Mate guarding and aggression by the crab spider Misumena vatia in relationto female reproductive status and sex ratio

Current theory predicts that males should guard prereproductive females if they will thereby achieve sperm priority and that the propensity of males t o guard prereproductive females, and aggression exhibited by both sexes at this time, should be related to their density and sex ratio. Low female den sity should select for males to guard prereproductive females, but a female -biased sex ratio should weaken any propensity for males to guard them. Low density and a female-biased sex ratio should favor a low level of aggressi on by both males and females. We tested these predictions with a low-densit y, female-biased population of the crab spider Misumena vatia. During exper imental introductions only 10% (n = 40) of the males guarded penultimate-st age females through their final molt to copulate with them, even though pen ultimate females seldom responded aggressively to males in these trials (10 %). Nearly all males mated soon after they found newly molted adult virgin females (94%: n = 31). Males guarded only 8% (n = 39) of the penultimate fe males censused in the field, but each male mated with the female it guarded as soon as that female molted. Only 22% (n = 23) of vials with two males a nd a penultimate female resulted in more than momentary physical contact be tween the males, and none resulted in injury. Thus, both predictions were b ased on the sex ratio, but only one of the two associated with population d ensity, were confirmed. These results suggest that sex ratio is more import ant than density in dictating precopulatory guarding and aggression. The st rongly female-biased sex ratio of Misumena may explain why males guard pl e -reproductive females less frequently and exhibit less aggression than male s in a population of the closely-related Misumenoides formosipes.