Presence Of Rival Males Research Articles

In the presence of rivals, males allocate less ejaculate per mating in Japanese pygmy squid with female sperm rejection

Abstract When mating is promiscuous, the ejaculate volume allocated to each female is expected (intuitively) to be linked with the presence and number of rival males. Previous theories have indicated that, in the absence of rival males, males will allocate the minimum ejaculate volume sufficient for fertilization of all available oocytes. However, it is unclear if this ejaculation strategy is still effective where females have a mechanism to remove sperm after copulation (“female sperm rejection”). In the Japanese pygmy squid, Idiosepius paradoxus, female sperm rejection was observed to occur frequently, but males were able to increase the remaining sperm volume available for fertilization, suggesting that there is no significant impact of female sperm rejection on male ejaculation strategy. However, males decreased ejaculate volume in the presence of rival males and increased it in their absence, a pattern counterintuitive to predictions from previous theories. Females reject sperm at every copulation, so after copulation, the amount of a given male’s sperm remaining with the female may decrease after each subsequent rival copulates with the female. Perhaps in this species, the presence of rivals signals the risk of further sperm rejection, so males choose to conserve their resources and move on.

Sensory perception of rivals has trait-dependent effects on plasticity in Drosophila melanogaster.

The social environment has myriad effects on individuals, altering reproduction, immune function, cognition, and aging. Phenotypic plasticity enables animals to respond to heterogeneous environments such as the social environment but requires that they assess those environments accurately. It has been suggested that combinations of sensory cues allow animals to respond rapidly and accurately to changeable environments, but it is unclear whether the same sensory inputs are required in all traits that respond to a particular environmental cue. Drosophila melanogaster males, in the presence of rival males, exhibit a consistent behavioral response by extending mating duration. However, exposure to a rival also results in a reduction in their lifespan, a phenomenon interpreted as a trade-off associated with sperm competition strategies. D. melanogaster perceive their rivals by using multiple sensory cues; interfering with at least two olfactory, auditory, or tactile cues eliminates the extension of mating duration. Here, we assessed whether these same cues were implicated in the lifespan reduction. Removal of combinations of auditory and olfactory cues removed the extended mating duration response to a rival, as previously found. However, we found that these manipulations did not alter the reduction in lifespan of males exposed to rivals or induce any changes in activity patterns, grooming, or male-male aggression. Therefore, our analysis suggests that lifespan reduction is not a cost associated with the behavioral responses to sperm competition. Moreover, this highlights the trait-specific nature of the mechanisms underlying plasticity in response to the same environmental conditions.

An Indirect Cost of Male-Male Aggression Arising from Female Response.

Animal behavior is often polymorphic between individuals within a population. A cost/benefit balance of a particular behavioral pattern may be influenced by social interaction with other individuals with different behavioral patterns. Males of a fruitfly, Drosophila prolongata, show genetically defined polymorphism in aggressiveness and boldness against rival males. Males of the H strain are highly aggressive, and their fights tend to escalate into boxing, the highest level of aggressive interaction. H males are also bold against sneaker males and do not hesitate to perform leg vibration (LV), a courtship behavior that is vulnerable to interception of the female by surrounding rival males. In contrast, males of the L strain rarely engage in boxing and do not perform LV in the presence of rival males. We examined their mating success in small experimental populations. The mating success of L males was higher in a pure L population than in a mixed population with H males, whereas that of H males was higher in a mixed population than in a pure H population. Notably, this 'cost of aggression' in a pure H population seemed not directly derived from the male-to-male interaction but was imposed by the female's response of escaping from fighting males, compromising the benefit of the resource monopolization as territory.

