Palasuberniam Kaliannan



Large size seems to be an advantage in a wide variety of ecological contexts in the flies mating system. The relationships between body size and superior fighting ability has been clearly shown in some species of flies. Research has shown that larger males and females exhibit higher mating success as compared to smaller individuals. In addition, fertilisation and reproductive success are positively correlated with the size. The larger flies have the advantage in a competitive resource based system, and the smaller ones always tend to drop their aggressive stance and retreat. However in some flies, the competitive advantages in size do not correlate with genetic variation. This review will explore the recent studies that examine the implications of size variation, especially with reference to dipteran mating bahavior.


I. Introduction

There are many factors that influence courtship behavior among animals. Size, shape, age, and other cues influence the behavior singly or in complex interaction. Many animals readily accept cues for courtship behavior, resulting in copulation and these behavior. serves as a reproductive mechanism for perpetuation of species.


Once an insect has emerged as an adult it must either call or search for a mate. Although in some cases both sexes may call, it is more usual for one sex to do the calling and the other do the searching. Although external stimuli such as olfactory, visual or auditory stimuli are some of the most obvious influences on courtship behavior but the physical characteristics such as size and shape seem to be the determining factor for successful courtship in some insects. Large males generally experience greater success in aggressive competition than smaller ones especially in species in which there is direct male-male competition for access to mates (Thornhill and Alcock, 1983; Otronen, 1984a; Mclain and Boromisa, 1987; Day et al., 1987 and Cook, 1988).


The relationship between large body size and success also holds for fighting, in everything from minute insects like aphids and thrips, to elephant seals (Alcock, 1993b). It is also striking that in many species, adult males are bigger than adult females. The differences among species in the degree of sexual dimorphism in body size are a possible measure of the investment males made in fighting capacity although there are other factors besides sexual competition that might result in sexual dimorphism in body size. Darwin (1859) realised that competition among males for the opportunity to mate would generate a strong evolutionary pressure on characteristic that contribute to a male’s success.


II. The size and mating in insects

The size of the mate seems to be important source of variation in reproductive success in individual insect species. Some insects behave discriminately in selecting their ‘mate’s choice. In particular, large females often have greater longevity, higher fecundity and larger males have enhanced mating success (Butler et al., 1984). A male preference for larger and hence more fecund females have been demonstrated in several species of insects (Hieber and Cohen 1983; Johnson and Hubbell, 1984). There is correlation between size and mating longevity across species (Bonner, 1965).


Various factors have been identified that causes difference in size of the offspring, including intraspecific and inter specific competition, and seasonal changes in food quality. Both theoretical (Dowhover, 1976; Searcy, 1980) and empirical (Brodie, 1975) studies suggest that if no food is available, larger animals will take longer to starve than small ones. The consequence of body size under conditions of food shortage is less clear and will depend partly upon the relationship between body size and rate of food intake. Other things being equal, the per weight energy expenditure of larger animals is lower than that of smaller ones, and it may be that this relationship means that smaller organisms are forced to put less energy into bodily repair. The main energetic trade off made by these animals must have been between sexual activities and general physiological maintenance, and this suggests that smaller males in absolute relative terms may have high repair costs or devote less energy in sexual activities.


III. Studies in the Diptera

1) Mating behavior in flies.

Courtship behavior exists for several reasons (Tinbergen, 1965). In some flies, sexual selection may operate in very cryptic ways. Auditory and visual stimuli are also not obvious in house flies or dung flies, yet female rejection prior to copulation occurs (Parker, 1970; Tobin and Stoffolano, 1973; Borgia, 1981). The effect of differences in female and male size has to be viewed separately, since their reproductive functions differ. Female fitness is selected for as a function of size. The impact of male size on reproductive behavior is somewhat different, since it is the effect of male size on their mating behavior that influences the population.


