One of the tricky things about studying the behavior of monkeys in the wild is that you have no control over what they are going to do, and therefore your observations are totally at the whim of the animals. If they decide to fly off through the trees over a swamp with water that goes up to your waist, that’s where you have to go to follow them. Or if they decide to sit still out of view high up in a tree tangle for hours upon end, then you’ve got to sit under that tree and wait with them. You’ll get good data when your animals let you get good data. Of course, the upside to that is that you get to see the animals do precisely what the animals want to do. Other than being habituated to a few clumsy researchers following them around on the ground all day, the animals being observed are living in the natural environment they evolved to survive in, allowing you to make conclusions about their natural behavior that cannot be made using only animals in captivity. For example, pictured here are a titi (left) and squirrel (right) monkey feasting on some crackers outside of the hotel where our first boat ride into Tiputini left. They are adorable and getting this close to them took way less effort than getting half as close to just one habituated monkey in the wild would. But all you can say about primate behavior from observing them is that monkeys who are regularly around humans who give them food are pretty much okay with hanging around humans who give them food. [Note: I unfortunately am not allowed to post pictures of the monkeys in Tiputini due to rules about data privacy, so this is the best I can do in terms of monkey pictures].
So primatologists and their field assistants go out into the field day after day to watch the monkeys in their natural habitats, and sometimes the animals do interesting things that provide a lot of data that helps us toward understanding their complex social lives, and sometimes they don’t seem to do much and you wonder how much you really learned anything that day. The contrast between two days I recently spent in the field, one with the sakis and one with the titis, exemplifies this variability quite well. Last week I went out early in the morning to find group L, who I had previously had my best days with, but this time the day was decidedly bad. In my first spotting of the group, I had Lucifer, Lulu, and their infant in my sights for maybe an hour, the majority of which was spent with the monkeys rustling about out of view in the top of the tree. They then decided to take a route through the trees which took them across a stream, forcing me to leave them to use the bridge several hundred meters upstream to get to the other side. I found them after another hour or so of searching, and had enough time to get one and a half twenty-minute observations periods before a downpour caused the monkeys to go high up into a tangle while I sought refuge under my umbrella. When the rain finally stopped, the group was nowhere to beseen, and while I found them and lost them several more times throughout the afternoon due to more rain and the titi’s talents as escape artists, I ended the day with focal samples that consisted largely of data points merely saying certain individuals were active but out of view.
Just two days ago, however, I spent the day following the saki group we are studying, and while the morning started similarly without a whole lot of recognizable activity, in the afternoon things started heating up. The group started spending a lot more time lower in the trees where I had a continuously clear view of their activity, and I began to notice Mendel, the adult male, engaging in some curious behavior. He started with a period of gruff vocalizations, almost like barks, and then began traveling from tree to tree rubbing his chest and sometimes his genitals on the branches he was occupying. He also began following Mona quite closely, not letting her get more than few meters away from him, and often chest rubbing on the spot she was in before. The following eventually stopped with Mona and Mendel settling down together and though I could not see entirely, it appeared that they began grooming eachother. Not long after, the two balls of fur I was looking at (which is generally what sakis look like up in the trees) shifted positions and seemed to meld into one, until I saw Mendel’s face poking out above the furry mass and realized Mendel was moving rhythmically back and forth with Mona below. They were having sex. It was rather brief and in some ways unspectacular, but it loaded the rest of the day’s activity with significance. Mating is obviously one of the most important activities to the animals’ survival, so getting data on everything surrounding such an episode, from the way Mona and Mendel acted throughout the day to the chest rubbing and vocalizing leading up to the sex, is invaluable and might offer insight in to the saki’s mating strategies.
So the lesson is that sometimes you’ll get a days’ worth of exciting and interesting data and sometimes you’ll get hardly anything, but you get to reassure yourself because you are studying monkeys in the wild, making the behavior you to see that much more meaningful. And thankfully those studying animal behavior in the wild are not completely at the whim of their animals, thanks to the possibility of playback experiments. Conducting a playback experiment involves playing a recording of a vocalization that a given group of animals might hear in the wild (their neighbors’ territorial calls, for example) from a hidden speaker, and observing the animals’ responses in order to gain an understanding of the significance of the call and what their responses might say about the animals’ behavioral tendencies. So while the circumstances are in fact artificial, the animals in theory are acting as they would in a completely natural circumstance. And as a researcher you are able to create a situation where you have increased your chances of getting high quality and interesting data that that has the potential to say something significant about the some aspect of the animals’ behavior.
