Finding water isn’t the easiest when studying in a hot desert. Ironically, we’re here studying waterways specifically, and how their seasonal activity affects one of Africa’s less-often-considered animal groups: the bats.
An Afternoon With the Ellies
Our first day on the Huab River went great. We turned upstream from the gravel road and found a huge pool of water a few kilometers later. Archie Gawusab, my local field assistant, and I then spent the afternoon climbing up and quietly sitting on the rocks above seven elephants.
There is nothing in the world quite like watching a family of desert elephants at water. The juveniles were playing, shoving a young male into a knee-deep pool. A mother doused herself in the cool liquid, a welcome relief I am sure from the afternoon heat. Meanwhile, another mother helped her newborn cross the water after he nearly lost his footing, steering him away from the curious juveniles. It is extremely humbling to spend a few quiet hours alone and unnoticed while seated above a family of elephants before they silently continue traveling along the dry riverbed.
That night, we caught only one bat in the cool evening breeze. We packed up quickly and set up camp, hoping the next evening downstream on the Huab would be more successful.
A Bit More on Where I Work
The major rivers in northwest Namibia run from east to west. They drain into the Atlantic Ocean if their variable, short, seasonal floods can make their way through the sand dunes of Skeleton Coast National Park. Average rainfall in my study area is about 1-4 inches per year with evaporation rates typically well above the average annual rainfall. Unsurprisingly, these riverbeds are dry throughout most of the year. Therefore, wildlife, livestock, and people rely on small natural springs and artificial waterholes.
My pilot study takes place along three of the ephemeral rivers—the Huab, Hoanib, and Hoarusib—in the Kunene Region of northwestern Namibia, including some of the smaller rivers in between. Over many years, they have carved canyons into the landscape, creating plenty of rocky habitat for the bats that prefer to roost in rock crevices and caves. Large trees similar to our cottonwoods in the United States, such as the Ana tree (Faidherbia albida), are also present sporadically in and along the riverbeds. These large trees with loose bark might be important for tree-roosting bats in the area.
Some Bat Facts
The past few nights, the moonlight has likely been interfering with our netting efforts, but we are still capturing a few bats and collecting more acoustic data. When we can see the net in the middle of the night due to moonlight, there is no doubt that a bat adapted to foraging in the darkness can see it too and avoid being caught. Now that the full moon has past, I am hoping for some darker skies and better luck with our capture efforts.The Egyptian free-tailed bat, Tadarida aegyptiaca, is widespread and abundant in southern Africa. The “free-tailed” bats, or molossidae family, are characterized by a tail that extends beyond the tail membrane, which allows for a degree of fine tuning in their flight maneuvers when flying at high speeds in pursuit of insects. (Photo by Theresa Laverty)
While bats do not need water at their roost sites, most insect-eating bats should require some form of water each day to maintain their high metabolic rates. The resting heart rate of a bat is about 450 beats per minute, which approaches 1,200 beats per minute when flying. In comparison, a normal resting heart rate for adult human beings is 60-100 beats per minute. To save energy and slow their water consumption, bats can enter torpor, or short bouts of hibernation, during which their heart rate may slow to one to two beats per minute.
Since bats rely on waterholes both for drinking and as hunting grounds for prey, I net over the water in attempt to target the widest diversity of insectivorous bats. From the time a bat enters the net, I try to have it released within 15 to 20 minutes. You can feel their hearts race as you untangle them from the mist net and process them afterward. Since some insect-eating bats consume up to 100 percent of their body weight each night, it’s important for me to do my work quickly and have them released as soon as possible, so they can continue foraging throughout the night.
In Search of Netting Sites
The presence of water does not unfortunately always indicate a good netting site. Since I work among elephants, rhinos, and lions, it is important to keep the bakkie (the regional term for pickup truck) close at riskier sites.
The past few days, Archie and I found five waterholes that we had to regrettably turn down as potential field sites for that very reason. We either could not get the bakkie close enough to the water due to the rough terrain or there was not adequate room for our bakkie and large wildlife to share the road and water site. Given the abundance of fresh elephant, rhino, and lion tracks, every day we assess each site to determine our best “safe” option for mist netting bats that evening.
Our first night off after two weeks of netting occurred yesterday because we were unable to find a field site by nightfall. (As a reminder, I am writing this post on January 5, 2015, to be posted when I return from the field). After a long day of driving, however, we did finally bump into our first pride of desert lions resting under a tree along the Springbok River (just north of the Huab River). A few minutes later, we also spotted a browsing black rhino. Good thing we opted not to net at a nearby water hole that night where we would have had to sit far from the vehicle and would have likely had some large mammal encounters.
Upcoming Plans and Overall Observations
With clouds building in the evenings, Archie and I are on our way north to Puros and the Hoarusib River. The wet season in the Kunene Region is typically reported to take place between January and April, although it is highly variable.
The Huab River felt much wider than the Hoanib River. The Huab River was also thick with reeds (often close to three meters high) and native tamarix, limiting our netting abilities. Therefore, with two weeks of fieldwork to go, I am excited for a glimpse at another river system. As an added bonus, I hear the Hoarusib River flows aboveground (!) for several kilometers in the area where I am hoping to work.