DOI: 10.5061/dryad.1vhhmgqq2
title: Data from: A life-history perspective on sexual selection in a polygamous species

description: Background: Ever since Darwin, evolutionary biologists have studied sexual
 selection driving differences in appearance and behaviour between males
 and females. An unchallenged paradigm in such studies is that one sex
 (usually the male) signals its quality as a mate to the other sex (usually
 the female), who is choosy in accepting a partner. Here, we hypothesize
 that in polygamous species these roles change dynamically with the mating
 status of males and females, depending on direct reproductive costs and
 benefits of multiple matings, and on sperm competition. We test this
 hypothesis by assessing fitness costs and benefits of multiple matings in
 both males and females in a polygamous moth species, as in moths not males
 but females are the signalers and males are the responders. Results: We
 found that multiple matings confer fitness costs and benefits for both
 sexes. Specifically, the number of matings did not affect the longevity of
 males or females, but only 67 % of the males and 14 % of the females mated
 successfully in all five nights. In addition, the female’s reproductive
 output increased with multiple matings, although when paired with a new
 virgin male every night, more than 3 matings decreased her reproductive
 output, so that the Bateman gradient for females fit a quadratic model
 better than a linear model. The male’s reproductive success was positively
 affected by the number of matings and a linear regression line best fit
 the data. Simulations of the effect of sperm competition showed that
 increasing last-male paternity increases the steepness of the male Bateman
 gradient and thus the male’s relative fitness gain from additional mating.
 Irrespective of last-male paternity value, the female Bateman gradient is
 steeper than the male one for up to three matings. Conclusion: Our results
 suggest that choosiness in moths may well change throughout the mating
 season, with males being more choosy early in the season and females being
 more choosy after having mated at least three times. This life-history
 perspective on the costs and benefits of multiple matings for both sexes
 sheds new light on sexual selection forces acting on sexual signals and
 responses.
description: <b>Aim, design and setting of
 the study</b> To assess how sexual
 selection dynamics change over the course of a lifetime in a polygamous
 moth species, we investigated the direct reproductive costs and benefits
 of multiple matings, as well as the role of sperm competition in
 reproductive success. To identify the factors that may affect reproductive
 success, we used two different setups: repeated matings and polygamous
 matings, described in detail below (experimental design). Using computer
 simulations, we also explored differences in absolute reproductive success
 between males and females. All experiments were conducted between November
 2014 and February 2016 in the laboratory at the Institute for Biodiversity
 and Ecosystem Dynamics, University of Amsterdam.
   <b>Description of
 materials</b>
 Populations of <i>H. virescens </i>moths
 have been laboratory reared at the Institute for Biodiversity and
 Ecosystem Dynamics, University of Amsterdam, since 2011. The rearings
 originate from North Carolina, where eggs were first collected from the
 field in 1988, supplemented with new field collections over the years, and
 reared at the Max Planck Institute for Chemical Ecology from 2003 until
 2011. Larvae were reared individually on
 artificial pinto bean diet under controlled conditions in a climate
 chamber (60% relative humidity (RH); 25 degrees C; 14 h light: 10 h dark
 with lights off at 11.00 am). Pupae were checked daily for eclosion. Newly
 emerged adults were sexed and placed individually into a plastic cup (37
 ml; Solo, Lake Forest, Illinois) with sugar water (cotton soaked with 10%
 sucrose). In all mating experiments, we used two- to three-day old virgin
 moths, and pairs were always from different families to avoid inbreeding
 effects. All experiments were conducted in clear plastic beakers (473 ml;
 Solo, Lake Forest, Illinois) covered with gauze, so that the moths could
 be observed but could not escape. <b>Experimental
 design</b> We determined the
 longevity and lifetime fecundity and fertility of individual moths, making
 use of the experimental design that compares repeated matings with
 polygamous matings to control for the factors partners and matings
 separately. Bateman gradients are defined as the slope of regressions of
 the number of offspring on the number of mates. To measure Bateman
 gradients, the data were combined from four treatments in our study.
 Specifically, four treatments were set up (see Table 1): a) one virgin
 male or female was placed per beaker and observed for five consecutive
 nights (treatment 1: no matings, females n = 30; males n = 30), b) virgin
 individuals were paired with a virgin mate on the first night, after which
 the sexes were separated into different beakers (treatment 2: mated once,
 females n = 29; males n = 31), c) individual moths were paired with a new,
 virgin mate every night for five consecutive nights (treatment 3:
 polygamous matings, females n= 59; males n = 63), d) individual moths were
 paired with the same mate every night (treatment 4: repeated matings,
 females n = 52; males n = 52) for five consecutive nights. To pair the
 individuals with a virgin mate every night, after each night the couples
 were separated into different beakers and given a new virgin mate.
