Digest: Female squid influence insemination success after mating to show a preference for smaller males *

In systems where females mate with multiple males, females can influence insemination success to favor the males they find most attractive. When a female influences insemination success, it is known as cryptic female choice (CFC) because her influence often occurs after mating (post copulation) inside the reproductive tract and out of sight. For example, female fowl show CFC by ejecting sperm from subdominant males after forced mating (Pizzari and Birkhead 2000), while female yellow dung flies can alter the paternity outcomes of their multiple partners (Ward 2000). Historically, it has been difficult to disentangle both the mechanisms and outcomes of CFC from those of male–male competition, such as sperm competition, as both influence sperm storage and paternity (Birkhead and Pizzari 2002). Furthermore, CFC is hard to observe, so studies of CFC have had to rely on indirect measures such as second male paternity, where CFC is inferred if offspring paternity does not reflect a male’s mating success (Ward 2000). In the Japanese pygmy squid, females are polyandrous and mate with multiple males. Male squid grasp females and ejaculate branching tubes (spermatangia) from spermatophores by a modified arm (hectocotylus). They then place several spermatangia on the female’s body where they release many spermatozoa that then migrate to her seminal receptacle within 24 h of copulation (Sato et al. 2014a). A female can remove the spermatangia from her body before spermatozoa transfer by blowing water on them with her siphon, or by removing and eating them. This behavior makes