Hiding in Plain Sight: Evidence of Hybridization between Cape Mountain Zebra (Equus zebra zebra) and Plains Zebra (Equus quagga burchelli)

Historically, Cape mountain zebras (Equus zebra zebra) were widely distributed along mountain ranges in the Eastern and Western Cape provinces of South Africa (Boshoff, Landman & Kerley, 2015). By the 1930s, excessive hunting and habitat loss resulted in a reduction in Cape mountain zebra numbers with populations being confined to only five localities. Two of these subpopulations subsequently became extinct. Three relic populations currently exist; the population in the Cradock district,was formally protected in 1937 by the proclamation of the Mountain Zebra National Park (MZNP; Lloyd 1984). The two other populations in the Kammanassie and Gamka Mountains, have been protected since 1923 and 1971, respectively. Cape mountain zebra numbers increased steadily from their critical status of fewer than 80 individuals in the 1950s, to an estimated minimum of 4791 individuals by 2015 (Hrabar & Kerley, 2015). Plains zebra (Equus quagga burchelli) were subsequently introduced, in sympatry with Cape mountain zebra into four formally protected areas, including the MZNP in 1999 and Karoo National Park in 1998. Until recently no cases of hybridization between plains zebra and Cape mountain zebra were known. Hybridization was not of great concern as a threat to Cape mountain zebra populations as fertile hybrids were thought to be unlikely, due to the relatively large difference in the number of chromosomal pairs between the two species (44 versus 32 in plains zebra and Cape mountain zebra, respectively; Ryder, Epel & Benirschke, 1978; Cordingley et al., 2009; Hrabar & Kerley, 2013). By 2013, the plains zebra population had increased substantially in the MZNP (estimated at 769 Cape mountain zebra and 124 plains zebra by aerial census (unpublished aerial census data, 2013) and were potentially competing with Cape mountain zebra for resources. A decision was thus taken to remove the plains zebra. This intervention resulted in a disruption in the social structure, and some of the small, fragmented groups or plains zebra individuals joined Cape mountain zebra herds. In addition, conservation officials observed ‘Cape mountain zebra’ with plains zebra characteristics. These included slight shadow striping, stripes extending all the way down to the ventral midline of the chest and abdomen, and, although they did have the reddish muzzle of mountain zebra, they did not have the characteristic mountain zebra gridiron pattern on their rumps (Fig. 1), but rather had absent or distorted patterns on the rump. They exhibited the distinct dewlap of the Cape mountain zebra and ear shapes were similar to plains zebra. This raised concerns of possible hybridization between the two species. Here, we report on a molecular evaluation using maternal, paternal and biparental markers to identify suspected hybrid Cape mountain and plains zebra in MZNP and Karoo National Park, South Africa.