Mean Size Of Females Research Articles

Predation-sensitive grouping and habitat use by eastern grey kangaroos: a field experiment

Eastern grey kangaroos, Macropus giganteus, are thought to respond to high risks of predation by foraging closer to refuge and by forming larger groups, but females with newly emergent offspring mostly forage alone, a response thought to reduce predation risk on the latter. I tested these hypotheses for wild kangaroo populations in southeastern Australia, using a field-based experimental reduction of a potential predator, the red fox, Vulpes vulpes. I monitored kangaroo foraging behaviour in two fox removal and two control sites by counting scats at 30 and 150m from refuge during September 1994 and 1995 (after 5 and 16 months of fox control). I also monitored kangaroo group sizes in all sites, using monthly or bimonthly direct observations over 22 months of fox reduction. In both years, fox removal caused kangaroos to deposit relatively more scats further from refuge than in control sites where fox numbers were more than 85% higher. This suggests strongly that kangaroos spend more time foraging close to refuge when predation risk is high. Group sizes were also smaller in fox removal sites although only during June–October when the majority of females had large pouch young. In November–March, group size decreased in all sites owing to the solitary behaviour of many females with young at foot. However, where foxes were removed, the mean group size of such females was smaller than in control sites and females were also more often alone where foxes were removed. These results suggest that solitary behaviour increases rather than decreases predation risk. This experiment highlights that grouping behaviour is risk sensitive and that some individuals accept higher risks of predation to mitigate other costs of group foraging.

Sex ratios in field populations of two parasitoids (Hymenoptera: Chalcidoidea) of Coccus hesperidum L. (Homoptera: Coccidae).

We tested several assumptions and predictions of host-quality-dependent sex allocation theory (Charnov et al. 1981) with data obtained for the parasitoid Metaphycus stanleyi Compere on its host, brown soft scale (Coccus hesperidum L.), in a California citrus grove and in the laboratory. Scales ceased growing after parasitization by M. stanleyi. Thus, M. stanleyi may gauge host quality (=size) at oviposition. Host size positively influenced adult parasitoid size, and parasitoid size in turn influenced adult longevity of M. stanleyi. However, parasitoid fitness gains with host size and adult size were similar in males versus females. Sex allocation to individual hosts by M. stanleyi depended on host size; females consistently emerged from larger hosts than males. Host size was important in a relative sense; the mean host sizes of females versus males, and of solitary versus gregarious parasitoids varied with the available host size distribution. The offspring sex ratio of M. stanleyi reflected the available host size distribution; the sex ratio of emerging parasitoids varied with the available host size distribution. We did not detect a "critical host size" below which males emerged, and above which females emerged; rather, only females emerged from hosts in the upper size range, and a variable ratio of males and females emerged from hosts in the lower size range. We conclude that the sex ratio of field populations of M. stanleyi is driven largely by the available size distribution of C. hesperidum. In addition, we tested predictions resulting from theoretical analyses of sex allocation in autoparasitoids with data obtained on Coccophagus semicircularis (Förster) parasitizing brown soft scale in the field. The sex ratio of C. semicircularis was consistently and strongly female biased (ca. 90% females). Based on available theoretical analyses, we suggest that this sex ratio pattern may have resulted from a very low encounter rate of secondary hosts coupled with a strong time limitation in C. semicircularis females. This explanation was the most plausible given constraints stemming from the detection of secondary hosts, their variable location within primary hosts, and their handling times. Finally, the size of hosts which yielded single versus multiple parasitoids, and the sizes of these parasitoids, were compared. These comparisons suggested that: (1) M. stanleyi females gauge host sizes precisely, and in terms of female offspring; thus a fitness penalty is not incurred by females which share a host, while males benefit from sharing a host, and; (2) instances where multiple C. semicircularis emerged from a single host were probably the result of parasitism by different females, or during different encounters by a single female.

