Search

Claw size of male fiddler crabs, Uca perplexa appears to be a target of female choice that increases the likelihood a female will initially approach a male. Here we show that a behavioral display trait, the maximum height that the tip of the claw reaches during a courtship wave, is a strong correlate of the subsequent likelihood that a female will visit a male's burrow (which is a prerequisite for a burrow mating). We experimentally manipulated claw mass, to test whether there is a trade-off between claw mass and wave height. Males with a metal weight added to their claw showed a large reduction in wave height, whereas control males (plastic added) showed no net change in wave height. There is therefore a trade-off between these two sexually selected traits (claw size and wave display). More importantly, the greater the initial wave height the smaller the subsequent decline in wave height. Assuming that variation in wave height is an index of quality, this variation in the cost-benefit trade-off is consistent with the requirements of a signaling system that conforms to the handicap principle when fitness is the multiplicative product of different fitness components. We conclude by discussing the ongoing difficulties in testing the handicap principle.

Few studies have examined genotype by environment (GxE) effects on premating reproductive isolation and associated behaviors, even though such effects may be common when speciation is driven by adaptation to different environments. In this study, mating success and courtship song differences among diverging populations of Drosophila mojavensis were investigated in a two-environment quantitative trait locus (QTL) analysis. Baja California and mainland Mexico populations of D. mojavensis feed and breed on different host cacti, so these host plants were used to culture F2 males to examine host-specific QTL effects and GxE interactions influencing mating success and courtship songs. Linear selection gradient analysis showed that mainland females mated with males that produced songs with significantly shorter L(long)-IPIs, burst durations, and interburst intervals. Twenty-one microsatellite loci distributed across all five major chromosomes were used to localize effects of mating success, time to copulation, and courtship song components. Male courtship success was influenced by a single detected QTL, the main effect of cactus, and four GxE interactions, whereas time to copulation was influenced by three different QTLs on the fourth chromosome. Multiple-locus restricted maximum likelihood (REML) analysis of courtship song revealed consistent effects linked with the same fourth chromosome markers that influenced time to copulation, a number of GxE interactions, and few possible cases of epistasis. GxE interactions for mate choice and song can maintain genetic variation in populations, but alter outcomes of sexual selection and isolation, so signal evolution and reproductive isolation may be slowed in diverging populations. Understanding the genetics of incipient speciation in D. mojavensis clearly depends on cactus-specific expression of traits associated with courtship behavior and sexual isolation.

We describe a combined phenotypic and quantitative genetic investigation of the traits that may contribute to reproductive success in the picture-winged fly, Drosophila silvestris. These were courtship behavior, aggressive success, and body size and shape. Behavioral tests were conducted on wild-caught sires and their laboratory-reared sons. Neither size, shape, nor aggressive success predicted mating success. In both generations, males that spent more time courting and in wing-vibration were more likely to mate. However, components of courtship, overall aggressive success, and overall mating success had very low and nonsignificant heritabilities. The genetic estimates did not depend on whether they were based on males reared in both environments or reared only in the laboratory.

Despite the importance of sexual isolation to speciation, few studies have analyzed the genetic basis of interspecific mating discrimination, particularly using hybrid males. In this study, I investigated the genetic basis of sexual isolation using male hybrids of Drosophila pseudoobscura and D. persimilis. Hybrid male mating success was caused by interactions between the X-chromosome and autosomes (or Y-chromosome), and different arms of the X-chromosome contributed to mating success with females of each species. Further, although there was an X-chromosome component to mating success, its magnitude was not disproportionately large when compared with the proportion of the genome contained on this chromosome. Some hybrid males courted with an anomalously low intensity, so I simultaneously mapped the genetic basis of this "courtship dysfunction." The courtship dysfunction was caused by an interaction between the left arm of the X-chromosome in D. persimilis with the autosomes or Y-chromosome from D. pseudoobscura. Anomalous courtship behavior in interspecific hybrids can obscure the conclusions of studies of the genetics of sexual isolation, so courtship intensity should be evaluated in all such investigations.

