The latest puzzling chimpanzee behavior

Young chimpanzee. Photo credit Sabine Bresse

Of the animal kingdom, chimpanzees, Pan troglodytes, are one of the well-known tool users. All populations of wild chimpanzees studied have been observed using leaves to obtain food and some populations have been observed modifying branches to forage for termites and even fashioning spears to hunt bushbabies (Shumaker et al, 2011; Boesch, 2012).

As is the case with studying many primates, our knowledge of tool use and how it is passed from generation to generation is limited by the number of field sites where long-term research occurs. Chimpanzees must first become used to human presence (or habituated) before their natural behavior can be studied, and this can take a great deal of time.

Theorizing that the repertoire of chimpanzee behavior is much wider than what is currently known, Kühl and colleagues used camera traps to study multiple populations of chimpanzees in West Africa that are not habituated. This is when they noticed some very interesting behavior. Researchers first noted unusual piles of rocks in trees and used camera traps to determine what was behind this puzzle. They observed chimpanzees picking up rocks from piles around or in trees and then throwing them at the tree.

Through the camera trap footage, researchers were able to determine that chimpanzees would throw the stones at the trees and vocalize a long-distance pant-hoot at the same time. The stones are mainly thrown by adult males and they seem to be independent of any sort of foraging behavior. The same chimpanzee was often observed returning to the tree and engaging in the behavior. This strange accumulative stone-throwing has only observed in West Africa and was seen in four populations of chimpanzees.

Stone throwing may be a type of male display, during which the individual attempts to draw attention to one's strength and impress other chimps. Kühl and colleagues theorize that using the stones in this manner may produce a loud sound resulting in a greater display. Another theory the authors present is that this may be a symbolic behavior. In either case, obviously more data is needed. Stone-throwing chimpanzees raises more questions than it does answers in terms of behavior and possibly cognition in the case of the latter theory.
 
This work goes to show that you can still learn a great deal about a comparatively well-studied primate species even if you're not Jane Goodall and haven't been working at the same research site for decades. Kühl and colleagues have captured some truly fascinating behavior that raise a series of new questions using technology to their benefit.

Links of potential interest:
Video of behavior
Pan African Programme: The Cultured Chimpanzee
How human are chimps?
Females more likely to use tools when hunting



Works cited:

Boesch, C. Wild Cultures: A Comparison between Chimpanzee and Human Cultures. (Cambridge University Press, 2012).

Kühl, H. S. et al. Chimpanzee accumulative stone throwing. Sci. Rep. 6, 22219; doi: 10.1038/srep22219 (2016).

Shumaker, R. W., Walkup, K. R. & Beck, B. B. Animal Tool Behavior: The Use and Manufacture of Tools by Animals. (JHU Press, 2011).

What effect temporally and spatially complex fruits may have on chimp cognition

Photo credit Sergio Morchon

A recent study by Janmaat and colleagues (2016) details how fruits in chimpanzee habitat are spatially and temporally complex (see that post here). As Janmaat and colleagues stated, this complexity in diet has implications for chimpanzee intelligence.

Fruits are a preferred food. It's worth hunting down these high-energy food items. That said, fruits aren't always easy to find. It's a waste of energy to travel to a specific fruiting tree with the thought of consuming a high-energy meal if that tree isn't producing. Wasting energy wandering around a forest looking for fruits that don't exist definitely isn't adaptive, and it's not something we would expect to see in chimpanzees or in any other species. It's in a chimpanzee's best interest to know where a fruit tree is and whether or not it will be fruiting. This type of processing  takes a certain amount of knowledge and brain power. 

The ecological intelligence hypothesis suggests that primates consuming foods that are fleeting in their availability and scattered geographically would require larger ranges and the cognitive capability to forage optimally for those ephemeral and scattered foods (Milton and May, 1976; Milton, 1980; Milton, 1981; Milton, 1988). Being able to remember where these scattered foods are and when they are available would be advantageous for the primate.