Bird predation as a sex‐ and size‐selective agent of the arboreal spider Pityohyphantes phrygianus

1. The solitary sheetweb spider Pityohyphantes phrygianus (Araneae: Linyphiidae) lives in canopies of coniferous trees. Earlier studies in Scandinavia have shown that bird predation affects the spider abundance negatively. 2. In a large‐scale, 2‐year field experiment, passerine birds were excluded from foraging on spruce branches. The experimental branches were net‐enclosed, which prevented bird predation but left spiders free to leave and enter. Samples were taken in the autumn and spring for 2 years. 3. Removal of bird predation increased the mean number of subadult P. phrygianus per branch between 2·1 and 10·6 times the control number for each sex. Bird predation had approximately similar effects on the sexes, except in the second experimental spring when males were affected twice as much as females, and there was a significantly lower proportion of males on control branches exposed to bird predation than on net‐enclosed branches. 4. The mean size of females was larger on net‐enclosed branches than on controls in both spring samples. In males, this was the case in the first spring sample only. 5. This study shows that there are also mortality costs for both sexes associated with large size in the non‐mating season. In certain winters, however, there is an additional mortality risk to males, presumably caused by a higher activity level than in females.

Reproductive biology of the antipatharian black coral Antipathes fiordensis in Doubtful Sound, Fiordland, New Zealand

The reproductive biology of Antipathes fiordensis Grange, a species endemic to south-western New Zealand, was followed from April 1994 to May 1995. Ten colonies were individually tagged in Doubtful Sound and sampled on a monthly basis in order to determine their reproductive activity. The fecundity of each of the five tagged female colonies was determined by estimating the total number of polyps per colony from photographs of each colony and by planimetry, the proportion of gravid polyps per colony, and the mean number of oocytes per gravid polyp. In addition, 56 colonies were sampled in March 1995 to estimate the sex ratio, height at sexual maturity, and mean sizes of females and males. A. fiordensis was found to be a dioecious species which, in the absence of gonads in the polyps, has no obvious external morphological differences between the sexes. Broadcast spawning of gametes is the likely mode of reproduction. Gametogenesis began in November 1994 and was highly synchronous within and between colonies, with spawning occurring in March 1995. The sex ratio in adults was 1:1. Colonies reached sexual maturity between the heights of 70 and 105 cm which, based on existing estimates of growth rate, corresponds to a minimum age for sexual maturity of about 31 yr. The largest oocytes measured ranged from 100 to 140 μm in size. Female colonies produced between 1.3 and 16.9 million oocytes, with the larger colonies dominating the reproductive output of the population.

Response of Mysis relicta population dynamics and productivity to spatial and seasonal gradients in Lake Ontario

Spatial and temporal patterns of production, density, reproductive status, and size composition of Mysis relicta in Lake Ontario were analyzed in relation to predation and physical conditions. Two whole-lake (May, November) surveys and one partial (August) survey were conducted in 1990. Spatial patterns were related to lake depth. Abundance increased with depth of sampling stations, particularly between 20 and 100 m. At station depths >100 m, the percentage of females carrying eggs and mean female size were constant, but the number of eggs per female increased with station depth. During the winter, the density–depth relationship weakened, apparently through the action of storms and currents, but strengthened again during the summer. Mysids >9 mm in length tended to move to deeper water during the summer. The distributions of predatory fish (alewife, smelt, and sculpin) suggest that depth-related patterns in the mysid population may be due to differences in predation rates with depth and season as well as to horizontal movement. Whole-lake production was estimated to be between 412.91 and 506.82 × 108 g dry weight: most production occurred between 100 and 200 m depth. Growth rate was similar in all regions of the lake.

Do we need harvest regulations for European crayfish?

Management of noble crayfish fisheries varies considerably between countries. Minimum legal harvest sizes range from 70 to 120 mm TL. National, regional and local crayfish harvesting regulations exist. The variations in regulations are strongly influenced by traditions for crayfish harvest and consumption. The current harvest of noble crayfish in Europe is approximately 220 tonnes; this is less than 10% of the pre-plague historic record. Improved fishery regulations may potentially increase the annual harvest. Because of allometric growth, noble crayfish increase by some 40–50% in weight by increasing in total length from 90 to 100 mm. Stock structure and stock characteristics vary between localities. Female noble crayfish mature at a size of 62–85 mm TL and the number of attached eggs is low, i.e. less than 200. Mean size of females in trap catches may be lower than mean size at maturity, indicating vulnerability to recruitment overfishing. There is a market for crayfish smaller than 90 mm TL, both for consumption and for stocking, and these often constitute more than 75% of trap catches. Regulations for catching crayfish are thus needed in order to prevent recruitment overfishing and to sustain high and stable yields. The authors therefore recommend a national minimum harvest size of 90–95 mm TL. The catching season should start after the first moult and after the brood have left their mother, and terminate when the mating period approaches in September. In localities with high exploitation a short harvest season (2–3 weeks) is recommended in the early August intermoult period. However, crayfish harvest regulations should also account for the variation that occurs between local stocks, and if necessary exemptions from the national regulations should be given where appropriate.

