Do Male Cook Strait Giant Weta Prudently Allocate Sperm?
Sperm production is costly and so males are expected to prudently allocate sperm to matings in a manner that maximizes their fitness. Sperm competition hypotheses predict that when facing increased sperm competition risk males should increase their investment in ejaculates. In contrast, when facing high future mating opportunities, males are expected to decrease their sperm investment in the current mating. This is because males should keep in reserve an amount of sperm proportional to their expected future mating opportunities. We experimentally tested whether male Cook Strait giant weta (Anostostomatidae: Orthoptera: Deinacrida rugosa) phenotypically adjust their investment in ejaculates in relation to their perceived risk of sperm competition and future mating opportunities. D. rugosa is a large flightless orthopteran insect in which males pass multiple spermatophores to females during a day-long mating bout. Contrary to expectation, we found that low female availability (i.e. increased sperm competition risk) had no effect on male resource allocation to sperm (i.e. number of spermatophores) compared to controls whereas, contrary to expectation, males experiencing high female availability increased their ejaculate investment by transferring significantly more spermatophores to their mates. Our results might be a consequence of males being insensitive to increased presence of rival males, reducing their allocation to sperm under increasingly risky circumstances, or due to females prolonging copulations when their perceived future mating opportunities are low.
KeywordsSexual selection spermatophore repeated mating sperm competition ejaculate expenditure sex ratio allocation
We thank Frank Higgott, Susan Caldwell, and Nio Mana (New Zealand Department of Conservation, Marlborough Sound) for assistance while this research was being conducted on Maud Island. We are grateful to Frank and Susan for collecting and dissecting the giant weta that were caught in mouse traps. Two referees provided valuable criticism and advice. This research was supported by Discovery Grants from the National Science and Engineering Research Council (NSERC) of Canada to CDK and DTG.
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