`Little Chipmunk' Syndrome? Male Body Size and Dominance in Captive Yellow‐Pine Chipmunks (Tamias amoenus)

In mammals, large males are often assumed to have higher mating success because they have greater success at contest competition. This relationship is often used to explain the prevalence of male-biased sexual size dimorphism in mammals. However, in many small vertebrates, large individuals are not always dominant. Using staged dyadic encounters, we examined the relationship between male body size and social dominance in captive male yellow-pine chipmunks ( Tamias amoenus ), a species with female-biased sexual size dimorphism. The yellow-pine chipmunk has a mating system in which males participate in mating chases and dominant males may have an advantage in acquiring matings with oestrous females. Captive male chipmunks were aggressive in only 28% of 144 paired encounters; however, several lines of evidence indicated that smaller chipmunks were dominant over large chipmunks: (1) small males were dominant in more dyads than large males; (2) within dyads, dominant males were smaller than subordinate males; and (3) small males performed more aggressive behaviour than large males. These results are not consistent with the prediction that large males are typically dominant. If large chipmunks are able to gain matings with females because of qualities other than dominance (such as the ability to successfully find and/or chase receptive females), then the costs of aggression to large chipmunks may outweigh any potential benefits. Small males, but not large males, may improve their mating success by being aggressive.     

Evolution of reproductive isolation as a by‐product of divergent life‐history evolution in laboratory populations of Drosophila melanogaster

We show that two complementary asymmetric isolating mechanisms, likely mediated by divergence in body size, underlie the evolution of incipient reproductive isolation between a set of Drosophila melanogaster populations selected for rapid development and their ancestral controls. Selection has led to great reduction in body size in the fast developing lines. Small males belonging to fast developing lines obtain few matings with large control females, both in presence and absence of large control line males, giving rise to unidirectional, premating isolation caused by sexual selection. Conversely, small selected line females suffer greatly increased mortality following mating with large control males, causing unidirectional postcopulatory prezygotic isolation. We discuss preliminary evidence for evolution of reduced male harm caused to females upon mating in the fast developing lines, and speculate that the females from these lines have coevolved reduced resistance to male harm such that they can no longer resist the harm caused by males from control lines. This potentially implicates differing levels of sexual conflict in creating reproductive barrier between the selected line females and the control males. We also show that a large difference in development time is not sufficient to cause postzygotic incompatibilities in the two sets of populations reaffirming the belief that prezygotic isolation can evolve much earlier than postzygotic isolation.     

Large males fight and court more across a range of social environments: an experiment on the two spotted goby Gobiusculus flavescens

The present study explored how male size relates to mating competition across a natural range of male and female densities in the two-spotted goby Gobiusculus flavescens. Across this range of social environments, large males were more than twice as likely as small ones to chase other males, to become nest-holders, and to court females, but large males were not significantly more likely to engage in agonistic fin displays. Overall, the study showed that large males court and fight more than small ones across a wide, yet natural, span of social environments. Having a large body size appears to confer competitive advantage for males in any social environment of the study species. Further studies are needed to disentangle whether the benefit of large size is mainly in competition over resources, over matings as such, or both.     

Male green frogs lower the pitch of acoustic signals in defense of territories: a possible dishonest signal of size?

In responses to broadcasts of conspecific advertisement calls,male green frogs (Rana clamitans) lower the dominant frequencyof their calls. Because dominant frequency is negatively correlatedwith male body size in green frogs, frequency alteration providesa means of potentially exaggerating size during territorialcontests. In field playback experiments, we broadcast syntheticstimuli representing small, medium, or large intruders to territorialresidents. We tested the hypotheses that males use frequency alteration to provide honest signals of their size or theirsize-independent fighting ability, or to dishonestly signalsize. Dominant frequency did not better predict male size inresponse calls than in unsolicited calls. The magnitude offrequency alteration was not related to body size, general condition, or an indirect measure of fighting ability. Thus,males did not use frequency alteration to provide honest informationabout body size or size-independent fighting ability. However,males significantly increased their apparent size by producinglower frequency calls. Small males produced relatively lowerfrequency calls in response to the large-male stimulus (comparedto the small-male and medium-male stimuli), but large malesdid not. Further, the magnitudes of frequency alteration weresignificantly greater in responses to the large-male stimulus,primarily because small males responded with a greater decreasein frequency to the large-male stimulus than to the small-maleand medium-male stimuli. These results support several predictions of the dishonest signal hypothesis and suggest that dishonestymay be a conditional strategy used by small males.     

