The Temporal and Spatial Availability of Chimpanzee (Pan troglodytes schweinfurthii) Prey Species in the Issa Valley, Tanzania
Introduction
Hunting is one of the most cost effective means of acquiring the necessary nutrients for an animal to maintain its fitness. Vertebrate prey is especially valuable due to its often year around availability and relatively large package size. Chimpanzees were first observed to hunt in the 1960s and over 40 vertebrate prey species have been recorded since then. Meat is an extremely valuable social resource which assists hunters most notably in building alliances and securing mates. Hunting strategies and prey preference differs between regions and communities. In the Issa Valley, Tanzania, chimpanzees hunt seven small mammal species.
Blue duiker
(Philantomba monticola)
Bushbuck
(Tragelaphus scriptus)
Checkered elephant shrew
(Rhynchocyon cirnei)
Greater galago
(Otolemur crassicaudatus)
Klipspringer
(Oreotragus oreotragus)
Red-tailed monkey
(Cercopithecus ascanius)
Yellow baboon
(Papio cynocephalus)
I use camera trap videos and the associated habitat and topography data to determine when and where these species are most available for the chimpanzees. My hypotheses were that (a) all species except for greater galagos will be available predominantly during the day, (b) subadult bushbucks may exhibit some seasonality, but all other species will be available evenly throughout the year and (c) blue duikers, bushbucks, checkered elephant shrews, greater galagos, and red-tailed monkeys will be more abundant in more densely forested areas while klipspringers and yellow baboons will more frequently inhabit more open landscapes.
Study Area
Greater Mahale Ecosystem Research & Conservation (GMERC) is based in the Issa Valley, located in the Katavi region of Western Tanzania. The Issa Valley, over 90 km east of Lake Tanganyika, is a highly seasonal and dry (annual rainfall <1250 mm) habitat. It consists largely of miombo woodland interspersed with thin riverine and thicket forest, swamp, and grassland type habitats. Research on chimpanzees in the region was first performed in 2001 and has been running continuously since 2005. A permanent station was established in 2008, and the chimpanzees became habituated in 2017. In addition to researchers on site, the area is monitored by an extensive network of camera traps.
Methods
Camera trap footage from the Issa Valley is sent by GMERC to UC Santa Cruz on an approximately yearly basis to be coded by an undergraduate team of video coders. Our team records the species, quantity, behavior and, when possible, demographic information for each video. I used the compiled data sheets from 2016 to 2018 to find all videos of the chimpanzee prey species, about 9000 videos. After checking species IDs, recording demographics for select species and manually recording the date and time, I added information associated with each tree tag.
Results
Blue duikers had to be excluded due to logistical problems. Additionally, the large size of adult bushbucks and yellow baboons prevents chimpanzees from successfully hunting them, so only subadult individuals of these species were considered in this study. This resulted in 713 videos total.
Habitat Type
Possible habitat types included closed forest, forest, open forest, swamp, woodland, and woodland rocky.
A X² test with 95% confidence revealed that the number of observations made in each category are significantly different (p = 3.56E-103) with the forest and woodland habitat types dominating over all.
None of the species had an even distribution throughout the six categories.
Topography Type
Topography types include moderate, plateau, sever, and valley.
Severe topography dominated the sample (n = 196, 52%) and every species was observed the most in this topography type.
A X² test with 95% confidence showed there was a significant difference between topography type (p = 4.25E-80) and distribution throughout the different topography types was different for each species.
Time of Day
Day (6:33-18:40), Twilight (6:02-6:32 and 18:41-19:11), and Night (19:12-6:01)
Most videos were recorded during the day (n = 594), some at night (n = 104), and very few during twilight (n = 15).
The number of observations in either category are significantly different, X² (2, 713) = 818, p = 2.32E-178.
The greater galago was the only species active predominantly during the night, X² (2, 41) = 76.15, p = 2.92E-17.
Season
There are two distinct seasons in the Issa Valley, the wet season, Nov-April, and the dry season, May until Oct (<60mm of rain)
Observations were split nearly evenly between the wet and dry season (n = 365, n = 348, respectively).
Subadult bushbucks were more abundant in the wet season, X² (1, 167) = 10.07, p = 0.00015.
Checkered elephant shrews were more available in the dry season, X² (1, 124) = 8.25, p = 0.0041.
Month
The number of videos recorded each month were significantly different from each other, X² (11, 713) = 148.29, p = 3.33E-26).
Only klipspringers were recorded evenly throughout the year, X² (11, 25) = 17.72, p = 0.088, but this could be due to the small sample size (n = 25).
September-December and April had the highest number of videos recorded.
Overall, the number of videos recorded peaked in the fall.
Discussion
The results of my study shed light on where the chimpanzee prey species are most available throughout the landscape, day and year. Most species were most available in woodland or forested habitats and in severe topography. With the exception of the greater galago, all species were observed during the day. Additionally, all but two species were observed evenly throughout the wet and dry seasons. These results suggest when and where the Issa chimpanzees would have the most success hunting vertebrate prey.
Bushbuck
(Tragelaphus scriptus)
Forested and woodland habitats
Severe, moderate and valley topography
More available during the day and in the wet season
Checkered elephant shrew
(Rhynchocyon cirnei)
Almost exclusively forest habitats
Severe topography
More available in the daytime and during the dry season
Greater galago
(Otolemur crassicaudatus)
Forested and woodland habitats
Severe topography
More available at night
Klipspringer
(Oreotragus oreotragus)
Almost exclusively observed in woodland habitat
Severe and moderate topography
More available during the day
Red-tailed monkey
(Cercopithecus ascanius)
Forested habitats
Severe and valley topography
Most available during the day
Yellow baboon
(Papio cynocephalus)
Woodland habitat
Severe or valley topography
Most available during the day
Limitations
Camera traps were not all deployed for the same amount of time, and the distribution of camera traps was not even throughout habitat and topography types. It is therefore likely that the data collected is biased towards the camera traps that were deployed for longer periods of time and the regions with more camera traps. Additionally, camera traps are deployed close together, meaning that it is possible for some animals to be caught on camera traps not necessarily in their preferred habitat which could potentially skew results.
Conclusion
My preliminary exploration into the availability of chimpanzee prey species may shed more light on the Issa chimpanzees’ hunting behaviors. Whether or not the Issa chimpanzees hunt entirely opportunistically or if they display forethought and planning by actively seeking out their prey may be discussed from comparing these results to the spatial and temporal distribution of the chimpanzees themselves to see where and when the species are in the same place at the same time.
Impacts
Research conducted on chimpanzees, especially in the field or in conjunction with a field site, provides a level of protection from threats of poaching for the chimpanzees and their sympatric fauna. As an umbrella species, protecting chimpanzee populations has an ultimately positive impact on the habitats in which they live, for example, by maintaining the forest with seed dispersal and limiting prey populations from expanding more than the ecosystem is able to support. These impacts all work together to keep the forest, and the countless species that are found there, healthy.