The reproductive success of plants depends to a very large degree on the quantity and quality of seed dispersal. If dispersal is by animals (zoochory), characteristics of fruits such as colour, size, shape and nutritional content have been shown to influence the likelihood of visitation by frugivores or the rate of fruit removal (e.g. Fuentes 1994, Howe 1983, Murray et al. 1993). While these characteristics function in the attraction of dispersers, plants may also evolve characters that manipulate the behaviour or the physiology of dispersers after the consumption of fruits and the ingestion of seeds. An example of such post-consumption manipulation of dispersers' behaviour by plants is provided by mistletoes. The viscous mistletoe seeds stimulate the disperser (usually birds) to either rub the cloaca (when seeds are passed through the gastro-intestinal tract) or the bill (when seeds are regurgitated) on a branch (e.g. Reid 1991). It has also been proposed that fruits might manipulate the disperser's physiology by including laxatives that determine the optimal passage time through the disperser's gut (Murray et al. 1994), although this view has recently been challenged (Witmer 1996).