Cheating is ubiquitous within mutualisms. Primary nectar-robbers, ubiquitous exploiters of pollination mutualisms, feed through holes they make in corollas. One commonly observed behavior is the addition of new holes to previously robbed flowers. Why flowers should be robbed repeatedly is difficult to understand: a hole signals that a nectar forager has already fed, perhaps very recently, likely reducing its value. Possibilities include: (1) Multiple holes appear only once every flower has been robbed; hence, they represent the only way to acquire nectar for robbers unwilling to re-use an existing hole and who cannot reach nectar by visiting flowers legitimately. (2) Individual foragers make multiple holes during single visits, perhaps as the most effective way to obtain the maximum amount of nectar. (3) previously robbed flowers contain more nectar than unrobbed ones, making them better choices for bees seeking high nectar rewards. During 2014-2016, inflorescences in a Colorado population of Corydalis caseana were tagged and flowers were marked with date-specific colors as they opened. Nectar volumes were recorded from flowers of known age that were protected and exposed to robbers. Holes were counted on exposed flowers as they aged. We also recorded population-wide rates of multiple robbing twice weekly.
Multiple holes began appearing when >75% of flowers lacked a single hole, allowing us to reject Hypothesis 1. Hypothesis 2 cannot offer a full explanation because >50% of holes appeared in flowers one or more days after the first hole appeared. Repeated sampling of bagged and exposed inflorescences revealed that flowers fill at a constant rate for 5-7 days and refill after being drained (either by a robber or a pollinator). As a direct consequence, consistent with Hypothesis 3, young, unrobbed flowers are of relatively low value to primary robbers because they contain little nectar. Rather, it is more profitable in many cases for bees to rob older flowers, even if they have previously been robbed. These results cannot explain why bees choose to create new holes rather than reuse existing ones. However, they do offer a simple explanation for the paradoxical clustering of cheater activity. Research on cheating in mutualisms has focused to date on the consequences of being cheated. Conversely, little is known about the choices involved in adopting cheating behaviors. We argue that nectar-robbing can serve as an ideal model system for the broader study of cheating in mutualism.