Genetic isolation of populations of the gammaridean amphipod, Corophium volutator, in the Bay of Fundy, Canada

Selection experiments suggest that evolutionary modifications in amphipod demography can respond to local environmental changes and that local races of amphipods may be common. We tested this hypothesis in mudflat populations of Corophium volutator in the Bay of Fundy, Canada. Due to the unique topography of the Bay of Fundy, distinctive environmental conditions are prevalent in different branches of the Bay, while the impact of shorebird predation has also been shown to vary between populations. Several hypotheses have been suggested to explain ecological evidence which indicates that Corophium volutator (Pallas), a common amphipod crustacean, exhibits extensive life history variation in Bay of Fundy populations. We used RAPD-PCR techniques to examine populations of C. volutator in an investigation of genetic isolation in marine environments. Our data suggest that variation in selection pressures have played a significant role in the genetic divergence of populations of C. volutator in the Bay of Fundy.     

Intense episodic predation by shorebirds may influence life history strategy of an intertidal amphipod

Life history theory suggests that selective predation on older or larger individuals in prey populations or a disturbance such as intense and episodic predation should lead to smaller size at maturity, and a tendency toward semelparity. Many populations of the intertidal amphipod, Corophium volutator , in the western Atlantic are subjected to an intense period of size-selective predation for about one month in summer, during the southward migration of shorebirds. We compared size at maturity and fecundity of populations of C. volutator from mudflats that are intensively used by shorebirds with populations that are visited by very few birds. We found that mature females were of similar size in May, but those from bird mudflats produced more offspring during the first reproductive episode. In July, females of the summer generation began to reproduce at a smaller size on bird mudflats, and as a consequence, produced fewer offspring that grew more slowly. The results of this correlative study suggest that shorebird predation has shaped C. volutator life history in two ways. First, females on bird mudflats concentrate their reproductive effort into a larger early brood, probably because later broods would come to maturity during the period of intense predation. Second, in summer, amphipods begin to reproduce at a smaller size so as to produce a brood before the arrival of the birds in mid-July. Predators are not present at the time that these responses occur, and thus they are probably not a plastic response to perceived predation risk. Thus the patterns we observed agree with the predictions of recent theories: individuals faced with a predictable disturbance alter their life cycles so as to minimize the effect of that disturbance.     

Reproductive synchrony in the intertidal amphipod Corophium volutator

Following logic of the mate-availability hypothesis, females are expected to show asynchronous reproduction in those species where operational sex ratios are female-biased and under circumstances where an individual female is sexually receptive only for short durations. We show that females of the intertidal amphipod Corophium volutator are receptive to mating only for a few days following their moult and are unable to hasten onset of moulting in the presence of a male. Despite meeting the conditions of the mate-availability hypothesis, reproduction was synchronous for female C. volutator across spatial and temporal scales relevant to mate-searching abilities of males. As such, some females are not expected to mate between moults, which coincide with their ability to mate. However, females do moult frequently (relative to males) which should increase their likelihood of mating over their lifetimes. It is unlikely that seasonal constraints, predation, or competition can account for the high degree of synchrony among breeding female amphipods. We suggest that dispersal of females or their offspring may constrain activity of females, as they moulted almost entirely during spring tides (although not always during the same set of spring tides). Female reproductive synchrony also has implications for reproductive behaviour of males, in particular, the possibility of harem-defence polygyny.     

Geographic variation in multiple paternity within natural populations of the guppy (Poecilia reticulata)

Mating can increase an individual''s risk of mortality by predation. In response to predation hazards, males in some species court females less often, but alternatively engage in coerced copulations more frequently and females become less selective. Such predator-mediated shifts in mating tactics may result in higher levels of multiple inseminations in females and, thus, in greater frequencies of females with broods of mixed paternity. We tested this hypothesis using two polymorphic microsatellite loci to estimate conservatively multiple paternity in broods of female guppies (Poecilia reticulata) originating from ten natural populations that have evolved under different fish predation regimes in Trinidad. The frequency of broods that were multiply sired was significantly greater on average in populations experiencing high predation pressure compared to populations experiencing a relatively low predation risk. These results suggest that the intensity of male sperm competition covaries geographically with predation pressure in this species and that the local risk of predation mediates the opportunity for sexual selection within populations.     

