PhD‐theses - Ethologische Gesellschaft

DETERMINANTS OF NATURAL MATING SUCCESS IN THE CANNIBALISTIC ORB‐WEB SPIDER
ARGIOPE BRUENNICHI
Stefanie Zimmer
zimmer.steffi@googlemail.com
Research Update
Diploma thesis, supervised by Prof. Dr. J.M. Schneider, Department of Biology, Zoological Institute &
Museum, University of Hamburg
Classical sex roles define the most common scenario of promiscuous males and choosy
females which result in polygynous mating systems. Low male mating rates are generally
associated with bi‐parental brood care but occur without paternal investment in various taxa
and have evolved several times independently in spiders. Low male mating frequencies are
associated with remarkable male mating strategies and are predicted to evolve under a
male‐biased effective sex ratio which means that more males than females get to mate.
While male reproductive strategies are well documented and male mating rates are easy to
quantify, especially in sexually cannibalistic species, female reproductive strategies, the
optimal female mating rate, and the factors that affect the evolution of female mating rates
are still unclear.
In this study, I examined natural female mating rates and tested the assumption of a male‐
biased sex ratio and female polyandry in a population of Argiope bruennichi in the field. An
experimental setup was designed that allowed to control female mating status prior to
observations and to monitor mating rates, as well as environmental factors. I predicted
variation in female mating frequencies as a result of (i) spatial and temporal heterogeneity
and (ii) female traits, such as female maturation date, adult weight, and competition with
other females. The possible influence of weather conditions was also assessed. Females had
a low average mating rate of 1.3 and the majority copulated only once. The low degree of
variation was not explained by any of the proposed factors. Male activity and the probability
of copulation were influenced by factors that affected pheromone dispersion such as virgin
female density and wind speed. I conclude that either the assumptions of the theory have to
be reconsidered or alternatively, A. bruennichi males appear to maintain monogynous and
bigynous mating strategies in the absence of obvious advantages over polygamy.
26