Intrasexual Competition For Mates Research Articles

Do male "long-distance calls" function in mate defense? A comparative study of long-distance calls in primates

Long-distance calls produced by males are common among vertebrate species. Several hypotheses have been proposed to explain features of male long-distance calls and their phylogenetic distribution in primates, but the putative functions of male long-distance calls have yet to be tested comprehensively. We used phylogenetic comparative methods to investigate hypotheses for the function of male long-distance calls. We focused on the mate defense hypothesis, which states that male long-distance calls function in intra-sexual competition for mates, but we also examined factors involving resource defense, mate attraction, and habitat. Phylogenetic reconstruction of male long-distance calls in 158 primate species indicates that the presence of male long-distance calls is the ancestral state. The carrying distance of male long-distance calls is correlated with the size of the home range, which is consistent with the role of male long-distance calls in defending mates, attracting mates, and defending resources. However, measures of male intra-sexual competition were not associated with the evolution of male long-distance calls. Evolutionary transitions were only partly correlated with factors related to mate attraction. Instead, the strongest correlates of male long-distance calls were activity period, body mass, home range, habitat and some measures of resource defense. Our results are consistent with long-distance call production as a costly signal, but detailed study within species is required to assess these costs and the functions of long-distance calls in individual cases. Electronic Supplementary Material is available if you access this article at http://dx.doi.org/10.1007/.

Sexual coercion in a territorial salamander: males punish socially polyandrous female partners

Clutton-Brock & Parker (1995, Animal Behaviour, 49, 1345–1365) proposed that sexual coercion is a third component of sexual selection, along with intrasexual competition for mates and intersexual mate choice. They stated that sexual coercion occurs when males subject females to forced copulation, harassment, or intimidation, and that aggression towards females may occur even when females merely associate (not mate) with other males. We tested this proposition in laboratory experiments with red-backed salamanders, Plethodon cinereus, that had been found in male–female pairs (we call members of a pair ‘partners’) and as single (nonpaired) individuals in the forest. We manipulated paired females so that they either had associated with another male (socially polyandrous partners) or had not associated with another male (socially monogamous partners) before returning to their partners' territories. We also manipulated single females so that, upon encountering a single male for the first time, they either had associated with another male (socially polyandrous strangers) or had not associated with another male (socially naı̈ve strangers). Males were significantly more aggressive towards and stayed farther away from female partners that were socially polyandrous relative to those that were socially monogamous, during both the summer noncourtship season and autumn courtship season. Socially polyandrous females increased escape behaviour relative to socially monogamous females. However, males that met females for the first time did not differ significantly in aggression towards or distance from them when those females were either socially polyandrous or naı̈ve, during both the noncourtship and courtship seasons. Thus, male aggressive behaviour was context dependent, with enhanced aggression only towards partners that were socially polyandrous. We infer that males attempted to manipulate female partners into social monogamy by increasing aggression towards socially polyandrous partners. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Causes and consequences of single-male and multimale mating in free-ranging patas monkeys, Erythrocebus patas

Patas monkeys and many forest guenon species live in one-male groups throughout the year. During some conceptive seasons, extragroup males enter the troop singly or in small groups to copulate with females, although this multimale state persists only as long as the conceptive season lasts. Our goals in this study were to (1) examine how the number of males in the group affected levels of intrasexual mate competition in males and females, (2) identify the proximate factors accounting for changes from single- to multimale mating, and (3) develop a model comparing the proximate factors leading to single- and multimale mating in patas monkeys with those for forest guenons. We studied the copulatory patterns of 23 female and 11 male patas monkeys during three breeding seasons in Laikipia, Kenya, and used rates and patterning of mate guarding, copulation harassment and wounding as indices of intrasexual competition. We observed multimale mating during the 1995 conceptive season, and single-male mating during 1997 and 1998. We found that moderately strong intrasexual competition in males led to infrequent high-intensity encounters across all mating seasons, and that males competed to control access to groups of females year round. Female intrasexual competition for mates was low, but slightly more vigorous during single-male mating seasons. Our model suggests that specific demographic and ecological conditions, combined with a single life history variable, interbirth interval, explain observed variation in mating systems of groups of patas monkeys and forest guenons.