Polygyny and its fitness consequences for primary and secondary female pied flycatchers

In polygynous species with biparental care, the amount of paternal support often varies considerably. In the pied flycatcher (Ficedula hypoleuca), females mated with monogamous males receive more male assistance during the nestling phase than females mated with bigynous males, as the latter have to share their mates with another female. Bigynous males, however, give more support to their primary broods than to their secondary broods. Using a long-term dataset (31 years), the present study revealed that direct reproductive success, i.e. number of fledglings, was lower in females that mated with bigynous males, especially in secondary broods without male assistance, than in females that mated with monogamous males. Secondary broods with male assistance were more affected than primary broods. Female survival was independent of mating status. In primary broods, a delayed compensation for inferior direct reproductive success was found in terms of the number of grandoffspring, a phenomenon that did not occur in secondary broods. Delayed compensation in primary broods refers to indirect effects, i.e. good genes. According to the sexy son hypothesis, genetically superior (i.e. sexy) males may have sons with a higher number of broods belonging to a polygynous breeding status than do sons from broods with a monogamous father. This was indeed the case for sons descending from primary broods, but not for sons descending from secondary broods.     

Mating system and sex allocation in the gregarious parasitoid Cotesia glomerata

The gregarious parasitoid Cotesia glomerata (L.) is often presumed to possess the characteristic attributes of a species that manifests local mate competition (LMC), as it commonly produces female-biased broods. However, our field surveys of sex ratio and laboratory observations of adult behaviour showed that this species is subject to partial local mate competition caused by natal dispersal. On average, 30% of males left their natal patch before mating, with the proportion of dispersing males increasing with an increase in the patch's sex ratio (i.e. proportion of males). Over 50% of females left their natal patch before mating, and only 27.5% of females mated with males emerging from the same natal patch. Although females showed no preference between males that were and were not their siblings, broods from females that mated with siblings had a significantly higher mean brood sex ratio (0.56) than broods from females that mated with nonsiblings (0.39). Furthermore, brood sex ratios increased as inbreeding was intensified over four generations. A field population of this wasp had a mean brood sex ratio of 0.35 over 3 years, which conformed well to the evolutionarily stable strategy sex ratio (r=0.34) predicted by Taylor's partial sibmating model for haplodiploid species. These results suggest that the sex allocation strategy of C. glomerata is based on both partial local mate competition in males and inbreeding avoidance in females. In turn, this mating system plays a role in the evolution of natal dispersal behaviour in this species.Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.      

Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus

In crustacean species with precopulatory mate-guarding, sexual size dimorphism has most often been regarded as the consequence of a large male advantage in contest competition for access to females. However, large body size in males may also be favoured indirectly through scramble competition. This might partly be the case if the actual target of selection is a morphological character, closely correlated with body size, involved in the detection of receptive females. We studied sexual selection on body size and antennae length in natural populations of Asellus aquaticus, an isopod species with precopulatory mate guarding. In this species, males are larger than females and male pairing success is positively related to body size. However, males also have longer antennae, relative to body size, than females, suggesting that this character may also be favoured by sexual selection. We used multivariate analysis of selection to assess the relative influences of body size and antennae length in five different populations in the field. Selection gradients indicated that overall body size was a better predictor of male pairing success than antennae length, although some variation was observed between sites. We then manipulated male antennae length in a series of experiments conducted in the laboratory, and compared the pairing ability of males with short or long antennae. Males with short antennae were less likely to detect, orient to and to pair with a receptive female compared to males with long antennae. We discuss the implications of our results for studies of male body size and sexual dimorphism in relation to sexual selection in crustaceans.     

Paramyxean–microsporidian co-infection in amphipods: Is the consensus that Microsporidia can feminise their hosts presumptive?

