OIKOS 55: 341-348. Copenhagen 1989 Ultraviolet reflectance of fruits of vertebrate-dispersed plants Mary F. Willson and Christopher J. Whelan Willson, M. F. and Whelan, C. J. 1989. Ultraviolet reflectance of fruits of vertebrate- dispersed plants. - Oikos 55: 341-348. We surveyed fleshy fruits of 53 species growing in east-central Illinois (40 native species) for the occurrence of a whitish, waxy "bloom" and ultraviolet reflectance. Fruits of 16 species had both well-developed bloom and strong UV reflectance, and one species had no evident bloom but reflected UV strongly. Bloom accounted for 39-70% of total reflectance in these 16 species, often increasing reflectance in the human-visible part of the spectrum as well as in the UV range. Aviary and field experiments with fruit-eating birds indicated that these consumers did not consistently discriminate between conspecific fruits with and without bloom. Lack of discrimination may have occurred because 1) the birds did not see a marked difference between fruits with and without bloom (an unlikely possibility), 2) discrim- ination is conditional upon particular contexts not present in our experiments, or 3) the development of bloom often varies within and among fruit crops and foraging birds learn to ignore such differences. Further experiments with other frugivores in other conditions would be valuable. M. E Willson and C. J. Whelan, Dept of Ecology, Ethology and Evolution, Univ. of Illinois, Shelford Vivarium, 606 E. Healey St., Champaign, IL 6I820 (present address of CJW: Morton Arboretum, Lisle, IL 60532, USA). Introduction The colors of fleshy fruits are generally considered to advertise the presence of ripe fruits to visually search- ing, fruit-eating vertebrates that disperse the enclosed seeds (Darwin 1859, Ridley 1930, Willson and Thomp- son 1982). In addition, Burkhardt (1982) reported that the whitish, waxy, cuticular "bloom" produced by many fruits reflects strongly in the near UV, just below the human-visible spectrum (about 400-750 nm). Because many bird species (including fruit eaters) are known or suspected to have color vision extending to the near UV (Burkhardt 1982, Chen et al. 1984), Burkhardt sug- gested that one function of the bloom of fruits is to enhance the signal character of these fruits for birds. We extend Burkhardt's findings from Europe to North America and examine the possible function of bloom in attracting frugivorous birds. Specifically, we had three goals: 1) To survey additional fleshy fruits (especially of North American native plants) for the presence of UV reflectance and bloom; 2) To quantify the effect of bloom on spectral reflectance; 3) To deter- Accepted 27 January 1989 0 OlKOS mine experimentally whether or not fruit-eating birds discriminate between conspecific fruits with and without bloom. Methods Survey of fruit color, bloom, and reflectance We sampled fruits of 53 species growing in east-central Illinois (40 native, 13 introduced). UV reflectance was examined in two ways. First, fruits (and associated leaves from the same plant) of 52 species were pho- tographed in full sunlight, once using Tri-X black-and- white film, and a normal 50 mm lens, then again with the additon of a Wratten 18A filter. This filter allows about 63% transmittance of available light at 350 nm and about 5% transmittance of available light and 725 nm (see Fig. 2). All other wavelengths are screened out. Second, at least two fruits of each of 43 species were examined with a Beckman DK-2A spectrophotometer calibrated to measure reflected light in the range of 32CL900 nm and plot the relative reflectance at each