Nocturnal bees learn landmark colours in starlight

Honeybees, like humans and most other vertebrates, are colour-blind in dim light. Bees are primarily day-active and have apposition compound eyes, the typical eye design of diurnal insects. Most bees are trichromats with photoreceptors sensitive in the UV, blue and green [1]. While their diurnal colour vision was established almost 100 years ago, honeybees are known to be colour-blind in moonlight [2]. Here, we present the first evidence that the only known obligately nocturnal bee, Correspondences the Indian carpenter bee Xylocopa tranquebarica (Fabricius), which flies even on moonless nights [3], uses colour vision to discriminate artificial landmarks at the nest in starlight. Humans, in contrast, are colour-blind at half-moon illumination. This finding, obtained using natural nests under natural illumination, is remarkable because insensitive apposition eyes were thought unable to support nocturnal colour vision. Hitherto, nocturnal colour vision was known only in nocturnal hawkmoths [4] and geckos [5], animals with eyes well adapted to nocturnality. Outdoor experiments were conducted at natural nests within Bhimashankar Wildlife Sanctuary, Maharashtra State, in the Western Ghats of India (see [3]), between December 2007 and March 2008. Experiments were performed each night when the bees started flying, approximately half an hour after sunset (which was between 18:00 and 19:00 hours). Experiments usually continued until 03:00 the following morning. Bees exiting and returning to the nest were observed using infrared-sensitive night-vision equipment and recorded on an infrared-sensitive Sony camcorder (TRV130E). At two nests, X. tranquebarica were trained to find their nest entrance centred behind a yellow square landmark on a large plywood wall (see Supplemental Figure S1 in the Supplemental Data available on-line with this issue). Each nest housed between one and four foragers at any given time. They were tested with four additional colour landmarks: Test 1: brighter and darker shades of yellow (Y1, Y2) and two shades of grey (Gr1, Gr2); Test 2: two shades of green (Ge1, Ge2) and the same two shades of grey (Figures 1A–D). The colours are named as they appear to the human eye; for example, the greys we chose look grey to humans but do not reflect ultraviolet (see Supplemental Figure S2 for spectral reflectances of all colours). Landmark positions were changed pseudo-randomly