Effects of size at metamorphosis on stonefly fecundity, longevity, and reproductive success

Many organisms with complex life cycles show considerable variation in size and timing at metamorphosis. Adult males of Megarcyssignata (Plecoptera: Perlodidae) are significantly smaller than females and emerge before females (protandry) from two western Colorado streams. During summer 1992 stoneflies from a trout stream emerged earlier in the season and at larger sizes than those from a colder fishless stream, and size at metamorphosis did not change over the emergence period in either stream. We performed two experiments to determine whether variation in size at metamorphosis affected the fecundity, reproductive success and longevity of individuals of this stonefly species and if total lifetime fecundity was affected by the number of matings. In the first experiment, total lifetime fecundity (eggs oviposited) was determined for adult females held in small plastic cages in the field. Males were removed after one copulation, or pairs were left together for life and allowed to multiply mate. Most copulations occurred in the first few days of the experiment. Females in treatments allowing multiple matings had significantly lower total lifetime fecundity and shorter adult longevity than females that only mated once. Multiple matings also reduced longevity of males. Fecundity increased significantly with female body mass at emergence, but only for females that mated once. While multiple matings eliminated the fecundity advantage of large female body size, number of matings did not affect the significant positive relationship between body mass at metamorphosis and longevity of males or females. In a second experiment designed to determine if body mass at emergence affected male mating success, we placed one large and one small male Megarcys in an observation arena containing one female and recorded which male obtained the first mating. The large and the small male had equal probabilities of copulating with the female. Copulations usually lasted all night, and the unmated male made frequent, but unsuccessful attempts to take over the copulating female. Our data suggest that selection pressures determining body size at metamorphosis may operate independently on males and females, resulting in evolution of sexual size dimorphism, protandry, and mating early in the adult stage. We emphasize the importance of interpreting the fitness consequences of larval growth and development on the timing of and size at metamorphosis in the context of the complete life cycle. Received: 1 July 1997 / Accepted: 12 November 1997     

Body size of virtual rivals affects ejaculate size in sticklebacks

Sperm competition occurs when sperm of two or more males competeto fertilize a given set of eggs. Theories on sperm competitionexpect males under high risk of sperm competition to increaseejaculate size. Here we confirm this prediction experimentallyin the three-spined stickleback (Gasterosteus aculeatus). Inthis species, sneaking (i.e., stealing of fertilizations byneighboring males) can lead to sperm competition. Sneaking malesinvade foreign nests, and the owners vigorously try to preventthis intrusion. In such fights, male body size is assumed tobe an important predictor of success. Consequently, the riskof sperm competition may depend on the size of a potential competitor.We experimentally confronted males before spawning with eithera large or a small computer-animated rival. We show that malesejaculated significantly more sperm after the presentation ofthe larger virtual rival than after the small stimulus. In addition,the time between the initiation of courting and the spawningwas shorter in the large virtual male treatment. The resultssuggest that stickleback males tailor ejaculate size relativeto the risk of sperm competition perceived by the size of apotential competitor.     