Male courtship behavior is generally thought to function prior to copulation, as an inducement to the female to allow the male to copulate with her; this study indicates however, that male courtship during and following copulation ("copulatory courtship") is common in insects and spiders (81% of 131 species in 102 genera and 49 families, mostly Coleoptera, Hemiptera, Diptera, and Araneioidea). Copulatory courtship is apparently evolutionarily labile, as expected if it is under sexual selection; intrageneric variation occurred in all 17 genera in which more than one species was observed. In 81% of 94 species with copulatory courtship, the male abandoned the female soon after copulation ended; thus, copulatory courtship appears not to function generally to induce acceptance of further copulatory attempts. The most likely explanation for copulatory courtship is that it represents attempts by males to influence cryptic female choice. This suggests that an aspect of sexual selection by female choice not considered by Darwin may be more important than previously appreciated and that the common practice in evolutionary studies of measuring male reproductive success by counting numbers of copulations may sometimes be misleading because of cryptic female choice during and after copulation.

We are interested in elucidating the extent to which lekking Hawaiian Drosophila species have diverged from their continental counterparts, which engage in sexual behavior at communal food sources, with regard to the chemical communication systems that the flies employ. Accordingly, we have analyzed flies from three closely related Hawaiian Drosophila species in the adiastola subgroup. These species are of interest because the males engage in a unique behavior: while courting, they raise their abdomens over their heads and emit anal droplets. Analysis of the flies' behavior, the hydrocarbons in males' anal droplets, and males' cuticular hydrocarbons suggest that females' responses to males may be mediated by cuticular pheromones and/or pheromones in males' extruded droplets that enable the females to distinguish conspecific from heterospeciflc males. Conversely, perception of cuticular hydrocarbons from conspecific females enables D. adiastola males to distinguish females from a closely related species from conspecific females. On the basis of these observations, we suggest that the adiastola subgroup species are unique among drosophilids in that they utilize an anal droplet-mediated pheromone communication system, some or all components of which are species specific. However, the lekking Hawaiian Drosophila species are similar to D. melanogaster and related continental species in that the Hawaiian flies employ a cuticular pheromone communication system, some components of which are sex and species-specific.

Male and female lacewings tremulate during courtship, establishing duets that always precede copulation. Three distinct courtship songs are found in populations of the green lacewing Chrysoperla plorabunda (P1, P2 and P3 song morphs). Analysis of five features of the songs for individuals collected from Connecticut, Idaho, Oregon and California showed few differences within song morphs, but sympatric song morphs differed significantly in temporal features of the songs and their mode of presentation. Playback experiments using recorded songs were performed on females with all possible sympatric and allopatric combinations of females and recorded songs. The results showed that females strongly prefer to duet with recordings of males of their own song type and usually showed no responses to songs of other types. Thus, song differences are effective barriers to reproduction between the sympatric morphs. Our results support the hypothesis that the three song morphs are true biological species.

The efficacy of bottlenecks to stimulate divergence in courtship behavior and consequent premating isolation was tested by serial founder-flush episodes of three sizes (one, four, or 16 pairs) on a population of houseflies established in the laboratory from a single field population. After the fifth founder-flush episode, intraline and interline crosses were performed to detect divergence in mating propensities and patterns of assortative mating. Videotapings of intraline courtships for the bottleneck lines and the control were evaluated for changes in courtship repertoire. All bottleneck lines showed significant divergence from the control in male and/or female mating propensity and in courtship behavior. Divergence from the control was bidirectional for both male and female mating propensities as well as for courtship element utilization. Out of 15 tests for assortative mating between bottleneck lines and between bottleneck lines and the control, only two cases of positive assortative mating and one case of negative assortative mating were detected. Because some bottleneck lines showed increased courtship element utilization and because decreased courtship utilization in some bottleneck lines was related to higher male mating success, the mechanisms behind the Kaneshiro model (which is based upon ancestral females discriminating against bottleneck males that had "lost" courtship elements) were not supported in general. A partitioning effect of the bottlenecks upon the intrinsic variation in the ancestral population for courtship pattern appeared to explain a large component of the directions of divergence from the control. Still, the pattern of divergence of some bottleneck lines apparently was not constrained by the intercorrelation structure of courtship behaviors detected in the control. Because previous studies showed that the bottleneck lines had rebounded from inbreeding depression to fitness levels of the control, this study documents nondebilitating differentiation in the courtship repertoire that can account for divergent mating propensities and premating isolation.

