Modification of the response to high density conditions in the guppy, Poecilia reticulata (Peters)

An examination was made of some of the factors which influence the response to density in the guppy. Water from high density fish was given to fish populations kept at high and low densities. This induced high density behaviour in low density fish by increasing aggression and decreasing courtship. But giving 'fresh' water to high density fish failed to alleviate the response to density. 'High density water' slightly reduced the numbers of young found in low density groups. High density fish kept in the dark had more young than high density groups in the light; and the number of young found was decreased when 'high density water' was given to low density fish. The number of ovarian stages was decreased in the dark in high density and was not overridden by the type of water. Giving 'high density water' to low density fish decreased the number of oocytes stage III, but the interaction between water and 'light' was confounded by the total numbers of ovarian stages. Increasing the 'visual space' of high density fish with mirrors partially decreased aggression and increased courtship and number of young found; but had little affect upon the number of ovarian stages. Decreasing the number of physical contacts by keeping the fish chronically tranquillized reduced the activity of the fish. This reduced the aggression of the high density fish and also reduced the differences between the high and low density fish on the basis of their courtship scores, number of young found and number of stages in the ovary. Fish from differing population sizes were given a choice between varying fish densities. The fish from a 'normal' sized population and with an extended visual field, tended to make a choice conforming to the overall selections compared with fish from populations of extreme size and with a reduced visual field.     

A quantitative analysis of social interaction of the guppy, Poecilia reticulata (pisces: Poeciliidae) as a function of population density

Various controlled densities of guppies, Poecilia reticulata, wer observed in order to determine the effects of population density on frequency of courtship and agonistic interactions. Populations with single male exhibit a low level of courtship activity; the presence of a second male increases this level of activity markedly. Populations with a 1:1 sex ratio in densities greater than one pair exhibit a constant mean number of courtship interactions but, with increased crowding, an increasing variance in level of courtship activity. This results in a frequency profile of courtship interaction characteristic of each observed density. Each population density exhibits a density-dependent pattern of social interaction defined quantitatively by frequencies of courtship aggressive interactions at the population level. The contribution of the behaviour of individual males and to this patterning of social interaction in each population was partially determined.     

Density-dependent refuge use among over-wintering wild Atlantic salmon juveniles

Sheltering behaviour of wild juvenile Atlantic salmon in an indoor stream was found to be density dependent; the proportion of fish sheltering decreased significantly with increasing population density. The mean number of fish occupying refugia was ≤1.5 fish per refuge even at very high densities (potentially 5 fish per shelter). These results suggest that shelter availability has potentially important consequences for the carrying capacity for natural populations of salmon in streams.     

Phenotypic plasticity in sex allocation and body size leads to trade-offs between male function and growth in a simultaneously hermaphroditic fish

Phenotypic plasticity in sex allocation enables organisms to maximize reproductive success in variable environments, and thus may generate different sex allocation patterns among populations that experience different mating opportunities. In this experiment, I test whether sex allocation is phenotypically plastic in Serranus tortugarum, a simultaneously hermaphroditic fish, by using reciprocal transplants among four reef study sites with populations at high and low densities and significant differences in sex allocation. Fish transplanted across different densities were predicted to alter sex allocation and body size through trade-offs in investments to somatic growth and male and/or female reproduction. As a control for effects of transplanting, I also transplanted fish across study sites with the same densities and marked and returned fish to their original study sites. As predicted, sex allocation and body size shifted significantly for fish transplanted across different densities but not for those transplanted across the same densities. Separate analyses revealed that the treatment effect on sex allocation was driven strongly by a reduction in male investment by fish transplanted from high to low density, and this reduction in male investment was accompanied by an increase in body size. Fish transplanted from low to high density did not appear to change either male or female investments, but they were smaller than transplants from low to low density. A trade-off between male and female function was not evident, but phenotypic plasticity in body size suggested a trade-off between growth and male function when sex allocation is adjusted. Large-scale empirical tests of sex allocation in the field are relatively rare, and the results of this experiment give novel insights into how animals respond to a change in mating opportunities under natural conditions. The effects of logistical problems associated with fieldwork, such as mortality of experimental animals, are considered in the discussion.     

