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At any given time, our field teams keep track of about 330 lions in 24 prides in the Serengeti, 50–60 lions in 5 prides on the floor of Ngorongoro Crater, 150–200 lions in 10 prides in Tarangire, and 100 lions in 15 prides in the Selous. This daily monitoring maintains demographic records that were started in the 1960s, yielding one of the most extensive datasets on any mammalian species anywhere in the world—over 5,000 lions have been included in the Serengeti and Crater studies over the past 40+ years, and genealogical data from these two populations extend over 12 generations. The daily records include information on the lions’ location, group size, diet, food intake, health and reproduction.
Only continuous long-term monitoring can address one of the fundamental issues in ecology: “What limits the size of a population?” This question is especially hard to answer in a changing environment. For example, the Serengeti woodlands have expanded and grasses have become taller over the past 40 years, providing the lions with more cover from which to ambush their prey, and the Serengeti lion population is currently at an all-time high.
However, East Africa has experienced a dramatic increase in the frequency of drought and flood in response to global climate change, and droughts have served to synchronize infections by canine distemper virus and a tick-borne disease that together killed over a third of the lions in the Serengeti in 1994 and Ngorongoro Crater in 2001.
Most importantly, the human population in rural Tanzania is increasing exponentially. More people means more habitat loss, more poaching, more retaliatory lion killing and more frequent disease outbreaks originating in domestic dogs.
Demographic monitoring provides an accurate count of the lions in each population and is essential for assessing the timing and impacts of these threats. Furthermore, the demographic data form the backbone for detailed studies of lion behavior and ecology, such as why lions have manes, why lions are social, and how lions coexist with other top predators in the ecosystem.
Monitoring consists of finding as many lions as possible as often as possible. Prides of lions are territorial, so we know approximately where to search, but locating lions is often very difficult. Between 1966 and 1984, we focused our efforts on known Serengeti “hot-spots” such as water holes, river banks and kopjes, scanned the horizon for hours with binoculars and went for days without seeing a single lion. Beginning in 1984, a female from each Serengeti pride was fitted with a radio-collar, and our success rates increased dramatically. Now our Serengeti, Tarangire and Selous research vehicles are outfitted with radio-tracking equipment, including headphones, a receiver and an antenna. A day in the life of each lion researcher is accompanied by the sounds of hissing and static punctuated by the ever-elusive beeps signaling the proximity of a collared lion.
Once a lion is located, we first identify every individual in the group. Each lion is born with a unique pattern of whisker spots that never changes, thus providing a natural “mark” for each animal in the population. Details of whisker spots, ear notches, eye coloration, coat color and persistent scars are recorded on an identification card for each individual. Since lions only associate with their pride-mates, we simply compare the spots of each individual in the group to the ID cards for that pride.
In addition to group composition, we record information about each lion’s belly size on a scale from 1 (full to the point of bursting) to 5 (emaciated), signs of illness, injury, pregnancy and lactation, as well as behavior and identity of mating partners. If the lions are feeding at a kill, we note the prey species, its age-sex class and whether the lions killed or scavenged it. Finally, we use a GPS unit to record their location.
Field data are transcribed onto “pride sheets” and entered into an Access database; the electronic data are then emailed to the Lion Research Center in Saint Paul where we rely on an army of enthusiastic undergraduates who assist in checking the data. The updates are added to the computer archives and used in statistical analyses of population fluctuations, fitness parameters and behavior.
If you go to the Serengeti, Tarangire or Matambwe, you might see a radio-collared lion. Collars are essential for monitoring each of these populations (no collars are necessary in Ngorongoro Crater where the lions are easily located by eye). Here are some of the more common questions about radio-collars.
A radio collar is a wide band of machine-belting fitted with a small radio transmitter and battery. The transmitter emits a signal at a specific frequency that can be tracked from up to 5 kms away. When trying to locate a particular collared lion, the researcher dials the appropriate frequency and drives while listening for the signal (“beep-beep-beep”). A directional antenna is mounted on top of the vehicle, and once the signal is detected, the researcher simply drives in the direction where the signal is loudest.
In most habitats, lions can be difficult to find. Pride territories may be as large as 400 km2, and lions are often hidden in dense vegetation or rough terrain. In order to see each lion every 2–3 days, we therefore rely on radio telemetry. Radio-collars permitted rapid detection of a major disease outbreak in the Serengeti in 1994 and enabled an eventual diagnosis for the cause of the die-off. Subsequently, a large-scale vaccination program for village dogs was established to protect Serengeti wildlife from canine distemper. Similarly, radio tracking revealed the extent to which the Tarangire lion population is affected by retaliatory killing by Maasai pastoralists outside the national park, thereby leading to the large-scale conflict-mitigation program currently underway.
A Serengeti veterinarian uses a dart gun to inject the lion with a drug called Telazol. Once the lion is immobilized, we fit the radio collar and also collect a set of biological samples (blood, urine, saliva, ticks) to measure physiological status. The lion is typically back on its feet after two hours, and the vet remains with the lion until it is fully recovered.
The lion takes a day or two to get accustomed to the collar, but she soon ignores it (as do domestic dogs). The collar is snug enough to prevent getting stuck in vegetation and loose enough to be comfortable whether the lion is upright, stretched out on its side or curled up in the rain.
Over the past 36 years, we have found no difference in life span or reproduction between collared and un-collared lions. Nevertheless, we have set the strength of the transmitters so that the batteries last for over four years, thus minimizing the number of times that the collar will need to be replaced on each animal.