Age-Specific Causes of Mortality among Male White-Tailed Deer Support Mate-Competition Theory

Mate-competition theory predicts that males will invest resources toward intrasexual competition for mates until reproductive benefits are outweighed by costs to future fitness. In populations that have a substantial proportion of mature males, theory predicts that young males will forego reproduction to reduce exposure to mortality that may result from breeding efforts. We examined age-specific mortality of males in a white-tailed deer (Odocoileus virginianus) population in which >50% of the males were ≥3.5 years old to determine whether patterns of male mortality conform to mate-competition theory. Annual mortality rates were relatively stable throughout adulthood (0.26-0.38), but causes of mortality changed with age. Young males (1.5 and 2.5 years old) were most susceptible to human-induced mortality (e.g., hunting and vehicle accidents), whereas males ≥3.5 years old tended to die from non-human causes (e.g., fighting, predation) more frequently than did younger deer. Proportional hazard models also indicated that as antler size increased, likelihood of mortality increased. Age- and cause-specific patterns of mortality in adult male deer from a population with >50% mature males reflected patterns predicted by mate-competition theory. When managing deer populations with high proportions of mature males, it is important to account for shifts in susceptibility to human-induced and natural mortality agents.

Natal dispersal of juvenile arctic ground squirrels in the boreal forest

Natal dispersal is assumed to be costly because of the risk of mortality, yet rarely are movement patterns and survival of dispersers observed directly. We determined the fates and dispersal distances of 150 radio-collared juvenile arctic ground squirrels from 1993 to 1995 at Kluane, Yukon Territory, Canada (61°N, 138°W). We tested the hypothesis that dispersal has a high mortality cost, and we also attempted to distinguish among three hypotheses to explain natal dispersal: competition for mates, competition for resources, and inbreeding avoidance. Juveniles were radio-collared at emergence from the natal burrow on five 9-ha grids nested within larger (1 km2) experimental manipulations: two controls, a predator exclosure, a food-supplemented grid, and a predator exclosure + food grid. In all years and on all areas, dispersing juveniles were more likely to die than philopatric squirrels, and the risk of mortality increased with distance from the natal burrow for both sexes. Overall, survival of philopatric squirrels was 73%, whereas survival of dispersing squirrels ranged from a maximum of 40% to a minimum of 25%. Juvenile females were strongly philopatric independent of population density, except on the predator exclosure + food grid in 1995, where population density was extremely high and resources other than food were probably limiting. Resource competition may explain patterns of philopatry and dispersal in female arctic ground squirrels. Juvenile males moved farther from their natal site than females and more of them died. Males also had a strong tendency to disperse that was independent of food availability or population density, which suggests that male arctic ground squirrels ultimately may disperse to avoid either inbreeding with female relatives or intrasexual competition for mates.

Evolution of Reversed Sexual Size Dimorphism in Skuas and Jaegers

Skuas and jaegers (Stercorariinae), like other birds with a raptorial lifestyle, display reversed sexual dimorphism in size (RSD) in which females are larger than males. This condition is commonly believed to be associated with the morphology or behavioral ecology of avian predators, and numerous theoretical explanations have been presented to account for its evolution. Most tests of these ideas have been theoretical or based on com- parative analyses. We present tests of specific hypotheses of RSD based on field studies of Great Skuas (Catharacta skua) and Parasitic Jaegers (Stercorarius parasiticus) breeding in Shet- land. We also present a comparative analysis of foraging behavior and degree of RSD within the Stercorariinae. We found no support for the popular theories that claim that RSD evolved as a result of specialized roles during breeding. Large females were not more fecund, nor were they the main defenders of the nest. Male body size was not correlated with any breed- ing parameter. In addition, interspecific comparisons did not support the idea that the de- gree of RSD is related to foraging behavior during the breeding season (as opposed to mor- phology). Female dominance within pairs did not affect any breeding parameters. Support was found for theories that RSD is linked to sexual selection. Large females had an advantage when pairing with older males, which could be a result of intrasexual competition for mates. There was assortative mating for size, indicating that small males could also have an advan- tage in pairing, because large males seem to be rejected by small females. Given the disparate results of several previous studies, we conclude that a large body of empirical evidence will be needed before we can draw firm conclusions about the main selective factors that act to maintain current levels of RSD in natural populations of raptorial birds. Received 24 July 1997,

Size, dominance and copulatory success in male northern elephant seals, Mirounga angustirostris