Reproduction ofAnnachlamys flabellata, Comptopallium radulaandMimachlamys gloriosa(Mollusca: Pectinidae) in the south-west lagoon of New Caledonia

In the south-west lagoon of New Caledonia, the first sexual maturity size is 45 mm for Annachlamys flabellata (Bernadi), 60 mm for Comptopallium radula (L.) and 50 mm for Mimachlamys gloriosa (Reeve). The global sex-ratio was not significantly different from 1:1 for C. radula unlike A. flabellata and M. gloriosa (1.3:1 for two species). The significantly greater mean size of females suggests a protandric sex reversal. The reproductive cycle was studied monthly for two years for M. gloriosa and one year for C. radula and A. flabellata, then weekly over periods of 12 months for M. gloriosa and 6 months for C. radula. Reproduction of the three species occurs throughout the year with a maximal activity during the warm season. M. gloriosa and C. radula are capable of spawning several times a month. Fluctuations of reproductive intensity are related to temperature and salinity fluctuations.

Sexual dimorphism inAnastrepha suspensa (Loew) and other tephritid fruit flies (Diptera: Tephritidae): Possible roles of developmental rate, fecundity, and dispersal

Larger male Caribbean fruit flies are more likely to be chosen as mates and defeat rivals in territorial contests. Yet males are smaller than females. Adaptive explanations for relatively small male size include (1) acceleration of male development to maximize female encounter rates, (2) selection for greater female size to increase fecundity, and (3) selection for body sizes most suitable for sexually dimorphic degrees of mobility, speed, and distance flight. None of these unambiguously accounts for the degree of sexual dimorphism. Male development is not accelerated relative to that of females. On average, males remain inside fruit longer than females and those males with extended development periods are smaller than more rapidly developing individuals. There is no evidence that female enlargement alone, presumably for greater fecundity, has generated the degree of dimorphism in the Caribbean fruit fly or other fruit flies. The relationship between dimorphism and mean female body size in 27 species of Tephritidae is the opposite of what would be predicted if differences in dimorphism were due to differences in unilateral female enlargement. Larger size in a species or in one sex of a species may be an adaptation for extensive flight. In general, among 32 species of fruit flies, as body size increases, wing shape becomes progressively more suited for distance flight. However, there are important exceptions to this correlation. Both sexual selection and nonadaptive allometries may contribute to the range of dimorphisms within the family.

Biometry of brooding in seven species of amphipod (Crustacea) from the Clyde Sea area

Brood size was found to be correlated linearly with female body length in all species studied, except Ampelisca macrocephala, which is usually a single brooded species. Of the six multi-brooded species, Lembos websteri, Stegocephaloides christianiensis, Echinogammarus pirloti and Gammaropsis nitida produced small numbers of large eggs in the winter and large numbers of small eggs in the summer. In Bathyporeia pilosa larger numbers of smaller eggs were produced by the overwintering generation. In Corophium bonnellii, both maximum and minimum brood sizes were produced by the summer generation. The largest broods relative to female body size were produced by the smaller species. In L. websteri, C. bonnellii and B. pilosa mean female size was smaller in the summer than the winter generation.

