Ecological character displacement caused by reproductive interference

We carried out a theoretical investigation of whether ecological character displacement can be caused by reproductive interference. Our model assumes that a quantitative character is associated with both resource use and species recognition, and that heterospecific mating incurs costs. The model shows that ecological character displacement can occur as a consequence of evolution of premating isolation; this conclusion is based on the premise that resource competition is less intense between species than within species and that the ecological character also contributes to premating isolation. When resource competition between species is intense, extinction of either species may occur by competitive exclusion before ecological character divergence. Some observational studies have shown that character displacement in body size is associated with not only resources use but also species recognition. We propose that body size displacement can occur as a consequence of evolution of premating isolation. Our results suggest that ecological character displacement results from reproductive character displacement.     

Winter and spring ecology of <Emphasis Type="Italic">Anaphes nitens</Emphasis>, a solitary egg-parasitoid of the <Emphasis Type="Italic">Eucalyptus</Emphasis> snout-beetle <Emphasis Type="Italic">Gonipterus scutellatus</Emphasis>

We investigated the effects of temperature, photoperiod, food and host availability, and body size on the overwintering abilities of the egg parasitoid Anaphes nitens Girault (Hymenoptera, Mymaridae) under natural conditions. Seven groups of eighty females received one of four treatments (n = 20): (i) honey and hosts, (ii) water and hosts, (iii) honey, or (iv) water. Seven groups of forty males received only honey or water (n = 20). To test if short day-length is the main cue for larval dormancy, the experiment was replicated inside a climate chamber at 20°C and under a winter photoperiod. A. nitens overwinters because of quiescence or oligopause inside the hosts and increased adult longevity. Mean pre-emergence mortality was up to 26% indoors and 15.2% outdoors, males being more affected. Development time had a significant and positive effect on body size. Honey-fed females without hosts had the highest longevity (53 days). Mother’s diet and size affected development time, body size, longevity, and fecundity of the progeny. The results confirm the good adaptation of the parasitoid to the environmental conditions of NW Spain and its ability to synchronize its life cycle with the phenology of the host. Handling editor: Drik Babendreier.     

Reproductive strategies of primates: The influence of body size and diet on litter size

The frequency of multiple births, life history parameters, body size, and diet characteristics were obtained from the literature for 70 primate species. The general pattern within the primate order is to have single infant litters, yet multiple births regularly occur in a number of species in specific phylogenetic groups. Primates which have large litters tend to be small, have short gestation periods and give birth to small infants, which are weaned quickly, and mature rapidly. Species in which multiple births are common also have short interbirth intervals and in the Callitrichidae have males which exhibit paternal care. In addition, they are commonly insectivorous. Although it is difficult to isolate the effects of diet on litter size, independent of body size, analyses suggest that after the influence of body size is statistically removed, as the proportion of insects in the diet increases, animals have larger litters. We suggest that by adopting a mixed diet of insects and fruit primates may be able to ensure access to a seasonally stable food resource that is not greatly restricted by the presence of toxins. This diet would allow a relatively high metabolism and facilitate large litters.     

Sex-specific life history patterns in bluegill (Lepomis macrochirus): interacting mechanisms influence individual body size

The ultimate body size that an individual fish achieves can be a function both of direct effects of growth or indirect effects associated with the timing of sexual maturation (and associated energetic tradeoffs). These alternatives are often invoked to explain variation in body size within and among fish populations, but have rarely been considered simultaneously. We assessed how resource availability and timing of maturation interact to influence individual body size of bluegill (Lepomis macrochirus). Resource availability (high and low food) and the social structure of the population (presence or absence of large, mature males) were varied in experimental ponds. Food ration affected growth (larger fish in the high food treatments) and the social structure of the population affected timing of maturation (early maturation of males in the absence of large males). Treatment effects, however, were sex-specific; males responded to the social structure of the population and females were more responsive to resource availability. We also found individuals that became sexually mature were smaller than those that remained immature, although results were sex-specific and resource dependent. For males, individuals that matured were smaller when resources were limited; mature and immature females showed no difference in body size regardless of food ration. We show that both resource availability and the processes that control timing of maturation interact in sex-specific ways to influence body size of bluegill. These results suggest that a more robust explanation for variable body size requires consideration of sex-specific interactions between ecological (food and growth) and evolutionary (timing of maturation) mechanisms.     

