Disentangling the costs of male harassment and the benefits of polyandry for females

Abstract

Many studies quantify how polyandry affects female fitness by allowing females to either mate with one or several males. But even if the number of matings is standardised, such studies conflate any costs of interacting with males with the potential benefits of receiving sperm from several males, obscuring the benefits of polyandry. We conducted a 2x2 factorial experiment on the mosquitofish, Gambusia holbrooki to quantify the independent effects of male harassment and polyandry. We artificially inseminated virgin females with sperm from either one or five males (monandry versus polyandry). Females were then housed in the presence or absence of reproductively ablated males who could harass, but not mate with 2 them. Our design ensured that the number of males inseminating a female was independent of the level of male harassment and the number of matings she received. Females who were not housed with males were instead housed with immature females to maintain densities constant across treatments. Unexpectedly, females that experienced sexual harassment were more likely to give birth, had shorter gestation periods and gave birth to larger broods. Furthermore, polyandrous females were more likely than monandrous females to give birth. Polyandrous females' sons also reached maturity faster than those of monandrous females. We therefore found no detectable costs to females producing their first brood of being harassed by males when the direct costs of mating were absent. We also showed that, in the absence of mating costs, there are direct and indirect fitness benefits of being inseminated by multiple males. If the costs of the act of mating are small or absent, polyandry will benefit female G. holbrooki producing their first brood.

An experimental test for body size‐dependent effects of male harassment and an elevated copulation rate on female lifetime fecundity and offspring performance

“…One potential weakness of our study is that, although experimental fish were fed ad libitum, females housed with males might have experienced greater feeding competition than those housed alone (i.e., control females). When designing the experiment, we deliberately decided not to place another female in the control female tank as has been done in studies on guppies (i.e., presence of competitor, but no sexual interactions, e.g., Gasparini et al , ) because previous studies suggest that social interactions between female mosquitofish can strongly affect their growth and fecundity (Smith, ; Fox et al , , Brookes, Iglesias‐Carrasco, Kruuk, & Head, in review). One of our results also militates against the argument that male presence increases feeding competition: larger males are likely to be stronger competitors, but there was no moderating role for male size on the effect of male presence on the measured female traits.…”

“…We allowed sperm to settle together at the bottom of the tube for several minutes, and then, we used a micropipette to transfer 3 μl of the sperm mixture into the oviduct of the anaesthetized female and returned her to her treatment tank. This method has been used to artificially inseminate females before with a high success rate (Fox et al , ). We kept females in the treatments for 23 weeks, which is the approximate length of the Gambusia breeding season in our study population (Kahn, Kokko, & Jennions, ).…”

“…In general, experimental studies show that females continuously housed with males have lower fitness than females that only have intermittent access to males (Edvardsson, ; Lew, Morrow, & Rice, ; Rönn et al , ). In most of these studies, however, researchers cannot determine the relative effects of the actual rate of copulation from those of male sexual harassment: the increased presence of males leads to both greater male harassment and a higher mating rate (but see Fox, Head, & Jennions, ; den Hollander & Gwynne, ; Partridge & Fowler, ; Zajitschek, Dowling, Head, Rodriguez‐Exposito, & Garcia‐Gonzalez, ). The distinction between these two processes matters.…”

“…Although males do not provide females with material benefits (i.e. no nuptial gifts), females mate multiply (Head, Kahn, Henshaw, Keogh, & Jennions, ; Zane, Nelson, Jones, & Avise, ), and recent evidence suggests that polyandry can both elevate female reproductive success and alter the phenotype of their offspring (Fox et al , ). In wild populations, adult male body size is highly variable despite minimal post‐maturation growth (Kahn, Mautz, & Jennions, ).…”

Dynamic phenotypic correlates of social status and mating effort in male and female red junglefowl, Gallus gallus

“…Evidence that these female behaviours incur immediate costs, however, remains ambiguous. For example, female resistance to male mating advances (sexual harassment) is thought to be energetically costly (Jormalainen, Merilaita, & Riihimaki, ; Perry, Sharpe, & Rowe, ), although measurements of these energetic costs are often indirect (reviewed in Fox, Head, & Jennions, ). Establishing how male and female phenotypes change within and among individuals in relation to intrasexual competition and mating effort is therefore important to understand mate preferences, the maintenance of variation in sexually selected traits, and sexual conflict (Andersson & Simmons, ; Arnqvist & Rowe, ; Dale, Dey, Delhey, Kempenaers, & Valcu, ; Foley et al., ; Joseph et al., ; Kokko, Brooks, Jennions, & Morley, ; Sánchez‐Tójar, Nakagawa, et al., ; Somjee et al., ).…”