Estimates of heritabilities and genetic correlations for seven reproductive attributes had previously been obtained from parent-offspring regression (Gromko, 1987, 1989). Copulation duration was shown to have a heritability of 0.23 and to be genetically correlated with courtship vigor (rA = -0.41) and with fertility (rA = 0.27). These observations form the basis for the prediction of direct and correlated responses to selection for increased and decreased copulation duration, which are reported here. The direct response corresponded closely to prediction, but the correlated responses did not provide consistent qualitative fit. A hypothesis is proposed to explain this difference in predictability of direct and correlated response to selection. The major postulate is that the different polygenes involved in the direct response to selection for copulation duration have different pleiotropic effects.

Mating behavior was studied in two laboratory populations of Drosophila sechellia and their reciprocal hybrids. The ancestral population was reared on a special medium, optimal for this species, while the derived population was reared on a standard Drosophila food, and underwent a bottleneck while adapting to this new medium, in a manner similar to the "founder-flush" process of Carson (1971). A significant tendency towards mating asymmetry was found, with ancestral females mating significantly less frequently with derived males than derived females with ancestral males. Analysis of hybrids suggested an important role for the male's X chromosome or for a maternal effect. No significant differences were found among parental types for their main female cuticular hydrocarbons, the proportion of courtship spent in various behavioral elements, body weight, or wing length. Significant differences were found in the structure of courtship, male locomotor activity, male cuticular hydrocarbon levels, and male courtship song inter-pulse interval (i.p.i.). None of these differences showed an X-linked effect in the reciprocal hybrids. Hypotheses put forward to explain interspecific mating asymmetries are discussed in the light of these results.

In many species of monogamous birds females copulate with males other than their social mates, resulting in extrapair fertilizations. Little is known about how females choose extrapair mates and whether the traits used to choose them are reliable indicators of male quality. Here we identify a novel male trait associated with extra-group mating success in the superb fairy-wren (Malurus cyaneus), a cooperatively breeding bird with one of the highest known frequencies of extra-group mating. Female fairy-wrens chose extra-group mates that molted earlier into breeding plumage. Males molted up to five months before the breeding season began, and only males that molted at least one month prior to its onset gained any extra-group fertilizations. This conclusion held after controlling statistically for the effect of age and social status on molt date. Once males acquired breeding plumage, they began courtship display to females on other territories. Thus, some males were displaying to females for several months before the breeding season began. This extraordinarily long period of advertisement by males may be facilitated by the long-term ownership of territories. We suggest that early acquisition of breeding plumage or the subsequent display behavior can be reliable cues for mate choice because they are costly to acquire or maintain.

Ethological studies in the 1940s and 1950s, most notably those of Lorenz and Tinbergen, emphasized a historical perspective. By the 1970s, the notion that behavioral traits are too plastic to retain historical information became prevalent, and evolutionary approaches in behavioral studies were largely abandoned. However, several recent studies have demonstrated that behavioral characters are remarkably consistent with phylogenies obtained from other data and not particularly prone to homoplasy. In this study, I coded descriptions of courtship display behaviors in stork species (Aves: Ciconiiformes: Ciconiidae) as a matrix of discrete characters. I mapped each behavioral character onto a phylogeny based on DNA-DNA hybridization distances to test the homology of individual characters. Generally, displays occurring early in courtship were congruent with phylogenetic relationships and showed little homoplasy, while displays occurring late in courtship were more homoplastic. I also performed a phylogenetic analysis of the behavioral data matrix using maximum parsimony. The strict consensus of the 24 most-parsimonious trees was congruent with the DNA-DNA hybridization tree in all nodes having greater than 70% bootstrap support.

