Early nutritional conditions, growth trajectories and mate choice: does compensatory growth lead to a reduction in adult sexual attractiveness?

It is well documented that poor early nutrition can have profound negative effects on adult life-history traits. However, it has also been demonstrated that organisms can undergo compensatory resource allocation strategies (such as an accelerated growth rate) if food availability improves, so as to mitigate the effects of the poor early conditions. Previous research has indicated that elevated growth rates can incur costs in the longer term, such as an increased rate of senescence and shorter lifespan. We tested whether a phase of compensatory growth after a period of reduced food availability earlier in life affected the sexual attractiveness of adult male green swordtails Xiphophorus helleri, a species in which it has previously been documented that females prefer larger-bodied and longer-tailed males. The experiment compared the attractiveness of size-matched brothers that had experienced contrasting growth trajectories as juveniles; the experiments were initially conducted in the middle of a male’s sexually reproductive life and were then repeated towards the very end of life. At both ages, males that had undergone compensatory growth were equally as attractive as their brothers that had grown normally. These results suggest that the growth compensation benefits males through an increase in their attractiveness over that which they would have had if they had remained on their original growth trajectory. The lack of change in relative attractiveness with age indicates that the compensation does not cause greater deterioration in secondary sexual characters at older ages than in continuously well-fed males.     

Do female bank voles (Clethrionomys glareolus) mate multiply to improve on previous mates?

Contrary to classical sexual selection theories, females of many taxa mate with multiple males during one reproductive cycle. In this study, we conducted an experiment on the “trade-up hypothesis”, which proposes that females remate if a subsequently encountered male is potentially superior to previous mates to maximize the genetic quality of their offspring. We presented bank vole females (Clethrionomys glareolus) sequentially with two males of known dominance rank in different orders, i.e., either first subordinate and second dominant, first dominant and second subordinate, or two males that were equal in dominance (high ranking) and observed their mating behavior. We found that 92% of the females mated multiply and did not base their remating decision on male social status. Therefore, polyandry cannot be explained by the “trade-up hypothesis” based on dominance rank in this species. However, we found that dominant males sired significantly more offspring than subordinate males. This varied according to mating order: dominant males sired more offspring when they were second than when they were first. Moreover, litter sizes were significantly smaller when the dominant male was first (smallest relative success of dominant males) compared to litter sizes when mating order was reversed or both males equal in status. Our results suggest that even though multimale mating includes males that are of poorer quality and thus potentially decreases the fitness of offspring, most of a female’s offspring are sired by dominant males. Whether this is due to cryptic female choice, sperm competition, or a combination of both, remains to be tested.     

Search costs influence the spatial distribution,but not the level,of extra-pair mating in tree swallows

Costs of searching for a mate are an important component of models of sexual selection, yet they have rarely been examined in wild populations of vertebrates. In this paper, we report an experiment in which we handicapped female tree swallows by clipping some flight feathers. This manipulation increased the costs of flight and searching for extra-pair mates. Despite these costs, handicapped females had the same level of extra-pair mating (percentage of extra-pair young, percentage of broods with extra-pair young, and the number of extra-pair sires per brood) as control females. However, handicapped females were more likely to have young sired by extra-pair males that lived closer to her nest than control females. This change in the distribution of extra-pair mating was most likely due to female choice rather than male coercion, and it suggests that extra-pair mating has significant benefits to females. One important implication of our study is that ecological and social factors that influence search costs could affect the spatial distribution of extra-pair sires and, consequently, the intensity of sexual selection. These effects may have been overlooked in previous studies that did not identify extra-pair sires.     

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Female mallard mating preferences for multiple male ornaments

Mating preferences were studied in captive wild-stock female mallards. Independent observers scored ornament brightness for the bill and eight plumage areas of male mallards on a scale of 1 to 6. Ornament scores were generally positively correlated with each other as indicated by univariate correlations and principal component analysis. Males with higher bill scores were significantly more likely to obtain pairings. None of eight individual plumage ornaments were significant predictors of pairing success, but males with higher average plumage scores had significantly higher pairing success. Males that completed the molt first were also significantly more likely to be paired. Thus females may have paid some attention to plumage brightness. Stepwise multiple regression produced a model that included only bill and molt, both of which contributed a similar degree (r ² = 0.39); plumage ornaments and size measurements did not enter the model. These results are interpreted in light of hypotheses for the evolution of multiple ornaments, and phylogenetic plumage patterns in dabbling ducks. Received: 18 December 1995 / Accepted after revision: 22 September 1996     