The current consensus is that Microsporidia belong to a select group of parasites capable of causing both intersexuality and complete sex reversal in their hosts. Indeed, species such as Nosema granulosis and Dictyocoela duebenum, which infect amphipod crustaceans, are regularly referred to as 'feminising microsporidians'. This categorisation is based on a combination of findings: that these species are vertically transmitted and occur at a high prevalence of infection in intersex and female amphipods, that infected amphipod populations are female-biased, and that infected females have significantly female-biased broods with no concurrent increase in mortality. In this study, we report on a population of the amphipod Echinogammarus marinus presenting both female bias and high levels of intersexuality, which are infected with D. deubenum. In keeping with its feminising classification, infection is prevalent in animals presenting female and intersex phenotypes. However, a further screen revealed the presence of a previously unknown paramyxean parasite related to organisms of the genus Marteilia, a group known to cause catastrophic sexual dysfunction in bivalves. We found that the paramyxean parasite was also vertically transmitted, with infections being more prevalent in females and intersex animals. Critically, every animal infected with D. deubenum was also co-infected with the paramyxean, with few animals presenting an independent paramyxean infection. In contrast, co-infection of E. marinus with a paramyxean and the non-feminising microsporidian Dictyocoela berillonum rarely occurred. These observations raise a new hypothesis, namely, that D. duebenum and other feminising microsporidians are not actually capable of host feminisation but instead 'hitch-hike' together with a feminising paramyxean parasite.     

Filial cannibalism in an assassin bug

Filial cannibalism has been described in many fish species with male parental care, and has typically been explained as a response to high energetic costs of brood defence and decreased feeding opportunities during the period of care. We investigated filial cannibalism in an insect, the assassin bug Rhinocoris tristis. In this species, males guard eggs of a number of females, cannibalizing some of their offspring within the brood. We monitored guarding males in both the field and the laboratory. Males typically ate eggs around the periphery of the brood, which were those most likely to have been parasitized by wasps. However, cannibalism persisted in the laboratory in the absence of parasites, and the number of cannibalized eggs was related to the length of care and overall brood size, suggesting that males use eggs as an alternative source of food. This conclusion was further supported by the fact that males in the field did not lose weight while guarding, despite being unable to forage efficiently while caring. Males were also observed to adopt broods, but in a laboratory experiment did not eat more eggs from adopted than from their own broods.     

Sex-specific costs of reproduction in Eastern Bluebirds Sialia sialis

In species with bi-parental care, individuals must partition energy between parental effort and mating effort. Typically, female songbirds invest more than males in reproductive activities such as egg-laying and incubation, but males invest more in secondary sexual traits used in attracting mates. Animals that breed more than once within a season must also allocate time and energy between first and subsequent breeding attempts and between current and future breeding seasons. To investigate strategies of reproductive investment by males and females and the consequences of such strategies, we manipulated the size of broods of Eastern Bluebirds Sialia sialis . Pairs with enlarged first broods were less likely to produce a second clutch or took longer to initiate one than pairs with reduced broods. After rearing enlarged broods, females were less likely than males to survive to the following year. Although plumage coloration is a sexually selected trait in Eastern Bluebirds that is influenced by nutritional stress, we did not detect an effect of brood-size manipulation on female coloration. Past research, however, demonstrates that, in males, plumage colour is negatively affected by increasing brood size. We suggest that there are sex-specific strategies of reproductive investment in Eastern Bluebirds, and that researchers should incorporate measures of residual reproductive value in studies of life-history evolution.     

