ing that the pseudoscorpions had almost 

 certainly mated on board their host. 



With the discovery that the beetles 

 served as mobile mating territories, our 

 previous observations began to make 

 sense. For several generations, pseu- 

 doscorpion populations thrive within the 

 decaying fig trees, until the trees' re- 

 sources are exhausted (about a year). As 

 new harlequin beetle adults emerge from 

 the rotting wood, large numbers of pseu- 

 doscorpions climb on board. A high pro- 

 portion of the female stowaways are sexu- 

 ally receptive. Males therefore compete 

 intensely to establish a mating territory on 

 a beetle's abdomen. When a harlequin lo- 

 cates a recently fallen fig tree, inseminated 

 female pseudoscorpions disembark to col- 

 onize the tree, and smaller males are 

 forced off by larger rivals. After the bee- 

 tle's maiden flight, it continues to search 

 the forest for suitable trees and mates, typ- 



ically carrying a single, large, male pseu- 

 doscorpion under its wing covers. Females 

 or challenging males may come aboard 

 when the harlequin visits dead trees that 

 are mosaics of old and new decay. The res- 

 ident male may disembark to reconnoiter 

 other beeties as its host beetle copulates, 

 but in the meantime he may be supplanted 

 by a larger intruder. 



In this cycle of population growth and 

 dispersal, we saw how sexual selection 

 could act to maintain the striking size vari- 

 ability among C. scorpioides males. In 

 essence, variabiUty persists because of the 

 two very different habitats in which male 

 pseudoscorpions must compete: on the 

 backs of beetles and within decaying trees. 

 During the pseudoscorpion's brief disper- 

 sal episodes on beetles, it pays to be large, 

 but during the several generations spent 

 living within the trees, big males seem to 

 have no advantage. In laboratory experi- 



ments, we found that big males were able 

 to monopolize matings only under 

 crowded conditions. In trees, where mates 

 are spread out, siring more offspring may 

 depend more on a male pseudoscorpion's 

 mobility and his ability to find mates 

 quickly. Selection may therefore favor 

 small size and rapid maturation. Thus, 

 rather than leading toward a single ideal 

 male, oscillating sexual selection alter- 

 nately favors small and then large males. 



Simply tallying the number of females 

 with which a male mated was not enough 

 to prove this hypothesis, however. Mating 

 itself does not guarantee the siring of off- 

 spring. As British biologist Geoffrey 

 Parker pointed out more than twenty years 

 ago, sexual selection does not necessarily 

 end with copulation. Female pseudoscor- 

 pions are able to store sperm. If a female 

 opts to mate with more than one male, the 

 sperm from each male may have to com- 



41 



