Competitive habitat utilization in the damselfly,Mnais nawai (Zygoptera: Calopterygidae) coexisting with a related species,Mnais pruinosa

Reproductive behaviors related to habitat utilization were studied in males of the damsefly,Mnais nawai, which has two male forms, territorial orange-winged males (nawai) and non-territorial pale-orange-winged males (sahoi), at the upper part of a mountain stream where they partiallycoexist with a related species,Mnais pruinosa, which also has two male forms, territorial orange-winged males (esakii) and non-territorial hyaline-winged males (strigata). These two species showed parapatric distribution; the lower part of the stream was occupied byM. nawai, and the upper part byM. pruinosa. In the present study, cross-matings occurred between bothMnais species, although normal intraspecific matings occurred more frequently than cross-matings. Territorial males of both species copulated with conspecific females that entered their territory and guarded the ovipositing females, probably to avoid sperm displacement resulting from subsequent copulations. Severe competition for oviposition sites by territorial males even occurred between the two species. On the other hand, non-territorial males of both species have alternative mating strategies (including several tactics such as sneaking, takeover and interception). The possible benefits from conflict among territorial males of both species is discussed.     

Male mating behavior in relation to spermatophore transfer in the white cabbage butterfly

The lifetime mating frequency of female butterflies is believed tobe dependent on the reproductive status of the males which they have mated. This report assesses those status usingPieris rapae L. Multiple mating females mated males with a short time interval after the last mating or males with many mating records. Such males, like small ones, produced small spermatophores during copulation, which may have resulted in high mating frequency of those females. The males with short time interval after the last mating or those with many mating records also showed a long mating duration. Alternative interpretations of the adaptive significance of this behavior for males are discussed.     

Mating strategies based on foraging ability: an experiment with grasshoppers

Female mate choice and the benefits of this behavior are criticalaspects of Darwinian sexual selection, but they are seldom documentedbecause it is difficult to identify the male trait(s) that femalesmay be seeking. We conducted experiments with grasshoppers (Melanoplussangutnipes: Orthoptera, Acrididae) to examine this behavior.Males that feed more intensively and select a diet mix thatpermits greater food intake (food intake per body mass per time)in laboratory trials were preferentially selected by females.These better foraging males on average provide greater paternalinvestment (greater spermatophore mass) to the female, whichincreases her reproductive rate (eggs produced per body massper time). However, paternal investment may not entirely explainfemale choice of better foraging males, because these maleswere still selected even if they had their food intake restrictedor had been allowed to recently mate, which reduces spermatophoreproduction. Furthermore, males change their mating strategyin response to female choice and the foraging abilities of surroundingmales. Poorer foraging males attempt forcible copulation ratherthan displaying and allowing female choice. A male will facultativelyswitch between these strategies depending on the foraging abilitiesof the surrounding males. While females attempt to reject forciblecopulation, forcible copulation reduces the frequency with whichfemales successfully copulate with better foraging males. Therefore,males that are less "attractive" to females adopt alternativemating strategies to counter female choice which would excludethem from mating.[Behav Ecol 7: 438–444 (1996)]     

Mating tactics in external fertilizers when sperm is limited

Among externally fertilizing animals in aquatic habitats, theproportion of a female's egg clutch that is successfully fertilizedoften falls below 100%. In many such species, particularly incoral reef fishes, males spawn daily at high frequencies, oftenwith little or no sperm competition. A major evolutionary problemfor such males is how best to allocate sperm over successivespawns. Females face the problem of ensuring complete fertilizationof their egg clutch. Here we model male and female mating tacticswhen daily sperm production is limited and with various assumptionsconcerning how differences in the number of sperm released duringa mating influence the number of eggs fertilized. The modelsreveal conditions under which males can maximize daily reproductivesuccess, either by releasing a fixed number of sperm duringall successive spawns or by matching sperm numbers to the clutchsize of their mates. These patterns of sperm allocation exertdifferent pressures on females, which may respond evolutionarilyby developing various mating tactics of their own.     

Excess carotenoids disturb prospective cell-to-cell recognition system in mating responses ofPhycomyces blakesleeanus

Carotenogenic mutants ofPhycomyces, which accumulate excess β-carotene or its intermediates, always failed in zygospore development. No improvement occurred when such mutants were mated together with a helper wild type of the same mating type against the wild type of the opposite mating type. Addition of excess synthesized pheromone, trisporin B, also failed to improve the zygospore development, though the mating response was significantly activated in the early stages and abundant zygophores were formed. Exceptional acceleration of the zygospore development under these experimental conditions occurred in a regulatory albino mutant (carA), which does not accumulate excess intermediate carotenoids. Chemically- or genetically-induced ovarproduction of β-carotene or lycopene also inhibited the zygospore development. These results imply that the zygospore development ofPhycomyces is maximal when the intracellular amount of β-carotene is optimal (=wild type), and that pheromones act mainly in the early stages of mating, while other factors such as the cell-to-cell recognition system may also be involved in the later stages. Intracellular accumulation of excess β-carotene or its intermediates probably disturb such later-stage factors.     