The evolutionary origin of reproductive isolation is still a source of considerable debate, although the consensus is that it develops by a gradual accumulation of genetic differences between populations, mostly in allopatry (Dobzhansky 1937; Muller 1942; Mayr 1963; Rice and Hostert 1993; Wu and Davies 1993; Templeton 1996). Quantitative studies of sexual isolation usually go no further than the calculation of the frequencies of homoand heterospecific matings; direct observations of courtship are rarely made (Spieth and Ringo 1983; Verrell and Arnold 1989). Consequently, relatively little is known about the role played by sexual behavior in maintaining reproductive barriers between species. Sexual isolation occurs when individuals tend to avoid mating with those of another strain, race, or species (Gilbert and Starmer 1985). Isolation estimates can be effected by mating propensity, or sexual vigor: the likelihood that a male or female will mate regardless of partner type. Moreover, mating propensity is likely to obscure estimates of asymmetry in sexual isolation unless controlled by experimental design or statistical analysis. Asymmetry is present when females from population A mate more frequently with males from population B than females from population B do with males from population A. Another source of error in both isolation and asymmetry estimates is the effect of social interactions. The most commonly used designs of mating experiments, in which individuals are given simultaneous choice of mates, do not control for intrasexual interactions and therefore can produce biased results (discussed in Arnold et al. 1996). We conducted a laboratory experiment with two species of European newts, the smooth newt, Triturus vulgaris, and Montandon's newt, T. montandoni, in which single males were paired with single females in a variety of crosses. This permitted a comparison of mating probabilities for four possible crosses. Our main goal was to determine the duration of courtships and identify the behavioral causes of sexual isolation. We also sought to demonstrate asymmetries at different stages of behavioral isolation between these two species. Although mating behavior of Triturus newts has been carefully described (Halliday 1977; Arntzen and Sparreboom 1989), surprisingly little information is available concerning the basis of sexual isolation between species. The genetic distance between T. vulgaris and T. montandoni is the lowest among species of the genus Triturus (DNei = 0.16; Rafin'ski and Arntzen 1987), and they readily hybridize in montane areas wherever they coexist (Hofmann 1908; Rafin'ski, unpubl. data). Old World newt species, including T. vulgaris and T. montandoni, possess elaborate and complex mating behavior that involves a prolonged male display followed by spermatophore transfer. Display consists of three distinct tail-movements produced by the male: whip, fan, and wiggle (detailed descriptions in Halliday 1974, 1975, 1976, 1977; Pecio and Rafin'ski 1985; Arntzen and Sparreboom 1989; Michalak 1996). Despite large differences in morphology of secondary sexual characters between the two species, there is only a slight difference in male courtship behavior, primarily in relative frequencies and duration of tail movements. In most cases the male initiates the courtship. The duration of display varies considerably and depends mainly on the reaction of the female. During spermatophore transfer, the female follows the male, walks over the spermatophore, and if her cloaca comes into contact with it, the spermatophore is then taken up into her cloaca. Cooperation on the female's part is an essential prerequisite both for spermatophore deposition and successful transfer. The onset of display, spermatophore deposition and transfer constitute distinctive events in the courtship of newts. Thus the composite courtship of newts provided us with an opportunity to analyze sexual isolation separately for the different mating phases.

During courtship and copulation, males of many insect species provide the female with a nuptial gift of a prey item or synthesized material. These gifts may be explained as a form of paternal investment by increasing female reproductive output, or in terms of mating effort by increasing male fertilization success. These explanations, while not mutually exclusive, are controversial. While experimental studies examine the maintenance of nuptial gifts in single species, comparative studies are required to indicate more general evolutionary trends. Male bushcrickets provide females with a nuptial gift, a spermatophylax, which is transferred to females at mating along with the sperm-containing ampulla. Analysis of comparative data of 28 species of bushcrickets (Orthoptera: Tettigoniidae), reveals that male spermatophore size (spermatophylax and ampulla weight) is positively correlated with female refractory period, which, in turn, correlates with male fertilization success. Moreover, gift size (the spermatophylax) covaries with ejaculate size (the ampulla), which is consistent with the hypothesis that it serves as a sperm protection device. In contrast, there is no significant correlation between any measure of female fecundity and male spermatophylax size. This indicates that the variation in spermatophore size among bushcrickets is better explained by a mating-effort function than a paternal investment function.

