The Science Behind Project Plains Zebra

While the African savannah is a vast region, encompassing almost 5% of the Earth’s terrestrial surface, at present we only have validated records of 4 mass migrations of large mammals across this ecosystem, although there are likely to be many more. These annual migrations of large herbivorous mammals are essential to the ecosystem’s health as these migrations move resources through the ecosystem, and while the Serengeti-Mara migration is very well-documented (e.g., Homewood et al., 2001), migrations also are known to occur in South Sudan (Morjan et al., 2018) and Botswana (Naidoo et al., 2016).

The one large mammal species that is a participant in all the known migrations occurring on the African savannah is the plains zebra: with the blue wildebeest, Grant’s and Thomson’s gazelles in the Serengeti-Mara (Homewood et al., 2001), the white-eared kob, tiang antelope and mongalla gazelles in two separate migrations in South Sudan (Cave and Cruikshank, 1940) and primarily by themselves in Botswana (Naidoo et al., 2016), with anecdotal reports indicate that blue wildebeest may also be involved in this migration. In addition, it has been noted that while the plains zebra present with a substantially smaller population than other migrating species, they appear to be consistently located on the flanks of the migrating herds (Cave and Cruikshank, 1940), indicating a role in directing the other migrating animals (link to video). Thus, we can reasonably conclude that, despite being only a small proportion of all migratory animals, these observations provide circumstantial evidence that the plains zebras are the essential species that allow for these migrations to occur in this ecosystem.

 When compared to all other equids, the plains zebras have by far the largest non-migratory reported home ranges (see Table below), encompassing areas from 100 to 500 km². In contrast, other equids have home ranges an order of magnitude smaller (see Table below) (Skinner and Chimimba, 2005; Wilson and Mittermeier, 2011). If we compare the non-migratory home ranges of sympatric migrating species including blue wildebeest, Grant’s gazelle, and Thomson’s gazelle, we again see that their non-migratory home ranges are an order of magnitude smaller than those of the plains zebra. Unfortunately, the non-migratory home ranges of white-eared kob, tiang antelopes, and Mongalla gazelles are not known. Focussing on non-migrating sympatric species such as the impala, waterbuck, nyala, and kudu have home ranges an order of magnitude smaller than that of the plains zebra, while the nomadic eland has home ranges of up to 360 km² (Wilson and Mittermeier, 2011).

Thus, the plains zebra is distinct to other equids, and indeed sympatric migrating and non-migrating artiodactyls, by having a non-migratory home range an order of magnitude larger. Thus, the large non-migrating home range and the annual migration, both support the notion that the plains zebra are the most navigationally competent species, and that they enable the migrations of large herbivores on the African savannah.

Table summarizing the differences in home range of the phylogenetically related Perissodactyls, sympatric migrating Artiodactyls, and sympatric non-migrating Artiodactyls with the plains zebra. Note, that apart from the nomadic eland, the home range of the plains zebra is an order of magnitude larger than the other species.

Grazing succession studies of the migratory populations have shown that the first species to arrive at any specific location on the migratory paths are the plains zebras, followed by other migratory species in the subsequent days and weeks (Vesey-Fitzgerald, 1960; Gwynne and Bell, 1968; Anderson et al., 2024).

The proposed reason for this temporal migratory sequence is the selective grazing of the plains zebra on the tallest and coarsest grasses (stems), which then exposes and makes more readily accessible the shorter dicotyledon material selectively grazed upon by other migratory species (Vesey-Fitzgerald, 1960; Gwynne and Bell, 1968), or that plains zebras are “nudged” into leading the migrations through competitive grazing (Anderson et al., 2024). This indicates that for the migrations to be successful, the plains zebras must arrive first, otherwise the foliage required for other migratory species will not be exposed and would be more difficult to access.

These observations provide further evidence that the plains zebras, out of necessity, lead the migrations.

A modelling study of the trajectories that plains zebra traverse for migration in Botswana indicated that memory, rather than sensory cues, are what are guiding the migrations (Bracis and Mueller, 2017). In this study it was shown that it is more likely that the plains zebras use their experience of previous resource availability to determine their migratory path, rather than using sensory cues to guide their long-range movements. By using their memory of the migration rather than sensory cues, the modelled plains zebras arrived closer to their migration destination when compared to modelled plains zebras that relied on sensory cues. One prediction that could be made from this modelling study is that the neural navigation system that directs the movements of the plains zebra might be specialized, and this is indeed what we have observed.

Plains zebras brains have an average mass of 537 grams, a mass that one would predict given their body mass (250-350 kg). Most of the brain of the plains zebra appears very similar to what one would see in their close relatives the mountain zebra and domestic horse. The one striking difference we have noted is that the hippocampal formation of the plains zebra is unusually large – in both absolute and relative terms. The absolute volume of the hippocampus in the plains zebra is 10.77 ml, being substantially larger than the 5.19 ml observed in the closely related mountain zebra and the 4.43 ml observed in the domestic horse, clearly exceeding what might be expected from a species with an average brain mass of 537 g compared to the 433 g brain mass of the mountain zebra and the 515 g brain mass of the domestic horse. The hippocampal volume of the plains zebra rivals that of the African elephant (10.84 ml), but with a brain mass of 4900 g the African elephant hippocampus is relatively smaller than that of the plains zebra. Hippocampal volumes reported for humans range from 8.35-8.46 ml (Maguire, 2000, 2006) up to 10.29 ml, thus the size of the plains zebra hippocampus exceeds that of humans, and given that human brains weigh around 1400 g, exceeds the human in size relative to brain mass. Indeed the hippocampus in the plains zebra is three times larger than we would expect for its brain mass.

In addition to this large size, our preliminary studies of the internal organization of the hippocampal formation of the plains zebra revealed a potential internal specialisation. While possessing the typical features of the hippocampal formation (such as entorhinal cortex, subicular complex, cornu ammonis regions CA1-4, and the dentate gyrus), one specific part, CA1, occupies a larger proportion of the hippocampal formation and has approximately 1.64 times the number of neurons in this region than seen in closely related species.

The CA1 region, especially the dorsal part, is of specific interest, as it is within this region that the “place cells” crucial for neural navigation are located. While the study of the plains zebra hippocampus requires a great deal more to be undertaken, what we can conclude from what we have observed is that it appears that a true cognitive specialization, related to navigation, has evolved in the plains zebra. This will provide the neural backing the plains zebras require to lead the migrations and the capacity to remember the migratory path and all associated landmarks needed to navigate the several hundred kilometres they traverse each year.

Our preliminary results indicate that the plains zebra is unique in this sense, with sympatric migratory species, such as the blue wildebeest, and non-migratory closely related species such as the mountain zebra, having hippocampal formations that could be considered quite typical for these species.

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