Fecundity compensation in the three‐spined stickleback Gasterosteus aculeatus infected by the diphyllobothriidean cestode Schistocephalus solidus

Causal explanations for host reproductive phenotypes influenced by parasitism fit into three broad evolutionary models: (1) non‐adaptive side effect; (2) adaptive parasitic manipulation; and (3) adaptive host defence. This study demonstrates fecundity compensation, an adaptive non‐immunological host defence, in the three‐spined stickleback fish (Gasterosteus aculeatus) infected by the diphyllobothriidean cestode Schistocephalus solidus. Both infected and uninfected female sticklebacks produced egg clutches at the same age and size. The reproductive capacity of infected females decreased rapidly with increased parasite : host body mass ratio. Body condition was lower in infected females than uninfected females and decreased with increasing parasite : host mass ratio. Females with clutches had greater body condition than those without clutches. A point biserial correlation showed that there was a body condition threshold necessary for clutch production to occur. Host females apparently had the capacity to produce egg clutches until the prolonged effects of nutrient theft by the parasite and the drain on resources from reproduction precluded clutch formation. Clutch mass, adjusted for female body mass, did not differ significantly between infected and uninfected females. Infected females apparently maintained the same level of reproductive allotment (egg mass as proportion of body mass) as uninfected females. Infected females produced larger clutches of smaller eggs than uninfected females, revealing a trade‐off between egg mass and egg number, consistent with the fecundity compensation hypothesis. The rapid loss of reproductive capacity with severity of infection probably reflects the influence of the parasite combined with a trade‐off between current and future reproduction in the host. Inter‐annual differences in reproductive performance may have reflected ecological influences on host pathology and/or intra‐annual seasonal changes. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Consistency of host responses to parasitic infection in the three‐spined stickleback fish infected by the diphyllobothriidean cestode Schistocephalus solidus

Among populations of the three‐spined stickleback fish in Alaska, females appear to show two forms of sterility tolerance to infection by the diphyllobothriidean cestode Schistocephalus solidus. In contrast to sticklebacks in other regions of the northern hemisphere, female fish are capable of producing clutches of eggs despite supporting large parasite burdens. Nonetheless, nutrient loss to the parasite, coupled with the energetic demands of host reproduction, eventually curtails spawning among infected females. Host females in Walby Lake experience ‘fecundity reduction’ resulting from nutrient theft as a side effect of infection. In Scout Lake, infected females show ‘fecundity compensation’, an adaptive, inducible response allowing them to increase current fecundity to compensate for reduction or loss of future reproduction. This multi‐year study of sticklebacks from each lake addresses two empirical questions for a better understanding of the dynamic interplay between host and parasite. First, is there is any annual variation within the two responses to parasitism in each host population; and, if so, is it related to parasite burden? Second, do the two host responses show consistent differences between the populations of sticklebacks despite any yearly variation in them? We found annual, intra‐population variation within the response shown by each population of stickleback which appears to have been influenced by the parasite : host mass ratio and possibly by unknown environmental conditions affecting the reproductive physiology of stickleback females. Moreover, the data support the hypothesis that ovum mass is more sensitive to parasitism (parasite burden) than clutch size in females from Walby Lake which exhibit fecundity reduction. Notwithstanding the intra‐population variation within each host response, the responses to infection occurred consistently within each respective stickleback population and appear to reflect stable, fundamental characteristics of the populations. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 958–968.     

Measuring temporal variation in reproductive output reveals optimal resource allocation to reproduction in the northern grass lizard, Takydromus septentrionalis

We measured the reproductive output of Takydromus septentrionalis collected over 5 years between 1997 and 2005 to test the hypothesis that reproductive females should allocate an optimal fraction of accessible resources in a particular clutch and to individual eggs. Females laid 1–7 clutches per breeding season, with large females producing more, as well as larger clutches, than did small females. Clutch size, clutch mass, annual fecundity, and annual reproductive output were all positively related to female size (snout–vent length). Females switched from producing more, but smaller eggs in the first clutch to fewer, but larger eggs in the subsequent clutches. The mass-specific clutch mass was greater in the first clutch than in the subsequent clutches, but it did not differ among the subsequent clutches. Post-oviposition body mass, clutch size, and egg size showed differing degrees of annual variation, but clutch mass of either the first or the second clutch remained unchanged across the sampling years. The regression line describing the size–number trade-off was higher in the subsequent clutch than in the first clutch, but neither the line for first clutch, nor the line for the second clutch varied among years. Reproduction retarded growth more markedly in small females than in large ones. Our data show that: (1) trade-offs between size and number of eggs and between reproduction and growth (and thus, future reproduction) are evident in T. septentrionalis ; (2) females allocate an optimal fraction of accessible resources in current reproduction and to individual eggs; and (3) seasonal shifts in reproductive output and egg size are determined ultimately by natural selection.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 315–324.     