We examined sexual selection by Drosophila littoralis, D. montana, and D. ezoana females on male courtship sounds to determine whether the females use absolute or relative criteria when choosing their mates. Behavior of the females was observed, when they were courted by a single male producing normal sounds, or by a single wing-manipulated male producing abnormal sounds; and when they were courted by one or both of these males in a choice situation. The females usually accepted short-winged (but not wingless) males producing abnormal sounds, if they had no alternatives. However, if they heard the sound produced by a normal male, they rejected the deficient male. Drosophila littoralis and D. ezoana females selected between two wing-manipulated males with different wing areas. Our results suggest that the females choose their mates on the basis of relative criteria if the signals emitted by the courting males are within the range of acceptable cues.

Sexual dimorphism has been a long-standing problem for evolutionary biology (Darwin, 1871; Selander, 1966, 1972; Ralls, 1977; Alexander et al., 1979; Slatkin, 1984; Bradbury and Andersson, 1987). It is generally thought that sexual selection is the major force leading to the evolution of sexually dimorphic traits (Darwin, 1871; Fisher, 1958; Trivers, 1972; Lande, 1980). This may be particularly true of lek-breeding species, because the potential for intense sexual selection should be quite high (Darwin, 1871; Fisher, 1958; Bradbury, 1981). Typically, there is extreme variance in male mating success on leks. As a general explanation for sexual size dimorphism in lek-breeding species, the hypothesis has been proposed that male-male competition for mates has led to larger male size (Darwin, 1871; Wiley, 1974). This may occur because larger males are more dominant and by the time mating takes place they can subtly control access of smaller males such that females are essentially unavailable to subordinates (Beehler and Foster, 1988). In contrast, females may directly and disproportionately choose dominant males as mates (Bradbury and Gibson, 1983). Dominant males may be chosen, for example, because of the potential for greater offspring viability (Wiley, 1973, 1974; Alatalo et al., 1991). Trail (1990) recently proposed that the degree of sexual (plumage) dimorphism can be explained by differences among lek-breeding species in the amounts and kinds of intrasexual competition. He hypothesizes that in addition to intense male-male competition, intrasexual competition among females in some lek-breeding species has led to a greater elaboration of characters in females and, hence, sexual monomorphism. It is clear that a variety of intraand intersexual mechanisms may explain selection for increased elaboration of characters in both males and females of lekbreeding species. One hopes to explain not only the elaboration of characters, however, but also constraints on further elaboration. One hypothesis is that the further elaboration of sexually dimorphic traits is dampened only when traits become so costly that their bearers are unlikely to survive to mate (Fisher, 1958; Selander, 1965; Lande, 1980; Arnold, 1983). This explanation was originally applied to male secondary sexual characters, but may apply to body size as well. I studied how female choice acts on male body size

i Present address: Department of Zoology, Field Museum of Natural History, Roosevelt Road at Lake Shore Drive, Chicago, IL 60605 USA. survive (Halliday, 1983). Octopuses appear to possess such a mating system (Wells and Wells, 1977); octopuses are not territorial in nature, and male octopuses are not known to actively compete for, or to guard mates. Thus female octopuses are predicted to choose mature males as mates although the mechanisms by which females might assess male maturity are unknown. The comparatively few observations of copulation

Spieth (1966, 1968a, 1968b) reported laboratory and field observations of the bizarre courtship behavior patterns displayed by Hawaiian drosophilids and suggested the role of ethological isolation as one of the major reproductive isolating mechanisms in the endemic fauna. Additionally, Spieth (1966) showed that qualitative analyses of these courtship patterns permit one to infer the phylogenetic relationships of species sharing elements of these patterns. Although preliminary observations of courtship patterns of the four species studied in this paper have been recorded by Spieth (pers. comm.), detailed analyses of these patterns have not been reported. Nevertheless, the data presented in this paper, measuring the amount of premating reproductive isolation among the four species in "malechoice" experiments, provide basis for the interpretation of phylogenetic relationships of these species. The subjects of this study are four closely related picture-winged species of Hawaiian Drosophila. Drosophila differens and planitibia inhabit the rain forests of Molokai and Maui respectively, while heteroneura and silvestris are sympatric over a wide range on the Big Island of Hawaii. The chromosomal relationships of all four species have been reported by Carson and Stalker (1968) and Craddock (1974). D. differens and planitibia are monomorphic and homosequential in banding patterns of polytene chromosomes while heteroneura and silvestris share a

Mise en evidence que les femelles de P. g. distinguent les mâles par leur coloration hors spectre UV et qu'elles solicitent preferentiellement les mâles de la couleur naturelle plutot que les mâles de forme mimetiques.

