IntroductionThis study aims to provide a greater understanding of the zebra (Equus quagga) migration that takes place between the Okavango Delta and the Makgadikgadi Pans National Park in Botswana. This migration is a key example of the connectivity between protected areas and will highlight the importance of conserving movement corridors and critical resources along the migration route.
Researcher: Dr Emily BennittRegion: Moremi Game Reserve and Makgadikgadi Pans National Park, Botswana
BackgroundIn 2007, it was discovered that zebra were migrating between the Okavango Delta and the Makgadikgadi Pans National Park (MPNP). It is one of the longest zebra migrations in the world, with these zebra joining a population of 15 000 other individuals that permanently reside in the MPNP.
Migrating zebra take 10-14 days to cover the distance between the two areas, which must put a substantial energetic strain on them. While the timing of the outward journey has been related to rainfall and vegetation, insufficient data were available from the previous study to identify factors triggering the return journey. Eight female zebra were collared in November 2014, but of the two that initiated migration movements, only one completed the migration to the Makgadikgadi.
In September 2015, these two individuals were used to target associated harems and nine female zebra were fitted with GPS-enabled collars. Shortly afterwards, the drop-off units on the first eight collars were triggered and the collars were recovered.
During the rainy season of 2015–2016, eight of the collared zebra initiated migration movements, but only one completed the route to the Makgadikgadi Pans. This rainy season was unusual, in that only small amounts of rain fell prior to March, by which time many zebra had given birth. It seems likely that unusual rainfall patterns, combined with strong maternal instincts to avoid exposing young to risks associated with long-distance movements, were responsible for the observed lack of complete migration. These collars are programmed to drop off in April 2017, and it is probable that the collared zebra will migrate during the 2016–2017 rainy season, as long as sufficient rain falls early in the season. Such data would provide key information about behavioural responses to environmental conditions and lead to greater understanding of the factors governing migration.
More research is required to understand the environmental triggers for the movement, and to quantify the proportion of the total population in the Delta and the MPNP that engage in this migration. This study will provide a greater understanding of the migration, a key example of connectivity between protected areas, and highlight the importance of conserving movement corridors and critical resources along the migration route.
The results from this research will provide information on a crucial natural process linking protected areas, which will benefit stakeholders such as safari operators and local communities, and will also help to advise government departments on the best conservation policy to ensure continued connectivity between Botswana’s national parks.
Objectives– Quantify the population of zebra migrating between the Delta and the MPNP
– Identify environmental factors triggering migration (outward and return journeys)
– Understand the metabolic costs of migration (energetics and muscle efficiency)
MethodologyNine female zebra are currently wearing GPS-enabled collars designed and manufactured by the Royal Veterinary College, London. These collars have been programmed to drop off following a remote trigger, removing the need to dart the zebra again to recover them.
a. Camera traps have been placed along the main game trails used by migrating zebra, as identified by previous research. These may not provide exact numbers, but they will allow us to identify annual changes in the number of animals migrating, temporal preferences, group structure, and route preferences. The Wilderness Wildlife Trust provided four camera traps in 2014, but two of those have been damaged by elephants and rainfall, one frequently over-exposes images and the fourth resets its time stamp often, reducing the accuracy of recorded data.
b. Use remote sensing technology, such as LIDAR and NDVI, in combination with climatic variables, such as rainfall, to relate movement patterns to environmental triggers.
c. Dr Wilson, a veterinarian registered in Botswana, has secured small muscle samples from collared individuals. These have been exported to his laboratory in the UK, where they are being subjected to in vitro experiments to quantify muscle power, velocity and efficiency under different conditions.
In 2011, Dr Hattie Bartlam-Brooks recorded a previously-unknown migration of plains zebra (Equus quagga) that spent the wet season in the Makgadikgadi Pans National Park (MPNP) and the dry season in the Moremi Game Reserve (MGR). This discovery did not form part of her original planned study, so she was not able to collect sufficient data to fully understand the triggers, and ecological costs and benefits of the migration.
In November 2014, Dr Bartlam-Brooks and I began a collaboration with Prof. Alan Wilson of the Royal Veterinary College, University of London. We agreed to work together on this migratory population, and he supplied us with eight GPS-enabled collars. We deployed them onto female zebra in different harems in the southern part of the MGR. The following wet season, only two of those animals showed any signs of migrating, and only one completed the migration. Her collar unfortunately stopped transmitting a VHF (Very High Frequency) signal, so we were unable to recover the data. All of the collars were fitted with drop-off devices that were programmed to allow the collars to fall off without further intervention in September 2015.