Regulation of local populations of a coral reef fish via joint effects of density- and number-dependent mortality

Density-dependent mortality can regulate local populations - effectively minimizing the likelihood of local extinctions and unchecked population growth. It is considered particularly important for many marine reef organisms with demographically open populations that lack potential regulatory mechanisms tied to local reproduction. While density-dependent mortality has been documented frequently for reef fishes, few studies have explored how the strength of density-dependence varies with density, or how density-dependence may be modified by numerical effects (i.e., number-dependent mortality). Both issues can have profound effects on spatial patterns of abundance and the regulation of local populations. I address these issues through empirical studies in Moorea, French Polynesia, of the six bar wrasse (Thalassoma hardwicke), a reef fish that settles to isolated patch reefs. Per capita mortality rates of newly settled wrasse increased as a function of density and were well approximated by the Beverton-Holt function for both naturally formed and experimentally generated juvenile cohorts. Average instantaneous mortality rates were a decelerating function of initial densities, indicating the per capita strength of density-dependence decreased with density. Results of a factorial manipulation of density and group size indicate that per capita mortality rates were simultaneously density- and number-dependent; fish at higher densities and/or in groups had higher probabilities of disappearing from patch reefs compared with fish that were solitary and/or at lower densities. Mortality rates were ~30% higher for fish at densities of 0.5 fish/m² than at 0.25 fish/m². Similarly, mortality rates increased by ~45% when group size was increased from 1 to 2 individuals per patch, even when density was kept constant. These observations suggest that the number of interacting individuals, independent of patch size (i.e., density-independent effects) can contribute to regulation of local populations. Overall, this work highlights a greater need to consider numerical effects in addition to density effects when exploring sources of population regulation.     

Observations on agonistic behaviour of Clarias gariepinus larvae and juveniles under different densities and feeding frequencies in a controlled environment

Ten-day-old African catfish larvae, Clarias gariepinus , were stocked in aquaria at 5, 13, 22, and 30 larvae per liter with fish at each density being fed three and six times per day over 35 days. Fish were fed with an artificial diet in excess of their satiation requirements at each feeding. Every week total length was determined for samples of 25 fish per tank and all fish were counted at weekly intervals. Behaviour patterns were categorized into timed behaviour (i.e. resting, swimming and browsing), and frequency events e.g. number of contacts, aggressive contacts and number of head or body attacks. Fish at the lowest density grew faster than fish at the other densities. Feeding frequency had no effect on growth. With increasing densities fish less frequently changed timed behaviour patterns while spending a greater percentage of the time swimming. Aggressive behaviour was highest at the lowest density but remained uninfluenced at densities greater than five fish per litre. The high feeding frequency provoked a higher rate of aggressive contacts than the low feeding frequency. A suggested explanation was that these fish were more often in anticipation of food and consequently more active between feedings. In summary, it was shown that density had an effect on duration behaviour patterns such as resting and swimming, while feeding frequency significantly influenced frequency events, in particular the rate of aggressive behaviour. The study contributes to an understanding of factors influencing C. gariepinus juvenile behaviour and suggestions for follow-up experiments are discussed.     

Involvement of different mesotocin (oxytocin homologue) populations in sexual and aggressive behaviours of the brown anole

The oxytocin (OT) family of neuropeptides are known to modulate social behaviours and anxiety in mammals and birds. We investigated cell numbers and neural activity, assessed as Fos induction, within magnocellular and parvocellular populations of neurons producing the OT homologue mesotocin (MT, Ile⁸-oxytocin). This was conducted within the male brown anole lizard, Anolis sagrei, following agonistic or courtship encounters with a conspecific. Both neurons colocalizing and not colocalizing corticotropin-releasing factor (CRF) were examined. Parvocellular neurons of the paraventricular nucleus exhibited a positive correlation between courtship frequency and Fos colocalization, regardless of whether they produce just MT or MT + CRF. Magnocellular populations showed only trends towards positive relationships with courtship and no cell populations showed aggression-related Fos induction. These findings are novel because they demonstrate the involvement of MT neurons in male social behaviour, especially in reptiles for whom the involvement of MT in social behaviour was previously unknown.     

The establishment of a ‘normal’ population and its behavioural maintenance in the guppy—Poecilia reticulata (Peters)*

Populations of the guppy—Poecilia reticulata (Peters)—were established with a single male and female fish per compartment. In spite of excess food, regular water changes, constant temperature and continuous aeration, the populations eventually reached a stable asymptote which was proportional to the size of the compartment, particularly when the populations were visually isolated from other populations. A weight asymptote was achieved some time after the population plateau. There was a tendency for females to exceed the numbers of males at the asymptote. Fish from the asymptotes were rearranged to relatively increase or decrease the populations, on the expectation that the populations would revert to the original asymptote. This did not happen. It was hypothesized that there was a learned behavioural response which prevented reproduction in the altered populations, but not an ovarian ‘carry-over’ for the ovaries of decreased and increased populations were distinguishable. The learning hypothesis was substantiated in fish of a single generation which were crowded, then uncrowded, and continued to show reduced courtship and increased aggressive behaviour.     

