Sexually dimorphic male horns and their use in agonistic behaviors in the horn-headed cricket <Emphasis Type="Italic">Loxoblemmus doenitzi</Emphasis> (Orthoptera: Gryllidae)

Sexual dimorphism, the difference between the sexes in secondary sexual characters, is in general driven by processes of sexual selection. The horn-headed cricket, Loxoblemmus doenitzi, exhibits sexual dimorphism in head shape. Males have flat heads and triangular horns on both sides of their heads, whereas females have rounded heads and no horns. We hypothesized that male horns have evolved due to intra-sexual selection, in which males use these horns as weapons in aggressive interactions. We tested two predictions of this hypothesis by conducting agonistic trials with field-caught males of L. doenitzi: (1) the horns should be used in agonistic interactions between males, and (2) the asymmetry in horn size or horn use may determine contest outcome. Horn length was significantly correlated with thorax length and hind femur length. During agonistic interactions, males aggressively used their horns by beating the opponent’s horns with their own or by poking the opponent’s body. However, logistic regression analysis revealed that neither horn length nor horn use were significant factors for contest outcome. Instead, body size was significant for determining contest outcome. We discuss possible scenarios for evolution of male horns in L. doenitzi.     

Sex-Specific Aggression and Antipredator Behaviour in Young Brown Trout

Sex differences in adult behaviour are often interpreted as consequences of sexual selection and/or different reproductive roles in males and females. Sex-specific juvenile behaviour, however, has received less attention. Adult brown trout males are more aggressive than females during spawning and juvenile aggression may be genetically correlated with adult aggression in fish. We therefore tested the prediction that immature brown trout males are more aggressive and bolder than immature females. Because previous work has suggested that precocious maturation increases dominance in salmonids, we included precocious males in the study to test the prediction that early sexual maturation increase male aggression and boldness. Aggression and dominance relations were estimated in dyadic contests, whereas boldness was measured as a response to simulated predation risk using a model heron. Independent of maturity state, males initiated more than twice as many agonistic interactions as females in intersexual contests. However, males were not significantly more likely to win these contests than females. The response to a first predator attack did not differ between sex categories, but males reacted less to a second predator attack than females. Sexual maturity did not affect the antipredator response in males. Since there is no evidence from field studies that stream-living immature male and female salmonids differ in growth rate, it appears unlikely that the sex differences demonstrated are behavioural consequences of sex-specific investment in growth. It seems more likely that sex-specific behaviour arises as a correlated response to sexually selected gene actions promoting differential behaviour in adult males and females during reproduction. Alternatively, sex differences may develop gradually during juvenile life, because a gradual developmental program should be less costly than a sudden behavioural change at the onset of sexual maturity.     

Spatial and Social Sexual Segregation Patterns in Indo-Pacific Bottlenose Dolphins (Tursiops aduncus)

Sexual segregation seems to be common in bottlenose dolphins, whereby males and females live in different pods that mix mainly for mating. Male dolphins often use aggressive behaviour to mate with females, while females with calves may have different activity and dietary requirements to males and different susceptibility to predation. We investigated the degree of spatial and social sexual segregation in Indo-Pacific bottlenose dolphins (Tursiops aduncus) in a subtropical estuary in Australia. Based on surveys completed over three years, dolphin groups were mostly mixed-sex or female. Mixed-sex groups were found in larger groups in mostly deeper water, whereas, female groups were foraging across all water depths in smaller groups. Aggressive coercive behaviour by males towards females was high, occurring mainly in deeper water, at higher tides, and outside the breeding season. Habitat use by female dolphin groups suggests that shallow tributaries may provide a sanctuary from aggressive males, access to suitable prey items and density for mothers and their calves, or a combination of these factors.     