Abstract. The relationship between size, dominance and copulatory success was investigated in male northern elephant seals to determine the importance of size in male intra-sexual competition for mates. The study was conducted over four breeding seasons at the Año Nuevo rookery in central California. Mass was estimated using a photogrammetrical technique in three seasons and measured directly in one. Copulatory success was positively correlated with size (mass and length). This correlation was mediated through the positive effect of size on dominance rank; variation in arrival mass, for example, accounted for 29-44% of the variation in dominance. Copulatory success was positively correlated with dominance rank for mid- to high-ranking males, but not for low-ranking males, which obtained few copulations. Copulatory success was not significantly correlated with size when the effect of dominance was removed statistically. These results support the hypothesis that male reproductive success in this species is principally determined by dominance rank, which, in turn, is affected by male size. However, they also suggest that large size is just one of a number of important factors that affect the outcome of male intra-sexual competition and hence, reproductive success.

Do the risks of predation and intermale competition affect courtship behavior in the salamander Desmognathus ochrophaeus?

Natural populations of mountain dusky salamanders, Desmognathus ochrophaeus (Amphibia: Caudata: Plethodontidae), may experience two types of risk during courtship encounters: interspecific predation, affecting both sexes, and intraspecific competition for mates, affecting males. We conducted two experiments in the laboratory to determine whether and how these two risks influence the outcome of male-female encounters. In experiment 1, courtship interactions resulting in spermatophore deposition were inhibited in the presence of a restrained species of salamander (Gyrinophilus porphyriticus) known to prey on D. ochrophaeus. Probability of successful courtship was not affected by the presence of a restrained species that is not predatory (Plethodon teyahalee). In experiment 2, courtship interactions resulting in spermatophore deposition were inhibited in the presence of two restrained conspecific males, but were unaffected in the presence of a single restrained male. We conclude that the dual risks of predation and intrasexual competition for mates have been significant selective factors in shaping the evolution of courtship decisions in Desmognathus ochrophaeus.

Male and Female Mating Competition in Wolves: Female Suppression Vs. Male Intervention

[The social organization of wolves is characterised by strong pairbond associations. Groups may consist of a mating couple and their remaining offspring or of an association of potentially reproductive adults. When a pack consists of several adults, intrasexual mating competition and intersexual partner preference are expected to play an important role in the establishment of sexual relationships. Data from the wild show that as a rule only one female in a pack gives birth. Female suppression is assumed to prevent the other pack members from breeding. Our main interest was to elucidate the proximate behavioural mechanisms by which the socio-sexual relationships are structured in times of the mating season. Special attention was paid to the influence of differences in dominance status and sex on the competitive interactions between pack members. From 1977 to 1985 we studied the social behaviour of wolves in and around the mating season at Burger's Zoo in Arnhem (the Netherlands) by means of detailed observations on two wolf packs. Together with a general rise in frequency of (sexuo-affiliative) interactions we observed an increase in general aggression and in the frequency of interventions during the mating season. Our results reveal certain differences in male and female mating strategies. 1) Separative interventions were observed predominantly in males and were directed against male-female sexual interactions, while such interventions by females were less apparent. 2) Males showed a high frequency of intrasexual aggression but only during the mating season. By contrast, females showed less intrasexual aggression, and instead, showed a high level of intrasexual dominance display, especially the α-female, and they did so both in and outside the mating season. 3) Males tended to intervene in especially those intersexual contacts in which their own preferred female was involved, whereas the intolerance of the α-female was more general, i.e. it was shown with respect to all manifestations of other females. These patterns clearly explain why in free nature wolf social organization tends towards a multi-male uni-female system in which the non-dominant males have little chance of sexual contact, but are generally tolerated as helpers in brood care. In conclusion, male competition was comparatively more context-related while female competition took the form of unprovoked hostility., The social organization of wolves is characterised by strong pairbond associations. Groups may consist of a mating couple and their remaining offspring or of an association of potentially reproductive adults. When a pack consists of several adults, intrasexual mating competition and intersexual partner preference are expected to play an important role in the establishment of sexual relationships. Data from the wild show that as a rule only one female in a pack gives birth. Female suppression is assumed to prevent the other pack members from breeding. Our main interest was to elucidate the proximate behavioural mechanisms by which the socio-sexual relationships are structured in times of the mating season. Special attention was paid to the influence of differences in dominance status and sex on the competitive interactions between pack members. From 1977 to 1985 we studied the social behaviour of wolves in and around the mating season at Burger's Zoo in Arnhem (the Netherlands) by means of detailed observations on two wolf packs. Together with a general rise in frequency of (sexuo-affiliative) interactions we observed an increase in general aggression and in the frequency of interventions during the mating season. Our results reveal certain differences in male and female mating strategies. 1) Separative interventions were observed predominantly in males and were directed against male-female sexual interactions, while such interventions by females were less apparent. 2) Males showed a high frequency of intrasexual aggression but only during the mating season. By contrast, females showed less intrasexual aggression, and instead, showed a high level of intrasexual dominance display, especially the α-female, and they did so both in and outside the mating season. 3) Males tended to intervene in especially those intersexual contacts in which their own preferred female was involved, whereas the intolerance of the α-female was more general, i.e. it was shown with respect to all manifestations of other females. These patterns clearly explain why in free nature wolf social organization tends towards a multi-male uni-female system in which the non-dominant males have little chance of sexual contact, but are generally tolerated as helpers in brood care. In conclusion, male competition was comparatively more context-related while female competition took the form of unprovoked hostility.]