Variation within and between Rivers in Adult Size and Sea Age at Maturity of Anadromous Brown Trout, Salmo trutta

I compared adult size and sea age at sexual maturity among nine populations of anadromous brown trout, Salmo trutta, in two Norwegian rivers to determine the extent of inter- and intrariverine variations. Variation in the mean length of spawners and in the mean sea age at sexual maturity were mainly dependent on the variations found within rather than between rivers. Mean lengths and mean age at maturity of males increased significantly with increasing altitude of the spawning area and with migration distance in freshwater. In females, positive significant correlations were found with mean lengths and altitude of the spawning area and with mean sea age at maturity and both spawning site altitude and migration distance. Mean lengths and ages of males and females were not significantly correlated with the rate of water discharge in the streams during spawning. The size of gravel substrate for spawning was of minor importance in explaining interpopulation variation in mean female size. The increase noted in mean length and in mean sea age at maturity of both males and females is probably an adaptation to greater energy expenditure to reach the uppermost natal spawning areas.

Effects of a sudden increase in natural mortality of adults on a population of the common snapping turtle (Chelydra serpentina)

A northern population of snapping turtles (Chelydra serpentina) centred around Lake Sasajewun in the Wildlife Research Area in Algonquin Park, Ontario, has been studied and individually marked since 1972. From 1972 to 1985, annual mortality and survivorship of adult females had been estimated at 1 and 96.6%, respectively, and only six dead turtles were found. Lake Sasajewun's population of C. serpentina was estimated in 1978–1979 and 1984–1985 at 38 and 47 adults, respectively. From 1976 to 1987, total number of nests found in the study area remained fairly constant and there were no significant changes in mean clutch size, mean clutch mass, or mean egg mass. On the main nest site, recruitment from 1976 to 1987 was 1.15 (1.8%) new females per year. From 1987 to 1989, we found 34 dead adult snapping turtles in the Wildlife Research Area. Observations of freshly dead animals indicated that most were killed by otters (Lutra canadensis) during the turtles' winter hibernation. A few uninjured turtles also died of septicemia in early spring shortly after emerging from hibernation. The estimated number of adults in Lake Sasajewun was 31 in 1988–1989, and the minimum number of adult residents known to be alive in the lake dropped from 47 in 1986 to 16 in 1989. In 1986 and 1987, annual adult female survivorship was estimated at 80 and 55%, respectively, and estimated numbers of nesting females declined from 82 in 1986 to 71 and 55 in 1987 and 1988, respectively. The actual number of nests found declined by 38 and 20% over the same periods. Although no significant differences occurred in mean egg mass or mean clutch size between 1987 and 1989 and earlier years, the mean clutch mass in 1988 was larger than in 1977 or 1978. This difference appeared to be due to a gradual increase in the mean age and body size of breeding females rather than to density-dependent changes. Recruitment into the adult breeding female population in 1987–1989 remained less than two individuals per year. Hatchling survival and number of juveniles were low throughout the study. Our observations support the view that populations of species with high, stochastic juvenile mortality and long adult life spans may be decimated quickly by increased mortality of adult animals, particularly if numbers of juveniles and immigrants are low. Recovery of such populations should be very slow because of a lack of effective density-dependent response in reproduction and recruitment.

Components of Density-Dependent Population Dynamics: Models and Tests with Drosophila

This study has tested a model of density-dependent population dynamics in food-limited environments. Parameters of the model were estimated using standard regression techniques and observations on viability and female size in eight populations of Drosophila melanogaster. The model was tested by comparing its predictions with observations of female size that had not been utilized in the estimation process. In general there was good agreement between observations and predictions of the mean size of females supplied with differing amounts of food. There were substantial deviations from the predicted and observed variance of female size. This difference may be attributed to certain simplifying assumptions of the model and factors other than food that contribute to mortality in these experimental systems. These results point to additional phenomena, such as tolerance to waste products, which may be important in the density-dependent population dynamics of Drosophila and should be incorporated in models describing this process.