Sexual reproduction of Daphnia pulex in a temporary habitat

Species of Daphnia (Crustacea: Cladocera) typically reproduce by cyclical parthenogenesis, in which a period of all-female parthenogenetic reproduction is followed by sexual reproduction. Sex in Daphnia is determined by the environment, with factors such as temperature, photoperiod and crowding stimulating the production of males and sexual females. Previous studies on Daphnia pulex from temporary pond habitats demonstrated the coexistence of male-producing and non-male-producing (NMP) females, as determined under crowding in the laboratory. A strong genetic component to this sex allocation variation suggested that sex expression in D. pulex is better described as a result of genotype-environment interaction. The present study examined the switch from parthenogenetic to sexual reproduction in two temporary-pond populations of D. pulex. Both populations showed a very early investment in sexual reproduction, independent of population density, by producing males very soon after the populations were reestablished from resting eggs in the early spring. Approximately 40% of the initial broods were male. Additional evidence for gender specialization was obtained by observing the sex of two or three successive broods for 85 individual females. Fifty-eight females produced successive broods of females, 13 females produced successive broods of males and 14 females produced successive broods which included both male and female broods. Females that produced successive female broods under natural conditions included a higher frequency of NMP females compared to a random sample of females, confirming the existence of NMP females. Sexual females were observed in both populations after the first appearence of males, suggesting that the presence of males may stimulate the production of sexual females. For D. pulex populations in a temporary environment, there appears to be an increased emphasis on sexual reproduction and a decreased influence of the environment on sex determination, compared to Daphnia populations in more permanent habitats. Received: 19 February 1996 / Accepted: 20 January 1997     

Population structure of pierid butterflies

Summary Mark-release-recapture techniques were used to study alfalfa pest populations of Colias philodice eriphyle. Two new methods for estimating relative catchability and residence rates were used to compare males to females. The results show that: (1) both sexes had limited dispersal, with mean individual ranges less than 100 m; (2) males were more abundant than females; (3) males and females had similar residence times; (4) males were more catchable than females in uncut fields, but not in cut fields. Explanations for the differences between the sexes are considered. Females may be less catchable in uncut fields because they spend less time in flight than males. Males may be more abundant than females because they develop faster, and may have lower pre-adult mortality. The differences between the sexes are discussed with respect to reproductive strategy. Comparisons with non-pest C.p. eriphyle show differences between pest and non-pest populations. Pest C.p. eriphyle were more sedentary. The residence times were similar for pest and non-pest populations, but pest C.p. eriphyle probably have longer reproductive life-spans. Mid-summer broods of the pest population were partially overlapping; the non-pest population has discrete broods. Pest population density varied less between years than non-pest population density. The differences between pest and non-pest C.p. eriphyle support the idea of ecological diversity among conspecific populations.     

Life cycle ofCorophium insidiosum (Crustacea,Amphipoda) in laboratory culture

The life cycle ofCorophium insidiosum (Crawford) was investigated for the first time in laboratory cultures. This amphipod is mainly a suspension feeder, but it can also switch to deposit feeding. It was cultivated in standing, unaerated finger bowls, feeding on detritus, living or dried algal matter, rotifers (Brachionus plicatilis), or large-sized phytoplankton (e. g. the flagellateScrippsiella faeroense). A mixture of these items gave best results and hence was used for long-term cultivation. Preliminary results are presented on ingestion rate (suspension feeding), digestion rate, starvation resistance, and elemental and biochemical composition. In the life cycle ofC. insidiosum, several factors were found to play an important role: temperature, individual age, sex, and in contrast to other amphipod species thus far studied also the age of the mother animal at the time of breeding. Increasing temperature reduces the total life span, the age and size at the time of attaining sexual maturity, and the duration of marsupial development. It increases growth and moulting rate. At increasing individual age, the growth rate, and in males also the moulting rate, decrease, while the number of offspring per brood and surprisingly also the duration of its marsupial development increase. Females generally have a longer life span than males, and they show a higher number of moults, higher moulting frequency and growth rate, and a larger maximum body size. Body length and age at the time of reaching sexual maturity are smaller in males than in females. Furthermore the age of the mother animal at the time of breeding proved to be of particular importance: Individuals from early broods have an apparently longer life span than those originating from late broods, and they have more moults, mostly a higher growth rate, higher number of broods, higher total number of offspring, and, surprisingly, also a longer period of incubating their broods. These major factors have differing combined effects.     