Smaller rival males do not affect male mate choice or cortisol but do affect 11-ketotestosterone in a unisexual-bisexual mating complex of fish

Male mate discrimination may be affected by the social environment (presence or absence of rival males or mates), which can also affect stress and sex hormones (e.g., cortisol and 11-ketotestosterone (11-KT)). The Amazon molly, Poecilia formosa, is an all-female fish species dependent on sperm from mating with male P. latipinna. We investigated male mate choice in P. latipinna between conspecific females and P. formosa with a rival male present and no rival male present. We measured cortisol and 11-KT release rates from all fish. The presence of a rival male had no effect on male mate choice for conspecific females nor overall mating effort. Male 11-KT decreased on the second day after exposure to a rival male on the first day. Focal male 11-KT is positively correlated with the size of the rival male. Both conspecific and heterospecific females released more 11-KT when in the rival male treatment than when not. Neither male nor female cortisol was affected by the presence or absence of the rival male. We did not find an effect of rival males on male mate choice in contrast to our prediction. Instead, our findings may indicate a hormonal response to social competition.

The effects of male social environment on sperm phenotype and genome integrity.

Sperm function and quality are primary determinants of male reproductive performance and hence fitness. The presence of rival males has been shown to affect ejaculate and sperm traits in a wide range of taxa. However, male physiological conditions may not only affect sperm phenotypic traits but also their genetic and epigenetic signatures, affecting the fitness of the resulting offspring. We investigated the effects of male‐male competition on sperm quality using TUNEL assays and geometric morphometrics in the zebrafish, Danio rerio. We found that the sperm produced by males exposed to high male–male competition had smaller heads but larger midpiece and flagellum than sperm produced by males under low competition. Head and flagella also appeared less sensitive to the osmotic stress induced by activation with water. In addition, more sperm showed signals of DNA damage in ejaculates of males under high competition. These findings suggest that the presence of a rival male may have positive effects on sperm phenotypic traits but negative effects on sperm DNA integrity. Overall, males facing the presence of rival males may produce faster swimming and more competitive sperm but this may come at a cost for the next generation.

Sociosexual environment influences patterns of ejaculate transfer and female kicking in Callosobruchus maculatus

There is growing evidence that many organisms adjust their physiology and behaviour during sexual encounters according to changes in their sociosexual situation. Selection tends to favour plasticity in males that can strategically ejaculate and females that can alter their resistance or remating behaviour adaptively. We investigated plasticity both in the ejaculate transfer patterns of males and in the pericopulatory kicking behaviour of female seed beetles, Callosobruchus maculatus . We altered the sociosexual environment of the beetles by (1) manipulating male mating history, (2) preventing females from kicking and (3) altering the risk of sperm competition. Males mating for the second time transferred half the weight of ejaculate of virgin males, but at the same rate. Males mating in the presence of rivals did not alter the weight of ejaculate, as might have been expected; however, the transfer of ejaculate began sooner. By looking at the effect of female kicking on male ejaculate transfer, we found that when females have their hindlegs ablated to prevent kicking, the rate of ejaculate transfer slows, but the weight remains the same. From the female perspective we found that the probability of females kicking was positively related to both copula duration and ejaculate weight, but also to male mating history; females were less likely to kick males that had mated more times previously even after controlling for copula duration and ejaculate weight. Finally, we found that removing a female's hindlegs increased the probability that females (attempted to) kick. Our results show that males are, to some extent, plastic in their ejaculatory responses to the sociosexual environment. More striking, however, and not previously documented, is that female kicking is affected by environmental variables, including male mating history. • Males can adjust ejaculate transfer dynamics to the sociosexual environment. • The presence of rival males did not affect the volume of ejaculate transferred. • Probability of female kicking increases with mating duration and ejaculate weight. • Probability of female kicking varies with male mating history.

The Heritability of Mating Behaviour in a Fly and Its Plasticity in Response to the Threat of Sperm Competition

Phenotypic plasticity is a key mechanism by which animals can cope with rapidly changeable environments, but the evolutionary lability of such plasticity remains unclear. The socio-sexual environment can fluctuate very rapidly, affecting both the frequency of mating opportunities and the level of competition males may face. Males of many species show plastic behavioural responses to changes in social environment, in particular the presence of rival males. For example, Drosophila pseudoobscura males respond to rivals by extending mating duration and increasing ejaculate size. Whilst such responses are predicted to be adaptive, the extent to which the magnitude of response is heritable, and hence selectable, is unknown. We investigated this using isofemale lines of the fruit fly D. pseudoobscura, estimating heritability of mating duration in males exposed or not to a rival, and any genetic basis to the change in this trait between these environments (i.e. degree of plasticity). The two populations differed in population sex ratio, and the presence of a sex ratio distorting selfish chromosome. We find that mating duration is heritable, but no evidence of population differences. We find no significant heritability of plasticity in mating duration in one population, but borderline significant heritability of plasticity in the second. This difference between populations might be related to the presence of the sex ratio distorting selfish gene in the latter population, but this will require investigation in additional populations to draw any conclusions. We suggest that there is scope for selection to produce an evolutionary response in the plasticity of mating duration in response to rivals in D. pseudoobscura, at least in some populations.