The effects of early and late androgen treatments on the behavior of Sarotherodon mossambicus (Pisces: Cichlidae)

Groups of Sarotherodon mossambicus were treated with androgen by immersion or oral treatment at various stages of development. Fish were allowed to mature and the effects of treatment on gonadal and behavioral differentiation were examined. The effects of treatment on gonadal differentiation were assessed by determining the sex ratio for each group. Three treatments resulted in sex ratios significantly different from the 1:1 sex ratio obtained in untreated fish. Behavioral differences were detected between groups of males in three measures of territorial defense and aggression, and differences were detected between groups of females in two measures of male-female courtship interaction. A second experiment determined that early-treated females were more sensitive to a second androgen treatment later in life than females not exposed to androgen during development. A number of sex-reversed genetic females functioning as males were detected in two treatment groups with predominantly male sex ratios. There were no differences in the behavior of sex-reversed and non-sex-reversed male fish from the same treatment group. This study establishes that hormone treatments administered during development influence behavioral differentiation in a teleost.     

用浅水湖泊型鱼探仪估算东湖鱼群数量

用浅水湖泊型鱼探仪估算了武汉东湖中的鱼类个体数。鱼探仪的探头可以横向或垂直放置,水平探鱼时得到的是从湖面到1.5米深的个体鱼探图像;垂直探鱼时得到的是从1.5米到水底的个体鱼探图像。按半减半角3°计算了鱼探仪探索的水团中鱼的密度。根据每100米距离的个体鱼探图像计数值的时间系列变化以及鱼类密度的两向方差分析,发现鱼类的栖息密度随湖区的不同而异。根据各湖区鱼类当时的“现存”尾数,估计东湖郭郑湖区(11平方公里)1983年10月“现存”的鱼数为44万尾,1984年3月为112万尾。     

Density dependent recruitment in the reef fish Chaetodon miliaris

Synopsis Recruitment of reef fish has generally been found to be unrelated to or positively related to adult densities. This paper reports an inverse relationship between the number of recruits of Chaetodon miliaris and the density of conspecific adults. C. miliaris are non-territorial, planktivorous butterflyfish. The study populations occurred on the flanks of patch reefs of about 30 m in diameter. At their peak densities populations consisted of 300–400 C. miliaris. Recruitment (the appearance in the stock of fish less than 3.5cm in total length) occurred primarily from April to June and corresponded to the new moon period in these months. Populations generally showed a steady decline in numbers during months of the year when recruitment was not occurring: the finite rate of decline was approximately 30% per month. Populations varied in the density of adults at the start of the spring-summer period of recruitment, and the magnitude of recruitment to a reef was inversely related to the density of these conspecific adults. Reduction of the densities of adult C. miliaris through trap fishing, resulted in increased recruitment to the fished stock. These results provide evidence that in some. species of reef fish, benthic processes may play an important role in determining the magnitude of recruitment to an adult stock, in contrast to the widely held view that recruitment is in large part a result of chance events in the plankton or that recruitment is directly related to adult densities.     

Genetic monogamy despite social promiscuity in the pot-bellied seahorse (Hippocampus abdominalis)

Sexual selection theory predicts a positive correlation between relative parental investment and mate choice. In syngnathid fishes (seahorses and pipefish), males brood offspring in specialized brooding structures. While female-female mating competition has been demonstrated in some pipefishes, all seahorses (genus Hippocampus) studied to date have been found to have conventional sex roles with greater male-male competition for access to mates despite possessing the most complex brood structures in the family. Although multiple mating is common in pipefish, seahorses are again exceptional, exhibiting strict genetic monogamy. Both demographic and behavioural explanations have been offered to explain the lack of multiple mating in seahorse species, but these hypotheses have not yet been explicitly addressed. We investigated mating systems and brood parentage of the pot-bellied seahorse, Hippocampus abdominalis, a temperate-water species that is socially promiscuous with conventional sex roles in laboratory populations. We observed promiscuous courtship behaviour and sex-role reversal in high density, female-biased field populations of H. abdominalis. We hypothesize that sex roles are plastic in H. abdominalis, depending on local population density and sex ratio. Despite promiscuous courtship behaviour, all assayed male seahorses were genetically monogamous in both laboratory and wild populations. Physiological limitations associated with embryo incubation may explain the absence of multiple mating in seahorses and may have played an important role in the development of the unique reproductive behaviour typical in these species.     

