Differential investment and size-related mating strategies facilitate extreme size variation in contesting male spiders

Sexually selected variation in male body size is often associated with alternative reproductive strategies that may persist under frequency-dependent selection or result from 'making the best of a bad job' in inferior individuals. The spider genus Nephila is well known for female gigantism and male dwarfism, serving as an established model system to investigate selection on male size and the still enigmatic sexual size dimorphism (SSD). Less attention has been paid to the enormous size variation among males that exists in some Nephila species despite broad evidence for large male advantages in contest competition. As the existing variation contradicts strongly biased fitness returns among different phenotypes, detailed investigations of fitness consequences related to different body sizes and their associated mating strategies are required. We used the African golden-silk spider Nephila senegalensis to test whether size-dependent alternative mating strategies yield equivalent reproductive success in dyads of size-mismatched males competing for a single female. Our results confirm that differently sized males achieve similar paternity and reveal complex interrelations between competing strategies, which mutually influence each other. Males integrate their own competitive ability, the intensity of sperm competition and female reproductive value to adjust their mating investment accordingly. Size-related mating strategies, combined with behavioural flexibility, may thus promote the maintenance of male size variation by balancing fitness returns and exemplify how changes in selective context can dissolve the principal large-size benefits. We suggest that flexible adaptive responses may significantly contribute to stabilizing SSD.