Monday, December 21, 2015

Chimpanzees at night

It’s the 21st of December – Winter solstice in the Northern hemisphere – and with over 16 hours of darkness the longest night here in Europe. These “dark times” motivated us to talk about chimpanzee activity at night and what you can see about it over at Chimp & See.

Chimps are not really known for any nocturnal behavior – in fact, not much is known about it at all in the wild, as research teams observing chimp communities during the day usually leave the group after the chimpanzees have made their nests for the night. Nightly vocalization (e.g., long distance calls) and other activity indicators had been recorded in the wild and in captive (zoo) apes, but the reasons for staying up late or even waking up and any possible activities lie mostly – well – in the dark.

Usually, it is assumed that chimpanzees mostly sleep through the night. In areas where chimpanzees live, sunset and sunrise varies not much seasonally and the length of the night (defined as the dark period between sunset and sunrise) is between 11 and 13 hours with only small changes during the year. Because of relatively poor night vision (like us humans) and with known predators (like leopards) active during the night, it is assumed that chimps generally stay in the safety of their nests through the nighttime hours from dusk until next day’s dawn. Given these considerations, we expected it to be very rare to see the chimpanzees at night.

An adult male balancing across a small creek at 5.33 a.m. (Muddy Frost 7)

Nonetheless, at almost each research site up at Chimp & See until now we have seen at least on one occasion what we call the “nightchimps” – chimpanzees that are caught on camera while it is dark. At most sites, these “nighttime” chimp activities (mostly traveling, some feeding) are during the early hours of the day (just before 6 a.m.) and are not considered very unusual. Depending on the distance to a desired feeding patch (e.g., fruit trees), chimps will get up early enough to reach them – or reach them first if feeding competition is an issue.

This chimpanzee infant is very tired on mom's back at 1.46 a.m. (Dry Lake 11)

But at the current site Dry Lake – the first in a dry savannah habitat in West Africa – we could collect significantly more “nightchimps” videos and several of them with time stamps very late at night. Currently, it is wild speculation why the chimps are active so late and why there are considerably more videos from the very early hours of the day than from other sites. One obvious reason might be the hot climate with day temperatures of over 40 °C during the dry season. Almost all “nightchimps” videos found until now have date stamps from March to May (in 2013) with hot temperatures and numbers of recorded “nightchimps” both peaking in April. The chimpanzees may just avoid the very hot day temperatures and feed and/or travel during the cooler morning and late evening hours. They might also need to forage longer or in a bigger area to find enough food. Other factors (like human pressure and domestic animals) may play a role but need to be analyzed and compared with other research sites. Certainly, something the science team will look into.

A family of three at 8.59 p.m. (Dry Lake 11)

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