Concurrent effects of sperm competition and female quality on male mate choice in the Trinidadian guppy (Poecilia reticulata)

Males are generally expected to be discriminating in their choice of mates when females vary in quality and when under sperm competition from rival males. However, how sperm competition and female quality interact to influence male mating decisions remains poorly understood. Here, we explored the concurrent effects of sperm competition and female body length on male mating decisions in the Trinidadian guppy (Poecilia reticulata) as a model system. We used female body length as a proxy for fecundity and manipulated the relative size difference of paired stimulus females concurrently with sociosexual cues (i.e., presence of rival males) that may predict sperm competition. When all else was equal, males preferred the larger female when the paired females differed considerably in body length. The presence of either 1 or 2 rival males near, and sexually interacting with, the initially preferred female reduced a focal male’s preference for that female and increased the probability that he would reverse his initial mate preference. However, focal males were more likely to reverse their initial preference when the females were similar in body length than when they differed considerably in body length after observing initially preferred females interacting with 1 rival male. Our novel results suggest that male guppies simultaneously evaluate sperm competition and female quality when making mating decisions. Key words: body size, male mate choice, mate quality, sexual selection, social information, sperm competition. [Behav Ecol]

Male-derived cuticular hydrocarbons signal sperm competition intensity and affect ejaculate expenditure in crickets

Female sexual promiscuity can have significant effects on male mating decisions because it increases the intensity of competition between ejaculates for fertilization. Because sperm production is costly, males that can detect multiple matings by females and allocate sperm strategically will have an obvious fitness advantage. The presence of rival males is widely recognized as a cue used by males to assess sperm competition. However, for species in which males neither congregate around nor guard females, other more cryptic cues might be involved. Here, we demonstrate unprecedented levels of sperm competition assessment by males, which is mediated via the use of chemical cues. Using the cricket Teleogryllus oceanicus, we manipulated male perception of sperm competition by experimentally coating live unmated females with cuticular compounds extracted from males. We found that males adjusted their ejaculate allocation in response to these compounds: the viability of sperm contained within a male's ejaculate decreased as the number of male extracts applied to his virgin female partner was increased. We further show that males do not respond to the relative concentration of male compounds present on females, but rather to the number of distinct signature odours of individual males. Our results conform to sperm competition theory, and show for the first time, to our knowledge, that males can detect different intensities of sperm competition by using distinct chemical cues of individual males present on females.

Timing of female sexual unreceptivity and male adjustment of copulatory behaviour under competition risk in the wolf spider Schizocosa malitiosa

Males can change their copulatory or sperm transfer patterns in response to sperm competition risk. Schizocosa malitiosa performs long copulations, which include two consecutive patterns (Patterns 1 and 2). Virgin females are very sexually receptive, but mated females diminish their receptiveness. Female unreceptivity has been attributed to the action of receptivity-inhibiting substances, mainly transferred during Pattern 1. We tested: (1) if females who mated only with Pattern 1 were immediately unreceptive; (2) male and female behaviours when the copulating couple was exposed to another male. For (1), we interrupted mating when Pattern 1 finished and immediately exposed the female to a second male. For (2), we introduced a second male when the couple was starting (Ei) or finishing copulation (Li). Females were unreceptive immediately after finishing Pattern 1. Males from Ei and Li dismounted and approached the second males. Ei males diminished the frequencies of insertion after perceiving the presence of a second male and dismounted less frequently when copulating with heavy females. The study provides insights about the timing of sexual unreceptivity in S. malitiosa under possibilities of sperm competition, discussing male adjustment of copulatory behaviour in the presence of rival males.