Microhabitat use by the larvae of cryptic lamprey species in <Emphasis Type="Italic">Lethenteron reissneri</Emphasis> in a sympatric area

Microhabitat use by the larvae of two cryptic lamprey species in Lethenteron reissneri, Lethenteron sp. N (northern form) and Lethenteron sp. S (southern form) larvae, as well as intraspecific variations on a size-related basis, were examined in a study area (500 × 5 m) located on a stream where they showed sympatric distribution. The total number of individuals collected from each of 31 quadrats (60 × 60 cm) ranged from 0 (0 n/m²) to 13 (36.1 n/m²). The distinct difference in habitat use was not observed between the two species in the present study area. Lamprey density was not significantly independent among the size groups (small and large) for each of Lethenteron sp. N and Lethenteron sp. S. The density of each size group of the two species was significantly correlated with some habitat variables, and stepwise multiple regression analysis revealed a positive association for a substrate with a smaller particle size and deeper depth for large individuals of Lethenteron sp. N and Lethenteron sp. S, respectively, suggesting that these conditions would be suitable for burrowing. On the other hand, small individuals of both species tended to occur in areas where there was a high content of medium sand and a faster current velocity, probably reflecting a preference for respiratory efficiency. Various condition of habitat should be maintained to conserve the cryptic lampreys.     

Male contest investment changes with male body size but not female quality in the spider Nephila clavipes

Animals use rules to adjust their level of investment in a contest. We evaluate male strategies during contests over females in the golden orb-web spider Nephila clavipes. We tested whether male behaviour changes with female value, and found that contests were similar in intensity and outcome whether the female was a juvenile or adult, virgin or non-virgin, or whether one male had invested sperm in the female. We found evidence that males use a self-assessment strategy when deciding to withdraw from a contest. Loser body size and contestant size difference were correlated with a higher frequency of contest escalation, and fights involving two large males were more likely to escalate than a fight in which one male was small. A multiple regression showed that loser body size had a stronger effect on contest escalation than contestant body size difference. More importantly, the size of the winning male had no effect on contest escalation, a key prediction of a self-assessment strategy. In N. clavipes, body size is the primary factor that determines the outcome of male contests, and males do not appear to assess their opponent or the quality of the resource when deciding to withdraw from the fight.     

Growth rates of rhizosphere microorganisms depend on competitive abilities of plants and N supply

Abstract

Plant‐microbial interactions under N‐limiting conditions are governed by competitive abilities of plants for N. Our study aimed to examine how two plant species of strawberry, Fragaria vesca L. (native species) and Duchesnea indica (Andrews) Focke (an invasive plant in central Europe), growing in intra‐specific and inter‐specific competition alter the functions of rhizosphere microorganisms in dependence on N availability. By intra‐specific competition at low N level, a 2.4‐fold slower microbial‐specific growth rate was observed under D. indica characterized by smaller root biomass and lower N content in roots compared with F. vesca. By inter‐specific competition of both plants at low N level, microbial growth rates were similar to those for D. indica indicating that plants with stronger competitive abilities for N controls microbial community in the rhizosphere. Since a high N level smoothed the differences between plant species in root and microbial biomass as well as in microbial growth rates under both intra‐specific and inter‐specific competition, we conclude that competitive abilities of plant species were crucial for microbial growth in the rhizosphere only under N imitation.     

Avoidance of reproductive interference causes resource partitioning in bean beetle females

Reproductive interference is any interspecific sexual interaction that adversely affects female fitness through indiscriminate reproductive activities. It can be a driving force of resource partitioning in conjunction with resource competition. We previously showed that the bean beetle Callosobruchus maculatus is superior in larval resource competition, but vulnerable in reproductive interference, compared with its congener C. chinensis. We hypothesized that these two species might use two resources differently if one of the resources modified the intensity of reproductive interference or resource competition. We observed that C. maculatus females often enter gaps between beans to avoid mating attempts of heterospecific males, and hypothesized that removing bean gaps would strengthen reproductive interference. Therefore, we provided normal beans with gaps and split beans without gaps to females of the two species housed with conspecific or heterospecific males or no males and compared the number of eggs on each bean type among treatments. Callosobruchus maculatus females housed with heterospecific males were more likely to oviposit on normal beans than C. chinensis females. As a result, more C. chinensis adults hatched from split beans and more C. maculatus hatched from normal beans when females and males of both species were housed together. Thus, oviposition resource partitioning resulted from the combination of female avoidance of reproductive interference and resource competition.     