Etude experimentale pour identifier le ou les moments specifiques, pendant la periode d'interactions male-femelle avant copulation, pour lesquels l'appariement entre individus de populations differentes se revele impossible

Drosophila heteroneura and D. silvestris are well-defined, sympatric species of the planitibia subgroup of Hawaiian Drosophila. D. silvestris can be subdivided into two allopatric morphotypes that differ in the number of bristle rows on the front tibia (two rows versus three rows). We measured courtship success of intraspecific and interspecific hybrids as the proportion of females inseminated during a two-week period with a single sib male. Proportions were arcsin-transformed so that the values were asymptotically normal in distribution, and tests of homogeneity and of mean differences were performed. Of key importance is the discovery of genetic variation for the proportion of inseminated females within both D. heteroneura and D. silvestris. The interspecific crosses and the D. silvestris intraspecific crosses also provide evidence for a coadapted gene complex with some dominance or heterosis. This coadapted gene complex correlates with the morphotypes of these flies, rather than with the D. heteroneura/D. silvestris contrasts per se. This observation stresses the importance of recognizing both behavioral and morphological components of the mate-recognition system. The incompatible coadaptation that separates the two-row from the three-row forms also supports recent molecular studies which indicate that the three-row form split from the two-row form prior to the split between D. heteroneura and two-row D. silvestris. The observations of intraspecific variability and coadaptation support the predictions of a genetic-transilience model which explains the origin of a new mate-recognition system in terms of sexual selection in the context of a founder-flush event.

Analyse du comportement de cour de D. a. pour determiner si des modifications du comportement de cour dependantes de l'experience apparaissent apres que des mâles matures aient courtises de jeunes mâles conspecifiques. De plus, les interactions des especes croisees de D. m. et D. a. sont etudiees pour savoir si le comportement de cour homosexuel est modifiee par l'experience des individus.

A number of investigations have shown that in competitive mating tests mutant Drosopkila are at a disadvantage when competing with wild type flies. Over long time intervals the mutant genes are either eliminated or maintained at a very low frequency in the population (Reed and Reed, 1948; Merrell and Underhill, 1956). This disadvantage has usually been attributed to a reduction in fitness on the part of the mutant when competing for the available food supply. Behavior is also a component of fitness, and there is evidence indicating that the disadvantage exhibited by many mutant genotypes is due to some impairment of their mating behavior. Reed and Reed (1950) found that males from the sex linked white eye mutation (w) of Drosophila melanogaster were at a considerable disadvantage when in competition with wild type males. Bastock (1956) demonstrated that the yellow mutant (y) of D. melanogaster gives rise to males which show a quantitative change in the wing vibration element of the courtship display. The slight reduction in mean vibration bout lengths of the mutants results in their being at a disadvantage. Petit (1959) found that wild type males had an advantage over wkite eye and Bar eyed males in a competitive situation, this she attributed to differences in the wing vibration component of the courtship display. Geer and Green (1962) were unable to confirm this effect on wing vibration in studies with vestigial mutant males. Geer and Green (loc. cit.) working with a series of eye pigment mutants obtained evidence of a relationship between the amount of eye pigmentation and mating success; the greater the pigment density the greater the mating success. This is a particularly interesting finding since it has been argued that vision plays only a minor role in the courtship of Drosophila melanogaster (Spieth and Hsu, 1950), although it has an important influence on the courtship of certain other members of the melanogaster species group (Grossfield, 1968). Geer and Green (loc. cit.) showed that the disadvantage suffered by the eye pigment mutants was no longer present when matings were allowed to take place in total darkness, indicating that vision may also be important in the courtship of Drosophila melanogaster. In none of the investigations where eye mutants have been used, has the behavior of the animals been observed, mating success being measured by progeny analysis or by the dissection of females to determine the presence or absence of sperm. Ewing and Manning (1967) in their review of behavior genetics in insects single out a difficulty in the interpretation of Geer and Green's results. They argue that although Geer and Green demonstrate a relationship between pigment density and mating success it is not possible to attribute the disadvantage shown by the eye pigment mutants solely to that sensory defect. An alternative hypothesis is that the eye pigment mutations produce some pleiotropic effect in the animal's central nervous system which affects some aspect of the male's sexual behavior. Since the behavior of the mutants has not been examined in detail this remains a plausible hypothesis. These two alternative explanations may be examined by using the phenocopying technique described by Burnet et al. (1968).

