Energy
requirements of migrating monarch butterflies. (adapted from dplex-l)
Gibo, David. September 26, 1997.
The question of the cost of flight for migrating monarch butterflies has come up a
couple times recently. There is no easy answer. But that shouldn't stop us from having
some fun examining the question from several different angles.
The cost of flight for Monarchs is determined largely by whether the butterflies proceed
by flapping flight or by gliding/soaring flight, and by the temperature of their bodies.
Flapping flight in this note refers to the somewhat leisurely flight mode most commonly
seen during migration, but not high speed escape flight, or vigorous flight against a
headwind. Flapping flight is much more costly than gliding/soaring flight. Alan Masters,
Stephen Malcolm, and Lincoln Brower (Ecology, 69(2), 1988, pp. 458 - 467) calculated that
flapping flight of monarch butterflies was about 28 - 31 times more costly in energy
expenditures than gliding/soaring flight (assuming that the cost of gliding/soaring flight
is the same as resting metabolism. By using a less rigorous approach, and making the same
assumption about the cost of gliding/soaring flight, Megan Pallett and I calculated that
flapping flight was about 24 times more costly than gliding flight (Canadian J. Zoology,
57(7), 1979, pp.1393-1401). Masters et.al. (1988) calculated that a Monarch butterfly
proceeding by flapping flight would burn lipid at the rate of 4.00 mg/hr (96 mg/d) if its
body temperature was 22 degrees C, but would only burn it at the rate of 1.67 mg/d (40.08
mg/d) if its body temperature was 15.5 degrees C. In contrast, a Monarch proceeding by
gliding/soaring flight would burn its lipid at the rate of 0.13 mg/hr (3.11 mg/d) at 22
degree C, and at a rate of only 0.06 mg/hr (1.52 mg/d) at a body temperature of 15.5
degrees C. A lucky butterfly that, once aloft, was able to migrate by gliding and soaring
for eight hours and maintain a body temperature of 15.5 degrees C would use only 8 x 0.06
= 0.48 mg of lipid (1.04 mg if body temperature = 22 degrees C. Another, less fortunate
individual, that had to flap along for 8 hours would burn 8 x 1.67 = 13.36 mg of lipid (32
mg if body temp = 22 degrees C). These preliminary calculations suggest that natural
selection favours migrating Monarchs that spend as much flight time as possible soaring
and gliding instead of flapping, and maintain as low a body temperature as possible during
the migration. It would be nice to know the actual proportions of the different flight
modes, and the actual range of body temperatures for the migrants. It also seems
reasonable that, if possible, natural selection will favour butterflies that minimize the
amount of energy expended per kilometer gained towards the overwintering sites. Flapping
butterflies will have straighter flight paths since, unlike soaring/gliding individuals,
they will never have to circle in a thermal to gain altitude. If both a flapping and a
gliding/soaring individual are flying in a tailwind, then the flapping individual will
make more rapid progress. Assuming an 18 km/hr (5 m/s) airspeed and a 25 km/hr tailwind,
the flapping individual will be be travelling (over the ground) at the rate of 43 km/hr.
At the 22 degrees C rate of lipid utilisation of 4.00 mg/hr, the flapping individual will
be gaining ground at the cost of 4/43 = 0.09 mg/km (0.15 mg/mile). What are the statistics
for the soaring and gliding individual? It depends. If it can soar and glide it
continuously in a straight line for eight hours (possible but unlikely) at an airspeed of
3.0 m/s (we measured 2.6 m/s for dried, under ballasted, specimens), or about 11 km/hr,
then it would cover 36 km. At the 22 degree rate of lipid utilisation for resting
metabolism, the 36 km would cost of about 0.13/36 = 0.004 mg/km (0.005 mg/mile). Gliding
and soaring is clearly the way to go. But is this true? What if the butterfly, more
realistically, had to spent half of the time drifting with the wind while it circled in
thermals gaining altitude and only flew straight while it glided between thermals? How
much would this would slow its progress and increase the cost per kilometer gained toward
the overwintering sites. What if the wind was a crosswind and the soaring individual was
drifted sideways, with respect to the overwintering sites, for the 50% of the flight time
it spent circling in thermals? What if the crosswind was not a pure crosswind but had a
headwind component of 3 km/hr? or 5 km/hr? or even 7 km/hr? In each case, energy
expenditures per hour per butterfly remain the same, but the cost for each kilometer
gained towards the overwintering sites changes greatly. Is soaring and gliding always the
best tactic? Finally, what would the results of the calculations be if the two butterflies
were operating at 15 degrees C instead of 22 degrees? Things can certainly get complicated
very fast.
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