Background & Aims

Foraging is a demanding task for many animals. According to optimal foraging theory, animals are adapted to minimize the cost of foraging (time, energy) while maximizing the benefits (energy intake). When animals pick a foraging strategy, they have to consider a variety of ecological, social, and spatial variables. There is always a trade-off between the costs and benefits of an animal’s foraging strategy.

When choosing a foraging strategy, an animal can use information that is gained and memorized through their own experience (internal information), and/or information from their environment (external information). Examples of the use of internal information are the use of spatial memory, temporal memory, and botanical knowledge. The use of external information is characterized by using information about the environment gained from locating food sources using sensory systems such as the olfactory system, visual system, auditory system, and touch, or using information emitted by other individuals.

Previous studies have usually focused on specific foraging strategies using either internal or external information. Still, few studies have investigated the use of both external and internal sources of information by one species to make foraging decisions. When studying foraging decisions, it is useful to look at primates since they are able to use cognitively demanding foraging strategies but also rely on their senses to find food. 

An example of a foraging strategy in primates wherein they rely on internal information is the use of knowledge about fruiting synchrony. Fruiting synchrony means that when one tree of a species is fruiting, most other trees of this species in the area will simultaneously fruit as well. If an animal can recognize a simultaneously fruiting tree species, the animal could know that there is a high probability that other trees of this species in the area will also produce fruit. This is expected to save a lot of energy and time since the primate can now focus on inspecting all trees of this species while disregarding other species of fruit trees. So far, the use of fruiting synchrony to make foraging decisions has been reported in Japanese Macaques (Macaca fuscata), grey-cheeked mangabeys (Lophocebus albigena), and chimpanzees (Pan troglodytes). Javan gibbons (Hylobates moloch) have been reported to visit trees of the same species after fruit discovery at the beginning of the fruiting season, even if they are not yet bearing fruit. However, there was no difference in their visitation rate between asynchronous and highly synchronous species.

When it comes to the use of external information in primates, primates have been shown to use all senses (vision, olfaction, hearing, touch, and taste) to acquire food. However, compared to the other senses, our knowledge of the extent to which olfactory information is used by primates to locate food is still lacking. One explanation for the assumption that olfaction is not important for primates is that primates have fewer olfactory receptors (OR) compared to other species and that the relative size of the olfactory bulb is on average smaller in primates than in other vertebrates. Nevertheless, recent research has shown that olfaction does play an important role in assessing the quality of food from a short distance. However, little is known about how primates use olfaction to locate food in the first place. The use of olfaction to locate food has only been reported in several species of new world primates, and in one strepsirrhine species. It has rarely been studied in old-world monkeys or apes, one study has reported that chimpanzees might use olfactory cues to find fruit-bearing trees but did not rely solely on olfactory cues. 

In this study, I wanted to investigate the use of internal information and external information for making foraging decisions in an old-world monkey species. To this end, I tested the use of synchrony cues and the use of olfaction to make foraging decisions in zoo-housed mandrills (Mandrillus sphinx). Mandrills have a mainly frugivorous diet and live in a rainforest with simultaneously fruiting trees, so the use of synchrony cues is expected to be an adaptive foraging strategy for wild mandrills. Furthermore, mandrills have demonstrated advanced cognitive abilities in previous research, making them suitable for studying cognitive mechanisms involved in foraging. Moreover, mandrills have been observed employing olfaction for social communication, implying that this sense may hold significance in other aspects of their lives, including foraging. 

The use of synchrony cues for making foraging decisions was tested in a group foraging context using a cognitive enrichment procedure. Prior to this study, mandrills had learned the locations of two different food rewards (grape and carrot) in their outside enclosure. To investigate if cues could trigger foraging at specific locations, I presented the mandrills with food and then buried that food at the learned locations. When grapes were buried in the outside enclosure, mandrills were also presented with grapes upon entering their outside enclosure. After this, the first choice of foraging location was recorded. Initially, only the scent of food in the form of juice was presented, but later whole pieces of food were used. 

The use of olfaction to locate food was also tested using an enrichment procedure in a group foraging context. Mandrills were repeatedly faced with a different spatial configuration of puzzle feeders that were spread out in their outside enclosure. Puzzle feeders would be either empty or have a food reward (dried mango) and an accompanying olfactory cue (mango juice). Hence, if a mandrill would want to increase their chance of getting to a puzzle feeder before conspecifics, it would have to make use of olfactory cues to locate the food source. I looked at all first visits at puzzle feeders and scored if the visit was a first foraging choice.  

Due to their reported cognitive abilities in previous research, I expect the mandrills to make significantly more first-foraging choices at foraging locations that contain the same food as the presented food cue in the internal information study, thus indicating that the mandrills use synchrony cues. Due to the importance of olfaction for mandrills’ social behaviour, I expect the mandrills to make significantly more first-foraging choices at olfactory feeders during the external information experiment, indicating that they use olfaction to locate food. 

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