This cutworm moth (Family Noctuidae) is the fattest animal in the world. In just two summer months of feasting on flower nectar, the migratory moths balloon from 20% to 80% body fat (1). Storing this much energy has a price though – no romance. Migratory moths put reproduction on hold to save up energy for their journey.
Thanks to all that fat, cutworm moths are a major food item for grizzly bears in the summer (2).
It got me wondering: These moths delay reproduction so they can migrate, but putting on fat makes them more delicious. Why not just stay put and make some babies instead? Since their migration is basically east-west, major temperature/seasonal shifts don’t require the move*. Local plants (food) don’t require the move either **. Parasites might. Army cutworm moths are highly parasitized. Moths with parasites stay in the Rocky Mountains longer, growing larger and fatter. So the bears may do the whole population of moths a favor by culling those with parasites.
* Altitude is a factor in temperature and season. But if moths stayed near the mountains, they could stay put moving up and down in altitude without flying a few hundred miles to and from the plains.
** Larvae eat a wide range of leaves and stems. Adults suck up flower nectar. So a large number of larvae may reduce some food for the adults.
- Kevan, PG and DM Kendall. 1997. Liquid Assets for Fat Bankers: Summer Nectarivory by Migratory Moth in the Rocky Mountains, Colorado, U.S.A. Arctic and Alpine Research 29(4):478-482
- French, SP, MG French and RR Knight. 1992. Bears: Their Biology and Management. p 389.