Background/Question/Methods   Most animals make use of color information, including wavelength, saturation, and intensity, as they explore their environments.  Butterflies in particular rely on color in a variety of behavioral contexts, and the range of their color perception is among the broadest known in the animal kingdom. The monarch butterfly, Danaus plexippus, is arguably the most widely recognized butterfly species in the US, well known for its intimate association with milkweed plants and incredible multi-generational trans-continental migrations. However, little is known about monarch butterflies' color perception or learning ability, despite the importance of visual information to butterfly behavior in the contexts of nectar foraging, host plant location, and mate recognition.
We have conducted behavioral tests on monarch butterflies in order address basic questions about their vision and to examine the relationship between their innate and learned color preferences. Specifically, we ask: (1) Can monarchs distinguish colors based on wavelength, independent of intensity? (2) What are monarchs' innate color preferences in the context of nectar foraging, and do relative preferences vary depending on choice of colors offered? (3) How do innate color preferences affect learning parameters? (4) Does a monarch learn a given color equally well, regardless of temporal sequence of training?
Results/Conclusions   Monarchs can discriminate between colors based on wavelength alone, and when offered a choice of six colors, show a strong innate preference for orange in the context of nectar foraging.  They rapidly learn to associate colors with sugar rewards, and learn non-innately preferred colors as quickly and proficiently as they do innately preferred colors.  Butterflies trained to green, red, and purple models confuse those colors with yellow, orange and blue, respectively, but the ‘confusion' is asymmetrical. Monarchs readily learn to associate a second color with reward, and in general, learning parameters do not vary with temporal sequence of training. Our results demonstrate that monarch butterflies are proficient and flexible color learners; these capabilities should allow them to respond rapidly to changing nectar availabilities as they travel over migratory routes, across both space and time.