Senescent widow spiders are poor mates but remain attractive to mate-seeking males through deceptive signalling
As fertility declines with age, unmated senescing female animals experience an increasing need to secure a mate. Elevated pheromone production and extended pheromone release are known mechanisms underlying deceptive sexual signalling in relation to the progressive age of signalers but both mechanisms incur elevated metabolic expenses. Here, we report an intricate new mechanism of deceptive signalling that allows unmated senescing female false widow spiders, Steatoda grossa, to conserve metabolic costs for silk and pheromone production while still achieving sustained attractiveness to mate-seeking males. Senescing females produced "honest" state-dependent signals, saving metabolic expenses by building webs with fewer silk strands and depositing less courtship-inducing contact pheromone on their web. However, senescing females concurrently engaged in deceptive signalling in that they remained as attractive to males as young(er) females by accelerating the hydrolytic conversion of web-borne contact pheromone components to air-borne mate-attractant pheromone components. Accelerated dissemination of mate-attractant pheromone from webs was correlated with an age-linked increase in web pH, which we posit enhanced the enzymatic activity of a web-borne carboxyl-ester-hydrolase. Essentially, senescing females concealed their low residual reproductive value by sustained high-level pheromone dissemination. Because the lifetime reproductive output of females declined with age at mating, old females are indeed poor prospective mates and thus deceptive signalers. Their deceptive mate-attractant signals seem evolutionary stable because they rarely remain unmated. Moreover, the males reproductive fitness benefit of mating with an old(er) female - and thus siring fewer offspring than mating with a young female - may still outweigh the costs of rejecting an old female and resuming mate search for a young female, a search which may not be successful.