- The first example is animal-based, but not strictly natural - during Inca times, vicuña (Vicugna vicugna, relatives of llamas) were sheared and then released during a herding event called a chacu, held once every four years. As vicuña produce about 500g of wool per year, a lot of animals were needed to make more than a few socks and scarves...
|A vicuña in Peru; thanks to Alexandre Buisse for putting this image in the public domain.|
- Female gorillas produce young about once every four years - about the same time that a young gorilla stays with its mother. A silverback male will look after an orphaned youngster as long as it has been weaned. More here.
- Similarly, female African bush/savannah elephants Loxodonta africana breed every four years...
- ...and so do North American bears.
- Lemming (rodents in the genera Dicrostonyx, Lemmus, Myopus and Synaptomys) populations crash every four years or so - the reasons are not fully understood (it may be to do with a combination of predation, food supply and seasonal conditions such as winter length - more info on the 'Lemming Cycle' here), but when lemming numbers are low, the spectacular Snowy Owl (Bubo scandiacus) is more likely to move south, being seen in areas where it is not usually found, including northern areas of Britain - very exciting for bird-watchers! Arctic fox (Vulpes lagopus) populations also appear to peak in reponse to high lemming prey numbers, but foxes can't fly to Britain when food is scarce.
- In the western states of the USA, wild horse herds can double in size around once every four years. For this reason, the Bureau of Land Management publishes and implements its Policy Handbook on Wild Horse and Burro Management.
- Salmon runs in North America can be impressive autumn events is one of the largest in North America. One of these, in the Adams River of British Columbia, Canada, is particularly large with peak 'dominant years' about every four years. These are marked by the quadrennial 'Salute to the Sockeye' festival.
There are other examples in nature, but in human society, many activities follow a four-year cycle for reasons of tradition and the practicalities of organising large events such as the Olympics and the Football World Cup. This tendency towards four-year cycles sometimes affect wildlife - for example, tiger surveys in India were undertaken every four years, but it has become clear that changes in tiger populations can occur over the course of a single year. This means that important population changes, and the causes of them, could be missed and there is now movement towards more appropriately frequent annual monitoring. Other event such as the Sockeye Salmon festival mentioned above are quite different, with human activities being driven by natural cycles - as it should be. Happy Leap Day!