Tim Laman; The Wildlife Collection 



After scratching at a beetle-damaged 

 Heliotropium leaf, two blue tiger males, 

 below, gain access to the pyrrolizidine 

 alkaloids within. These butterflies must 

 also continue the regular business of 

 feeding on nectar, right. 



Michael Boppre 



feed on, which could be thought of as gro- 

 cery stores, and those they gather sec- 

 ondary chemicals from, which could be 

 cbnsidered pharmacies. 



Why do males engage in these efforts? 

 Some twenty years ago, biologist Miriam 

 Rothschild studied moth larvae feeding on 

 fresh PA plants and proposed that insects 

 are capable of stockpiling the alkaloids to 

 protect themselves from predators. In the 

 years since her suggestion, chemical 

 analyses conducted by several separate re- 

 search groups have revealed that pyrro- 

 Uzidine alkaloids gathered by adult butter- 

 flies from dry plants are used for the same 

 purpose. The insects' storage capacity is 

 impressive: up to 15 percent of a butter- 

 fly's dry weight may be made up of un- 

 converted pyrrolizidine alkaloids ex- 

 tracted from dry plants. 



Behavioral tests of butterfly predators 

 have shown that the stockpiled PAs can 

 provide the insects with protection from 

 many enemies. These chemicals, which 

 become toxic once ingested, taste bad and 

 have been found to be repellent, to varying 

 degrees, to some mice, bats, lizards, spi- 

 ders, birds, and all unadapted insects. 



Some members of the milkweed butter- 

 fly family — monarchs and queens — are 

 protected by other chemicals unpalatable 



to predators. Unlike PAs, fliese chemicals, 

 known as cardenolides, have an immedi- 

 ate effect on heart rate and blood pressure. 

 Neither egg-laying females nor larvae 

 specifically seek out cardenoUdes, but if 

 the larval host plant contains them, fliey 

 are ingested along with food. Stored in the 

 larval body and retained into adulthood, 

 these cardenolides deter several predators, 

 as has been well documented during the 

 last twenty-five years. Film footage based 

 on Lincoln Brower's studies with blue jays 

 provided the most memorable proof: blue 

 jays eating with gusto and then immedi- 

 ately vomiting up monarch butterflies that 



had been reared as larvae on cardenoUde- 

 containing plants. 



For certain milkweed butterfly species 

 or individuals, then, pyrrolizidine alka- 

 loids add another dimension to their un- 

 palatability, while for others, the alkaloids 

 may be the only defensive compounds. In 

 all cases, however, these plant chemicals 

 play a dual role in the lives of danaines: 

 they help males seduce females, and they 

 act as potential lifesavers. Thus, males 

 have good reason to pursue pyrroUzidine 

 alkaloids. But why is a female so inter- 

 ested in whether or not a suitor smells of 

 the PA-derived dihydropyrrofizines? And 



30 Natural History 1/94 



