The Cyanogenic Glycoside Amygdalin Does Not Deter Consumption of Ripe Fruit by Cedar Waxwings
Cyanogenic glycosides are common secondary compounds in ripe fruits that are dispersed by birds. These substances are toxic to some mammals. We examined the repellent effect of amygdalin, a cyanogenic glycoside, on Cedar Waxwings Bombycilla cedrorum. Amygdalin did not reduce food ingestion in Cedar Waxwings, even at relatively high concentrations. In addition, these birds did not exhibit preference for amygdalin-free over amygdalin-containing fruit. Cedar Waxwings given artificial food that contained four times the amount of amygdalin found in some wild fruits ingested the equivalent of 5.5 times the oral lethal dose for rats in 4 hr without exhibiting any external signs of toxicity. Amygdalin ingestion appeared to have a negative effect on nitrogen retention and food assimilation. However, when nitrogen retention and food assimilation were recalculated assuming that all amygdalin ingested was excreted intact, these negative effects disappeared. The presence of large amounts of unhydrolyzed amygdalin in the excreta of waxwings fed on amygdalin-laced food confirmed our conjecture that amygdalin was excreted intact. We hypothesize that in Cedar Waxwings, amygdalin is absorbed in the intestine but is not hydrolyzed by endogenous enzymes and thus is excreted intact in urine. The apparent lack of repellent effects of amygdalin in Cedar Waxwings suggest that toxicity data for rats and humans may be a poor predictor for the deterrent effect of fruit secondary compounds on frugiverous birds. Many hypotheses that have been posed to explain the presence of secondary compounds in ripe fruit assume that these substances have repellent/toxic effects on avian seed dispersers. For some compounds, such as amygdalin and other cyanogenic glycosides, this assumption may not be valid.
Heather M. Struempf et al. The Auk Vol. 116 No. 3 July 1999