 Individuals paired with different and same mates that died prior to the
 fifth night were excluded from the analyses (5 females and 2 males in
 treatment 3, four couples in treatment 4). Female fecundity was defined as
 the lifetime egg production of a female and female fertility was defined
 as the total  percentage of eggs that hatched. Male fecundity and
 fertility was measured indirectly by counting the total number of eggs
 laid and the total percentage of eggs that hatched from his female
 partners. Successful copulations were determined as
 follows. Moths are completely inactive during the daytime and become
 sexually active at specific hours at night. <i>Heliothis
 virescens</i> is sexually active between three and 6-7 hours after
 sunset and matings last on average 3 h. Therefore, for five consecutive
 nights individual moths of all four treatments were observed every half
 hour with a red light to note copulating pairs, starting at the onset of
 the dark period (scotophase), in a climate chamber with the same
 conditions as described above. As spermatophores are formed during mating
 and remain in the bursa of the female, the number of successful matings
 can be determined by dissecting the female at the end of her life. To
 measure their longevity, all moths from the experiments were kept
 separately and fed with sugar water (cotton soaked with 10% sucrose) every
 two days until death. Therefore, after death adult females were dissected
 under a microscope to count the number of spermatophores inside each
 female, which was taken as the number of successful copulations.
 <b>Effects of mating history
 </b> To assess the effects of
 successful matings with virgin partners or partners with different mating
 histories, we first determined the number of successful copulations, as
 described above, after which we assessed the successful mating incidences
 of males and females in treatments 3 and 4. To evaluate the effect of
 previous successful male mating history on the reproduction of singly
 mated females, only the males from the “polygamous matings” (treatment 3)
 were used, because in the “repeated matings” treatment we could not be
 sure in which night(s) a male had mated successfully with the paired
 female (i.e., when a mating resulted in a spermatophore)  – we could only
 ascertain her cumulative mating status afterwards by dissection (see
 above). To determine the fecundity of each moth, during
 the 5 days of the mating experiments, the eggs laid were collected and the
 moths were transferred into new beakers every day. After the experiments,
 all eggs from each cup were collected every second day and the number of
 eggs was counted using a grid under the microscope. Fecundity was
 calculated as the total number of eggs laid by each female during her life
 span. Fecundity of males could also be determined in this way, at least in
 treatment 3, by keeping track of each female that one male had mated with.
 In this way, we analyzed the fertility and fecundity of females and males
 that had mated with 0, 1, 2, 3 or 4 previously mated partners.
 To determine the number of fertile eggs, the collected eggs were
 kept in separate cups (250 ml) until the eggs hatched. The number of
 hatched eggs (i.e., black eggs, which indicate developing larvae, and
 larvae) were counted 3 days after egg hatch started. Fertility was
 calculated as the percentage of eggs hatching.
 <b>Reproductive success with
 sperm precedence</b> Using data from
 the mating experiments described above and available in the literature, we
 simulated various sperm precedence scenarios by modeling the mating
 dynamics in a population of moths. Specifically, we assumed that mating
 probabilities were proportional to the available males and females in each
 night, that mating probability deterministically determines mating rate,
 and that 60 % of the females that mated in one night were available for
 mating the next night, as was found to be the case in the “polygamous
 matings” treatment  (see Fig. 3b). We assumed relative cumulative
 reproductive output as a function of the number of matings as fitted for
 our data (Fig. 3a), and division of reproductive output over days since
 first mating as measured for singly mated females. To determine the effect
 of remating on reproductive success for males, we used four paternity
 scenarios: a) 100 % first-male sperm precedence: all offspring is sired by
 the first male a female mated with, and thus 0 % is sired by any
 subsequent male, b) 0.33 last-male paternity: fertilization probability of
 sperm from earlier matings decreases by 33 % each time a female mates with
 a new male, c) 0.66 last-male paternity, and d) 100% last-male sperm
 precedence: 100 % of offspring is sired by the last male a female mated
 with. For females, we assumed that additional matings would result in more
 offspring as measured in the mating experiment, using the fitted relation
 between relative offspring production and relative mating success (see
 Fig. 3a and explanation below).        
 <b>Statistical
 analysis</b> All statistical
 analyses were performed in R software version 3.4.1 {R, 2018 #6053}.
 Survival analysis was conducted on the longevity data using Cox
 proportional hazards model (package: survival in R). The following
 covariates and their interactions were considered to be incorporated in
 these models: treatment, sex, number of matings. These models were
 simplified using analysis of deviance and AIC criteria. The model that was
 used to compare the longevity between males and females contained
 treatment, number of matings and sex without interactions as covariates. A
 separate Cox regression model was fitted for males and females to
 investigate the effect of multiple matings and treatment. These models
 contained treatment and the number of matings without interactions as
 covariates. To compare the effect of treatment on longevity, we used a
 Tukey <i>post-hoc</i> test at the 5 % probability level for
 multiple comparisons (package: multcomp in R). Separate
 Bateman curves were fitted for males and females. We regressed the number
 of live offspring on the number of matings (mating success) using  linear
 models. To generate conventional Bateman curves that are comparable to
 results reported for other species, we only used the data obtained from
 treatment 3, i.e., individuals receiving a different virgin partner every
 night for five consecutive nights. The effects of
 mating history on fecundity and fertility of singly mated females were
 analyzed using a generalized linear model (GLM) with a quasi-Poisson
 distribution and the number of previous matings as factor. Tukey
 <i>post-hoc</i> tests at the 5 % probability level for
 multiple comparisons were conducted to assess differences between previous
 male mating histories. To calculate the Coefficient of Variation (CV) in
 reproductive success, only the data of treatment 3 were used, where
 individuals received 5 virgin partners on five consecutive nights. The CV
 in reproductive success was calculated as the ratio of the standard
 deviation of the number of offspring and the mean number of
 offspring.
description: All data are present in the excel file. Missing data are noted as
 NA or empty cells