Gonopodial Thrusts Research Articles

Inter-population variation in male mating behaviours in the sailfin mollie, Poecilia latipinna

Abstract. Male sailfin mollies show size-dependent variation in sexual behaviour. The level of variation between six north Florida populations in rates of condition-dependent behaviours was estimated and whether behavioural variation is ordered with respect to male body size distributions was determined. In five of six populations, courtship display rates increased with male length, supporting previous evidence. Several results were not consistent with those reported elsewhere. Rates of gonopodial thrusting and gonoporal nibbling were not related to male body length. Courtship display rates adjusted for male body size were not ordered with respect to male body size distributions. High adjusted courtship rates were characteristic of populations where males were predominantly large and where they were predominantly small. No consistent pattern of variation existed between populations in the relationships of the three behaviour patterns to one another. Variation between populations in size-dependent behaviour is therefore far more extensive but less patterned than previously reported. These results imply that evolution can adjust the rates of the three behaviour patterns both independently of one another and of body size, and that the direction of behavioural evolution is not likely to be constrained by covariances within any single natural population.

Sexual selection and sexual size dimorphism in the eastern mosquitofishGambusia holbrooki(Pisces Poeciliidae)

We have previously shown (BISAZZA & MARIN 1991) that female eastern mosquitofish, Gambusia holbrooki, did not discriminate among males of different size, although they showed a preference for the dominant male in a group of males. In this study we examined the relationship between body size and the success of gonopodial thrusting, a mating tactic characterisitc of the poeciliids that enables a male to achieve copulation without the female's cooperation. Without competitors the probability that a mating attempt was successful decreased exponentially with male length while it increased with the size of the female. When tested in a group, males competed for access to females. The largest individual monopolized access to females and accomplished the majority of mating attempts, whatever the number of its competitors. We used these results and demographic data (length distributions and composition of groups) of a natural population to simulate how the different mechanisms of sexual selection may interact under natural condition. The results of computer simulations indicated that the size of successful males always ranged well below the average length of females. Small male advantage in copulation was generally more effective than intrasexual competition and female choice in determining male mating success. A simulation which accounted for seasonal variation in body size and in the pattern of male aggregation indicated that small males had a reproductive advantage for most of the time, and that the correlation between body size and mating success became positive only at the end of the reproductive season, when population density peaks and the sex-ratio becomes male-biased. We suggest that the mating advantage of small males may be the cause of the reverse sexual size dimorphism of the poeciliids.

Environmental conditions and male morphology determine alternative mating behaviour in Trinidadian guppies

Abstract. Individual male guppies ( Poecilia reticulata: Poeciliinae) exhibit two distinct types of mating behaviour: sigmoid displays used in courtship, and gonopodial thrusts used in circumventing female choice. To test how these alternatives are influenced by sexual selection and natural selection, ambient light levels were manipulated to vary potential predation risk, and mating behaviour was examined in relation to male morphology. At high light intensity, larger males displayed significantly less often than smaller ones, and did not compensate with increased thrusting. In contrast, at low light, display frequency was unrelated to male morphology, but thrusting behaviour became correlated positively with the length of the male's intromittent organ (the gonopodium). No colour pigment was correlated with either display or thrusting behaviour at either light level when correlations with body size were removed. The decreased courtship by larger males under high light is consistent with individual differences in a trade-off between predation risk and mating success. The correlation between thrusting and gonopodium length parallels an interspecific pattern, and suggests that gonopodium length may be coadapted with male behaviour to circumvent female choice. Thus, covariation between alternative mating behaviour and male morphology is mediated by changing environmental conditions influencing the strengths of sexual and natural selection.

Aggressive, Sexual, and Foraging Behaviour in Poecilia velifera (Pisces: Poeciliidae) during Captivity1

AbstractForaging, agonistic, and sexual behaviour in heterosexual groups of Poecilia velifera were examined during conditions of captivity. Social organization was found to resemble what was previously found in other poeciliids, hierarchies being established within each sex. Males showed considerably more aggressive activities than females, and alpha males tended to dominate all individuals.Aggressiveness is the only measure where dominating individuals exceed submissives. Levels of aggressiveness higher than a specific value may have negative influences in gaining access to resources: Those individuals with highest or lowest scores for aggressive behaviour were low scoring with respect to sexual and foraging behaviour.Attempts were made to separate factors determining aggressiveness in competition for food and mates by observing in periods with and without food being presented. Only beta individuals changed levels significantly: females decreased both chasing and biting during non‐feeding periods, whereas males only decreased biting.Sexual behaviour in terms of butting tended to decrease in alpha and increase in omega males during non‐feeding periods, whereas beta males in the same periods showed significantly decreased levels of both butting and gonopodial thrusting.

