This is a quick post on a paper just published in the Journal of Zoology, in collaboration with Jodie Martin and Norman Owen-Smith. The idea came to me during a field excursion in Kruger NP, when I noticed the influence of the full-moon rise on nightjars (fiery-necked nightjar from what I recall). This got me thinking about African ungulate activity over the full moon phase, and then quite quickly an idea on a paper came about given that we had access to GPS-collar data for wildebeest, zebra and lion.
Anyhow, I assumed a priori that both zebra and wildebeest would be more active over the full moon period given the additional light provided – as I’m sure many people would. When I got around to reading the published literature, it transpired that indeed, (mostly anecdotal) evidence suggested that African herbivores such as kudu, gemsbok and warthog were more active over the full moon period. A recent meta-analysis on the influence of moon phase on mammals showed that species dependent principally on vision are more active over the full moon period. This ‘visual acuity hypothesis’ stated that such species would be more active over the full moon period given that the increased light would increase opportunity to forage and/or detect predators.
We don’t know to what extent zebra and wildebeest rely on vision to detect predators, but one would assume that these animals would be more active over full moons right? Wrong. When we looked at the data, we found that when lion are not around (as in further than 1 km from zebra or wildebeest), there is very little difference in the night-time movement patterns of zebra and wildebeest, over moon phase. So moonlight does not matter. But when lion are within 1 km of these animals, then there is a big difference (See Figure), with prey animals being much more active over the new moon phase, when lions are most likely hunting.
Interesting no? We think there may be quite simple ecological reasons for this; wildebeest in our study area (Orpen gate in Kruger NP), use open grazing areas, coined ‘grazing lawns’ that provide not only forage but also opportunity to detect ambush predators, like lion. At night, the wildebeest are effectively ‘anchored’ to these sites given the over-riding danger of lion lurking in the surrounding thicket. And so regardless of moon phase – they stay put. They are more restless, or disturbed when lions are nearby over the new moon phase, but not by much.
Non-ruminant, hind gut fermenters such as zebra however require greater forage intake, even overnight, and so their forage needs over-ride the risks posed by lurking lions. Thus, they move at an equal rate regardless of moon phase – until they encounter lion, and then they run like hell. Well, that is what we think!
Our article can be viewed here, or e-mail me for a copy.