Differences in adult male size and age at sexual maturity in the Río Coy (Mexico) population of Xiphophorus nigrensis (Pisces; Poeciliidae) are controlled by genetic variation at a Y-linked locus. Four genetic size-classes have been identified. The mating behavior of the males of the three largest size-classes consists exclusively of an elaborate courtship display, whereas that of the genetically small males ranges from display to a sneak-chase attempt at copulation. In the presence of large males, small males switch to the sneak-chase behavior. Females prefer the display of large males. In mating-competition experiments (two females with one large male and one small male), the large males are dominant and deny the small males access to females. From 20 such experiments, 601 large-male and 200 small-male progeny were obtained, indicating that the switch to sneak-chase behavior by small males is not particularly effective in overcoming the large-male advantage. By using the largest males of the genetically smallest size class and the smallest males of the genetically next-larger size-class, size was kept constant, whereas genotype was varied. When these males were tested in competition with genetically large males, only the males of the genetically smallest size class showed sneak-chase behavior. These observations suggest that the difference in mating behavior is not an indirect developmental effect of size but, rather, is under direct genetic control.

It is tempting to assume that behavior patterns have evolved in the same manner as other features of animal populations, by the selection of inheritable differences. This idea is supported by those comparisons which have been made between the behavior patterns of closely related animals, one of the best examples of which is the review by Spieth (1952) of the courtship behavior of 101 species and subspecies of the genus Drosophila. Spieth compares the degree of morphological with that of behavioral divergence and he finds that the two roughly agree so that mating behavior confirms the validity of the existing classification as presented by Patterson and Wheeler (1949). Yet it is impossible to be certain, however strong the implication about the evolution of behavior, unless one can demonstrate the existence of 'genetic' variations in behavior upon which selection could act. It is true that some gene mutations are known which affect behavior in some way. Thus one mutation in Panaxia dominula has influenced its mating 'preferences' (Sheppard, 1952), the ebony mutant in Drosophila melanogaster mates more successfully in the dark than in the light (Rendel, 1951), and there is a mutation which affects tameness in rats (Keeler and King, 1942). But the differences in the courtship behavior of the Drosophila species studied by Spieth and also by Milani (1951a, b) are of a more specific nature. Courtship is a complex behavior pattern and the variations often occur only in the relative frequency of the elements concerned, some elements occurring more often in one species than another. Sometimes elements are omitted or new ones are added and sometimes they may change in form, but the most common difference between the most closely related species concerns this difference in frequency. The investigations reviewed by Moynihan (1953, 1955) on the differences between closely related gulls and the descriptions given by Clark, Aronson and Gordon ( 1954) on the differences between the platyfish (Xiphophorus (Platypoecilus) maculatus), and the swordtail (Xiphophorus helleri), suggest changes of the same nature, and yet no single gene mutation is known which alters a behavior pattern in this way. However since considerable detailed investigation is necessary to reveal differences in frequency or patterning, this is not so very surprising. Indeed some of the mutations listed above might well have such effects; they simply have not been investigated. It seemed worth while therefore to examine more closely one example of a gene mutation affecting behavior and to ask two questions, ( 1 ) how does it bring about its effect? (this is of considerable genetical interest), (2) what part might it play in evolution? (this is of considerable evolutionary interest).