Mate preferences by male guppies ( Poecilia reticulata ) in relation to the risk of sperm competition

We investigated male mate preferences in relation to the perceived risk of sperm competition in the guppy (Poecilia reticulata), a freshwater fish with a promiscuous mating system. Our laboratory experiments revealed that male mate choice behaviour is not influenced by the presence of rival males that are merely in close proximity to a potential mate, as there was no significant difference in the amount of time that males spent with females that were recently either alone or in close proximity to four rival males. Male mate choice behaviour was, however, strongly influenced by the presence of rival males in a second experiment, where those rivals were permitted to copulate with one of the females. In that situation, males spent significantly more time with, and directed significantly more sigmoid courtship displays toward, females that they had recently seen alone compared to females they had seen receiving forced copulations from up to four rival males. Our results therefore indicate that male guppies are sensitive to the risk of sperm competition and alter their mate choice behaviour in an adaptive fashion.

Response of territorial males to the threat of sneaking in the three‐spined stickleback (Gasterosteus aculeatus): a field study

AbstractAlternative mating tactics are found in many species, and may have important implications for population genetics and speciation. The existence of such alternative mating tactics is well‐documented in the three‐spined stickleback Gasterosteus aculeatus, and sneaking and egg‐stealing may occur in a significant proportion of matings under natural conditions. Sneaking can impose high costs on territorial males, both in terms of reduced reproductive output and caring for unrelated offspring. We ask whether territorial males adjust their behaviour in response to the risk of sneaking. In a field study of three‐spined sticklebacks on the Isle of Arran, Scotland, territorial males were presented with glass bottles containing either a male, a female or neither, to address whether territorial males were more aggressive to other males in the presence of a female, and whether territorial males courted females less in the presence of another male. Behavioural observations showed that territorial males did not behave more aggressively towards rival males in the presence of a female, but did reduce their rate of courtship towards females in the presence of rival males. We conclude that territorial males adopt behavioural strategies that may reduce their risk of reproductive parasitism.

Male copulation behaviour and the risk of sperm competition

When females mate with more than one male during their reproductive cycle, males may increase their share of paternity by copulating repeatedly with the same female. Accordingly, males should mate repeatedly with the same female more frequently when the risk of sperm competition is greater. We examined this idea experimentally in the orb-web spiderNephila edulis , which is characterized by both extreme sexual size dimorphism and extreme male size variation. Comparison of the mating behaviour of solitary and pairs of males on the webs of virgin and mated females revealed that males adjust the frequency and duration of copulation according to the mating history of the female and the presence of rival males. Males copulated more frequently and for longer with virgin than mated females. The copulation behaviour of males in the presence of rivals depended upon their relative size. Typically, larger males prevented smaller rivals from gaining access to the female and therefore were able to copulate more frequently. Smaller males copulated less frequently, but for longer periods, which may have increased their share of paternity. The size of male N.edulis can vary by an order of magnitude, and our results suggest that this variation may be maintained by the alternative size-dependent strategies of preventing or winning sperm competition. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.

Female preference for multiple partners: sperm competition in the hide beetle, Dermestes maculatus (DeGeer)

Explanations for the evolution of polyandry emphasize either direct benefits or indirect genetic benefits. The former class of explanation predicts that females may seek multiple copulations but not necessarily multiple partners, while the latter predicts that females will seek multiple copulations with different partners. We investigated these predictions experimentally in the hide beetle, in which females mate more than once during their reproductive cycle. Female hide beetles remated more readily with novel males than with their previous partners, suggesting they mate multiply in order to obtain indirect, genetic benefits. In double-mating experiments with sterile males, the proportion of eggs fertilized by the second male was highly variable; this could not be explained by variation in male body size, duration of copulation or mating order. Male hide beetles form postinsemination associations by riding on the backs of females. This behaviour was more prevalent in the presence of rival males and less prevalent in the presence of additional females, suggesting that the behaviour serves to increase paternity, perhaps by obtaining additional copulations.