Asymmetric size effect of sexes on reproductive fitness in an aphid parasitoid Aphidius ervi (Hymenoptera: Aphidiidae)

Most studies on size–fitness relationships focus on females and neglect males. Here, we investigated how body size of both sexes of an aphid parasitoid, Aphidius ervi Haliday, affected the reproductive fitness. Reproductive fitness was generally positively correlated with body size for both sexes in this species. Large individuals of both sexes had greater longevity, large males fathered more progeny, and large females had higher fecundity, parasitism, and greater ability in host searching and handling. We demonstrated in this study that size effects of males and females were asymmetric on different reproductive fitness parameters. With increasing body size females gained more than males in longevity and fecundity while males gained more than females in the number of female progeny. Regardless of female size, large males sustained a female-biased population longer than small males. These results suggest that male body size should also be considered in the quality control of mass-rearing programs and the evaluation of parasitoid population growth.     

Contribution analysis of body mass dynamics in <Emphasis Type="Italic">Daphnia</Emphasis>

The concept of body mass dynamics can be viewed as part of life history theory, but its potential has remained largely untapped due to a lack of analytical methodology. We therefore propose a method, called contribution analysis, which enables us to decompose a change in body mass into contributions associated with variations in individual egg mass, clutch size, and standard somatic mass (somatic mass adjusted to body length). The advantage of contribution analysis is that various contributions are expressed in the same units (units of mass) and show the amount of resources committed to changes in the individual traits, while the traits themselves are measured in different units and thus incomparable on their own. The method is tuned to study zooplankton, and is applied to examine body mass dynamics in Daphnia galeata. We found that when recovering from a poor-resource environment just above the threshold food concentration, Daphnia primarily increase their standard somatic mass, that is, restore body condition. When the trophic environment improves further but remains below the incipient limiting level, resources are invested equally to enhance body condition and reproduction in terms of clutch size. Finally, when food is no longer a limiting factor, almost all resources are committed to increase clutch size. While individual egg mass also varies, it never attracts more resources than the shift in the most prioritized trait. We suggest that the significance of this shift in resource allocation priorities is to keep an adult female alive in a poor environment and thus to allow her to retain her reproductive potential for better conditions in the future. Contribution analysis of body mass dynamics may allow us to detect flexible allocation strategies in a changing natural environment.     

Contrasting effects of large density changes on relative testes size in fluctuating populations of sympatric vole species

Across species, there is usually a positive relationship between sperm competition level and male reproductive effort on ejaculates, typically measured using relative testes size (RTS). Within populations, demographic and ecological processes may drastically alter the level of sperm competition and thus, potentially affect the evolution of testes size. Here, we use longitudinal records (across 38 years) from wild sympatric Fennoscandian populations of five species of voles to investigate whether RTS responds to natural fluctuations in population density, i.e. variation in sperm competition risk. We show that for some species RTS increases with density. However, our results also show that this relationship can be reversed in populations with large-scale between-year differences in density. Multiple mechanisms are suggested to explain the negative RTS–density relationship, including testes size response to density-dependent species interactions, an evolutionary response to sperm competition levels that is lagged when density fluctuations are over a certain threshold, or differing investment in pre- and post-copulatory competition at different densities. The results emphasize that our understanding of sperm competition in fluctuating environments is still very limited.     

The differential use of cheek pouches in a troop ofPapio ursinus

Cheek pouches, one of the distinguishing characters of the Cercopithecinae, are structures used for the temporary storage of food. Their size and frequency of use within a given species are related primarily to the amount of conspecific competition for food. In relation to total body size, members of the genusPapio are considered to have relatively small cheek pouches which are said to be used only occasionally to maximize food harvest when local clusters of food are encountered. This investigation represents 165 hours of observation on a troop ofPapio ursinus at Mkuzi Game Reserve in Natal, South Africa. At the time of observation the choice of foods in the home range was restricted and usually found in abundance only in small clusters of trees. Given that conspecific competition would have been pronounced under these conditions, it was thought that any differences in the frequency of cheek pouch use related to age, sex, rank, or the reproductive state of an animal would be readily recognizable. The results indicated that cheek pouches were used differentially throughout the day, with virtually all ages of each sex displaying the same general pattern of maximal cheek pouch use during the middle of the day. In each sex there was an age graded diminution of the frequency of cheek pouch use from juveniles to adults. This trend was more pronounced in males resulting in a substantial sex difference in the use of cheek pouches between adult males and females. In addition, there was some indication that differences in the frequency of cheek pouch use between adult males were correlated with rank. Although a pattern of cheek pouch use and rank was not evident amongst adult females, there was an association between reproductive state and the frequency of cheek pouch use. Overall, body size, dominance, and energetic demands appeared to be the most significant factors underlying the differences in cheek pouch use in this troop.     

