Evaluating cryptic female choice in highly promiscuous tribolium beetles

Abstract

Tribolium flour beetles have been an important model organism for the study of postcopulatory sexual selection. These beetles are representative of many insects that live in high-density aggregations and therefore experience frequent mating opportunities. However, unlike many such insects, Tribolium females are extremely polyandrous and seemingly promiscuous as they engage in frequent matings without any apparent pre-mating courtship ritual. Instead, intrasexual competition as well as intersexual mate/paternity choices occurs primarily during and after copulation. Here, we review the evidence for at least four cryptic female choice (CFC) mechanisms in Tribolium that may affect how offspring paternity is apportioned among a female’s mates. By comparing live to freshly euthanized females (both readily mated by males), studies reveal that during mating, females reject spermatophore transfer or limit sperm numbers transferred by inferior males. Re-mating with additional males is another CFC mechanism, and mated Tribolium females will more readily accept a spermatophore from males that are more attractive than their previous mate. Finally, Tribolium females may also use the timing of spermatophore ejection after mating to bias offspring paternity. Tribolium life history and mating system traits (including long adult life, continuous egg-laying, and frequent re-mating) suggest that females are unlikely to be harmed by multiple matings. Indeed, there exists little experimental evidence for the cost of polyandry to females, and the Tribolium mating system appears to lack many elements that would indicate sexually antagonistic coevolution. The multiple CFC mechanisms demonstrated in Tribolium may stem from females’ inability to effectively assess males before mating. Such mechanisms may provide an effective strategy for females to adjust their paternity decisions at different stages during and after mating, as they receive more information about their current and future mates and about their environment.