Sexual Dimorphism in Size, Relative Size of Testes, and Mating Systems in North American Voles

Sexual body-length dimorphism and relative size of testes were compared among 16 species (25 subspecies) of North American Microtus and Clethrionomys by use of data collected from museum specimens. Species with polygynous mating systems and territorial males had the greatest degree of sexual size dimorphism with males larger than females, and relatively small testes. Promiscuous species tended to show little or no sexual size dimorphism and relatively large testes. Species considered monogamous had no size dimorphism, and also had large testes. These data support the hypothesis that sexual dimorphism in size is correlated with the intensity of intrasexual competition for mates. However, the hypothesis that species with single-male breeding systems (single-male polygyny and monogamy) have relatively smaller testes than species with mating systems characterized by a high potential for sperm competition (promiscuity) was supported only for polygynous species.

Infanticide by male and female house sparrows

Of 211 breeding attempts of house sparrows, Passer domesticus, 9–12% were affected by successful or apparently attempted infanticide. Among males, infanticide appeared to be committed by territorial individuals that had recently lost their mates. Since there was a large population of female floaters, such males had access to mates, suggesting that intrasexual competition for mates was not a necessary precondition for male infanticide in this population. Infanticidal males bred with the females whose chicks they killed sooner than replacement males that refrained from infanticide. Females that mated with bigamous males lost eggs and nestlings due to infanticide by females more frequently than did monogamous females. Moreover, primary females (the only ones assisted by the male) tended to suffer from infanticide by females more often than did secondary females. This suggests that females commit infanticide in order to monopolize paternal investment.

Aromatase, 5 alpha- and 5 beta-reductase in brain, pituitary and skin of the sex-role reversed Wilson's phalarope.

While intrasexual competition for mates is generally considered to be an androgen-dependent characteristic of reproductively active males, in the Wilson's phalarope (Phalaropus tricolor) it is the female that acquires the brighter nuptial plumage and aggressively competes for access to the less aggressive males. Despite this pronounced sex-role reversal, circulating sex steroid hormones of breeding phalaropes are similar to those of avian species displaying traditional male-female reproductive roles. To investigate whether these behavioural and morphological steroid-dependent differences may be due to differences in target organ metabolism of circulating androgen, [3H]androstenedione in the presence of an NADPH-generating system was incubated with homogenates of brain, pituitary and skin of male and female Wilson's phalaropes collected from a naturally breeding population. Oestrone, 5 alpha-androstanedione and 5 beta-androstanedione were measured as endpoints of aromatization, 5 alpha-reduction and 5 beta-reduction respectively. Aromatase activity in the anterior hypothalamus/preoptic area (AHPOA) and posterior hypothalamus was greater in breeding males with high circulating concentrations of testosterone than in females, and activity in the AHPOA was greater in breeding than in non-breeding males (with low circulating testosterone). Aromatase levels did not differ in septum, archistriatum, hyperstriatum or pituitary. 5 alpha- and 5 beta-reductase were detected in all neuroendocrine tissues sampled and although there were no significant male-female differences, 5 alpha-reductase was greater in the AHPOA of breeding than of nonbreeding males.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex ratios and intrasexual competition for mates in a sex-role reversed shorebird, Wilson's phalarope (Phalaropus tricolor)