Behavioural allometry and interdemic variation in sexual behaviour of the sailfin molly, Poecilia latipinna (Pisces: Poeciliidae)

Sailfin mollies, Poecilia latipinna, exhibit remarkable levels of intraspecific variation in reproductive behaviour. Larger males exhibit higher rates of courtship and lowered rates of gonoporal nibbling and gonopodial thrusting (forced copulation attempts). Larger males have relatively longer and higher dorsal fins than smaller males. The dorsal fin is a prominent component of the courtship display. Variation in fin measurements, behaviour patterns, and body size of mature males is continuous, and fin shape and behaviour patterns are allometrically related to body size. The allometric pattern is displayed by individual traits as well as by the morphological or behavioural traits in ensemble. Eight populations of mollies from markedly distinct habitats exhibited similar consistent levels of intrademic variation in the size of mature males. Detailed studies on three populations showed that dorsal fin shape could be described by the same regression relationship in all populations, and indicted that a male's shape was determined by his absolute body size. By contrast, there was some variation among populations in the relation of behaviour patterns to male body size. The pattern of this interdemic variation indicated that a male's behaviour patterns were influenced by his relative size in a population. Successful inseminations following forced copulations were rare. The average size of successful males was smaller than the average size of unsuccessful males in all three populations. These results indicated that successful insemination through forced copulation was, like behaviour patterns generally, more a function of the relative size of the male, than his absolute size.

Geographic Variation in the Sexual Behaviour of the Guppy, Poecilia Reticulata (Peters)

Abstract The guppy, Poecilia reticulata, exhibits marked intraspecific variation in behaviour and morphology and occurs over a wide range of natural environments in Trinidad, West In-dies. In this paper we examine the hypotheses that: 1) in clear mountain headstream waters in Trinidad, where Rivulus hartii is the only major aquatic predator, sexual selection has favoured the evolution of visual components of courtship behaviour in the guppy, and 2) in turbid lowland rivers with numerous aquatic predators selection has resulted in greater dependence upon non-visual components of courtship. Guppies were observed in four rivers in Trinidad. In two mountain headstreams guppies were evenly distributed across and along the river. Guppies in a lowland river and a 'midstream' river occurred in small schools close to the water's edge. Males guppies in the headstreams performed more frequent sigmoid displays and displays of longer duration than males in the turbid lowland river and the midstream river. Males in the turbid lowland river were found to exhibit higher frequencies of gonopodial thrusts than males in the other three rivers. Offspring of guppies from the four rivers were raised under identical conditions in the laboratory, free from predation and other selective agents. The results of observations of courtship behaviour in laboratory-raised fish are consistent with those obtained in the field. The overall similarity of results in the field and the laboratory provides evidence that there is a genetic basis to the observed behavioral differences between populations.

Social Behavior Patterns as Determinants of Reproductive Success in the Guppy, Poecilia Reticulata Peters (Pisces: Poeciliidae) an Experimental Study of the Effects of Intermale Competition, Female Choice, and Sexual Selection

AbstractThe reproductive success of competing male guppies, Poecilia reticulata, was measured and compared to various features of social behavior and secondary sexual coloration. The most important determinant of reproductive success was the rate at which a male courted females relative to those of other males. Males with higher display rates have a greater chance of encountering a receptive female and are preferred by females. Males adjust their display rate in such a way as to be just noticeably more active than a competitor, and inbred strains differ in the maximum rate at which males can court females. No other factor is able to offset the disadvantage of displaying at a relatively low rate. Sexual selection has resulted in the maximization of courtship activity in natural populations. If males court females with equal frequencies, those which also inseminate females through gonopodial thrusting without female cooperation have a selective advantage. Because poeciliid females store sperm, inseminations through gonopodial thrusting can reduce the reproductive success of competitor males which copulate only following a display. A mixed strategy of displaying and gonopodial thrusting is more successful than either pure strategy. The result is a mating system which partially ignores female choice mechanisms. Intermale aggresssion was found to be maladaptive. Males which displayed at higher rates than competitors were less successful if they were also more aggressive than the competitors, than when they were non-aggressive or the competitors were more aggressive. Males were not able to reduce a competitor's courtship display rate through aggression. It was hypothesized that the low level of aggression in natural guppy populations is attributable to the fact that variance in size of males is low and fights would be lengthy before a winner could be determined. This would subtract from time available for courtship, and female preference for high-displaying males would select against aggressive phenotypes. There was little evidence that conspicuousness of male coloration influences female choice of males. Dull males with high courtship display rates were significantly more successful than conspicuous males with low display rates. It was concluded that females prefer conspicuous males only if all males exhibit equal courtship display rates. The frequency of male color patterns in a population did affect reproductive success. Males with rare color patterns sired more offspring than expected given their frequency. The mechanism by which a rare male effect was achieved depended on the relative mating success of phenotypes in control populations (all phenotypes occurring with equal frequency). If a normally preferred phenotype was rare, females continued to prefer that phenotype over the common phenotype. If a normally unpreferred phenotype was rare, females mated with that phenotype in addition to the preferred phenotype, and the rare male effect was thus achieved by multiple inseminations. Females also tended to mate with more than one male in polymorphic populations, and it was concluded that certain female choice patterns with frequent multiple inseminations can maintain a polymorphism in addition to a pure rare male effect. It was hypothesized that courtship displays and conspicuous male coloration are sexually selected characters which evolved in response to the occupation of invariable habitats or specialized ecological niches by a sexually monomorphic ancestor with only gonopodial thrusting as a means of inseminating females. A subsequent reinvasion of variable habitats resulted in female choice mechanisms which maximize the level of heterozygosity of their offspring, thus resulting in polymorphic populations. The coloration of male guppies is a phenotypic cue which influences female choice in such a manner that they mate with those males with whom they most probably have the fewest genes in common.