Male and female secondary sexual traits show different patterns of quantitative genetic and environmental variation in the horned beetle Onthophagus sagittarius

The expression of secondary sexual traits in females has often been attributed to a correlated response to selection on male traits. In rare cases, females have secondary sexual traits that are not homologous structures to secondary sexual traits in males and are thus less likely to have evolved in females because of correlated selection. In this study, we used the dung beetle Onthophagus sagittarius, a species with sex‐specific horns, to examine the environmental and quantitative genetic control of horn expression in males and females. Offspring subjected to different brood mass manipulations (dung addition/removal) were found to differ significantly in body size. Brood mass manipulation also had a significant effect on the length of male horns; however, female horn length was found to be relatively impervious to the treatment, showing stronger patterns of additive genetic variance than males. We found no correlations between horn expression in males and females. We therefore conclude that the horns of O. sagittarius females are unlikely to result from genetic correlations between males and females. Rather, our data suggest that they may be under independent genetic control.     

Effect of paired encounters on plasma androgens and behaviour in males and females of the spiny damselfish Acanthochromis polyacanthus

Sexually mature male and female spiny damselfish, Acanthochromis polyacanthus (Pomacentridae) were introduced to resident fish of both sexes in paired encounter trials and the effect on activity, agonistic behaviour and plasma levels of testosterone (T) and 11-ketotestosterone (11-KT) in males, and T and 17β-estradiol (E₂) in females, and cortisol in both sexes was determined. Introduced males showed suppressed levels of T in the presence of both resident males and females, whereas resident males showed elevated T in response to introduced males but not females. Plasma 11-KT levels were unchanged in either resident or introduced males. Resident males showed increased activity in the presence of males but not females. Resident females showed an increase in activity when paired with introduced females and greater aggressive behaviour towards females than males. There were no differences in plasma hormone levels in either resident or introduced female fish. Plasma cortisol levels were low in all fish and stress effects did not appear to account for differences in T levels in males. The results of the present study show that elevations in plasma T levels in resident male spiny damselfish are associated only with male presence but that suppression of T in introduced fish occurs irrespective of the sex of the resident. In contrast resident females, which show stronger evidence of aggressive behaviour do not show changes in plasma androgen levels. This suggests that aggressive behaviour in male and female fishes may be mediated by different endocrine pathways.     

Horn asymmetry and fitness in gemsbok, Oryx g. gazella

The relationship between fluctuating asymmetry in horns of gemsbok(Oryx g. gazella) and a number of fitness components was determinedin a field study in Etosha National Park, Namibia. The lengthand width of horns and skull length demonstrated fluctuatingasymmetry. Both males and females with asymmetric horns werein poorer condition than symmetric individuals. Individualsof both sexes with symmetric horns more often won aggressiveinteractions at waterholes. Although symmetric individuals spentmore time in dense vegetation, their vigilance rate was nothigher than that of asymmetric individuals. Territorial, singlemales had more symmetric horns than males in herds, suggestingthat mating success was inversely related to horn asymmetry.Females with symmetric horns more often had calves than asymmetricfemales. Horn asymmetry thus appears to reliably reveal phenotypicquality as demonstrated by a suite of fitness components.[BehavEcol 7: 247-253 (1996)]     

Male dwarf chameleons assess risk of courting large, aggressive females

Conflict between the sexes has traditionally been studied in terms of costs of mating to females and female resistance. However, courting can also be costly to males, especially when females are larger and aggressively resist copulation attempts. We examined male display intensity towards females in the Cape dwarf chameleon, Bradypodion pumilum, in which females are larger than males and very aggressive. We assessed whether aggressive female rejection imposes potential costs on males and whether males vary their display behaviour with intensity of female rejection, female size or relative size differences. Males persisted in courtship after initial female rejection in 84% of trials, and were bitten in 28% of trials. Attempted mounts were positively associated with males being bitten. Males reduced courtship with increased intensity of female rejection. Male courtship behaviour also varied with female size: males were more likely to court and approach smaller females, consistent with the hypothesis that larger females can inflict more damage. These results suggest that, in addition to assessing female willingness to mate, male dwarf chameleons may use courtship displays to assess potential costs of persistence, including costs associated with aggressive female rejection, weighed against potential reproductive pay-offs associated with forced copulation.     

