Biased sex ratio and low population density increase male mating success in the bug Nysius huttoni (Heteroptera: Lygaeidae)

Demographic factors such as operational sex ratio (OSR) and local population density (LPD) are temporally and spatially dynamic in the natural environment but the influence of these variables on male mating success and the mechanisms behind it are still poorly understood and highly controversial. Here, we manipulated the OSR and LPD of a seed bug, Nysius huttoni, and carried out a series of mating trials to test how these variables affected male mating success. The two demographic factors had no significant interactions, suggesting that they affect male mating success independently in N. huttoni. In this species male mating success was significantly higher in both male- and female-biased OSR than in even OSR. It is suggested that, in male-biased OSR, the increased intensity of competition and interference does not result in lower male mating success; rather, males may make more effort in courting and females may have more chance to encounter better males, resulting in higher male mating success. In female-biased OSR, females may become less choosy and less likely to reject male mating attempt, leading to the higher male mating success. Lower male mating success in N. huttoni in high LPD may be due to increased interference between males and/or delayed female receptiveness for mating. OSR had a stronger effect on male mating success than LPD in N. huttoni, suggesting that OSR and LPD affect mating success in different ways and intensities.     

Alpha-male paternity in elephant seals

The aim of this study was to assess paternity of males that dominated mating in harems at northern (Mirounga angustirostris) and southern (M. leonina) elephant seal rookeries using DNA fingerprinting and microsatellite DNA analysis. Southern alpha males had greater reproductive success than most northern alphas at similar-sized harems. Comparison of the relatedness between pups within harems also suggested that fewer males achieved matings in the southern elephant seal population. This was consistent with behavioral observations that suggest greater competition for mates in northern elephant seal harems. Reproductive success was consistent with estimates of mating success in some cases, but lower than expected for some northern elephant seal alpha males. A lower reproductive success than predicted from mating behavior may arise from a variety of factors including sperm competition, male sperm depletion from frequent mating, or reduced fertility. The alternatives are discussed in the context of environmental and historical factors. Received: 17 August 1998 / Received in revised form: 28 March 1999 / Accepted: 30 May 1999     

Female encounter rate and the calling behavior and mating propensity of male Requena verticalis (Orthoptera: Tettigoniidae)

Mating effort, the energy exerted in finding and persuading a member of the opposite sex to mate, may be influenced by how frequently potential mates are encountered. Specifically, males that frequently encounter females may reduce calling effort and be less eager to mate than males that infrequently encounter females. An experiment was set up to test this hypothesis, using the tettigoniid Requena verticalis. We examined the song structure, calling activity and mating propensity of individual males exposed to one of five different encounter rates with virgin females. Song structure and calling effort were significantly altered by an encounter with a female. After an encounter, males significantly increased chirp rate and decreased variability in interchirp interval. Encounters also stimulated a male to call and to continue to call for up to two hours. The elapsed time since mating affected mating propensity but not calling activity. Mating propensity asymptotically increased to reach a maximum by day 17 since last mating. However, neither the frequency of encounters, nor the number of previous encounters experienced by a male, influenced calling activity or the propensity of a male to mate. The significance of changes in song structure and calling activity following an encounter, and of increasing male mating propensity over time, are discussed. Correspondence to: G.R. Allen     

Function of the mating plug in Drosophila hibisci Bock

The mating plug in Drosophila hibisci Bock is a firm, gelatinous structure that forms within the female’s uterus during copulation. Two non-mutually exclusive hypotheses for the function of the plug were evaluated. The plug may serve as a nutritional gift that females digest, using the constituents for somatic maintenance or to provision eggs as they mature within the ovaries. Alternatively, the plug may act as a chastity enforcement device by preventing subsequent copulations, and thereby reducing sperm competition. Plug size did not decrease within females over a period of 2 days, and dietary treatment in females did not affect plug size. The extent of ovarian provisioning was also not related to plug size. These results weaken the nutritional gift hypothesis. In contrast, the probability of a second copulation increased sharply with an experimental decrease in plug size. Moreover, females with plugs experimentally reduced in size were courted significantly more and mated significantly faster than females with larger plugs. These results support the chastity enforcement hypothesis. The plug retains the ejaculate and concentrates sperm at the anterior end of the uterus near the apertures of the sperm storage organs. The presence of the plug thus probably facilitates the movement of sperm into storage by retaining sperm at the anterior end of the uterus near the apertures of the sperm storage organs, which may be especially important for D. hibisci, in which sperm length is nearly twofold greater than ventral receptacle length. Matings with newly eclosed virgin females were significantly shorter than with older virgins, and copulations with the younger virgins ended more often without any sperm having yet entered into storage. The effectiveness of the plug in safeguarding a male’s ejaculate may have favoured the evolution of shortened copula durations with young virgins. One fitness advantage of shortened copula duration could be time liberated for the pursuit of further mating opportunities. Received: 12 May 2000 / Revised: 22 September 2000 / Accepted: 15 October 2000     