Sex‐Specific Life History Tactics and Risk‐Taking in Coho Salmon

Differences in size and abundance between males and female at sexual maturity are common in vertebrates. Evolutionary explanations for these differences generally assume a consistent pattern within a species. Among Pacific salmon (Oncorhynchus spp.) and coho salmon (O. kisutch) in particular, however, sexual size dimorphism and sex ratio are not consistent among or within populations, making extant explanations unsatisfactory. In this paper we describe the variation size dimorphism and sex ratio within and among coho salmon populations and propose a model to account for this variation. Our data consist of a 16—yr study of coho salmon in carnation Creek, British Columbia, and shorter term studies of 30 other coho populations ranging from California to Alaska. Among mature coho salmon in Carnation Creek males outnumbered females but females were larger, and the male/female sex ratio varied directly with the female/male size ratio. Mean female size was constant over 16 yr, while male size varied inversely with the male/female sex ratio. Females apparently had higher mortality rates than males, and the difference in mortality rates appeared to increase as the animals neared maturity. Sex— and size—ratio data from other coho salmon populations indicate that there are two general population types. In one type males outnumber females, and females are larger at maturity. In the other type the sexes are equally abundant and equal in size. We propose that the range of size for successful reproduction in coho salmon is set by conditions in the breeding environment, and that the range differs for males and females because of the different roles that the sexes play in reproduction. The sex—specific constraints on reproductive size lead to sex—specific foraging strategies and risk—taking. Interpopulation differences in size and sex ratio are determined by differences in physical characteristics of the breeding environment, spawning population density, and the relative riskiness of ocean nursery areas. Within populations, variation is determined by the interaction of sex—specific foraging strategies with interannual variation in ocean feeding conditions and predator abundance.

Co-existence within a group of congeneric species of Acartia (Copepoda Calanoida): sexual dimorphism and ecological niche in Acartia grani

Components of dimorphic pairs may differ in several important aspects and they may develop divergent strategies in response to environmental factors. To test this theory, the coexistence within a group of congeneric species of Acartia was analyzed in the context of the niche theory. Sexual size dimorphism was characterized and expressed as the ratio of mean female size to mean male size. The dimorphic and seasonally present species A.grani showed some features typical of species which inhabit temporary waters. The sex-size ratio varied little over the year but, in general, the observed temporal variation of the ratio values is similar to those of abundance of both males and females. Annual variation in adult body size showed a direct relationship with both temperature and seawater chlorophyll concentrations, but the covariance between these environmental variables was important. The fit was improved when body size was correlated with environmental data recorded 15 or 30 days before the sampling of the copepod populations to be measured. The degree of sexual size dimorphism shown by this species is, perhaps, a dynamic compromise between a survival strategy which seeks to maximize the range of the different classes of variables occupied by each sex and the reciprocal effect of slight interspecies competition tending to reduce the sexual size difference. In this context, the analysis of the ecological niches showed some differential interactions of males and females of A.grani with the other congeneric species. The females were more generalist with wider niches and higher probabilities of interspecies encounters than the males. However, the results suggest that sexual dimorphism does not confer an overall increase in niche breadth for A.grani from the joint activities of males and females in comparison with the female niche breadth.

Growth and Survivorship in a California Population of Gerrhonotus coeruleus, with Comments on Intraspecific Variation in Adult Female Size