Male larval nutrition in Choristoneura rosaceana (Lepidoptera: Tortricidae): an important factor in reproductive success

This study examines how Choristoneura rosaceana male quality, as determined by larval diet, age and mating history, affects the reproductive success of both sexes. While the size of the spermatophore produced at first mating increased linearly with male age, the frequency of mating was significantly higher for middle-aged males (2–4 days old) than younger (0–2 days old) or older (6–8 days old) individuals, when both sexes were fed on artificial diet. However, the duration of copulation was longer in couples with older than younger males. The observed age-related changes in spermatophore size had no significant effect on female longevity, fecundity or fertility, suggesting no direct relationship between male investment and spermatophore size under these experimental conditions. Different larval food sources (artificial diet, maple and hazelnut) did not affect the proportion of 2-day-old virgin males that mated; however, the proportion that remated was significantly higher for males reared on high-quality food (maple and artificial diet) than those on hazelnut, a poorer food source. There was a 5-fold decline in spermatophore size between the first and second matings on all diets, but female reproductive output was reduced by only 25%. In contrast, while the first spermatophore produced by males on hazelnut was 1.5 times smaller than those produced on maple and artificial diet, the fecundity of their mates was 40% less than those mated with high-quality virgin males. These results provide additional support to the idea that spermatophore size is not a valuable indicator of male quality. Most tethered females placed in the field during the first flight period mated with virgin males (based on the size of the spermatophore), suggesting that female choice exists in this species. These results are discussed in relation to the incidence of polyandry in naturally occurring populations of Choristoneura and the potential use of size and/or chemical cues by females to assess male quality.     

Structure of the exocrine pancreas of flounder (Paralichthys olivaceus): immunological localization of zymogen granules in the digestive tract using anti-trypsinogen antibody

We determined the fertilization rates achieved by males of the polygynous wrasse Xyrichtys novacula in the northern Mediterranean sea, by collecting all sperms and eggs released in individual spawns from three adjacent male territories that encompassed different numbers of female home ranges. Two males with territories including four and six females respectively, released similar average numbers of sperm/spawn, whereas a male holding at the same time two territories with six females each, released significantly fewer sperm/spawn. Although fertilization rates decreased with absolute numbers of sperm released, the male with the largest harem achieved the highest total daily number of fertilized eggs, as a result of the high number of females in his territory. These results demonstrate a strategy of a male's sperm economy that is in conflict with the female's interests.     

Precedence of the first male sperm in successive matings in the HymenopteraDiadromus pulchellus

In most insect species where double matings occur, sperm from the second male preferentially fertilize subsequent eggs. However, we demonstrate here that, as already shown for some other hymenopteran species, this is not the case in the ichneumonid waspDiadromus pulchellus (Wesmeal): sperm from the first male usually father all the female progeny. This precedence of the first male sperm is also observed in double matings involving an haploid male and a diploid sterile male, whichever is the first mating male. We discuss the consequences of this phenomenon from an evolutionary point of view.     

MUTATIONAL MELTDOWNS IN SEXUAL POPULATIONS

Although it is widely acknowledged that the gradual accumulation of mildly deleterious mutations is an important source of extinction for asexual populations, it is generally assumed that this process is of little relevance to sexual species. Here we present results, based on computer simulations and supported by analytical approximations, that indicate that mutation accumulation in small, random-mating monoecious populations can lead to mean extinction times less than a few hundred to a few thousand generations. Unlike the situation in obligate asexuals in which the mean time to extinction (t?_e) increases more slowly than linearly with the population carrying capacity (K), t?_e increases approximately exponentially with K in outcrossing sexual populations. The mean time to extinction for obligately selfing populations is shown to be equivalent to that for asexual populations of the same size, but with half the mutation rate and twice the mutational effect; this suggests that obligate selfing, like obligate asexuality, is inviable as a long-term reproductive strategy. Under all mating systems, the mean time to extinction increases relatively slowly with the logarithm of fecundity, and mutations with intermediate effects (similar to those observed empirically) cause the greatest risk of extinction. Because our analyses ignore sources of demographic and environmental stochasticity, which have synergistic effects that exacerbate the accumulation of deleterious mutations, our results should yield liberal upper bounds to the mean time to extinction caused by mutational degradation. Thus, deleterious mutation accumulation cannot be ruled out generally as a significant source of extinction vulnerability in small sexual populations or as a selective force influencing mating-system evolution.     

Seasonal migration promoting assortative mating in Littorina brevicula on a boulder shore in Japan

Littorina brevicula Philippi is one of the most common snails found in the upper intertidal zone of Japan. In Amakusa, some of the population of L. brevicula migrate to the lower zone in the winter, while the rest stay in the upper zone. Thus, during the winter, which is its reproductive season, the population of L. brevicula divides into two sub-populations. This leads to a hypothesis that the migration pattern in winter is genetically controlled and this behavioural dimorphism is maintained by reproductive isolation between the two sub-populations. In order to test this hypothesis, the following three points were investigated: (1) whether the same snails migrate in a similar way every winter, (2) whether there is a significant tidal level preference in snails, and (3) whether reproductive isolation occurs between the two sub-populations. The results showed (1) the migration behaviour of each snail was consistent over two successive winters, i.e. the same group of snails migrated downward every winter and the same group of snails stayed in the upper zone every winter, (2) transplanted snails moved toward the original zones where they were caught, suggesting that the snails actively selected their tidal zone in winter, and (3) most of the snails copulated within each sub-population. Therefore, reproductive isolation between the two sub-populations was considered to be established to some extent by the dimorphic migration behaviour. In conclusion, the migratory behaviour of L. brevicula is determined separately for each individual and might be genetically controlled, and the behavioural dimorphism may be maintained by partial reproductive isolation between the two sub-populations.