The evolution of empty nuptial gifts in a dance fly, Empis snoddyi (Diptera: Empididae): bigger isn't always better

Adaptive female choice is thought to have led to the evolution of nutritionally valuable nuptial gifts in many insect species. However, in several dance fly species, males offer and females accept “empty gifts” with no nutritional value. In the species studied here, Empis snoddyi Steyskal, males produce empty balloons comprised of hundreds of silk bubbles and form mating swarms that females approach to investigate males. Males within the swarm engage in agonistic interactions. The empty balloon has been hypothesized to be an indicator of male condition such that males with larger balloons are predicted to have higher mating success and be more successful in male-male interactions than males with smaller balloons. We examined the role of male body size and balloon size in the context of intersexual and intrasexual selection. We found that neither male body size nor balloon size affected the outcome of pairwise male-male interactions. Using multiple-regression techniques, we found significant linear selection for increasing male body size and decreasing balloon size associated with mating success, a surprising result given a positive relationship between male body size and balloon size. A visualization of selection showed the highest peak of male mating success for larger males with intermediate-size balloons. These results can be explained by a trade-off between long-range attraction of females using large balloons and close-range attraction of females via improved flying efficiency associated with smaller balloons. Both male body size and balloon size are important components in determining male mating success; however, the empty balloon does not appear to play a typical role as a sexually selected ornament. Received: 29 December 1997 / Accepted after revision: 7 October 1998     

Mating strategies and mating success of fallow (Dama dama) bucks in a non-lekking population

The rutting behaviour of bucks in an enclosed population was investigated between 1988 and 1990. A substantial proportion of the matings were observed. After preliminary observations in the 1987 rut we categorised bucks into one of four rutting strategies based mainly on their degree of territoriality. We investigate the effects of age, dominance and mating strategy on mating success. Territories were aggregated in an area of oak woods and mating success was highly skewed. Bucks of between 5 and 7 years old achieved the majority (over 90%) of observed matings. Mating success was highly correlated with dominance but only weakly related to fighting success. The possession of a territory was crucial to achieving high reproductive success, with a 38-fold difference between the most and least successful strategies. Bucks pursuing the different strategies also differed in the time they commenced groaning, timing of matings, mating interference and the locations where they achieved their matings. Although high-ranking males devoted considerable effort to obtaining and defending a territory only 36% of each buck's matings were achieved on his territory and males tended to abandon these sites when the tendency of females to visit them decreased.     

Variation in homeward orientation performance in the sleepy lizard (Tiliqua rugosa): effects of sex and reproductive period

Sleepy lizards are monogamous skinks which show high pair fidelity. This study reveals inter- and intrasexual differences in homeward orientation performance in this lizard. Male and female lizards were displaced during three phases of the spring activity period, the pre-pairing, pairing/mating, and post-pairing periods. All groups (with the exception of post-pairing males) were significantly oriented homewards, but males were significantly better oriented towards home than females during the pairing period. Furthermore, males were significantly better homeward oriented during the pre-pairing and pairing periods than in the post-pairing period. Similar results were observed for rate of movement away from the release site. In sleepy lizards, sex-based differences in homing behaviour are unlikely to be attributable to differences in the area of familiarity, or availability of orientation mechanisms. However differences in homing motivation may explain these differences. Males may miss mating if absent from the home range during the pre-pairing and pairing periods, while females may still be able to obtain a mating even when absent. Females however may be more motivated than males to return to the familiar home range during the post-pairing period to ensure efficient feeding during internal embryo development. Received: 16 February 1998 / Accepted after revision: 28 March 1998     

Alternative reproductive tactics in male common shrews: relationships between mate-searching behaviour,sperm production,and reproductive success as revealed by DNA fingerprinting