Gerrhonotus coeruleus is a viviparous lizard that ranges into high latitudes and altitudes in western North America. Females in a population on the N coast of California (Mendocino Co.) reach sexual maturity in 32 months and produce a single litter each season. Relatively low clutch sizes (X = 3.8 i 0.2) contribute to low fecundity. Annual survivorship was approximately 73 % for adults and 54 % for juvenile lizards. Female survivorship exceeded male survivorship during 1974 and mean female size (87.7 i 1.0 mm) was significantly larger than mean male size (83.6 i 1.2 mm) the following year. Growth of females in a second coastal population (Monterey Co.) was more rapid and these lizards were larger at sexual maturity than Mendocino County females. Females in a montane population (El Dorado Co., Calif.) reached reproductive maturity when larger and older (44 months) than females in either coastal population although they attain a size in 32 months equal to Mendocino County females at the same age. Litter size, newborn size and relative clutch mass were significantly higher in the El Dorado County population. The evolution of delayed maturity and larger adult size in this species may result from selection for both larger litters and larger young. INTRODUCTION Viviparous reproductive habits superficially suggest such a fundamentally different pattern from egg-laying modes of reproduction that dramatic differences in life history components might be expected. The existence of a pattern of life history traits associated with reproductive mode and the relative position of viviparous species within this general pattern were first proposed for lizards by Tinkle (1969) and Tinkle et al. (1970). Life history traits predicted to be associated with viviparity include later maturity, a single litter per reproductive season, larger eggs, greater life expectancy and reduced reproductive risk. Studies of viviparous lizards (Alcala, 1966; Zweifel and Lowe, 1966; Bustard, 1970; Ballinger, 1973, 1979; Lee, 1975; Pilorge and Barbault, 1981) reveal that these species do not all share a unique complex of life history characteristics. Viviparity occurs commonly within squamate reptiles and the diversity of lineages with viviparous species is evidence for the polyphyletic evolution of this reproductive mode from oviparous species (Packard et al., 1977; Tinkle and Gibbons, 1977; Shine and Bull, 1979; Blackburn, 1982, in press). A variety of selective advantages of viviparity have been suggested (Fitch, 1935; Weekes, 1935; Neill, 1964; Packard, 1966; Goin and Goin, 1971; Packard et al., 1977; Tinkle and Gibbons, 1977; Shine and Bull, 1979). Comparisons among viviparous species representing diverse genera should be useful for the identification of general features associated with viviparity, but an intrinsic risk to this method of analysis is that it may be insensitive to the life history attributes that favor the evolution of viviparity in a particular lineage. If the selective advantages that culminate in the acquisition of viviparity are as diverse as the number of ecological benefits predicted for viviparous reptiles, comparisons with oviparous congeners will be most useful in characterizing the distinctive attributes and evolutionary trends within each lineage (Ballinger, 1973; Guillette et al., 1980; Guillette, 1981). Twenty-four genera of lizards are known that contain both oviparous and viviparous species (Blackburn, 1982, in press). Unfortunately, life history studies of

The apparent rôle of temperature in breeding initiation and winter population structure in Hyale nilssoni Rathke (Amphipoda): Field observations 1972–1983

Data are presented on the population structure of Hyale nilssoni Rathke (an amphipod inhabitant of high littoral seaweeds) over the period of breeding initiation (late February) of 11 yr. By sampling, as near as possible, the same date annually, photoperiodic effects are controlled. The percentage of ovigerous females on this date was significantly correlated with post-winter solstice mean sea temperature. February sex ratios (M : F) did not correlate with either preceding August or September mean sea temperature such as would have been expected was sex determination temperature sensitive. Sex ratio, however, was significantly positively related to mean winter sea temperatures, with warmer winters having male-dominated Hyale populations in late February. Behavioural hypotheses involving amphipod migration and/or size specific elimination of larger males (c.f. females) during harsher winters are offered in explanation. Population size-frequency distributions over 11 yr were remarkably consistent. Male size was more heterogeneous between years than female size. Mean male size (late February) proved to be significantly negatively correlated with mean winter sea temperatures. Mean female size showed no significant relationship. Given that large males are necessary for precopulatory carrying behaviour it is important that, as shown, they are available to initiate spring breeding, even after harsh winters.

Population Ecology of the Lizard Japalura swinhonis formosensis (Sauria: Agamidae) in Taiwan

The population ecology ofJapalura swinhonisformosensis in Taiwan was studied by capture-recapture method from February 1979 to April 1980. The characteristic growth rate of the male lizards is 0.005 mm per day. Male hatchlings of early clutches require 142 days to reach their mature snout-vent length (SVL), those of late clutches 262 days. The characteristic growth rate of the female lizards is 0.004 mm per day. Female hatchlings require 347 days to reach adult size. Growth rates after the third year for both sexes practically cease. Mean home range size of males is 33.47 m2 and is not significantly different between the breeding and non-breeding season. Few home ranges of males overlap. Only large adult males over 73 mm SVL establish territories. Male lizards appear to carry out rout defense concurrently with normal feeding as well as specific site defense. Mean home range size of females is 11.46 m2. Most of the home ranges of females are entirely or partially within those of males. The mating system appears to be polygynous. Monthly population size and survivorship of adult lizards are fairly stable. Mean population sizes of adult males and females are 210.3 and 224.7 lizards per hectare, respectively. Survivorship of males and females is 82.3% and 82.5%, respectively. On the average, second year adult males make up 48.4% of the total male individuals, the third year adult males, 39.0%, and fourth year adult males, 4.1%. Second year adult females make up 28.8% of the total female individuals, third year adult females, 47.1%, and fourth year adult females, 13.4%. Sex ratios indicate that males predominate over females in the first two years, and the opposite is true for third and fourth year individuals. Males and females utilize the habitat horizontally but at different heights; the same is true of the different SVL classes of males. This habitat partitioning results not so much from specialization for food resources, as from the social interactions among males. Perch diameter does not appear to play any significant ecological role.

