SEXUAL SELECTION AND THE ENERGETICS OF COPULATORY COURTSHIP IN THE SIERRA DOME SPIDER, LINYPHIA LITIGIOSA


ABSTRACT

Mating in the Sierra dome spider, Linyphia litigiosa (Linyphiidae) begins with 2-6 h of aspermic copulatory courtship, termed pre-insemination phase copulation. Male intromission rate, that is, the speed of repeated genitalic insertions, withdrawals and re-insertions, and body mass are related to fertilization success via cryptic female choice (i.e. female preferences are expressed in their sperm use patterns; Watson 1991b, Anim. Behav., 41, 343-360). Carbon dioxide respirometry indicated that pre-insemination phase copulation is energetically demanding. Male intromission rate and body mass (which are uncorrelated traits) each were positively associated with different subsets of six complementary measures of metabolic competence made during copulatory courtship; (1) efficiency; (2, 3) maximum metabolic rate and scope; (4, 5) rate and linearity of metabolic increase; (6) the consistency of metabolic output during sustained courtship. Composite ranks of males based on the six metabolic and the two overt phenotypic measures of quality were positively correlated. Body mass and intromission rate also were associated with two aspects of male fighting ability, strength and aggressiveness (i.e. willingness to escalate when at a modest size disadvantage), respectively. Fighting is important for defence of webs, prey and mates. Thus metabolic competence, a fundamental and possibly heritable component of viability, is favoured by sexual selection; this may help to yield offspring that fare better under both natural and sexual selection.