Effects of flowering plant's patch size on species composition of pollinator communities,foraging strategies,and resource partitioning in bumblebees (Hymenoptera: Apidae)

Summary In 4 common Middle-European mainly bumblebee-pollinated plant species (Impatiens glandulifera, Echium vulgare, Aconitum napellus, Symphytum officinale) the influence of patch size on species composition of the pollinator community was studied. Short-tongued species were most dominant in large patches, while small patches were frequented by middle- and long-tongued bumblebees. This phenomenon was extremely obvious in Symphytum officinale and Aconitum napellus, where short-tongued species had bitten a hole in nearly every flower of large patches. Long-tongued species were forced to small patches, where nectarrobbing occurred only exceptionally. In small patches visitationrate (Number of visits per flower per hour) was not lower but either equal or even higher then in large patches. Nectar measurements in Echium vulgare showed, that not only the mean quantity of nectar but also the variance was lower in small patches. As a result, the possible gain can be predicted much more precisely in a small patch than in a large one, and bumblebees have less difficulties in making the right foraging decisions. According to this, foraging strategies depend on patch size. This was confirmed by a computer simulation. The conclusion can be drawn, that many bumblebee species are able to share the same resource by using different patch sizes. Since large flower patches occur mainly in man-made habitats, the dominance of short-tongued species in many bumblebee communities studied by other authors may be unnatural.Supported by the Landesgraduiertenförderung Baden-Württemberg     

The impact of limb autotomy on mate competition in blue crabs Callinectes sapidus Rathbun

Summary This study is the first to demonstrate experimentally that autotomy (self-amputation of a body part) adversely affects competition for mates. Experiments were conducted using blue crabs Callinectes sapidus Rathbun to examine the consequences of limb loss and pairing precedence on mate acquisition by males. Two adult males of equivalent size were introduced sequentially into pools containing a sexually-receptive female and observed after 24 h and 48 h. One male in each pair was left intact, while the other experienced: (1) no autotomy, (2) autotomy of one cheliped, or (3) autotomy of both chelipeds, one walking leg, and one swimming leg. In the absence of a competitor (first 24 h), both intact and injured males established precopulatory embraces with females. Intact males were highly successful (84–95%) in defending females from intact or injured intruders in the second 24 h period. Both autotomy treatments, however, significantly reduced the ability of males to defend females from intact intruders. Females in experiments suffered greater frequency of limb loss than did males. In the field, paired blue crabs showed significantly higher incidence of limb loss than unpaired crabs. Limb loss frequency increases with body size, and field observations indicated that larger males may be more successful than smaller males in obtaining females. Both experimental manipulations and field studies provide strong evidence for mate competition in this ecologically and commercially important portunid species.     

Comparative survivorship and life history of painted greenling (Oxylebius pictus) in Puget Sound,Washington and Monterey Bay,California

Synopsis The painted greenling, Oxylebius pictus, a hexagrammid fish of shallow, northeastern Pacific rocky habitats was observed and collected near the center of its range at Monterey Bay, California, and near the northern limit of its distribution at Puget Sound. Life history characteristics were compared for breeding stocks from the two areas. Mean and maximum adult body sizes (lengths and wet weights) and longevities of both sexes are greater at Puget Sound. Also, both males and females reach sexual maturity at a smaller body length at Monterey. Age group 0 and I juveniles are twice as abundant as adults at Monterey but rare at Puget Sound. Monterey females have greater relative fecundities. Egg development rates are similar at equivalent temperatures; eggs are smaller, however, at Monterey, and thus embryos are smaller at hatching. Length increase is faster for Puget Sound juveniles; adult fish in the two areas grow in length at comparable rates, although weight per body length is consistently greater for Monterey fish. These findings reflect a poorer survivorship at Monterey, where natural predation on fertilized eggs and the mortality rates of adult males are greater.     