Playbacks are particularly helpful for the work done under the Monogamous Primates Project because they provide an opportunity to observe potential differences in male and female investment in territoriality and mate guarding. Titis, for example, often call together (called dueting) both naturally on their own and in response to the vocalizations of surrounding groups. But males have still been observed to show a stronger response to potential intruders, leading movement toward other groups’ vocalizations and displaying more aggressive behavior than the females. Very little is known about male and female saki vocalizations, but the coordinated displays usually seen in titis has not observed. Seeing precisely how the males and females respond to such encounters can provide hints as to what exactly each sex is getting out of a monogamous social relationship with their partner.
In the playback experiments I have been helping run, we play a recording of the territorial vocalizations often heard in inter-group encounters from a speaker about 40-50 meters away. Ideally, the animals respond to the vocalizations we play with vocalizations of their own, so each time we do a playback we bring the recording equipment and my job is to make the recording while Amy does the behavioral data. The recording needs to be from a neighboring group so that the individuals being observed are actually hearing a call that means something to them, meaning that we need one good quality recording for each group’s neighbors. But even in a controlled experiment like a playback, my experience so far has further shown the variability you get in terms of animal cooperation with your data taking needs. Group B is a group of titis whose territory sits right between the two other titi groups, so getting a recording of their calls is essential for quality playbacks with the other groups, but both times that we tried a playback with them, the group simply didn’t respond with obvious vocalization or other sort of behavior. Its hard to say at this point whether it’s specific to the recording we are using or the group itself, but the recording used was at least good enough to cause group B’s neighbors to call though so it really is rather puzzling
Thankfully, the two other playbacks we have run thus far did in fact lead to some noteworthy behavior. With the sakis, as soon as the recorded call began playing, male Mendel oriented himself toward the speaker, becoming piloerect (meaning his hair was standing on end) and eventually barking in the same direction. The rest of the group meanwhile moved out of view away from the sound of the call, and about 10 minutes after the sound of the call Mendel also moved out of view in the same direction. About 45 minutes later Mendel began calling again, starting out as a bark and eventually leading to a loud trill, which was soon joined by Mona (the adult female). Just before this, however, we heard a different Saki group calling, and Amy spotted them heading away from the group we were following, which adds an interesting but also somewhat complicated twist to group M’s behavior. Mona and Mendel continued calling intermittently for the next fifteen minutes before beginning a foraging bout on a group of swarming army ants, and then their day seemed to progress rather normally.
With the titi group K, the adult male Kong similarly turned directly toward the call and started chirping and moaning. He then took off in the direction of the call with Katta (the adult female) and Katta (the juvenile female) trailing him, first solo calling before Katta joined him in a duet. The sheer volume of the duet is quite impressive from such small animals, and it really is quite amusing to see the titis’ entire bodies shake with each note they bellow in an attempt to intimidate potential intruders. The dueting started and stopped several times throughout the next twenty minutes, with Kong continually moving closer to the speaker with Katta in tow. Eventually they stopped, and spent the next few hours at the edge of their territory in the area where the recording was played.
So what can we say about the significance of these responses? At this point not a whole lot, as we need a lot more data to see what sort of patterns emerge (Amy is hoping to run a total of eight playbacks witheach group over the course of her study), but I do think its interesting and possibly significant that the male titi went directly toward the source of the vocalization and the rest of the group followed, whereas with the sakis, Mendel stayed put while the rest of his group left, and eventually he followed them. Perhaps that had something to do with the presence of the other group of Sakis that we saw and heard later, or perhaps there was some feature of the calls themselves that caused the different responses, but it also seems possible that it reflects a greater concern of sakis with mate-guarding while male titis are less concerned about protecting access to their female and more worried about territory. Going along with line of thinking, I found it curious that throughout the rest of the day, Kong seemed to spend much more time in proximity (including two grooming bouts) with his juvenile offspring Kia, rather than Katta. Even though the events of the morning were clearly quite alarming for Kong, they didn’t appear to lead to increased mate-defense or any real change in the interaction of the Kong and Katta. It would be easy to continue hypothesizing interpretations for the significance of the events surrounding the playback, but so far its only two data points, and the difficult thing with behavioral data is that there a large number of explanations for any given action. Nonetheless, its exciting and refreshing to be able to run a real experiment with the animals that ensures at least a pretty good chance of seeing some truly interesting and significant behavior.