Abstract
We present evidence of a possible case of self-medication in a lekking bird, the great bustard Otis tarda. Great bustards consumed blister beetles (Meloidae), in spite of the fact that they contain cantharidin, a highly toxic compound that is lethal in moderate doses. In addition to anthelminthic properties, cantharidin was effective against gastrointestinal bacteria that cause sexually-transmitted diseases. Although both sexes consumed blister beetles during the mating season, only males selected them among all available insects, and ingested more and larger beetles than females. The male-biased consumption suggests that males could use cantharidin to reduce their parasite load and increase their sexual attractiveness. This plausibly explains the intense cloaca display males perform to approaching females, and the meticulous inspection females conduct of the male's cloaca, a behaviour only observed in this and another similar species of the bustard family. A white, clean cloaca with no infection symptoms (e.g., diarrhoea) is an honest signal of both, resistance to cantharidin and absence of parasites, and represents a reliable indicator of the male quality to the extremely choosy females. Our results do not definitely prove, but certainly strongly suggest that cantharidin, obtained by consumption of blister beetles, acts in great bustards as an oral anti-microbial and pathogen-limiting compound, and that males ingest these poisonous insects to increase their mating success, pointing out that self-medication might have been overlooked as a sexually-selected mechanism enhancing male fitness.
Highlights
In polygynous birds, where competition among males for access to females is strong, parasite load of males is assessed by females before mating and may affect male breeding success [1,2,3,4,5,6]
These insects are avoided by most animals because they contain cantharidin, a bitter-tasting and highly toxic defensive chemical with high immunogenicity [17] that acts in blister beetles as fungicide and nematocide [18]
Great bustards are distributed in fragmented populations through the Palaearctic from the Iberian Peninsula and Morocco to Eastern China [38], whilst B. majalis is present in north-western Africa, Iberian Peninsula and southern France [39,40], and P. corallifer is endemic to the Iberian Peninsula [39].The season when adult blister beetles are present coincides with the display period of great bustards
Summary
In polygynous birds, where competition among males for access to females is strong, parasite load of males is assessed by females before mating and may affect male breeding success [1,2,3,4,5,6]. Self-medication is used by some animals to prevent infections, fight against parasites or pathogens, or improve a variety of suboptimal physiological states [7,8,9,10,11,12]. It involves the consumption of plants, animals or minerals that have a prophylactic or therapeutic effect against disease agents which should lead to an increase in host fitness [7,8,9,10]. Cantharidintolerant foragers would benefit from its anti-microbial and anthelminthic properties [24], and could enhance their health and their attractiveness to potential mates during the mate selection process
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