We used the remaining animal that had attempted to migrate to fit another collar to a female in her harem in September 2015. We then used that animal to identify associated harems that were likely to migrate, and deployed eight additional collars in October 2015. All of those nine animals attempted at least partial migrations, but only one completed the route. We suspect that this is linked to the unusual rainfall patterns that saw very little rain before the end of February, by which time many zebra had given birth. The collars are scheduled to drop off at the end of April 2017, so we hope that a more usual rainfall pattern this rainy season will trigger a migration in our collared animals, although one of the collars has broken and been recovered in the meantime.
Camera trap summary
In partnership with the data from the collars, we deployed four camera traps in November 2014. Two each were placed at points along the migration route, identified by Dr Bartlam-Brooks, where zebra passed close to fence corners. These were later supplemented by two additional camera traps, one in each location.
We have visited each station every 1-2 months since deployment to change batteries and download images. Unfortunately, some of the camera traps were damaged by water or elephants, but we were allocated new funds from the Wilderness Safaris Wildlife Trust to replace them and continue the study.
Since the camera traps were deployed, we have recorded many images of different species, including zebra. Elephant have been most numerous, but we have also had good numbers of buffalo, sable, roan, kudu and steenbok, as well as African wild dog, lion, leopard and hyaena.
Surprisingly, we have recorded zebra sightings throughout the year, although we do see certain trends. We have split the data according to the direction of movement, towards MPNP (outbound) and towards MGR (return). We saw a peak in the number of outbound migrants in November 2014, when rainfall was in line with the usual temporal patterns. Since then, we have seen relatively few outbound migrants. These records do not show how many animals completed the migration, only the number of individuals who passed in front of the camera traps in a particular direction.
Overall, there were more individuals observed in the return direction, and these had more definite peaks. Some individuals in December 2014 would have initiated the migration, but, perhaps because of insufficient water availability, returned to their dry season range, which would explain the peak at that time. The second peak corresponded to the months following the end of the rainy season, when animals would have been making their way back to their dry season range. The last peak corresponded to the unusual rainfall patterns in the last rainy season, which may have caused several harems to initiate and abort migration journeys, a hypothesis that appears to be supported by our collar data.
We will shortly be visiting the camera traps to download images and replace the batteries, in anticipation of the first rains, which could be as early as the end of September.
We believe that the data from the camera traps provide a broader, population-level understanding of the movement patterns, which complements the data provided on individuals by the collars. This rainy season, in addition to our collars on this migration, we will also have zebra collared on the Boteti. This population migrates to the same area as our existing study population during the rainy season, but their migration route covers approximately 50 km, as opposed to the 290 km travelled by the zebra from the MGR. We therefore expect to see some differences between the two populations in terms of their migration triggers and the movement patterns along the route.
We hope to collect sufficient data from the collared animals to submit a scientific article for publication in a peer-reviewed journal next year.
Annual Report, February 2016
Of the eight zebra that were collared in November 2014, only one individual (Z596) completed the migration from the Okavango Delta to the Makgadikgadi Pans. A second (Z593) began migrating but turned around before reaching the halfway point. Z596 departed several days before Z593, and her journey coincided with several large storms, which she appeared to move towards (Figure 1). We think that there was insufficient water for Z593 to complete the migration when she attempted it several days later, and that she would have migrated under different conditions.
The other six collared zebra remained in the Okavango Delta until their collars dropped off on 17 September 2015. As the VHF components of some of the collars ceased functioning, not all were recovered. In October 2015, a further eight collars were deployed onto zebra identified as potential migrants. Unfortunately, this year has also seen unusual rainfall, and in January 2016, the collared zebra were still in the Okavango Delta.
Six camera traps are currently deployed along the zebra migration route, strategically placed at optimal locations for capturing images of the maximum number of migrants. The camera traps were last checked in December 2015, at which point only five zebra harems had passed in front of them in the direction of the Makgadikgadi, and within eight days, five zebra harems passed in front of the camera traps in the opposite direction. This suggests that some zebra began the migration but probably turned around and returned to the Okavango because of insufficient water availability.