Braod Habits of Two Subspecies of a Freshwater Shrimp, Paratya compressa (Decapoda, Atyidae) and their Geographical Variations

The concept of'brood habit'recently proposed has been introduced in the examination of two subspecies of Paratya compressa. The pattern of brood habit within a population in compressa shows that egg size, relative brood size (RBS) and relative brood weight (RBW) remain almost similar regardless of body size, whereas in a population of improvisa egg size and RBW slightly increase with body size and RBS remains constant.The remarkable feature in brood habit among populations of the subspecies compressa is the presence of two distinct population groups that exhibit a varying series separately for mean egg size, mean RBS and mean brood size in relation to mean female body size, and the subspecies improvisa is included at the remote end of one series. The validity of these subspecies is doubted, by taking into account their distributional and morphological features as well, and two forms, improvisa-compressa complex (large-egg form) and the part of compressa (small-egg form) are shown recognizable. A geographical trend is noticeable in the variation of brood habit for each form, that can be considered as related to the adaptive strategy of increasing egg and body size near the border of species distribution range.

POPULATION STRUCTURE OF MALAYSIAN PRAWNS, Macrobrachium rosenbergii (DE MAN), REARED IN EARTHEN PONDS IN SOUTH CAROLINA, 1974–19761

AbstractPopulation structures of Malaysian prawns, Macrobrachium rosenbergii, reared in earthen ponds in South Carolina are presented and various pond management strategies discussed. Sex ratios were generally biased toward females and overall, the total biomass of females was 20.6% greater than the total biomass of males. Mean sizes of males and females at harvest were usually similar (within one gram); however, their respective population structures showed distinct and characteristic differences. In general, females exhibited unimodal approximately normal distributions over a relatively narrow size range whereas the male populations exhibited markedly skewed, bi‐ or multimodal distributions over a comparatively broad size range.The size distributions of prawns produced in the 15 grow‐out studies were influenced by prawn size at stocking, stocking density, and duration of grow‐out (4 to 6 months). Of these, initial prawn size was clearly the most important factor controlling size distribution. Stocking juveniles resulted in shifting the prawn biomass at harvest into the larger size classes and the lower the stocking density or longer the rearing period, the more pronounced was this shift in biomass distribution. In contrast, regardless of density (8.4–20.1 m‐2) and grow‐out time (137–181 days), the stocking of postlarvae resulted in 46 to 96% of the harvested biomass being contained in the small (<15.0 g), less valuable size category.Although stocking juveniles results in a more valued crop, the cost of producing these juveniles may offset much of this economic gain. Our data suggest that an alternative stocking strategy which consists of stocking a mixture of postlarvae and juveniles may in fact result in a greater net return. Pilot‐scale demonstrations coupled with economic analyses are needed to test this pond stocking strategy.

The effects of population density and food surface area on body weight of Acheta domesticus (L.) (Orthoptera: Gryllidae)

Experiments were performed to determine the effects of population density and food surface area on individual size (wet weight) of the house cricket, Acheta domesticus (L.). Three initial population densities (50, 200, and 400 hoppers per microcosm) were used to study the effects of population density, three food surface areas (one, two, and three petri dishes, 9.0 cm in diameter) were used to assess the effects of food surface area, and each treatment was replicated with three populations. Mortality increased and individual weight (at 53 days) decreased with increasing density. Both percentage survivorship and mean body weight increased with increasing food surface area. In both sets of experiments the mean size of emerging adults (0–24 h) decreased with increasing nymphal development time, and nymphal development time increased with increasing density and decreasing food surface area. On a daily comparison the mean size of emerging adults of both sexes decreased with increasing density, and the mean size of females increased with decreasing food surface area.