ENERGETIC COSTS OF SIZE AND SEXUAL SIGNALING IN A WOLF SPIDER

A prerequisite for honest handicaps is that there are significant cond ition-dependent costs in the expression of sexual traits. In the wolf spider Hygrolycosa rubrofasciata (Ohlert), sexual signalling (drumming ) is costly in terms of increased mortality. Here we investigated whet her this mortality may be caused by increased energy expenditure. Duri ng sexual signalling, metabolic rate was 22 times higher than at rest and four times higher than when males were actively moving. Metabolic rate per unit mass was positively related to absolute body mass during sexual signalling but not during other activities. This positive rela tionship is novel to any studies of metabolic rates. :Indeed, it seems that the largest males can drum only 12 times per minute before reach ing the maximum sustainable metabolic rate, whereas the smallest males may drum up to 39 times per minute. However, there is no relationship between body mass and drumming rate, indicating that larger males are able to compensate for the higher cost of drumming. There was a quadr atic relationship between relative abdomen mass and overall body mass, which may provide a partial explanation fbr the increased energy expe nditure of largest males while drumming. Altogether, our results indic ate that sexual signalling is highly energetically demanding, which ma y be the main reason for the honesty of signalling in this species. In addition, the energetic costs are surprisingly strongly size dependen t, which may compensate any disadvantage of small male size.