Operational sex ratio (OSR) was proposed by Emlen and Oring (1977) as an empirical measure of the intensity of sexual selection. Few studies, however, have examined the link between OSR and levels of intrasexual competition, which may influence selection. We studied the seasonal relationship between OSR and female-female competition for mates in Wilson's phalarope (Phalaropus tricolor), a sex-role reversed, non-territorial shorebird. Positive correlations between four measures of OSR (Scan Ratio, Day Ratio, Focal Ratio, and Chase Ratio) indicated seasonal changes in the availability of mates for females. Changes in mate availability resulted from the interaction between paternal care and female emancipation, asynchronous spring arrival schedules of the sexes (Reynolds et al. 1986), and the effect of clutch failure on renesting opportunities. Measures of intrasexual competition (courtship chases, percent males defended, rate and intensity of mate defense, and female-male proximity) varied significantly within and among years. Univariate and multivariate correlations indicated general agreement between measures of OSR and estimates of intrasexual competition. Our results suggest that OSR may provide a useful estimate of the opportunity for sexual selection, especially in species with matedefense mating systems.

Dorsal fin scars as indicators of age, sex, and social status in humpback whales (Megaptera novaeangliae)

We compared the number of linear marks and the shapes of dorsal fins of different classes of humpback whales during the breeding season. Male humpbacks tend to have more marks than females. Such marks are probably scars resulting from physical contact during intrasexual competition for mates. Not all males have marks; therefore, an unmarked adult humpback could be a male or a female. An adult with many marks is likely a male, however. Differences in the dorsal fins of principal and secondary escorts in large groups of competing males suggest that not all escorts have the same chance of attaining principal escort status. Therefore, male – male competitions in large groups should not necessarily be viewed as contests between equals. Principal escorts showed significantly more of some types of marks than did lone escorts of cows with calves, suggesting that not all lone escorts may retain their position if challenged.

Sexual Size Dimorphism and Disassortative Mating in the Dunlin Calidris alpina schinzii in Southern Sweden

In a South Swedish population of Dunlins, males were on average smaller than females. Differences were most pronounced in bill length and body mass. Males arrived at the breeding sites in spring on average 12 d earlier than females, with a tendency for large and long-billed females to arrive earlier than small and short-billed females. A significant positive correlation between date of arrival and start of egg-laying was found in females, but not in males. Previously mated pairs started egg-laying on average 4 d earlier than newly formed pairs. Disassortative mating in relation to size was found in new pairs, so that small and short-billed males, and large and long-billed females nested earlier than averagesized birds. A significant positive correlation was also found between female size and mean egg volume. To explain the observed patterns of mating and sexual size dimorphism, I suggest a combination of sexual and natural (ecological) selection. Small size in males may be energetically advantageous during aerial display and male parental care. Short bills may be selected for when males are feeding in terrestrial habitats during broodrearing. In females, selection for large size and long bill is probably associated with the production of larger eggs and more efficient feeding in marine littoral habitats, respectively. Selection may also favour large females in intra-sexual competition for mates and food resources.

Why are males bigger than females in pre-copula pairs of Gammarus pulex?

In natural streams, males are bigger than females in precopula pairs of the freshwater amphipod Gammarus pulex. A set of experiments is described, the results of which are consistent with the hypothesis that males are larger as a result of a mechanical constraint and not intrasexual competition for mates. Where the male in a pair is relatively larger than the female the swimming performance is superior to those pairs in which the male and female are of similar sizes. This minimises the risk of being washed downstream by the current.

Sexual Selection in Plants

The concept of sexual selection (intrasexual competition for mates and mate preference) is used in revising the classical explanation of dioecy in plants. Male and female functions of hermaphroditic flowers can be subject to different sexual selection pressures, which may conceivably lead to the separation of male and female structures. This suggestion does not exclude genetic advantages that may accrue from outcrossing; the two aspects of selection probably operated together in the evolution of dioecy. The hypothesis regarding the importance of sexual selection could be tested by monitoring variation in individual fitness via pollen donation and via ovule maturation. The evolution of pollinia and other large pollen dispersal units is also viewed in terms of sexual selection. Availability of a reliable pollen vector is a prerequisite to the evolution of pollen packages. From the point of view of a flower functioning as a male, large PDU's ensure fertilization of more ovules and possible preemption of stig...