Mating behavior in seaweed fly, Coelopa frigida seems to be very simple (Day et al., 1987). C. frigida seems to possess no ornamentation or weaponry, nor is any dancing or singing been served before mating. The male mounts the females with no obvious courtship behavior. Gilburn et al., (1991), identified at least 3 reasons for mounting becoming prolonged when the female is large; large females somehow are attractive to males, they might be more readily seen by the male, or they might emit some pheromones in larger amounts. Alternatively large females might more rarely reject males. Thirdly, males might dismount large females less frequently than small ones. Svensson and Peterson (1989), found that among the dance fly Empis borealis, mated females tended to be relatively larger than refused females in swarms. They also found that mated females had lower wing wear indices than refused females in swarms, indicating that female size may be an important cue in the male chance process.


In the yellow dung fly, Scathophaga stercoraria, the size is highly variable in both sexes, and there is clear evidence males competition for females. The gravid female chooses large males both by moving towards larger males on the pad, and by moving to the prime oviposition site near the centre of the pad where they are mated by the larger males. Males searching strategies are also known to be influenced by body size; larger males search on the pats, while smaller males are generally found in the surrounding grass, apparently ousted from the pats by the larger males (Borgia, 1982). Additionally, Sigurjonsdottir and Parker (1981) and Borgia (1980) showed that larger males are more successful than smaller ones in fights between males and females. Besides the relative size of the opponents, the relative size of the female and the resource value (the egg content of the female) also played an important part in male struggle for female. The larger or heavier females produce more eggs. Borgia (1981) argued that females passively choose large males as mates by moving to areas where large males are more likely to be encountered. These differences give large males a mating advantage over smaller ones.


In the fly Dryomyza anilis, the larger average size males as compared with females show that selection among males favors larger size and presumably, for better fighting ability (Otronen, 1984b). Males do not discriminate against small females. The important feature of a female for a mate seeking male seems to be her egg content rather than size. It is advantage for a male to distinguish between females of different reproductive status because of the considerable time he devotes to each female. Males also copulate with females that have partly developed eggs.


In Mediterranean fruit fly, Ceratitis capitata, size is an important factor in mating success (Stanlard et al., 1986). The mating frequency was reduced when females were larger than males. However, mating frequency was higher when males were of equal or larger sizes than females. Mating duration for larger males was significantly longer than smaller males. The female showed shorter mating duration when the size is large compared with other size. The percentage of adult eclosing, flying and mating; as well mating speed were positively correlated with increased pupal size. The larger male C. capitata also have a larger wing surface (which could be a means of displacing more air) and hence produce a longer, more attractive sound to attract female (Webb et. al., 1984).


In Australian blow fly, Lucilia cuprina, virgin males preferred to mate with non virgin females larger than themselves and males paired with females smaller than themselves adopted mate rejection behavior typical of unreceptive females (Cook, 1991).

Female fruit flies in other genera show varying response to males of different sizes. For the Caribbean fruit fly, Anaestrepha suspensa, the form and intensity of male produced songs that a female uses to judge the size and vigor of a potential mate are reported to be an important selection criteria (Burk & Webb, 1983, Sivinski,, 1984). Besides mating more often it also mated longer (Webb et al., 1984). In the laboratory, female A. suspensa showed a ca. 3.1 preference for larger males. The larger, more sexually successful males were found to have a higher calling propensity; win more flights, and repeat calling songs pulses more quickly than rejected males (Burk, 1983).


A lab study on the Drosophila melanogaster by Partridge and Farquha (1983), found that larger males had mating advantages over smaller males for two reasons: they had a longevity advantage and they also had higher mating success for a given age.


Silvinski & Webb (1985), found that larger acouistically signalling male papaya fruit flies, Toxotrypana curvicauda, are also more sexually successful and the sound pressure level of their approach songs is louder. Mating propensity, however reduced in the European cherry fruit fly, Rhagoletis cerasi L. (Boller, 1976) and the oriental fruit fly, Dacus dorsalis (Iga, 1981), when the male is larger than the female.


2) Resource base mating system.

When mate choice is a resource based system, large males sometimes gain a mating advantage over small size for example in dance fly, E borelis. The male provides the female with some sort of nutrition at courtship. In E borealis, males usually come to swarms one by one, carrying a captured prey as a nuptial gift (Svensson and Peterson, 1987). In a swarm the male approaches one of the large females from below and takes off with her in an ascending flight, the pair couples in the air and then descends to the ground, where they copulate while the female feeds on the nuptial gift. Mate choice is controlled by the males.


Males of Blow fly, L. cuprina contribute both sperm and accessory gland material to females at mating (Cook, 1991). Male size is likely to be important in determining the absolute amount of accessory gland material transferred to females with successive mating and hence the duration of switch off. By choosing to mate with females larger than themselves, males may increase their own reproductive success (Day et al. 1990), especially as the fecundity of L. cuprina females is positively correlated with size (Foster et al., 1975)


In a study on Dryomyza anilis fly by Ostronen, (1984b), found that males defend carcasses, the ovipositional sites for females. On carcasses, the males establish territory. Territorial males are larger than other males on average. Large carcasses attract more males than small ones but the largest males do not choose the largest carcasses. A territorial male is more likely to capture an arriving female, but the advantage of territorial bahavior decreases with increasing density of males. Copulating males are significantly larger than average male, but smaller than territorial males. Males seem to be able to assess a female’s egg load, and adjust the duration of copulation accordingly.


Alcock and Smith, (1995) found that tachinid fly, Microtopesa sinuata engage in landmark territoriality with individuals defending small areas at local ‘hilltops’ along a ridge line. Territory ownership is established through ritualized aerial contests. Different sizes of males rank perch sites similarly. Some males exhibit strong fidelity to a given site over a period of as much as 2 weeks. Differences among males in site fidelity are not related to body sizes. Males of Rutila micans exhibit scramble competition, with individuals flying quickly from one tree to the next, inspecting trunks for perched females. Male aggression and territoriality are absent. The difference between the mate locating tactics of the two species is possibly related to the greater size and density of the male population in R. micans, raising the cost of territorial defense for the tachnid. In some insect species, age or fat reserves are more important than body size in affecting territorial ability (Marden and Waage, 1990).


Males of bombyliid fly, Comptosia sp. occupy traditional territories on a South East Queensland hilltop, to which females come solely for the purpose of mating. Territorial fights between males involved aerial collisions during which modified spines on the wing margins produce scars on the bodies of opponents. Territory owners and mating males are not different in size or age from the remainder of the male population. Hilltop male lacked the extensive population variation typically found in territorial species, and thus the presumed advantages of traits such as large size may be suppressed. Hilltop males were larger than that males at a non hilltop, resource based mating site and the possibility of alternative mating tactics is present (Dodson and Yeates, 1990).


However, even in non resource based systems females often prefer to mate with large males. In many such cases female choice appears to be reinforcing the effects of male- male competition (Burkhardt and de’la Motte, 1988). Male visiting time per female decreased with increasing size difference among females because males decision making process becoming easier when female size increased (Svensson, et. Al., 1988). They also noted that males mated with relatively larger females with less worn wings indicating that mating done with less effort.


3) In-flight mating

Large male body size in caddis fly, Athripsodes cinerene, may be adapted for flight during pairing, that is, larger males are more likely to be able to carry larger loads (Petersson, 1995). Alcock, (1990) found that in a stratiomyid fly, Hermetia comstocki, large males enjoy an advantage in fight with small rivals for possession of landmark mating. territories. Males of the fly compete to perch on certain agaves and yuccas during the middle of the day in the summer flight season. Receptive females occasionally visit these plants, where they are grasped in flight by the resident territorial large males. The pair then lands usually on the defended perch site, and copulation takes place. Female usually depart within minutes after termination of copulation, showing that females use the defended landmark only as a mating rendezvous site and not as an oviposition or feeding resource.


In some species which have aerial agility, favours a high ratio of flight muscle mass to body mass, plays a critical role in establishing the winner of mating contests (Alcock, 1990).


4) Fertilisation success

Many studies of sperm competition have demonstrated large individual variation in male fertilisation success and this variation can relate to male characteristics such as size (Simmons and Parker, 1992) or genotype (Dewsbury 1982; Gromko, et. al., 1984). Simmons and Parker (1992) showed that in the yellow dung fly, S. stercoraria, the fertilisation success of the last male is related to his size because the number of sperm per ejaculate is correlated with male size. In addition, the timing or relative duration of consequtive matings can influence individual variation in fertilization success (Simmons and Parker, 1992).


In the fly, D. anilis, a male’s fertilisation success varies according to his size as well as to the different mating situations in which he may find himself at female oviposition sites (Otrenen, 1994). He found that large males enjoy a higher fertilisation success than small ones when they are the last to mate with the female after ordinary pair formation. The higher fertilisation success of large males in ordinary matings, due to, large males may produce larger ejaculates as in dung fly (Simmons and Parker, 1992). Large males may be preferred as mate, may be because their tapping sequences are more effective and may have several copulation bouts per mating (Eberhard, 1985). Females maybe able to choose sperm from the large male to be used in fertilisation (Ward, 1993). The copula duration was dependent on male size with smaller males copulating for longer (Simmons and Parker, 1992).


5) Genetic Variation

Genetical experiments have suggested that there is hierarchy of decision being made prior to mating in C. frigida (Crocker and Day, 1987) and studies indicate that opportunities do exist during the mating process for both female and male choice that may be intra-sexual selection favouring large males. Mating in C. frigida is positively assortative with respect to size and negatively assortative with respect to a substantial proportion of their genes (Crocker and Day, 1987; Day and Butlin, 1987). Butler et al., (1985), found, in laboratory trials with one female and two males, that the larger male was significantly more successful at siring progeny. However, in the more natural mass mating situation, it was the smaller males that consistently exhibited greater mating success (Day et. al., 1987).


The body size of D. melanogaster is known to be closely related to a number of traits with important life history consequences, such as fecundity, dispersal ability and mating success. Studies indicated that the means of several size measures differ significantly among populations while the genetic correlation’s among these traits generally do not differ (Thomas and Barker, 1993). Males from the dimorphic species tended to respond to a wider range of hydrocarbons which act as contact hormones and also showed high levels of courtship with young flies of both sexes and all species whose hydrocarbons had some structural similarities to those of homotypic females (Cobb and Jallon, 1990). "Courtship genes" that seem to play more particular roles were originally identified as sensory, learning, or rhythm mutations; their reproductive abnormalities have been especially informative for revealing components of male or female actions that might otherwise have gone unnoticed (Hall, 1994).


Female mating preferences for large size in C. frigida appear to exert strong directional selection in favor of large males. There was also an association between female acceptance rate and female alcohol dehydrogenase (Adh) geneotype.


In dung fly, S. stercoria, males with larger testes copulate for longer than do larger males and copula duration remain constant even though the testes shrivel with successive copulas (Ward and Simmons, 1991).


IV. Conclusion

Studies of animals show that the competition for mates component of sexual selection is not restricted to males but may apply to females as well (Daly and Wilson, 1983). The male preference for larger and hence more fecund females has been demonstrated in several species of flies. Larger males have been shown to have increase reproductive success though in some species small males are at an advantage. When mate choice is resource based, large males sometimes gain a mating advantage over small ones. Studies have shown that, larger male flies in some species enjoy a higher fertilisation success as compared to small ones. Larger flies seem to possess stronger muscle and better adapted for flying, thus also increasing their chance of mating. One of the recent advances in the study of size importance in dipteran mating has been to study the genetics of this interaction, to date the means of several size factors differ significantly among populations but the genetic correlations among these traits generally do not differ.



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