Social control of terminal phase transition in primary males of the diandric wrasse, <Emphasis Type="Italic">Halichoeres poecilopterus</Emphasis> (Pisces: Labridae)

In many diandric fishes, large territorial males with bright body coloration (terminal phase (TP) males) are derived either from initial phase (IP) females that change sex to male or from IP primary males that change color and behavior, but do not change sex. The mechanism controlling the transition of IP primary males into TP males is not well understood. We conducted cohabitation experiments to examine social conditions favoring TP transition by primary males in the diandric wrasse, Halichoeres poecilopterus. IP primary males always started TP-specific sexual behavior in the presence of a smaller subordinate, and subsequently acquired TP body coloration. In contrast, primary males under subordinate conditions often performed female-like sexual behavior. In pairs with similar body sizes, both individuals initiated TP male behavior. The results suggest that TP transition in primary males may be closely related to a dominance relationship (or size order) within social groups, as it is in the case of sex change by females.     

Sexual cannibalism, competition, and size dimorphism in the orb-weaving spider Nephila plumipes Latreille (Araneae: Araneoidea)

The degree and direction of sexual dimorphism varies widely,but in several taxa of orb-weaving spiders, including Nephila,males may be less than one-tenth the size of females. This differenceis commonly attributed to selection through precopulation sexualcannibalism: females may either fail to detect very small males,or ignore them as potential prey items. However, there is oftenthe potential for male-male competition in these species becauseseveral males can be found on the web of a single female. Weinvestigated experimentally the effects of sexual cannibalismand male-male competition on male body size and hence sexualdimorphism in the Australian golden orb-weaver (Nephila plumipes).Small males were less likely to be detected and cannibalizedthan larger males. However, larger males excluded small malesfrom the central hub of the web, where mating takes place. Theconflicting effects of sexual cannibalism and male-male competitionmay be responsible for the relatively large variation in malebody size in this species.     

Is male plumage reflectance correlated with paternal care in bluethroats?

Although it is now well established that the conspicuous maleplumage colors of many birds have been subject to sexual selectionby female choice, it is still debated whether females matewith colorful males to obtain direct or indirect benefits.In species where males provide substantial parental care, femalesmay obtain direct benefits from mating with the males that are best at providing care. The good parent hypothesis suggeststhat male plumage coloration signals a male's ability to provideparental care. Alternatively, the differential-allocation hypothesissuggests that colorful males reduce their care in responseto increased investment by females mated to attractive males.We tested these hypotheses on the bluethroat (Luscinia s. svecica),a socially monogamous, sexually dichromatic bird, in which males have a colorful throat patch consisting of a structurallyderived blue area surrounding a melanin-based chestnut spot.Male plumage coloration was objectively quantified by use ofreflectance spectrometry. We found no evidence of a relationshipbetween male coloration of either the blue patch or the chestnutspot and the level of paternal care. Nor were there any correlationsbetween male coloration and body size or body condition. Thus, our study does not support the hypothesis that male colorationsignals male parental quality (the good parent hypothesis)or the hypothesis that colorful males reduce their care inresponse to increased investment by females (the differential-allocationhypothesis).     

Fitness correlates of male coloration in a Lake Victoria cichlid fish

Sexual selection by female choice has contributed to the rapidevolution of phenotypic diversity in the cichlid fish speciesflocks of East Africa. Yet, very little is known about the ecologicalmechanisms that drive the evolution of female mating preferences.We studied fitness correlates of male nuptial coloration ina member of a diverse Lake Victoria cichlid lineage, Pundamilianyererei. In this species, male red coloration is subject tointraspecific sexual selection by female mate choice. Male nuptialcoloration plays a critical role also in reproductive isolationbetween this species and the closely related sympatric speciesP. pundamilia. Here, we show that P. nyererei male colorationis carotenoid based, illustrating the potential for honest signalingof individual quality. In a wild population, we found that variationin male coloration was not associated with variation in a setof strongly intercorrelated indicators of male dominance: malesize, territory size, and territory location. Instead, the 2male characters that predominantly determine female choice,territory size and red coloration, may be independent predictorsof male quality: males with bright red coloration and largeterritories had lower parasite infestation rates. As a result,female preferences tended to select against heavily parasitizedmales. Consistent with parasite-mediated sexual selection, maleshad higher and more variable parasite loads than females.     