Ornamentation predicts reproductive success in female pipefish

In the sex-role reversed pipefish Syngnathus typhle females compete for access to males and males are choosy. Females develop a temporary ornament when competing over mates with other females and when performing nuptial dances with males. This ornament is an amplification of the normal striped pattern in these fishes. We here show experimentally that (1) the contrast of this normal pattern forecasts the extent to which the ornament is shown, (2) contrast and ornamentation honestly signal female quality (egg numbers), (3) contrast and ornamentation accurately predict female mating success, (4) contrast is a phenotypically plastic trait specifically exaggerated under situations of female – female competition, and (5) neither contrast nor ornament are energetically expensive to the females (i.e., they are independent of short-term nutritional status). Hence, as predicted in sex-role reversed species, ornament design is constrained by costs to female fecundity: an energetically demanding ornament would impair on a female's ability to produce eggs. The type of ornament described here is the expected one, costly for reasons other than being energetically expensive to produce. Received: 4 April 1996 / Accepted after revision: 27 October 1996     

Repeatability and heritability of sperm competition outcomes in males and females of <Emphasis Type="Italic">Tribolium castaneum</Emphasis>

Differences among males in their success in achieving fertilisations when females mate with more than one partner are now recognised as an important target of sexual selection. However, very few studies have attempted to determine whether particular males are consistently successful in sperm competition and whether success in sperm competition is a heritable trait. Additionally, the potential heritability of female traits that influence the outcome of sperm competition has received only limited attention. Using the polyandrous beetle Tribolium castaneum, we examined repeatability of male success in sperm competition by mating pairs of males carrying different visible genetic markers to a string of different females. Males showed consistency in their ability to successfully transfer sperm to females, but not in their success in sperm competition. Furthermore, when we independently compared success in sperm competition of fathers with their sons, we found no evidence for heritability of this trait. Similarly, females that exhibited high or low first male sperm precedence did not tend to have daughters that showed the same pattern. Our results suggest that we should be wary of assuming that success in sperm competition is heritable through either sex.     

No evidence for female mate choice based on genetic similarity in the túngara frog Physalaemus pustulosus

In most sexually reproducing animals, the behavior of one or both sexes during courtship critically influences the success at mating of the opposite sex. This behavior is often interpreted as “mate choice,” and there is great interest in why such choices are exercised. The explanation for the evolution of mate choice that has received the most attention and generated the most controversy is based on assumed genetic effects. In this study, we investigated whether female túngara frogs, which choose mates based on acoustic cues, have a preference for genetically less related males. Specifically, we determine if there is disassortive mating based on microsatellite markers, if there is information in the advertisement call that could be used to assess genetic similarity, and if females exhibit acoustic-based mating preferences that would promote choice for genetic diversity. Using seven microsatellite markers, we found no correlation of male call similarity and male genetic relatedness. Female choice experiments showed no female preference for calls of less related males, and there was no evidence for inbreeding avoidance in the field. Our results do not support the hypothesis of mate choice based on information about genetic relatedness conveyed by acoustic signals in túngara frogs.     

Sperm exchange in a simultaneous hermaphrodite

Many simultaneously hermaphroditic animals cross-inseminate when copulating, even though unilateral sperm exchange is functionally possible. The question is why reciprocity is the rule. This study addresses sperm exchange in a hermaphroditic planarian flatworm, Dugesia polychroa, in which self-fertilisation does not occur. We investigated the availability of self sperm for inseminating a partner, how self sperm affects the likelihood of sperm donation, and whether sperm donation depends on reciprocation by the partner. The amount of self sperm depended on body size, duration of isolation and experimental date, and animals that had more self sperm were more likely to mate. Depletion of allosperm (sperm received from partners) played only an indirect role, suggesting that animals copulate more to donate sperm rather than to replenish allosperm reserves. Among 60 copulating pairs, reciprocal insemination was more common (63.3%) than unilateral transfer (18.3%). A surprising 18.3% did not exchange sperm in either direction. Individuals were more likely to behave like their partners, resulting in more symmetrical matings (either reciprocity or no sperm exchange) than asymmetrical matings (unilateral sperm donation). The amount of self sperm donated during a copulation depended mainly on the amount available before the copulation and not on the amount received from the partner. The results suggest that conditional sperm exchange, or sperm trading, takes place in D. polychroa. Received: 31 May 1997 / Accepted after revision: 1 December 1997