Nature scrapbook: Monarch butterfly

Though a few eager monarch butterflies may reach Ontario from Mexico by late May, most arrive in early July, the products of eggs laid during the return trip of parents that winter in the south. Between 200 million and one billion monarchs huddle for five months in colonies on just a dozen cool, fir-tree-covered mountains in central Mexico’s transvolcanic plateau. With much of those wintering grounds threatened by logging, increasingly exposing the butter­flies to fatal cold rains and freak snowstorms, much depends on rebuilding numbers each summer in the north.

Sex with benefits

Monarch mating takes 16 hours or more, during which a male gradually forms and transfers a gel­atinous capsule, called a spermatophore, that equals between five and 10 per cent of his weight. Along with sperm, the capsule includes protein, which allows his partner to produce substantially more eggs. Because the nectar that butterflies slurp from flowers has little protein, they’re otherwise limited to what’s left in their bellies from their leaf-nibbling youth.

Serious weight gain

Females lay between 20 and 80 pinhead-sized green eggs a day, averaging 700 in total, each attached to the underside of milkweed leaves, their only larval food. Tiny caterpillars hatch several days later and eat constantly for up to two weeks, shedding skin five times, and developing black, white, and yellow bands as they fatten to 2,700 to 3,000 times their hatching weight.

Packing poison

The bright colours of monarch caterpillars and butterflies warn birds of their unpalatability. The insects’ milkweed staple contains toxic sap that protects the plants from larger browsers. Monarchs tolerate the toxins, which they store in their bodies. Once most birds eat one, they steer clear of others in the future.

Feats of flight

After 10 to 15 days suspended from branches or leaves as gold-speckled green pupae, early-summer butterflies emerge and live for only two to six weeks, laying their own eggs in turn. The season’s fourth monarch generation, which transforms between mid-August and mid-September, migrates back to Mexico on an eight- to 12-week autumn journey and survives into spring. Circling upwards as high as 1,500 metres on rising columns of warm air, the southbound travellers collect along the north shores of the Great Lakes as they search for
the shortest water crossings. It’s estimated that up to 350,000 have passed through Point Pelee in a 90-minute span.