Department of Physics, Chemistry and Biology (IFM)

The results of the present study show that spider monkeys significantly preferred ethanol solutions between concentrations of 0.5% and 3% when tested against water. This result is remarkable as to my knowledge, no primate species tested so far has been found to prefer ethanol solutions when tested against water, except for Rhesus macaques (Macaca mulatta).

Ethanol has been found to interact with sweet taste receptors and activating a sucrose-responsive gustatory neural pathway. Nevertheless, this interaction also happens in primates that do not prefer ethanol over water and therefore, it is unclear whether this contributes to the taste responsiveness to ethanol displayed by different primate species. Also, spider monkeys might recognize ethanol as an additional source of calories besides fruit-sugars, as other researchers suggested.

Impurities in sugarcane alcohol did not affect the taste preference threshold for ethanol nor the taste preference for ethanol at all tested concentrations above the taste preference threshold. An increase in preference could have been expected as sugarcane alcohol contains a small amount of sugars and ethanol has been shown to increase gustatory nerve response to sucrose.

Furthermore, small impurities in drinks have repeatedly been shown to affect human taste perception, for example minerals in drinking water, and such a difference in taste perception, and thus taste preference, could have been expected in the present study.

The results of the present study show that spider monkeys significantly preferred ethanol-spiked sucrose solutions over non-spiked sucrose solutions of the same sucrose concentration. These findings are in line with previous electrophysiological findings, as they found that ethanol increased sweetness sensation when mixed with sweet solutions.

Nonetheless, when the non-spiked stimulus was sweeter than the ethanol-spiked alternative, the animals in the present study significantly rejected the ethanol-spiked stimulus with all tested ethanol concentrations. This indicates that spider monkeys have a stronger preference for sucrose than for ethanol and that they choose the sweeter solution regardless of total caloric value.

Previous research has shown that in food preference tests, spider monkeys preferred the more caloric food available. In the present study, spider monkeys chose sweet taste over calories when presented with sucrose-ethanol solutions.

Thus, the findings of the present study do not support claims that a presumed predilection of non-human primates for alcohol-containing overripe fruits would reflect the evolutionary origins of human alcoholism, as in the present study, the spider monkeys clearly preferred the sweeter, non-alcohol-containing stimuli over the less sweet, alcohol-containing ones.

The results of the present study show that the spider monkeys preferred the pureed fruit spiked with ethanol over the non-spiked alternative. However, the preference was not as strong as in the first two test series of the second experiment. The ethanol-spiked fruit puree was preferred to a higher degree in fruits with a higher sugar content and a lower water content (See Table 1).

These results might simply reflect the monkey’s taste preference regarding the different fruits, which are in line with previous findings regarding the spider monkeys' diet preferences. Otherswise, these findings could indicate that a higher sugar and /or calorie content increased the preference for ethanol in spider monkeys, which is in line with previous findings indicating that ethanol increased sweetness sensation when mixed with sweet solutions. Both options could explain the preference for ethanol increasing with the sugar content of the fruits.

Primates

The voluntary consumption of ethanol in sweet-tasting substance has been observed, described, and/or tested in many primate species, such as squirrel monkeys (Saimiri sciureus), rhesus monkeys (Macaca mulatta), chimpanzees (Pan troglodytes verus), vervet monkeys (Chorocebus aethiops), slow lorises (Nycticebus coucang), and aye-ayes (Daubentonia madagascariensis). Similarly, spider monkeys in this study strongly preferred the ethanol-spiked sucrose solution over the non-spiked alternatives.

The spider monkeys' taste preference threshold determined in this study (0.5% ethanol) is similar to the average human ethanol detection threshold (0.43% ethanol, from the results summarized in L.J. Van Gemert’s “Flavour thresholds”), which would indicate a similar sensitivity to ethanol in both species.

Nonetheless, taste preference thresholds are only a conservative approximation of an animal’s ability to perceive a given taste and are usually higher than the taste detection thresholds which are commonly determined using sophisticated signal detection methods.

Non-primate mammals

The results from this study indicate that spider monkeys have a lower taste preference threshold than rats (Rattus norvegicus domesticus); have a higher tolerance to ethanol than egyptian fruit bat (Rousettus aegyptiacus); and have a higher sensitivity to ethanol than sheep (Ovis aries) and possibly other ruminants.

The spider monkeys in the present study have shown a preference for ethanol concentrations between 0.1% and 3% diluted either in water, sucrose solutions, or pureed fruit, when tested against alcohol-free alternatives of the same diluent. The spider monkeys had a stronger preference for sucrose than for ethanol, regardless of the total caloric content of the stimuli. Spider monkeys are one of the few tested primates so far that show a preference for ethanol solutions over water. Their sensitivity appears to be in line with previous findings in other species, including humans. Finally, the spider monkeys in the present study had a lower preference threshold than previously tested rats, and a higher sensitivity than previously tested sheep.