Shorebird predation may cause discrete generations in an amphipod prey

We examine the possible impact of intense, periodic predation by the semipalmated sandpiper Calidris pusilla, on the life history patterns of its amphipod prey, Co-rophium volutator We compare populations from two mudflats, one of which is visited by shorebirds on their annual migration south, and one which physically appears very similar but is not visited by the birds The Corophium population exposed to intense predation had two distinct peaks in density within the season, corresponding to two generations, and the two cohorts had constrained size distributions, and relatively synchronized timing of reproduction On the mudflat not visited by sandpipers, densities increased m spring and then remained constant through summer Reproduction was continuous The mid-summer decline in amphipod density on the mudflat used by sandpipers could not be attributed directly to sandpiper predation as had been previously argued We interpret the decline as the result of a synchronized natural die-off after reproduction Selective predation on large amphipods may contribute to the second peak in density by increasing juvenile survivorship due to the removal of competing adults Size-selective predation by sandpipers causes the overwintering cohort to have a restricted size range m the autumn, and this synchrony persists through to the reproductive period of the following spring We do not exclude the possibility that these differences in life history have been influenced in an evolutionary sense by the long history of intense periodic predation     

The function of extrapair paternity in blue tits and great tits: good genes or fertility insurance?

DNA fingerprinting of an island population of blue tits andgreat tits in southeast Norway revealed that extrapair paternityaccounted for 36% (17/47) and 27% (15/55) of broods and for7% (31/466) and 8% (33/408) of young in the two species, respectively.Cuckolded males did not differ from noncuckolded males withrespect to morphology, age, or survival. There was no seasonalpattern in the frequency of extrapair paternity, and males showedno individual consistency in paternity loss over multiple broods.Extrapair offspring did not grow faster, they did not fledgewith a higher body mass, and they did not show a higher localsurvival rate than their half siblings. Hence, there was noevidence of any association between extrapair paternity andmale phenotypic or genotypic quality. Extrapair offspring wererandomly distributed among broods, with the only exceptionsof one blue tit and two great tit broods in which all young(six to nine) were sired by an extrapair male. This patternis best explained by a small proportion of males (2%–4%)being infertile and by most females performing a few extrapaircopulations as insurance against laying infertile eggs. We concludethat the results suggest a role for fertility insurance butthat alternative functional explanations to extrapair paternityin these populations cannot yet be ruled out.     

Mechanisms of sexual selection operating on body size in the mosquitofish (Gambusia holbrooki)

Collections in two natural populations of the mosquitofish,Gambusia holbrooki, were made to determine whether the sexesassociated nonrandomly based on body size, and laboratory experimentswere performed to identify mechanisms that could generate theobserved field patterns of association. In both populationslarger females were associated with larger males. However, thesize distributions of males caught with and without femaleswere not different. Both gravid females and females that hadreleased broods within the previous 24 h spent more time nearlarger males in laboratory choice experiments. In contrast,males did not discriminate between females on the basis of sizeor reproductive status in laboratory choice experiments, andmales did not attempt relatively more copulations with largerfemales when one male was allowed to freely interact with twofemales. Body size differences and aggressive interactions betweenmales did influence the access of males to females, with largermales attempting more copulations with females when two maleswere allowed to freely interact with one female: aggressiveinteractions between males were frequent when males differedby < 1.0 mm standard length but were rare when males differedby>5.0 mm standard length. These results indicate that femalechoice for larger males could contribute to generating the nonrandomassociations between the sexes that were observed in these twopopulations and that interactions between males in the vicinityof females may additionally increase the mating advantages oflarger males.     

Assortative pairing in the amphipod <Emphasis Type="Italic">Paracalliope fluviatilis</Emphasis>: a role for parasites?

The potential impact of parasitism on pairing patterns of the amphipod Paracalliope fluviatilis was investigated with regard to the infection status of both males and females. Two helminth parasites commonly use this crustacean species as second intermediate host. One of them, Coitocaecum parvum, is a progenetic trematode with an egg-producing metacercaria occasionally reaching 2.0 mm in length, i.e. more than 50% the typical length of its amphipod host. The amphipod was shown to exhibit the common reproductive features of most precopula pair-forming crustaceans, i.e. larger males and females among pairs than among singles, more fecund females in pairs, and a trend for size-assortative pairing. Although effects of parasitism were expected a priori, no major influence of the two trematodes was shown on the host reproductive patterns herein investigated. In particular, the occurrence of pairs of infected individuals conformed exactly to what can be expected under the null hypothesis of completely random pair formation. The absence of parasite effect was also illustrated by a similar fecundity of infected and uninfected females. This study thus tends to moderate the widespread view of a strong influence of parasitism on host mating patterns, as suggested by the recent literature on this specific subject.     