Individual differences in aggressiveness of female hamsters: response to intact and castrated males and to females

The aggressive behaviour of female hamsters was studied while they were housed in large enclosures with males and in brief tests with males or females. Some females are not aggressive with any male, whereas others are very aggressive toward all males in both testing conditions. Females that are not aggressive toward intact males may be very aggressive toward castrated males or females. When the animals are housed together for long periods of time, males dominate only if they are much heavier. Male dominance takes a relatively long time to establish and often there is an equivocal period characterized by reversals of dominance. Female dominance is rapidly established. Unless the male is much heavier, the female determines the presence or absence of agonistic behaviour.     

The Haremic Mating System and Mate Choice in the Wide-Eyed Flounder, Bothus podas

The social structure and reproductive behaviour of the wide-eyed flounder, Bothus podas, was studied in the coastal waters around the Azorean Islands. Both sexes are territorial throughout the year. Adult males defend large territories, which include several smaller female territories. Intraspecific agonistic behaviour was frequent and differed between sexes: males were more aggressive towards other males, while females were only aggressive towards each other and juveniles. During the reproductive season and only at dawn, territorial males court and mate successively with females in their territories, and females seem to show mating fidelity to their dominant male. Such territoriality and mating patterns indicate a haremic social system in the wide-eyed flounder. In order to identify potential factors influencing female mate choice acting on this haremic system, we examined male mating success and some of its potential correlates. We found no evidence for female preference for any of the males' physical or territory characteristics. However, courtship effort was strongly correlated with the total number of attempted and successful spawnings, indicating that females seem to mate preferentially with males that court them more vigorously. Thus, our data suggest that courtship plays an important role in determining male mating success in the wide-eyed flounder and, that it may possibly serve as an honest indicator of male `quality' for female choice.     

Function of weaponry in females: the use of horns in intrasexual competition for resources in female Soay sheep

In many species, females show reduced expression of a trait that is under sexual selection in males, and this expression is thought to be maintained through genetic associations with the male phenotype. However, there is also the potential for the female trait to convey an advantage in intrasexual conflicts over resources. We tested this hypothesis in a feral population of Soay sheep, in which males and females have a polymorphism for horn development, producing either full (normal horned), reduced (scurred) or no (polled, females only) horns. During the lambing period, females who possessed horns were more likely to initiate and win aggressive interactions, independent of age, weight and birthing status. The occurrence of aggression was also context dependent, decreasing over the lambing period and associated with local density. Our results demonstrate that a trait that confers benefits to males during intrasexual competition for mates may also be used by females in intrasexual competition over resources: males use weaponry to gain mates, whereas females use weaponry to gain food.     

Differential movement and activity patterns of sexes in a biparental beetle during the reproductive season

1. Biparental care is stabilised if parents perform different tasks during care. Specialised parental roles may require different time and energy budgets that in turn are expected to influence the activity and space use of sexes. 2. Here we investigate movement patterns of the biparental Lethrus apterus beetle using a grid of pitfall traps in their natural habitat. 3. Sexes of the burrow building L. apterus perform different roles during caregiving, as females collect most of the leaves, which serve as food for the offspring while paired males stay mostly in the burrow. We hypothesised that sex differences in mate search and parental activities are reflected in movement patterns. 4. We found that females frequently travelled short distances, whereas males were detected less often but when detected, they travelled significantly longer distances than females. 5. Our results are consistent with the notion that efficient parental food provisioning requires more localised movement and activity patterns. Furthermore, the long distance movements of some males may indicate active mate searching behaviour.     