Janmaat and colleagues (2016) found substantial variation between fruiting species in regards to the timing of fruit production, and the authors suggest that chimps would benefit from learning species-specific fruiting patterns to locate these foods. There was also significant variation within a species in the monthly percentage of fruiting trees across years and between forests. Rather than this knowledge being genetic or something all chimpanzees are born with, it is more likely that chimpanzees learn about synchronicity of fruiting.

In regards to remembering trees that produce large amounts of fruit, the authors used existing literature and their own observations of great variation in fruit tree production histories to hypothesize that chimpanzees use their ranging patterns to monitor trees that are likely to produce large crops of fruit. Chimps would need to store information on fruit production histories over many years, particularly for species that fruit every few years rather than every few months, providing further evidence of how chimpanzees use their brains and intelligence to survive in their environment.

The forests chimpanzees inhabit clearly provide challenges for our closest relatives in terms of finding their preferred foods, ripe fruits. However, these intelligent animals have the brain power needed to master this environment and the challenges forests present. Their intelligence not only helps them navigate living in a social group and managing complex relationships but it also allows them to navigate the complex ecology surrounding them.

Links of possible interest:
Chimpanzees and long term memory
NOVA's Ape Genius


Works cited:

Milton, K., & May, M. L. (1976). Body weight, diet and home range area in primates. Nature, 259(5543), 459-462.

Milton, K. (1980). The foraging strategy of howler monkeys: a study in primate economics. Columbia University Press.

Milton, K. (1981). Distribution patterns of tropical plant foods as an evolutionary stimulus to primate mental development. American Anthropologist, 83(3), 534-548.

Milton K. 1988. Foraging behaviour and the evolution of primate intelligence. In: Byrne RW, Whiten A, editors. Machiavellian intelligence: social expertise and the evolution of intellect in monkeys, apes and humans. Oxford: Clarendon Press. p 285–305.

Gorillas and humans diverged from one another earlier than thought

Studying ancient humans and primates tells us more about our owns selves and what it means to be human, what it means to be of the species modern Homo sapiens. While we can look at our closest living relatives, apes and other primates, to theorize a great deal about how our ancestors acted  5 million years ago (mya), we also rely on fossil evidence and genetic studies. Studying human ancestors presents one very large problem: lack of fossil evidence. The fossilization process requires just the right conditions at time of death and the right conditions afterwards.  Thus, there are many gaps in our knowledge because the data simply isn't available. The fossils haven't been found.

Juvenile mountain gorillas, Photo credit: Philip Milne

Scientists are particularly interested in fossils from 12-7 mya because this is around the time when African apes (gorillas, chimpanzees, and bonobos) and humans split. Yet, few fossils from this time have been discovered.

Katoh and colleagues (2016) studied fossilized teeth found in Ethiopia's Afar Rift from an ancestral gorilla, called Choroapithecus abyssinicus, and the surrounding layers of earth these fossils were found in. Their results that suggest C. abyssinicus is older than previously believed. The teeth have been dated to 8 mya, which would mean that these ancestral gorillas and ancestral humans had to have diverged earlier in time than previously thought.

Previous estimates of the split between African apes and humans suggested the two lines diverged more recently. Data from genetics suggests that humans and gorillas split somewhere between seven to eight million years ago.  When C. abyssinicus was first discovered, it was found in deposits that are older than 8mya. The geology previously suggested that this species lived 10-10.5 mya (Suwa et al., 2007). Thus, this study provides some needed clarity on the age of these fossils.

With these new results, we can place the age of these fossils at 8 mya, meaning this ancestral ape and humans likely split around 10 mya rather than 8 mya and suggesting the mutation rate between the two was slower than previously thought. We can also confirm that ancestral great apes evolved in Africa, as opposed to Europe or Asia.

It's always impressive how a few extra teeth and a lot of hard work can transform what we know about our own origins.

Works cited:

Katoh, S., Beyene, Y., Itaya, T., Hyodo, H., Hyodo, M., Yagi, K., ... & Nakaya, H. (2016). New geological and palaeontological age constraint for the gorilla–human lineage split. Nature, 530(7589), 215-218.

Suwa, G., Kono, R. T., Katoh, S., Asfaw, B., & Beyene, Y. (2007). A new species of great ape from the late Miocene epoch in Ethiopia. Nature, 448(7156), 921-924.