Ecology of fruit-colour polymorphism in Myrtus communis and differential effects of birds and mammals on seed germination and seedling growth

1 The fruit-colour polymorphism. of Myrtus communis, a common Mediterranean shrub, is examined. We investigate whether frugivores affect the maintenan ce of the polymorphism, whether morphs differ in germination or seedling gr owth, and whether passage through frugivores' gut affects seed behaviour an d seedling growth. 2 Blue berries are very similar in morphological traits and nutrient compos ition to the rarer white morph. Rates of fruit removal by birds, the main d ispersers of this species, did not differ between morphs, suggesting that a colour preference is not involved in maintenance of the polymorphism. 3 Seeds of the two morphs showed the same germinability (final percentage g ermination) as well as similar rates of germination under controlled condit ions (in growth chamber and greenhouse). Outdoors, seeds from blue berries tended to germinate slightly faster (which might give them an early advanta ge) but differences between morphs disappeared after several weeks of growt h. 4 The germination of myrtle seeds is differently affected by passage throug h the digestive tract of birds, which appeared to significantly increase ge rminability, and of the carnivorous pine marten, Martes martes, which did n ot. The different seed retention time in the gut or the chemical compositio n of the food ingested along with the seeds may be responsible for such dif ferences. 5 The detection of a positive effect of bird ingestion in outdoor condition s, but not in the growth chamber or in the greenhouse, demonstrates the imp ortance of performing germination tests in the natural habitat. 6 Seed size, irrespective of morph, affects the response to passage through the bird gut. Large seeds, which are more rapidly ejected, and are thus le ss abraded, germinated at a similar speed to non-ingested controls. Variati on in seed size within a single species should thus be considered in future studies.