How Consistently are Good Parents Good Parents? Repeatability of Parental Care in the House Sparrow,Passer domesticus

In many avian species, substantial individual variation occurs in parental food‐provisioning levels, which often is assumed to reflect variation in parental quality. Parental quality also has often been invoked as a key element in mate choice among biparental species, and many sexually‐selected traits have been investigated as potential predictors of parental quality. In recent studies of house sparrow (Passer domesticus) parents, we found that individuals behaved remarkably consistently across time, regardless of temporary manipulations of the nestling provisioning of their partners. This suggests that variation in parental competence may be attributable to quality differences among individuals. One prediction of the ‘parental quality differences’ hypothesis is that individuals also should show consistency in their provisioning behavior across broods. To test this, we compared the parental delivery rates of individual house sparrows across broods. Parents of both sexes reduced their per‐chick delivery rates as the season progressed; parents of both sexes were also responsive to changes in their brood sizes. Despite these sources of environmental variation in provisioning rates, the parental care of individual males was highly repeatable across broods. By contrast, female parental care showed extremely low repeatability, and standardized measures of among‐individual variation in parental behavior revealed females to be much less variable than males. These results indicate that females in this multi‐brooded species have much to gain from mate‐choice decisions predicated on male parental quality or accurate indicators of such, whereas males are less likely to profit from being highly selective about the ‘parental quality’ of their partners.     

Effect of the snail Ilyanassa obsoleta (Say) on dynamics of the amphipod Corophium volutator (Pallas) on an intertidal mudflat

We investigated interactions between two dominant invertebrate species of intertidal soft-sediment environments of the northwest Atlantic, the mud snail Ilyanassa obsoleta and the burrowing amphipod Corophium volutator, on a mudflat of the upper Bay of Fundy, Canada. Distribution of I. obsoleta on the mudflat was highly patchy and negatively correlated with density of C. volutator. Manipulation of snail density in cages showed that I. obsoleta influences C. volutator; specifically, increasing density of snails reduced density, increased patchiness in distribution, decreased recapture rates and decreased immigration of C. volutator. Ilyanassa obsoleta seems to be affecting C. volutator through an influence on survival rate and emigration rate, although temporal variation in these effects was observed. Given that both I. obsoleta and C. volutator show a preference for tide pools, an important microhabitat on mudflats, snails might have a profound impact on C. volutator population dynamics.     

Experimental tail elongation in male Barn Swallows Hirundo rustica reduces provisioning of young, but only in second broods

At the beginning of the breeding season, the outermost tail feathers of 31 male Barn Swallows Hirundo rustica were either shortened by 20 mm, elongated by 20 mm or left unmanipulated. In first broods, the number of feeding bouts (per nestling per hour) by males and females did not differ significantly among experimental groups. However, in second broods, males with elongated tails fed their nestlings less often than males with shortened or unmanipulated tails. Male tail elongation may have been detrimental to flight, making capture of insects more difficult. Females paired to long-tailed males did not compensate for this reduction in feeding by males and their nestlings received less food. Neither feeding rates nor brood size differed significantly between first and second broods. Variation in abundance of large insects at different times of the day (high around noon and low in the morning and afternoon) matched variation in feeding rates, supporting the importance of large insects in the Barn Swallow diet. Different feeding rate patterns in first and second broods by males with elongated tails could have at least three explanations: (1) such males were able to adjust their parental effort to some extent, temporarily compensating for an imposed handicap; (2) they were more sensitive than males in the other experimental groups to late season food shortage (a decrease in numbers of large insects, the main Barn Swallow prey, was observed as the season progressed); and (3) they were more sensitive than males in the other experimental groups to the deterioration in their physical condition after first broods.