesis, which purported that the tree-limb-to-sleeping platform transition in Miocene apes improved sleep quality.