A life-history perspective on strategic mating effort in male scorpionflies

In species with high male mating effort, there is a trade-offbetween mating effort spent in a current mating and resourcesleft for future matings. Consequently, to maximize their reproductivesuccess, males have to invest strategically, saving resourcesin matings with low reproductive gain for future, more valuablematings. However, as males age, the expected future reproductivesuccess constantly declines. Thus, the importance of resource rationing may drastically change during a lifetime. Males ofthe scorpionfly Panorpa cognata offer females a costly nuptialgift before copulation, which functions as male mating effort.Resources for the production of these salivary masses are severelylimited for males in poor condition. We found that males investedmore in copulations with high-quality females than in copulationswith low-quality females. However, males ceased to discriminateas they became older. Old males, with a relative small numberof expected future matings, did not invest differentially incopulations with high- versus low-quality females. In copulationswith low-quality females, males invested more in late thanin initial matings, whereas in matings with high-quality females,time of mating had no influence on mating effort. These resultsimply that males adaptively change their resource allocationstrategy during the course of the season. Initial matings seemto be characterized by male prudence; in later matings, malesseem to adopt a more opportunistic mating strategy.     

Structural coloration and sexual selection in the barn swallow Hirundo rustica

Structural coloration has been hypothesized to play a role insexual selection, and we tested whether this was the case ina field study of the barn swallow Hirundo rustica. The dorsaliridescent plumage of barn swallows has a strong reflectancein the ultraviolet (UV) region, with adult males on averagereflecting 8-9% more than adult females, as revealed by a 2-yearstudy in southwestern Spain. The correlation between structural coloration (described by the reflectance in the UV part of thespectrum, UV chroma and blue chroma) and three other secondarysexual characters significantly associated with male matingsuccess (tail length, tail asymmetry, and red facial coloration)was weak and generally nonsignificant. Nor was there a significantrelationship between color parameters and body condition. Wetested for an association between structural coloration of the dorsal plumage and sexual selection in a number of independenttests. Arrival date of males was not significantly relatedto color; there was no significant relationship between colorationand probability of survival or age; mated males did not havestronger reflectance than unmated males; and the duration ofthe premating period was not significantly related to color.Reproductive success was not significantly correlated withplumage coloration in males, nor was the feeding rate of offspringby brightly colored males higher than that of males with lessbright plumage. Given that sample sizes were large, and the power of statistical tests high, we conclude that current sexualselection on the coloration of the dorsal plumage in the barnswallow is, at best, weak.     

Male distribution and interactions at female oviposition sites as factors affecting mating success in the flyDryomyza anilis (Dryomyzidae)

Summary Dryomyza anilis males gather around carcasses where females arrive to lay their eggs and mate with them before oviposition. Distribution and mating success of males of different size at these sites were studied. The position of males of different size on the carcass and in ten circular zones around it was recorded every 10 min during 2–3 h observation periods. The position of mating pairs, mating initiations and the number of take-overs in different zones were also recorded. It was expected that males would distribute themselves according to the ideal free distribution, i.e. their gain rate in different zones in terms of number of mating initiations would be equalized.Males in different zones reached a stable distribution after 30 min from the beginning of observations. Although males, females and mating pairs were distributed over several zones, most matings were started and take-overs took place on the carcass or in the first zone (C+Z1) because arriving females usually reached the centre before being intercepted by a male. Therefore, males in C+Z1 attained a much higher mating rate than males further away: the percentage of matings started there was significantly higher than the percentage of males present there at the same time. The frequency of male-male fights was highest in C+Z1 which is likely to prevent more males from entering the central area. In this respect the observed male distribution bears resemblance to the ideal despotic distribution.However, the distribution of males of different size did not differ, and there were many small males in C+Z1. Small males in C+Z1 had a higher likelihood of starting a mating than small males further away but suffered from frequent take-overs like small males in all zones. For example, small males started 38% of all matings but there were only 8% of them among males guarding an ovipositing female. The corresponding figures for large males were 35% and 77%, respectively, suggesting that kleptoparasitic interactions and not aggressions between males were the main reason for differences in mating success between males of different size. Since small males do not have any alternative mating sites where large males would be absent, being the second last male to mate with a female is the best small males can achieve. Large males may even benefit from the presence of small males in the centre: they intercept females which large males can then easily take over.     

Bluegill Coloration as a Sexual Ornament: Evidence From Ontogeny,Sexual Dichromatism,and Condition Dependence

In aquatic environments, visual communication is expected in animals that inhabit clear, shallow waters. Here, we investigate variation in the colorful traits of bluegills, Lepomis macrochirus, to elucidate their possible function. Bluegills use alternative mating tactics whereby males develop into one of two irreversible phenotypes termed parental and cuckolder. Parentals build and defend nests and care for offspring whereas cuckolders obtain matings by sneaking copulations. We hypothesized that bluegill coloration might function as a sexual ornament in parental males and that ornamental coloration might serve as an honest indicator of male quality. We predicted that coloration should be more pronounced in parental males than in females and immature males and should be more pronounced during the breeding season. We also predicted that males in better condition should be more intensely colored than fish in poor condition. To test our predictions, we sampled 510 bluegills during the breeding and post‐breeding seasons at nine lakes in southern Ontario, Canada, in 2007. We used reflectance spectrometry to quantify the coloration of five body regions, aged and sexed each fish, and calculated Fulton’s condition factor from morphological measurements. A separate experiment showed that color did not fade several minutes post capture, suggesting that coloration could be measured reliably and consistently. We found that color was influenced by maturity, sex, and season, in the predicted direction, for three body regions (breast, cheek, and opercular flap). We also found that color varied with the condition of males such that males in better condition were darker for the sexually dichromatic ventral and facial regions. Our findings therefore suggest that some colorful traits in bluegills may serve as condition‐dependent sexual signals during the breeding season. Our research contributes to a growing appreciation of the importance of visual signaling in aquatic environments.     

Influence of male competition on male mating behaviour in the cave molly, Poecilia mexicana

In many species, male mating behaviour is correlated with male body size, with large males often being preferred by females. Small surface-dwelling Poecilia mexicana males compensate for this disadvantage by being more sexually active and using sneaky copulations. In a cave-dwelling population, however, small males do not show this behaviour. Do small males alter their behaviour in the presence of a large rival? Here, we investigated the influence of male competition on male mating behaviour in the cave form. Two males of different sizes were mated with a female either alone or together with the other male. No aggressive interactions were observed between either fish. There was no statistically significant difference in the frequency of sexual behaviours between the two treatments. In both treatments, large males were more sexually active than small males. Thus, small cave molly males do not switch to an alternative mating behaviour in the presence of a larger rival. Possibly, the extreme environmental conditions in the cave (e.g. low oxygen content and high levels of hydrogen sulphide) favour saving energetic costs, resulting in the absence of alternative mating behaviour in small males.     

The effect of male size on receptivity in femaleLucilia cuprina (Diptera: Calliphoridae)

The first five matings by both large and small L. cuprinamales switched off receptivity in over 90% of females for 7 days after their initial mating. Small males were virtually unable to switch off females after five matings, whereas the fifth to tenth matings by large males rendered nearly two-thirds of females unreceptive. Mating duration of large and small males was about 11 min even after numerous matings. Given the rarity of field virgins and the temporal and spatial variability of female-required resources, larger males may enjoy greater mating success due to their ability to switch off many females in succession. Virgin males preferred to mate with nonvirgin females larger than themselves; males paired with females smaller than themselves adopted mate-rejection behavior typical of unreceptive females.     

Females decide whether size matters: plastic mate preferences tuned to the intensity of male-male competition

Recent studies have indicated that mating success of large malesmay improve under increasing levels of mating competition. Thisoutcome is explained 1) if male mating competition is overridingfemale preferences for male traits that are unrelated to, ornegatively correlated with, male size and dominance and, inso doing, dictates the distribution of matings or 2) if femalesalter their preferences with respect to large males when male–malecompetition is intense. Under both hypotheses, one could expectlarge, dominant males to be more successful under intense competitionthan under weak competition. However, only the first explanationpredicts that male mating success under intense competitionshould be determined by dominance; traits that are unrelatedto male dominance should be uncorrelated to mating success.In contrast, if females change their preferences (explanation2), males with traits beneficial to females independent of thecompetitive environment can maintain a high mating success underall levels of male–male competition. We tested these alternativesusing a small marine fish, the sand goby, Pomatoschistus minutus.The mating success of large males increased under conditionsallowing intense male competition, whereas females showed apreference for good nest building independent of the level ofcompetition. These findings suggest that females are in controlof their choice by altering their preference for male size inresponse to the intensity of male–male competition ratherthan female preference being overshadowed by male dominance.This plasticity of preferences implies that the strength ofsexual selection is not constant at the population level.     

Mate preference for multiple cues: interplay between male and nest size in the sand goby, Pomatoschistus minutus

Female mating preferences are often based on more than one cue.In empirical studies, however, different mate choice cues aretypically treated separately ignoring their possible interactions.In the current work, we studied how male body size and sizeof the male's nest jointly affect mate preferences of femalesand gobies, Pomatoschistus minutus. The females were givena binary choice between males that differed either in body sizeor size of their nest or both. We found that neither body sizenor size of the nest alone affected male attractiveness, buttogether these 2 cues had a significant effect. Specifically,large males were more popular among females when they had alarge nest than when they occupied a small nest. The resultssuggest that if interaction effects between multiple mate choicecues are not considered, there is a danger of ignoring or underestimatingthe importance of these cues in sexual selection by female choice.     

Predation by red-jointed fiddler crabs on congeners: interaction between body size and positive allometry of the sexually selected claw

The enlarged (major) claw of male fiddler crabs is used in contestsover breeding burrows and is waved to attract females. We recentlydiscovered that males of the red-jointed fiddler crab, Uca minax,also use the claw to kill smaller-sized fiddler crabs, U. pugnaxand U. pugilator, with which they co-occur in Atlantic coastsalt marshes. Large U. minax males use walking legs or the enlargedclaw to capture prey feeding on moist sand flats. On sand flats,small U. minax males and females are much less common than largemales, suggesting that large males move onto sand flats to seekprey. Males of prey species use the major claw against attackingpredators and, consequently, are more likely than females toescape. In laboratory experiments, large U. minax males weremore likely to attack and kill small-clawed males and femalesthan large-clawed males, consistent with a preference for morevulnerable, less threatening prey. The size of the major clawis a positive allometric function of body size. The allometricfunction varies little among species. Also, the mechanical advantageand indices of closing speed and closing force of the majorclaw, when corrected for body size, are not consistently greaterin U. minax relative to prey species. Thus, predation by U.minax males may reflect the opportunity afforded by larger bodysize and positive allometric growth, which result in a majorclaw that is more massive than the prey it is directed against.     

Group spawning results from the streaking of small males into a sneaking pair: male alternative reproductive tactics in the threespot wrasse Halichoeres trimaculatus

In many protogynous wrasses, large males with bright coloration (terminal phase males, TP males) establish mating territories and pair-spawn with females. In contrast, small primary males with drab coloration (initial phase males, IP males) are non-territorial and adopt three alternative reproductive tactics—group spawning, streaking, and sneaking. We investigated how IP males of the threespot wrasse Halichoeres trimaculatus use these tactics in different situations. The mating frequency of the IP males was positively correlated with their courtship frequency, but not with their body size. Larger IP males tended to attack the smaller ones at the mating sites. This indicates that the larger IP males attempted to exclude the smaller ones from mating with the intention of minimizing the number of IP males involved in group spawning and ultimately leading to pair spawning (sneaking). However, the larger IP males were unable to completely exclude the smaller males because the intensity of the attack by the larger IP males was weak. Consequently, the smaller IP males could easily streak into the sneaking of larger IP males, thereby resulting in group spawning.