Biol. Rev. (2009), 84, pp. 259–276. 259 doi:10.1111/j.1469-185X.2008.00074.x Consumptive emasculation: the ecological and evolutionary consequences of pollen theft Anna L. Hargreaves 1 *, Lawrence D. Harder 1 and Steven D. Johnson 2 1 Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary Alberta, Canada, T 2N 1N4 2 School of Biological and Conservation Sciences, University of KwaZulu-Natal, P. Bag X01 Scottsville, Pietermaritzburg 3209, South Africa (Received 23 April 2008; revised 17 December 2008; accepted 20 December 2008) ABSTRACT Many of the diverse animals that consume floral rewards act as efficient pollinators; however, others ‘steal’ rewards without ‘paying’ for them by pollinating. In contrast to the extensive studies of the ecological and evolutionary consequences of nectar theft, pollen theft and its implications remain largely neglected, even though it affects plant reproduction more directly. Here we review existing studies of pollen theft and find that: (1) most pollen thieves pollinate other plant species, suggesting that theft generally arises from a mismatch between the flower and thief that precludes pollen deposition, (2) bees are the most commonly documented pollen thieves, and (3) the floral traits that typically facilitate pollen theft involve either spatial or temporal separation of sex function within flowers (herkogamy and dichogamy, respectively). Given that herkogamy and dichogamy occur commonly and that bees are globally the most important floral visitors, pollen theft is likely a greatly under- appreciated component of floral ecology and influence on floral evolution. We identify the mechanisms by which pollen theft can affect plant fitness, and review the evidence for theft-induced ecological effects, including pollen limitation. We then explore the consequences of pollen theft for the evolution of floral traits and sexual systems, and conclude by identifying key directions for future research. Key words: pollen limitation, pollen parasitism, pollen robbery, pollination efficiency, cheating, toxic pollen, sexual systems. CONTENTS I. Introduction ...................................................................................................................................... 260 (1) Who are pollen thieves? ............................................................................................................. 260 (2) Effects of pollen theft on plant reproduction ............................................................................. 261 ( a ) Direct effects .......................................................................................................................... 261 ( b ) Indirect effects ....................................................................................................................... 261 II. Ecological consequences of pollen theft ........................................................................................... 266 (1) Pollen limitation .......................................................................................................................... 266 (2) Measurement of the impact of pollen theft ............................................................................... 266 III. Evolutionary consequences of pollen theft ....................................................................................... 268 (1) Adaptation of floral traits ........................................................................................................... 268 ( a ) Tolerance ............................................................................................................................... 268 ( b ) Resistance .............................................................................................................................. 269 (i ) Escape in time. .............................................................................................................. 269 (ii ) Escape in space: cryptic pollen. ................................................................................... 269 (iii ) Escape in space: inaccessible pollen. ............................................................................ 269 (iv ) Deterrents: pollen structure. ......................................................................................... 269 (v ) Deterrents: chemical defence. ...................................................................................... 270 * Address for correspondence: (email alhargreaves@gmail.com) Biological Reviews 84 (2009) 259–276 Ó 2009 The Authors Journal compilation Ó 2009 Cambridge Philosophical Society Cambridge Philosophical Society BIOLOGICAL REVIEWS