Evidence for sex-related differences in phototactic bahaviour ofStreptocephalus proboscideus (Crustacea: Anostraca)

The phototactic behaviour of adults of the Sudanese fairy shrimpStreptocephalus probiscideus was studied under laboratory conditions. Males were less negatively phototactic than females. This was also evident when colour filters were used. females only became little less negatively phototactic under yellow light, whereas males showed a strong positively phototactic response. The response to the positioning of a yellow filter was stronger than to the use of a red or blue filter for both sexes. The laboratory findings were compared with casual field observations onStreptocaphalus torvicornis that indicate differential vertical distribution between the sexes and a nocturnal vertical migration. Migratory behaviour with ascent starting at dusk is also predicted forS. proboscideus. This behaviour may reduce common stress factors in desert pools such as photodamage, visual predation pressure, and high surface temperatures.     

Female aggression and the maintenance of monogamy: female behaviour predicts male mating status in European starlings

Any reduction in the fitness of a breeding female induced by the settlement of additional females with her mate creates a conflict between the sexes over mating system. In birds, females are often aggressive towards other females but few studies have been able to quantify the importance of female-female aggression for the maintenance of monogamy. This study of the European starling, Sturnus vulgaris, quantifies male and female behaviour towards a potential prospecting female, presented in a cage during the pre-laying period, and relates it to the subsequent mating status of the male. A solitary breeding male was given the opportunity to attract an additional mate, which almost half of the males did. No biometric characters of the male or female were related to the subsequent mating status. Males demonstrated mate-attraction behaviour towards the caged female but the behaviour of the male did not predict the likelihood to attract an additional female. However, the proportion of time that the female spent near the potential settler was related to mating status, indicating that females that reacted more strongly towards a potential female competitor maintained their monogamous status. These results suggest that female behaviour may play an important role in shaping the mating system of facultatively polygynous species.     

Spatial and temporal habitat use of kob antelopes (Kobus kob kob, Erxleben 1777) in the Comoé National Park, Ivory Coast as revealed by radio tracking

Spatial and temporal habitat use of kob antelopes (Kobus kob kob) have been investigated in the Comoé National Park (Ivory Coast, West Africa) by use of radio telemetry. A total of 23 kob were equipped with radio collars and radio tracked for up to 15 months. Home ranges of males were smaller and those of females larger than expected from theoretical models. Adult males used smaller areas than adult females and did not show seasonal home range shifts. Daily distances travelled did not differ between sexes. Kob walked less during the night than by day and covered shorter distances in the wet season. Whereas an increase in home range overlap between females resulted in higher rates of association among individuals, association of adults of mixed sexes was not correlated with the degree of home range overlap. Territorial behaviour of males and predator avoidance by females are suggested to explain the sex‐specific differences in home range size of adults and the deviation from the predicted sizes. Predator avoidance is presumed as the main reason for the reduced walking distances at night as well as in the wet season. Reproductive behaviour and feeding ecology are assumed to determine the degree of association of conspecifics.     

`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.     

Sexual conflict and speciation.

We review the significance of two forms of sexual conflict (different evolutionary interests of the two sexes) for genetic differentiation of populations and the evolution of reproductive isolation. Conflicting selection on the alleles at a single locus can occur in males and females if the sexes have different optima for a trait, and there are pleiotropic genetic correlations between the sexes for it. There will then be selection for sex limitation and hence sexual dimorphism. This sex limitation could break down in hybrids and reduce their fitness. Pleiotropic genetic correlations between the sexes could also affect the likelihood of mating in interpopulation encounters. Conflict can also occur between (sex-limited) loci that determine behaviour in males and those that determine behaviour in females. Reproductive isolation may occur by rapid coevolution of male trait and female mating preference. This would tend to generate assortative mating on secondary contact, hence promoting speciation. Sexual conflict resulting from sensory exploitation, polyspermy and the cost of mating could result in high levels of interpopulation mating. If females evolve resistance to make pre- and postmating manipulation, males from one population could be more successful with females from the other, because females would have evolved resistance to their own (but not to the allopatric) males. Between-locus sexual conflict could also occur as a result of conflict between males and females of different populations over the production of unfit hybrids. We develop models which show that females are in general selected to resist such matings and males to persist, and this could have a bearing on both the initial level of interpopulation matings and the likelihood that reinforcement will occur. In effect, selection on males usually acts to promote gene flow and to restrict premating isolation, whereas selection on females usually acts in the reverse direction. We review theoretical models relevant to resolution of this conflict. The winning role depends on a balance between the ''value of winning'' and ''power'' (relating to contest or armament costs): the winning role is likely to correlate with high value of winning and low costs. Sperm-ovum (or sperm-female tract) conflicts (and their plant parallels) are likely to obey the same principles. Males may typically have higher values of winning, but it is difficult to quantify ''power'', and females may often be able to resist mating more cheaply than males can force it. We tentatively predict that sexual conflict will typically result in a higher rate of speciation in ''female-win'' clades, that females will be responsible for premating isolation through reinforcement, and that ''female-win'' populations will be less genetically diverse.     

