613 INTRODUCTION Many animal species show preferences for certain food types (Ayala- Berdon et al., 2008). The amount of food consumed by the animal is usually reduced as levels of available energy are increased (Collins, 1981; Slansky and Wheeler, 1992). Studies of this ‘intake response’ (Castle and Wunder, 1995) have mostly focused on nectarivorous birds (Collins, 1981; López-Calleja et al., 1997; McWhorter and Martínez del Rio, 2000) but these issues have seldom been addressed in frugivores (see Levey and Martínez del Rio, 1999). According to Baker and Baker, hummingbird-pollinated plants produce nectar that is rich in sucrose, whereas passerine-pollinated plants produce nectar in which hexose sugars are dominant (Baker and Baker, 1983). Some choice tests have shown that several hummingbird species prefer sucrose to glucose or fructose (Hainsworth and Wolf, 1976; Martínez del Rio, 1990a), whereas some passerine species prefer glucose and fructose to sucrose (Martínez del Rio and Stevens, 1989; Martínez del Rio et al., 1989). However, Lotz and Schondube suggested that birds will generally prefer hexose sugars to sucrose at low equicaloric concentrations but will prefer sucrose to hexose at high equicaloric concentrations (Lotz and Schondube, 2006). A possible explanation for this switch at high concentrations is that sucrose nectar contains double the energy content per unit of osmotic concentration than hexose nectar (Beuchat et al., 1990). This reduced osmotic concentration may increase post-ingestional intestinal energy delivery rate by increasing gastric emptying rate (Karasov and Cork, 1994; Schondube and Martínez del Rio, 2003). If the delivery of high osmotic, concentrated solutions is not regulated birds may experience osmotic diarrhoea (Lotz and Schondube, 2006). Sucrotic birds may therefore lessen their osmotic stress by choosing sucrose rather than hexose at high concentrations but at low concentrations this effect may have less consequence (Lotz and Schondube, 2006). However, Johnson and Nicolson recommend that a more useful distinction would be between specialist and generalist bird pollination systems (Johnson and Nicolson, 2008). They suggest that generalist bird-pollinated plants are characterised by large volumes of nectar (probably as a result of occasional avian nectarivores having a much large body size) with low concentrations and low sucrose proportions whereas specialist bird- pollinated plants are characterised by smaller volumes of nectar with higher concentrations and high sucrose proportions. It has also been suggested that preferences are based on energy yield for relatively small specialist nectarivores (Downs and Perrin, 1996). However, experimental procedure may affect the results of choice tests (Brown et al., 2008). Previous studies have used solutions that are equicaloric (Fleming et al., 2004), equimolar (Downs, 1997; Downs, 2000) or solutions that are equivalent by mass (Lotz and Nicolson, 1996; Blem et al., 2000). Equimolar solutions offer equal amounts of molecules per solution, but a sucrose solution at a particular concentration has approximately double the energy of an equimolar hexose solution (Hixon, 1980; Schoener, 1983; Downs and Perrin, 1996; Downs, 1997). Sucrose solutions that are equivalent in sugar mass to a hexose sugar solution have approximately 5% more available energy (Fleming et al., 2004). It has therefore been suggested that in choice tests sugar solutions that are energetically equivalent should be used (Fleming et al., 2004). The Journal of Experimental Biology 214, 613-618 © 2011. Published by The Company of Biologists Ltd doi:10.1242/jeb.047217 Food preferences of Knysna and purple-crested turacos fed varying concentrations of equicaloric and equimolar artificial fruit Amy-Leigh Wilson and Colleen T. Downs* School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Pietermaritzburg, 3209, South Africa *Author for correspondence (downs@ukzn.ac.za) Accepted 27 October 2010 SUMMARY The effects that different fruit sugar types and concentrations have on food preferences of avian frugivores have been relatively poorly studied. Although it has been recently advocated that preference is based on equicaloric energy it is also important to note whether preferences change as energy content changes. Therefore, sugar preferences of equicaloric and equimolar artificial fruit of different sugar types at varying concentrations and molarities were investigated in two relatively large South African frugivores, Knysna (Tauraco corythaix) and purple-crested (Gallirex porphyreolophus) turacos. Artificial fruits containing 6.6, 12.4 or 22% sucrose or glucose, and artificial fruits containing 0.42, 0.83 or 1.66 mol l –1 sucrose or glucose, were used to determine sugar preferences. Knysna turacos preferred the sucrose to the glucose equicaloric artificial fruit diet at low concentrations whereas purple-crested turacos showed no preference for either diet. Both turacos species preferred the sucrose equimolar artificial fruit diet to the glucose at low concentrations. At high concentrations neither species showed a preference for either equicaloric or equimolar artificial fruit diets. This suggests that energy requirements influence food preferences more than sugar type and that birds will select fruit that is higher in energy irrespective of sugar type. This complements an earlier study on digestion of differing equicaloric and equimolar artificial fruit sugar types. It again emphasizes the need for future studies looking at the composition of indigenous forest fruit sugars in order to obtain insight into the role of these avian frugivores as potential seed dispersers of fruiting tree species. Key words: sugar type, sugar preference, energy requirement, frugivory, Knysna turaco, purple-crested turaco, artificial fruit. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY