The effects of male body size on territorial and mating success in the landmark-defending fly Hermetia comstocki (Stratiomyidae)

Abstract.

• 1 Males of Hermetia comstocki Williston compete for territorial control of certain agaves and yuccas. Winners copulate with females that visit these plants solely to acquire a mate.
• 2 Males vary in body weight by more than an order of magnitude and larger flies almost always defeat smaller ones in aerial contests for control of landmark territories.
• 3 The mean body size (as measured by wing-length) was significantly greater for males retaining residency at a site for at least one hour compared to males unable to do so. Likewise, males able to return to a perch site in the study area on more than one day were larger on average than males unable to do so.
• 4 Male preferences for landmark territories remained similar across years. Large males dominated the perch landmarks most likely to be occupied by males and most likely to be visited by females.
• 5 Despite the fighting and territorial advantages enjoyed by large males, the mean size of males found mating with females was not significantly larger than that of the general population.
• 6 The apparent failure of large males to secure a statistically significant mating advantage may be a statistical consequence of the small sample size of males observed mating. On the other hand, any mating advantage of large males may be reduced because (a) receptive females visit many different landmarks, (b) females mate with the first male they encounter at a landmark, regardless of his size, (c) there are usually many vacant landmarks available for smaller males, and (d) even popular territories are often open to small males, thanks to the low site-tenacity of territory owners.

     

Male contesis for territories and females in the fly Dryomyza Anilis

Dryomyza anilis males defend the small carcasses on which females oviposit as well as the females themselves during copulation and oviposition. Contests over territories and females, were compared to assess the way different asymmetries affect the outcome of a struggle. In contests over females with a lot of eggs the rate of take-over was lower than in contests over immature females, but the size of the female or her size relative to the owner did not affect the outcome. In territorial disputes and territories was the size difference between the opponents. Fifty-three per cent of the contests over females and 39% of territorial struggles resulted in a take-over. In take-overs, owners were smaller than copulating males or territory owners on average. Take-overs are part of a process in which resource owners continue to change until one is strong enough to defend the resource successfully. Contests over females lasted significantly longer than those over territories, which suggests that for males females are more valuable than territories. This is to be expected as territories are of value because they ultimately increase the probability of obtaining females. A positional advantage or the higher payoff of the owner in disputes over females may be the reason for the lower rate of take-over in female defence than in territorial struggles between opponents of similar size.     

The effect of hunger on mating behaviour and sexual selection for male body size in Gerris buenoi

The set of mating behaviours expressed by an individual may depend upon the state of that individual and local environmental conditions. Understanding how these factors affect mating behaviours may elucidate how a mating system operates, and its consequences for the form and strength of sexual selection. We conducted two experiments on the water striderGerris buenoi to (1) determine the effect of hunger on the mating behaviour of both sexes and (2) examine female choice for large males. In our first experiment, we manipulated hunger (20 h starvation) in both sexes and recorded mating, male harassment, copulation duration and guarding duration. We predicted that hunger would increase female reluctance to mate because mating conflicts with foraging. Female hunger (20 h starvation) decreased mating rate by two-thirds but had no significant effect on male mating behaviour. In a second experiment, we examined the effect of female hunger, and resulting reluctance, on sexual selection for large male size. Hungry females (5 h starvation) were placed with two fed males (one large, one small) and we recorded male premating and mating behaviours. We observed significant large-male mating advantage when females were hungry, but not when satiated. Mating efforts (harassment, premating struggles) were similar for both male phenotypes in both female hunger treatments, suggesting that the mating advantage of large males resulted from increased reluctance of hungry females to mate. Neither male body size nor female hunger explained a significant amount of variation in copulation duration or guarding duration. We discuss our results in light of two competing hypotheses for female choice (active and passive) on male body size and suggest that passive choice for large males acts in this system. Copyright 2002 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour     

Prior residence, body size and the dynamics of territorial disputes between male freshwater angelfish

Territorial interactions between pairs of size mismatched, sexually mature male angelfish Pterophyllum scalare were investigated in three different conditions: with the larger fish resident (the large resident condition), with the smaller fish resident (the small resident condition) and in a neutral territory (the neutral condition). In the two resident conditions, approximately half of the intruders had previously held territories and half had not. In all categories of fight, one fish showed submissive postures and lost the fight; eventual losers performed both attack and threat at a lower rate than eventual winners. Attack rate declined as the encounter progressed, while rate of performance of threat postures increased. In fights on neutral territories, the larger fish won all fights. In all fights with a resident-intruder asymmetry, the resident fish won the encounter, regardless of relative size. In eventual winners but not in eventual losers, levels of attack were lowest in the neutral encounters. In the small resident condition, levels of attack (corrected for activity of the resident) were lower in intruders that had previously held a breeding territory. Relative size influenced behaviour shown during fights, in that overall intensity was correlated negatively with size differential in all conditions. Thus although prior residence is the primary determinant of the outcome of territorial encounters in this species, both relative body size and prior possession of territory also influence the nature of the interaction.     

The effect of body size on male courtship success in a plethodontid salamander

Two different sets of courtship encounters between female Desmognathus ochrophaeus and males of large (X¯=58) and small (X¯=38 mm) body size were staged in the laboratory. In the first set, encounters were arranged between male-female pairs so that each of five females individually encountered each of 10 males. In these encouters, small and large males were equally likely to court with and inseminate females. In the second set, each encounter was staged for a trio fo one female and two males (one large and one small); each female had a courtship opportunity with five large-small pairs of males, and each pair of males encountered five different females. In the 25 trio encounters, the large male always chased away the small male before proceeding to court the female. Small males never deposited spermatophores if a large male was present. Male body size should have a significant effect on courtship success in natural populations of D. ochrophaeus if male-male competition affects access to females.     

The effect of temperature and body size on digestive efficiency in Fundulus heteroclitus (L.)

The digestive efficiency of temperature acclimated mummichogs, Fundulus heteroclitus (L.), was determined using the amphipod Orchestia grillus Bosc as prey. Experiments were conducted on three size groups of mummichogs (<1 g, 1–3 g, > 3 g) at 5, 13, 21, and 29 °C. No difference was found in digestive efficiency by different sizes of mummichogs. There was, however, a statistically significant difference in efficiency over the range of acclimation temperatures, with the efficiencies being temperature independent from 13 to 29 °C and dropping slightly at 5 °C. From 13 to 29 °C, digestive efficiencies were the maximum possible. Temperatures in this range are normal late spring, summer, and early fall habitat temperatures in Maine estuaries. The ability to maintain a maximum efficiency of digestion over this 16°C temperature range allows mummichogs to get the maximum amount of energy from their prey during the time of year when they are utilizing substantial energy for growth (somatic and gonadal, and for activity (foraging and mating). The digestive efficiency at 5 °C was only about 13.5% less than at 21 and 29 °C. This drop is probably of little ecological or energetic significance, so that mummichogs are actually able to absorb food energy across their alimentary tract relatively independent of acclimation temperature over a 24 °C range.     

Island/mainland body size differences in Australian varanid lizards

Island varanids seem to be an exception to the rule that territorial vertebrate taxa often become gigantic relative to mainland relatives when on islands, whereas non-territorial species become dwarfed (Case 1978). However, no systematic island/mainland studies have examined the empirical size trends in this group of carnivorous lizards. We perform such an analysis for the Australian region and critically evaluate various selective agents that might be responsible for size changes in several island populations. Insular gigantism occurs at least four times among the island populations examined. The magnitude of size change is positively correlated to prey abundance on the islands (as indirectly measured through a condition index of the lizards, essentially a measure of how fat they arc) and the size of prey: islands with large prey have large varanids and vice versa. Since the island population with the largest size change, the Reevesby Varanus rosenbergi, was introduced less than 100 years ago, these size changes can be quite rapid. This might indicate that selective coefficients are strong; however, we can not exclude the possibility that these size differences have no genetic component and simply reflect environmental differences in growth rate and shifts in age structure between island and mainland locations.     

Interactions among mechanisms of sexual selection on male body size and head shape in a sexually dimorphic fly

Abstract Darwin envisaged male-male and male-female interactions as mutually supporting mechanisms of sexual selection, in which the best armed males were also the most attractive to females. Although this belief continues to predominate today, it has been challenged by sexual conflict theory, which suggests that divergence in the interests of males and females may result in conflicting sexual selection. This raises the empirical question of how multiple mechanisms of sexual selection interact to shape targeted traits. We investigated sexual selection on male morphology in the sexually dimorphic fly Prochyliza xanthostoma , using indices of male performance in male-male and male-female interactions in laboratory arenas to calculate gradients of direct, linear selection on male body size and an index of head elongation. In male-male combat, the first interaction with a new opponent selected for large body size but reduced head elongation, whereas multiple interactions with the same opponent favored large body size only. In male-female interactions, females preferred males with relatively elongated heads, but male performance of the precopulatory leap favored large body size and, possibly, reduced head elongation. In addition, the amount of sperm transferred (much of which is ingested by females) was an increasing function of both body size and head elongation. Thus, whereas both male-male and male-female interactions favored large male body size, male head shape appeared to be subject to conflicting sexual selection. We argue that conflicting sexual selection may be a common result of divergence in the interests of the sexes.     

The effect of female body size on male mounting behaviour in Dermacentor variabilis and D. andersoni

Abstract. This study was undertaken to determine if body size of female ticks was an important factor in eliciting the mating behaviour of Dermacentor variabilis and D. andersoni males. Dummy female ticks (DFTs) representing varying sizes of feeding females were prepared from plastic beads. Size of DFT was shown to be important in determining both the time spent by males in contact with DFTs and the extent of male mating response released. Dermacentor varlabilis males preferred the smaller-sized DFT, whereas D. andersoni males preferred the larger sizes. Males of both species were able to discriminate between size of DFT only when mounting sex pheromone (MSP) was present.
Size of the DFT was more important than the composition of the MSP extract in determining the time males spent in contact with DFTs.
Males of both species were sensitive to variations in concentration of extract, and spent 2–6 times longer in contact with DFTs when the concentration was optimal than when it was not. No significant difference in the male's mating response was seen in response to variation in concentration of heterospecific extract.
Male mating response, i.e. the behaviour of the male as it progresses through the initial contact, climbing onto the dorsal surface and turning onto the venter, was shown not to be dependent upon how long the male spent in contact with the DFT. Male D. andersoni had a much lower mating response to DFTs than D. variabilis males, suggesting that some further stimulus may be required.     

The mating system of a bee fly (Diptera: Bombyliidae). II. Factors affecting male territorial and mating success

Males of an undescribed bombyliidfly (Comptosia sp.)occupy traditional territories on a Southeast Queensland hilltop, to which females come solely for the purpose of mating. Territorial fights between males involve 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. Although residency is related to fighting success, the strength of the effect is ambiguous. Consequently, our data do not appear to fit predictions from game theoretical models for fighting protocol. Hilltop males 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 males at a nonhilltop, resource-based mating site and the possibility of alternative mating tactics is discussed.     

Sexual selection for large male size in a polyandrous butterfly: the effect of body size on male versus female reproductive success in Pieris napi

Sexual selection theory predicts that the larger sex shouldbe that for which fitness increases at the faster rate withsize. In butterflies, as in most invertebrates, females areusually the larger sex, but previous comparative analysis hasshown that relative male size increases with female polyandryamong butterflies. In agreement with this pattern, males arelarger than females in the strongly polyandrous green-veinedwhite butterfly, Pieris napi L., and in this article we assessthe size dependence of reproductive success in both sexes. Inan experiment where virgin males and females were released inthe field, we found no strong association between size and malemating success. However, laboratory experiments showed thatthere was a strong correlation between size and the ejaculatethat the male delivered to the female at mating and that largeejaculates delayed female remating for a longer time comparedto small ejaculates. Moreover, female P. napi utilize male-derivednutrients received at mating to increase their fecundity. Hence,large males sire more offspring both by way of donating morenutrients to female egg production and by way of delaying femaleremating (given that the last male to mate with the female willfather most of the offspring). Laboratory experiments showedthat the association between size and fecundity was low, ornonexistent, among P. napi females allowed to mate only once.However, weak size dependence was found for polyandrous females.We hypothesize that size dependence of female fecundity maybe especially weak among polyandrous butterflies because a fundamentalsource of variation in fecundity relates to their ability tofind nutrient giving males, an ability which may be unrelatedto female size. According to this hypothesis there is a causalassociation between weak size dependence of female fecundityand polyandry, and a strong size dependence of male reproductivesuccess that may underlie the comparative pattern of positivecorrelation between relative male size and polyandry.     

Sex differences in the lipolytic activity of Galleria mellonella fat body

The sexual differences in the intensity of production and release of free fatty acids (FFA) as well as the differences in the absolute amount of FFA produced and released from the fat body are most conspicuous at the sixth day after the larval-pupal ecdysis. Furthermore, it is demonstrated that a significant difference exists in the lipolytic activity of the fat body between the two sexes.     

Spatiotemporal variation in selection on body size in the dung fly Sepsis cynipsea

Standardized measures of the strength of selection on a character allow quantitative comparisons across populations in time and space. Spatiotemporal variation in selection influences patterns of adaptation and the evolution of characters and must therefore be documented. For the dung-breeding fly Sepsis cynipsea, we document patterns of variation in sexual, fecundity and larval and adult viability selection on body size at several spatiotemporal scales: between-populations, over the season, over the day and between dung pats. Adult viability selection based on residual physiological survivorship in the laboratory was nil or weakly negative. In contrast, larval viability selection in two laboratory environments was weakly positive for males at low competition and females at high competition. Fecundity selection was positive and strong at all times and in all populations. Sexual selection reflecting pairing success was overall strongly positive (about three times stronger than fecundity selection), while selection reflecting male reproductive success via the clutch size of his mate (i.e. assortative mating) was essentially nil. Only sexual selection varied significantly at coarse (between populations and seasonally) but not at fine (within a day or between pats on a pasture) spatial and temporal scales. Quadratic and correlational selection differentials were low and inconsistent in all episodes except for fecundity selection, where there was some evidence that clutch size reaches an asymptote at large body sizes, implying weaker selection for large size as females get bigger. Implications of these results for the evolution of body size and body size dimorphism are discussed.     

The effect of body coloration and group size on social partner preferences in female fighting fish (Betta splendens)

Females of the fighting fish Betta splendens have been shown to associate with other B. splendens females in a manner reminiscent of shoaling behavior. Since body coloration varies dramatically in this species, and since body coloration has been shown to affect shoalmate choice in other species of fish, we examined the influence of body coloration on association preferences in female B. splendens. In dichotomous choice tests, B. splendens females spent more time swimming near groups of females (regardless of coloration) than swimming near an empty chamber, and chose to swim near fish of similar coloration to their own when choosing between two distinctly colored groups of females. When examining the interplay between body coloration and group size, focal fish spent more time swimming near larger groups (N = 5) of similarly colored fish than swimming near an individual female of similar coloration. However, focal fish showed no preference when presented with an individual female of similar coloration and a larger group of females of dissimilar coloration. These results suggest that association choices in B. splendens females are strongly affected by both body coloration and by group size.     

Distribution of male yellow dungflies around ovipasition sites: the effect of body size

Abstract.

• 1 Spatial and temporal variation in body size of yellow dungflies, Scatophaga stercoraria, gathering on and around cow droppings was studied in an Icelandic population in order to elucidate the effect of male and female size on male mating tactics.
• 2 Males copulating on droppings were on average larger than males copulating in the grass, but of similar size to males guarding ovipositing females. Males searching on droppings were smaller than males copulating or guarding females on droppings but larger than males copulating in the grass. No such differences were found in female size.
• 3 Resource-holding power of males (RHP, i.e. male: female size ratio) differed between the three mating groups and was highest for males on the droppings. Size and RHP clearly affect the tactics of copulating males. Males with low RHP tend to copulate in the grass in spite of the cost of longer copulation duration. We argue that this is caused by risk of takeovers from large searching males.
• 4 There was no change in male size with the age of individual droppings. Contrary to what might be expected, large searching males are not predominantly found at fresh droppings when the probability of catching unpaired females is highest. We suggest instead that good prospects in taking females over from other males must make the strategy to search for females on older droppings profitable.
• 5 RHP did not change with age of dropping in the three mating groups. The size of ovipositing females increased with age of dropping, probably reflecting longer copulation and egg-laying times of large females.
• 6 We found an overall positive relationship between sizes of male and female partners. This correlation was highly significant for copulating pairs in the grass. This is probably a consequence of males with low RHP copulating in the grass and fights in which larger males take over females from smaller males. A weaker, but significant, correlation was found amongst ovipositing pairs. This must be due to take-over effects. No size correlation was found for pairs copulating on droppings.

     

Quantity of the male ejaculate influenced by female unreceptivity in the fly, Culicoides melleus

The number of sperm placed in the spermatophore by virgin Culicoides melleus males mated with virgin females was compared with the number deposited in matings with previously once or twice copulated females. When compared to an all-virgin mating, there was a significant reduction (P<0·001) in sperm complement (of 15 per cent) when the female had already been mated once as compared to a virgin, and a greater reduction (of 21 per cent) when the female had already been mated twice. The mean numbers (± S.E.) of sperm ejaculated with virgin, once mated, and twice mated females were 857 ± 24, 732 ± 23, and 673 ± 25, respectively.When a major activity in the unreceptive behaviour of females (kicking at the male in the early stages of copulation) was prevented by amputation of the hind legs, the mean number of sperm ejaculated (788 ± 29) was significantly larger (P<0·01) compared with twice mated females having both legs intact, and also larger, though not significantly so, than with intact once mated females. In addition to the kicking activity, there was evidence that other forms of unreceptive behaviour contributed to reducing the quantity of sperm ejaculated.