Evolution of female remating behaviour following experimental removal of sexual selection

The relatively small number of ova produced by a female can be fertilized by a single ejaculate in most species. Why females of many species mate with multiple males is therefore enigmatic, especially given that costs associated with remating have been well documented. Recently, it has been argued that females may remate at a maladaptive rate as an outcome of sexually antagonistic coevolution: the evolutionary tug-of-war between manipulation by one sex and resistance to being manipulated by the other sex. We tested this hypothesis experimentally for the evolution of the female remating interval in a naturally promiscuous species, Drosophila melanogaster. In two replicate populations, sexual selection was removed through enforced monogamous mating with random mate assignment, or retained in polyandrous controls. Monogamy constrains the reproductive success of mates to be identical, thereby converting prior conflicts between mates into opportunities for mutualism. Under these experimental conditions, the sexually antagonistic coevolution hypothesis generates explicit predictions regarding the direction of evolutionary change in female remating behaviour. These predictions are contingent upon the mechanism of male manipulation, which may be mediated biochemically by seminal fluids or behaviourally by courtship. Levels of divergence in female remating interval across lines, and in male ejaculatory and courtship effects on female remating, were quantified after 84 generations of selection. Data refute the hypothesis that the evolutionary change in female remating behaviour was due to sexually antagonistic coevolution of courtship signal and receiver traits. The data were, however, consistent with a hypothesis of sexual conflict mediated through ejaculate manipulation. Monogamy-line males evolved ejaculates that were less effective in inducing female non-receptivity and monogamy-line females evolved to remate less frequently, symptomatic of lowered resistance to ejaculate manipulation. The consistency of the results with alternative hypotheses to explain female promiscuity are discussed.     

Male courtship preferences demonstrate discrimination against allopatric colour morphs in a cichlid fish

Whether premating isolation is achieved by male‐specific, female‐specific or sex‐independent assortative preferences often depends on the underlying evolutionary processes. Here we test mate preferences of males presented with females of different allopatric colour variants of the cichlid fish Tropheus sp., a Lake Tanganyika endemic with rich geographical colour pattern variation, in which the strength of sexual isolation varies between populations. We conducted two‐way mate choice experiments to compare behaviour of males of a red‐bodied morph (population Moliro) towards females from their own population with behaviour towards females from four allopatric populations at different stages of phylogenetic and phenotypic divergence. Males courted same‐population females significantly more intensely than females of other populations, and reduced their heteromorphic courtship efforts both with increasing genetic and increasing phenotypic distinctness of the females. In particular, females of a closely related red‐bodied population received significantly more courtship than either genetically distinct, similarly coloured females (‘Kirschfleck’ morph) or genetically related, differently coloured females (‘yellow‐blotch’ morph), both of which were courted similarly. Genetically and phenotypically distinct females (Tropheus polli) were not courted at all. Consistent with previous female‐choice experiments, female courtship activity also decreased with increasing genetic distance from the males’ population. Given successful experimental and natural introgression between colour morphs and the pervasive allopatry of related variants, we consider it unlikely that assortative preferences of both sexes were driven by direct selection during periods of secondary contact or, in turn, drove colour pattern differentiation in allopatry. Rather, we suggest that sexual isolation evolved as by‐product of allopatric divergence.     

Population density,resource patterning,and territoriality in the Everglades pygmy sunfish

The means by which pygmy sunfish compete for food are influenced by the density of the population and the dispersion of the prey as well as by the sex and dominance status of the individual. At all densities when the prey were predictably located in a central clump, males established territories. When prey were dispersed randomly in both the high and low density population, males abandoned territorial behaviour and, like females, swam freely about the aquaria. Only in the intermediate density populations did the males maintain territories and continue to defend resources. Development of a simple cost-benefit model shows that male territorial behaviour is governed to a large extent by economic considerations. Despite these overall patterns, differences in competitive strategies were observed within populations. Dominant individuals tended to possess territories nearest the central patch of prey, and use the most intense and physiologically exhausting displays. Only under the most stressful conditions did they acquire significantly more food than subordinates, and even then these benefits were not translated into increased growth, largely because dominant fish engaged in disproportionately more energy-consuming contests.     