Behaviours of collared and white-lipped peccaries (Tayassu tajacu andT. pecari) in relation to sexual receptivity of the female

Changes in behaviours of the two peccary species,Tayassu tajacu Linnaeus, 1758 andT. pecari Link, 1795 between non-receptive and receptive periods were followed by presenting females to males daily for 15 minutes. InT. tajacu, the rank order of behaviours, similar in both sexes during the non-receptive period, differs during receptivity. Contact behaviours decrease in males, whereas sexual ones progress. The same tendency appears in females. Inhibited bites replace markings of partner as the most common behaviour in both sexes. InT. pecari, the rank order of behaviours always differs between sexes. When females become receptive, the differences from the non-receptive period are neither numerous nor significant. The most common behaviour of males, previously markings of partner, becomes mounts, whereas in females agonistic behaviours reinforce their dominance. In this species, the only behaviours that increase are those leading directly to copulation or those of an agonistic nature. In both species, females show more agonistic behaviours than males (mainly inhibited bites inT. tajacu, truly aggressive ones inT. pecari). When females are receptive, males ofT. pecari become less active, contrary toT. tajacu where both sexes double their activity. InT. tajacu, most behaviours vary significantly in relation to the progesterone level, contrary to the other species. These pecularities appear correlated to herd composition and organisation.     

Males with short horns spent more time mating in the Japanese horned beetle Allomyrina dichotoma

We examined the relationships between male body and horn sizes and mating duration in the Japanese horned beetle, Allomyrina dichotoma. Smaller males possessing shorter horns spent more time for copulation with a female and mounting the female without copulation. The results of multiple regression analyses indicate that the horn length is a determining factor for the time spent by the males during mating. A previous study has documented that the horn length of male A. dichotoma primarily determined the outcomes of aggressive male–male interactions; hence, predicts access to females. Therefore, instead of fighting for females, males possessing short horns may maximize their fertilization success by mating longer with the few females they have access to.     

Sex differences in the trade-off between feeding and mating in the guppy

Male guppies Poecilia reticulata court almost continuously and often indulge in sneaky mating attempts. Female guppies, on the other hand, devote much of their time to foraging. This paper demonstrates that both sexes reduce their feeding rate in mixed-sex groups relative to single sex ones and suggests that the decrease is less pronounced for females than for males. Thus males make feeding and mating decisions that are dependent on hunger while female foraging behaviour is constrained by sexual harassment. The observed sex differences in behaviour are a consequence of asymmetrical mating costs in males and females.     

Sex differences in song perception in Bengalese finches measured by the cardiac response

Birdsong may be perceived and processed differently by males and females because the production and function of this behaviour are often sexually dimorphic. However, examination of this hypothesis has been difficult, since different behavioural measures have been used to describe the perceptual process for each sex. We analysed changes in heart rate as an index of song perception in Bengalese finches, Lonchura striata var. domestica. In this species, only males sing, and song is used exclusively for mate attraction. This species is not territorial and songs are never used in aggressive contexts. In both sexes, repeated presentation of a song resulted in a waning of the heart rate response. Presentation of heterospecific songs did not increase the heart rate. When a novel conspecific song was presented, the heart rate increased only in females with each presentation of the stimulus, not in males. These results correspond to the sex differences in song usage in this species. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.