Body size, rarity, and phylogenetic community structure: insights from diving beetle assemblages of Alberta

Although it is implicit that interactions between species depend on their traits, studies on the probability of finding related species in a community are in their infancy. Community composition and species richness of predaceous diving beetles (Dytiscidae: Coleoptera) have been used as indicators of freshwater ecosystem function yet no incorporation of phylogenetic relationships of coexisting dytiscids has been attempted to date. Improved knowledge of phylogenetic relationships and phylogenetic community structure analysis methods may provide additional insight into the relationships between community composition and species richness, thus impacting our interpretation of aquatic indicator species metrics. Here, we use museum records of dytiscid beetles in 53 lakes of Alberta, Canada to: (1) compile a supertree of dytiscid beetles that live in the province, (2) examine whether coexisting dytiscids tend to be more or less related than expected by chance, and (3) examine whether phylogenetic structuring depends on species richness or mean size of coexisting species. We find that, although the majority of dytiscid assemblages exhibited phylogenetic clustering, the extent to which this occurred depended on the mean size of dytiscids. We discuss the potential mechanisms and implications of the observed patterns in phylogenetic clustering, along with data that would further improve our understanding of community dynamics in dytiscid beetles.     

PHYLOGENETIC EVIDENCE FOR COMPETITIVELY DRIVEN DIVERGENCE: BODY-SIZE EVOLUTION IN CARIBBEAN TREEFROGS (HYLIDAE: OSTEOPILUS)

Understanding the role of competition in explaining phenotypic diversity is a challenging problem, given that the most divergent species may no longer compete today. However, convergent evolution of extreme body sizes across communities may offer evidence of past competition. For example, many treefrog assemblages around the world have convergently evolved species with very large and small body sizes. To better understand this global pattern, we studied body-size diversification within the small, endemic radiation of Caribbean treefrogs ( Osteopilus ). We introduce a suite of analyses designed to help reveal the signature of past competition. Diet analyses show that Osteopilus are generalist predators and that prey size is strongly associated with body size, suggesting that body-size divergence facilitates resource partitioning. Community assembly models indicate that treefrog body-size distributions in Jamaica and Hispaniola are consistent with expectations from competition. Phylogenetic analyses show that similar body-size extremes in Jamaica and Hispaniola have originated through parallel evolution on each island, and the rate of body-size evolution in Osteopilus is accelerated relative to mainland treefrogs. Together, these results suggest that competition may have driven the rapid diversification of body sizes in Caribbean treefrogs to the extremes seen in treefrog communities around the world.     

EVOLUTION OF GIGANTISM IN NINE‐SPINED STICKLEBACKS

The relaxation of predation and interspecific competition are hypothesized to allow evolution toward “optimal” body size in island environments, resulting in the gigantism of small organisms. We tested this hypothesis by studying a small teleost (nine‐spined stickleback, Pungitius pungitius) from four marine and five lake (diverse fish community) and nine pond (impoverished fish community) populations. In line with theory, pond fish tended to be larger than their marine or lake conspecifics, sometimes reaching giant sizes. In two geographically independent cases when predatory fish had been introduced into ponds, fish were smaller than those in nearby ponds lacking predators. Pond fish were also smaller when found in sympatry with three‐spined stickleback (Gasterosteus aculeatus) than those in ponds lacking competitors. Size‐at‐age analyses demonstrated that larger size in ponds was achieved by both increased growth rates and extended longevity of pond fish. Results from a common garden experiment indicate that the growth differences had a genetic basis: pond fish developed two to three times higher body mass than marine fish during 36 weeks of growth under similar conditions. Hence, reduced risk of predation and interspecific competition appear to be chief forces driving insular body size evolution toward gigantism.     

AgRP-deficiency could lead to increased lifespan

The Agouti-related protein (AgRP) is a central orexigenic peptide leading to increased food intake when ubiquitously overexpressed. AgRP-deficient (AgRP(-/-)) mice have either no phenotype or present an age-related leanness. In this study, AgRP(-/-) mice were fed alternate high fat or low fat diets in an effort to determine whether AgRP is a mediating factor for the effects of dietary fat on metabolic parameters. There were no striking metabolic differences between AgRP(-/-) and the equally obese wild type littermates but AgRP(-/-) mice displayed a significantly longer lifespan. The point estimate of median survival for the AgRP(-/-) group was 9.8% greater while the significantly low hazard ratio (0.494) suggests that mortality incidence of AgRP(-/-) mice is less than one-half that of the wild type reference population. It is concluded that although AgRP(-/-) mice become morbidly obese consuming a high fat diet (a landmark feature for a shortened lifespan), they seem to overcome obesity- and age-related pathologies and live significantly longer than their metabolically similar wild type littermates.