The common shrew (Sorex araneus) is a solitary small mammal with a promiscuous mating system. Previous studies of this species suggest that females typically mate multiply, and that males may adopt alternative mate-searching tactics. We studied two generations of common shrews in a population near Oxford, England. Males were found to adopt two different mate-searching tactics. Those classed as type A occupied relatively small exclusive ranges during March, and made repeated long-distance movements to visit female ranges around the time of first oestrus in April. Males classed as type B established large overlapping ranges in areas of relatively high female density during March, and maintained these ranges throughout April. Type B males were larger than type A males at an early stage of sexual maturation, but there was no difference in the adult body size of the two types of male. Type A males had significantly higher epididymal sperm counts than type B males. Paternity analyses of litters born during the first year of the study reveal that the mean number of offspring fathered by type B males was greater than the mean number fathered by type A males. It is concluded that different mate-searching tactics may be conditional upon the timing of sexual maturation. Differences in sperm production are discussed in relation to sperm competition theory.     

Effect of sperm depletion and starvation on female mating behavior in the water strider,Aquarius remigis

The resolution of intersexual conflict over mating should be dependent on the current state of each individual. In this study, I used a factorial design to examine the influence of two physiological factors, sperm depletion and food deprivation, on resistance to mating by females of the water strider, Aquarius remigis. Females employ several different mate-resisting tactics during an encounter with a male. Five measures of female resistance to mating were identified: jumping, rolling, dunking, time spent dunking, and struggle duration. Jump, roll, and dunk rates were highly correlated with each other and combined into one metric of resistance to mating (PC1) using principal components analysis. Time per dunk (T/D) and struggle duration were also analyzed. Discrete male behaviors during the struggle could not be identified. Two measures of female resistance, PC1 and T/D, were significantly lower in sperm-depleted females than in sperm-replenished females. Struggle duration did not differ between the two treatments. Starvation had no effect on any of the measures of resistance. Sperm depletion significantly enhanced the probability of mating (54% vs. 24% for replenished females), while starvation had no effect on the probability of mating. I pooled all the females and compared females that mated with those that did not mate. Nonmating females resisted significantly more than mating females in all three measures of resistance. Path analysis indicated that PC1 was the only measure of resistance that was significantly negatively related to the probability of mating. Almost half (46%) of sperm-depleted females showed no resistance to males, while only 3% of sperm-replenished females were nonresistant. When nonresisters were removed from the analysis, sperm depletion had no effect on any of the measures of female resistance to mating and no effect on the probability of mating. In A. remigis, female resistance appears to be a yes/no phenomenon with respect to sperm depletion and not affected directly by starvation. Received: 2 September 1994/Accepted after revision: 9 September 1995     

Egg mimicry and allopaternal care: two mate-attracting tactics by which nesting striped darter (Etheostoma virgatum) males enhance reproductive success

In a variety of fish species with paternal care of offspring, females prefer to spawn in nests that already contain eggs. This female preference has been hypothesized to explain egg thievery in male sticklebacks, allopaternal care of eggs in minnows, and the evolution of egg-mimicking body features in male cichlids and darters. Here we employ microsatellite-based parentage analyses to evaluate the reproductive success of striped darter (Etheostoma virgatum) males that appear to utilize two of these functionally related tactics to entice females to spawn in their nests. In an isolated population (Clear Creek, Ky.), we observed that breeding males develop conspicuous white spots on their pectoral fins. If these spots are egg mimics, as we suspect, then this represents the fourth independent evolutionary origin of egg mimicry documented to date in darters, the first based on pigmentation (as opposed to physical structures), and the first in which the egg mimics vary greatly in number among males. From direct counts of microsatellite genotypes in clutches of embryos, at least 3.8 females contributed to the progeny within a typical nest, and females tended to spawn preferentially with males that were larger and displayed more egg-mimic spots. In another population (Hurricane Creek, Tenn.) without egg mimics, the multi-locus genetic data document that allopaternal care is common, especially among the smallest males who sometimes tend nests containing their own as well as an earlier sire's offspring. Thus, these foster males had adopted egg-containing nests and then successfully spawned with subsequent females. Overall, the genetic data on paternity and maternity, in conjunction with field observations, suggest that egg mimicry and allopaternal care are two mate-attracting reproductive tactics employed by striped darter males to exploit female preferences for spawning in nests with 'eggs'. Electronic Publication