Male mating behaviour and sperm production characteristics under varying sperm competition risk in guppies

Since natural populations of guppies, Poecilia reticulata, often differ from one another in social structure, the intensity of sperm competition is likely to vary between localities. Guppies are promiscuous, with female choice for colourful males playing a central role in the mating system. In addition, male guppies use forced copulations to circumvent female choice. Both methods of copulation are used interchangeably by individual males, but the degree to which either is used may depend on the social environment into which males are born. Here we show that male mating behaviour varies according to the rearing sex ratio: when reared in male-biased groups, males performed more forced copulations and fewer courtship displays but showed the opposite pattern of behaviour when reared in female-biased groups. Our prediction, based on sperm competition theory, that stripped sperm number would reflect social structure was not supported by our results. Instead, the overall level of sexual activity (gonopodial thrusts+sigmoid displays) was a better predictor of sperm number in the different groups of males. Rearing density, where sex ratio was controlled, did not significantly affect male mating behaviour or sperm traits. Males reared under the different sex ratios continued to show their characteristic behaviour patterns when placed in equal sex ratio tanks. We conclude, therefore, that males adopt mating strategies to suit their social environment, and that these strategies remain fixed, for short periods at least, if population structure changes. Copyright 1999 The Association for the Study of Animal Behaviour.     

Population density influences male-male competition in guppies

This study tested the general prediction that population density affects male-male competition, female mate choice and the opportunity for sexual selection. By manipulating the density of guppies, Poecilia reticulata, while keeping the sex ratio constant, I found that male mating tactics were phenotypically plastic with respect to density. As density increased, males decreased their courtship displays. Male-male competition and mate searching were highest at intermediate densities. Population density had no significant effect on the total number of copulations, copulatory tactics or the percentage of postcopulatory guarding. Female preference for males with a higher percentage of orange coloration was similar at all density levels. The 'opportunity for sexual selection', which estimates the upper limit to which a selected trait can shift if directional selection is operating and was calculated as the variance in number of copulations per male divided by the square of the mean number of copulations, was negatively associated with population density. This may be due to the decrease in male-male competition at high density rather than female preference which was similar across density treatments. Copyright 1999 The Association for the Study of Animal Behaviour.     

Effects of pollution on reproductive behaviour of fishes

This review attempts to integrate pollution research with behavioural ecology by focusing on reproductive behaviour of fishes. A search of Aquatic Sciences and Fisheries Abstractsand other sources showed that only 0.1% of 19 199 studies of aquatic pollution and fishes during the past 20 years have made this link. Effects on parental care and courtship have been investigated using a variety of pollutants (e.g. acidification, herbicide, thermal effluent) in several fish families (e.g. Cichlidae, Poeciliidae, Gasterosteidae, Cyprinidae).Eleven of the 19 studies found a change in behaviour from the norm. Effects on courtship included decreases or increases in frequency of displays, increased courtship duration, or performance of male-like behaviour by masculinized females. Studies of parental care have found decreased nest-building activity, decreased offspring defence, or changes in division of parental care between the sexes. Few studies have measured reproductive success or extrapolated their results to effects on populations.We develop a framework for exploring links between pollution and behavioural ecology which suggests potential impacts on life history trade-offs in reproduction, genetic changes in populations, and population sizes. Many reproductive behaviours of fish species are readily quantifiable and behaviours such as courtship by male guppies and other members of the Poeciliidae show some promise for pollution monitoring and behavioural toxicity tests. Choice of such assays would have to compete with the sensitivity and practicality of more traditional methods but may serve as useful complements. There is considerable scope for further research into conservation. A synthesis between behavioural ecology and toxicology should thus provide useful insights for both fields     

Variation with reproductive status of PGE₂receptor immunoreactivities in the Bostrichthys sinensis olfactory system

The role of PGE(2) as a putative sex pheromone in Chinese black sleeper Bostrichthys sinensis was investigated, using immunocytochemistry and how the immunoreactivities of the prostaglandin E(2) (PGE(2) ) receptor subtypes EP(1) , EP(2) , EP(3) and EP(4) varied with reproductive status in the olfactory system was determined. The results showed that PGE(2) receptors were present in the whole of the olfactory system of B. sinensis, and that the number of receptors was linked to the reproductive status of the fish. The densities of EP(1) immunoreactivity in the olfactory epithelium of mature fish were significantly (P < 0·01) higher than those in immature fish of both sexes, and the densities of EP(2) and EP(3) immunoreactivities in mature fish were higher (but not significantly) than those in immature fish of both sexes. In the olfactory nerve, the density of EP(2) immunoreactivity in mature fish was higher (but not significantly) than that in immature fish in both sexes. In the olfactory bulb, the densities of EP(1-4) immunoreactivities in mature females were significantly (P < 0·05 or <0·01) higher than those in immature females, and the density of EP(4) immunoreactivity in mature males was significantly (P < 0·01) higher than that in immature males. As far as is known, the present study is the first report of the immunoreactivities of PGE(2) receptor subtypes in the olfactory system of a teleost, and offers new findings regarding the role of PGE(2) as sex pheromone and hormone in the reproductive behaviour and pheromonal communication of B. sinensis.