The remarkable life history of the Monarch butterfly in North America (Danaus plexippus) is very well-known not least because the entire population from North America east of the Rockies overwinters at a few sites in the Mexican mountains where they constitute one of the great wildlife spectacles of the world. How and why the Monarch does what it does raises all sorts of fundamental biological questions. In short, in spring the butterflies leave the mountains of central Mexico and fly north. Once in the USA they breed rapidly; the caterpillars which feed on milkweed which is toxic to most animals then pupate and hatch. Those new adults then fly north and repeat the whole process again and again, until the 5th generation is reached. All the members of that generation which by then have been hatched in Canada and the northernmost parts of the USA then fly all the way back—up to 2,800 miles (4,500 km)—to the same areas of the same mountains in Mexico which their great-great-great grandparents left months earlier. The whole winter is spent closely packed on the branches of oyamel fir trees over small areas. Each hectare of occupied forest is estimated to hold over 20 million Monarchs. On fine and not-too-windy days some butterflies will take flight. They are said not to consume much nectar during the winter but do take on water.
It is no wonder that the brain teems with questions ranging from how the return flight to the USA is spring is fuelled through the method of navigation to the genetic and environmental factors controlling the whole process.
The pine forests in Mexico used by the Monarchs to overwinter are at an altitude of approximately 15,000 feet. The altitude chosen is said to be increasing as temperatures have risen. Overwintering in a cool place is clearly an important mechanism of preserving energy supplies. Why they pack themselves so closely in the trees is an interesting question.
In Mexico we visited two Monarch sites: Piedra Herrada Monarch Reserve on one morning and Sierra Chincua on another. Both were well set up for visitors. Getting to the butterflies involves an initial climb on horseback and then a climb on foot, with much puffing and panting initially by the horses and then by their riders to nearly 11,000 feet (3,300 metres).
Here are a some photographs:
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| At both sites the ascent to the Monarchs began on a horse |
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| Piedra Harreda. The cluster of Monarchs can be seen high in the trees |
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| At Sierra Chincua the Monarchs were in oyamel firs on a windy ridge at ca 11,000 feet |
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| Butterfly watching at Sierra Chincua |
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| Densely packed Monarchs on an oyamel fir tree |
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| Deep in shad it is hard to take in the fact that every 'leaf' is in fact a Monarch with the branches of the oyamel fir weighed down by the extra weight being carried |
After our return we read that the population was estimated to have doubled compared with the previous winter. The overall trend has been downwards. However, the method used to estimate the total population is very indirect (the number of hectares occupied by the overwintering Monarchs). All sorts of factors have been suggested to account for the apparent decline, from logging in Mexico (although the sites occupied are very small) to a fall in the abundance of milkweed. As with all butterflies there seems to be large variation depending on the weather.
I read that the common name Monarch for this orange butterfly is after none other than the Prince of Orange, King William III of England, Ireland and Scotland 1689-1702.
Here is a video showing the Monarchs in flight during our two days at the sanctuaries in Mexico:
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