Monarch or Viceroy?

Monarch

Wingspan - 2 3/4 to 4 inches

Caterpillar - Black, yellow and cream bands with black tentacles behind the head.  Feeds on milkweed.

Range - Common in the Americas

Habitat - Open land, fields, roadsides, prairies and gardens

Family - Brushfooted Butterflies

Subfamily - Milkweed Butterflies

Viceroy (note additional black line running across hindwing)

Wingspan - 2 3/4 to 3 inches

Caterpillar - Olive-green and brown body with bristles behind the head.  Feeds on trees such as poplar and willow.  Looks a little like bird droppings for camouflage.

Range - Common from Canada down into Mexico

Habitat - Open, damp areas, river edges, streams, marshes

Family - Brushfooted Butterflies

Subfamily - Admirals and Relatives

Monarch caterpillar

Viceroy caterpillar

Copyright Whitney Cranshaw, Colorado State University, Bugwood.org

Used with Permission = cc-by-3.0-us

OK, now for the nitty gritty part.  For many years, it was believed that because Monarchs ate toxin-rich milkweed, birds (predators) learned to avoid them.   Scientific myth said that Viceroys evolved to have a similar appearance to Monarchs, and so by default, birds would also avoid them.  However in 1991, experiments were conducted feeding butterfly bodies of Viceroys, Monarchs and other butterflies to red-winged blackbirds.  Without the wings, the bird had no warning of toxicity and would eat any of the bodies.   Interestingly, Viceroy bodies and Monarch bodies were equally distasteful to the birds.  

Why are these butterflies toxic?  The monarch is full of toxic substances known as cardiac glycosides, acquired as a result of the caterpillar feeding on milkweed.  The Viceroy, though not as foul-tasting to birds as the monarch,  is full of toxic substances (like salicylic-acid) as a result of the caterpillar feeding on the leaves of willows and poplars.

Why does any of this matter?  Well, it probably doesn't matter all that much, but I thought it was interesting!   

1.  Batesian mimicry is when one harmless species adopts the appearance of a harmful species, to gain the advantage of the predator avoidance.  This was the original understanding of the relationship between Monarch and Viceroy, but was flawed.

2.  Müllerian mimicry is when two or more poisonous species, that may or may not be closely related,  and share one or more common predators, have come to mimic each others' warning signals.  This mimicry is mutually beneficial to the species involved.  This is the new understanding of the relationship between Monarch and Viceroy.

I know I learned something new about both Monarchs and Viceroys (and mimicry) and hope you did too! 

Question:  Do you wonder how a human would be able to determine that one butterfly was more palatable than another to a red-wing blackbird?   Sounds like a challenge to me!

Answer:  I don't know.  Quantities, I suppose.  They did say that after eating the toxic bodies, the birds shook their heads, guzzled water and acted agitated.  Poor things!

Sources:  
"A Pocket Guide to Butterflies & Moths" by Elizabeth Balmer
"Encyclopedia Britannica Blog, Facts Matter, "Mutual Mimicry:  Viceroy and Monarch" by Kara Rogers, May 18, 2011