Reduction of egg size in natural populations of threespine stickleback infected with a cestode macroparasite

Manifestations of infectious disease may represent host adaptations to avoid or reduce the effects of infection on host fitness, parasite manipulations that benefit the pathogen's fitness, or nonadaptive side effects of parasitism. Threespine stickleback fish (Gasterosteus aculeatus) from Alaska and the cestode macroparasite Schistocephalus solidus provide an excellent system for study of the effects of parasitism on host egg size because females in populations there are capable of producing clutches of eggs in the face of substantial infection, contrary to the inhibition of reproduction that has been observed in other stickleback populations or other species of fish. A side effect resulting in reduction of mean ovummass among infected females was predicted based on the egg production process in female stickleback, the considerable energy and resource demands of S. solidus, and the chronic and progressive nature of the effects the macroparasite should have on the host fish. In each of 9 populations of G. aculeatus representing replicate natural experiments in lakes scattered across the Matanuska-Susitna Valley and the Kenai Peninsula of south-central Alaska and among all populations combined, the mean ovum mass of infected female fish is significantly reduced in comparison with that of uninfected females taken from the same population at the same time. Reduction in mean female egg mass ranged from 8 to 32% across all populations. To examine whether reduction in mean female ovum mass was a nonadaptive side effect or an adaptation, relatively large data sets from 2 of the populations were used. Mean ovum mass of infected females was predicted to decrease directly in relation to parasite index (PI) if the diminution in mean egg mass were the result of a nonadaptive side effect resulting from host nutrient loss. Alternatively, the absence of a relationship between PI and reduction in ovum mass is predicted if decreases in mean female ovum mass result from host or parasite adaptation (or both) because lightly infected hosts should show a response similar to that of heavily infected ones. In each of the 2 populations, there is a significant, negative relationship between mean female ovum mass and PI, demonstrating a correlation between the decrease in ovum mass and the level of infection. Thus, the results are consistent with the hypothesis that the reductions in mean female egg mass represent side effects of parasitism involving nutrient theft. Moreover, the proportional decline in egg mass with increasing PI apparently differed between the 2 populations, and there was no significant relationship between mean percent decrease in mean female ovum mass and mean PI across populations. These observations suggest that unknown ecological and evolutionary factors influence the degree of reduction in mean ovum mass in a population-specific manner.     

Decreased reproductive investment of female threespine stickleback Gasterosteus aculeatus infected with the cestode Schistocephalus solidus: parasite adaptation,host adaptation,or side effect?

Parasitic infections may cause alterations in host life history, including changes in reproductive investment (absolute amount of energy allocated to reproduction) and reproductive effort (proportion of available energy allocated to reproduction). Such changes in host life history may reflect: 1) a parasite tactic: the parasite adaptively manipulates energy flow within the host so that the host is induced to make a reduction in reproductive effort and reproductive investment, making more energy available to the parasite; 2) no tactic: there is no change in host reproductive effort and reproductive investment simply decreases as a side effect of the parasite depleting host energy stores; 3) a host tactic: the host adaptively increases reproductive effort in the face of infection and loss of body condition, reproductive investment possibly being reduced despite the increased reproductive effort. Females in Alaskan lake populations of threespine sticklebacks ( Gasterosteus aculeatus ) are capable of clutch production when parasitized by the cestode Schistocephalus solidus despite large relative parasite masses. We analyzed the somatic energy reserves, maturation stage and ovarian mass of female sticklebacks collected from an Alaska lake during a single reproductive season. We found that parasitized females were less likely to carry fully-matured gametes, had smaller ovarian masses, and had lower somatic energy stores than unparasitized females. The relationship between reproductive investment and energy storage did not differ between parasitized and unparasitized females. Thus, reproductive effort did not change in response to parasitic infection. We conclude there was no indication of either a parasite tactic or a host tactic. Simple nutrient theft is involved in the parasite's influence on host reproduction, consistent with an earlier hypothesis that reproductive curtailment in threespine sticklebacks is a side effect.     

Reproductive potential of Schistocephalus solidus‐infected male three‐spined stickleback Gasterosteus aculeatus from two U.K. populations

Male three‐spined stickleback Gasterosteus aculeatus from two U.K. populations with endemic infections of the cestode Schistocephalus solidus were brought into the laboratory prior to the breeding season and transferred to nesting tanks under conditions designed to stimulate sexual maturation. Nesting and courtship behaviours were scored over a 35 day period, after which fish were euthanized and the liver, spleen, kidney and gonads were weighed. Among G. aculeatus from a park pond in Leicester, U.K., infected males rarely engaged in reproductive behaviours and exhibited reduced indices of sexual development, body condition and general health, with effects being largely independent of relative parasite mass (parasite index, I_P). In contrast, the reproductive behaviour of infected fish from Kendoon Loch in Dumfriesshire, U.K. appeared to be less severely affected, with infected fish regularly building nests and courting females under laboratory conditions. This was paralleled by a more limited effect of infection on physiological indicators of development, condition and general health. Furthermore, behavioural and physiological variables typically correlated with I_P among infected fish from this population. Although comparing the performance of infected fish from the two populations directly was difficult due to potentially confounding factors, the results support the findings of recent studies showing that the effects of S. solidus on host reproduction are unlikely to be uniform across G. aculeatus populations. One possibility is that variation in the effects of infection arises from differences in the co‐evolutionary association times of G. aculeatus with the parasite.     

Reproductive performance of a lacertid lizard at the core and the periphery of the species' range

The range boundaries of organisms are frequently interpreted in terms of a decline in the extent to which the life histories of outer populations are able to adapt to local environmental conditions. To test this hypothesis, we compared the reproductive characteristics of two Iberian populations of the lizard Psammodromus algirus (Linnaeus, 1758). One of them (Lerma) is close to the northern edge of the species' range, whereas the other one (El Pardo) occupies a typical core habitat 200 km further south. Gravid females were captured in the field and transported to the lab for egg laying. Second clutches were less frequent at Lerma (where clutch size and clutch mass were larger for first than for second clutches) than at El Pardo. The total mass of both clutches combined was similar at both sites. Thus, the higher frequency of second clutches at El Pardo appeared to balance the between-sites difference in energy allocation to the first clutch. Females from Lerma laid more but smaller eggs than those from El Pardo. When incubated at the same temperature, eggs from Lerma hatched sooner even when controlling for between-sites differences in mean egg size. These differences are interpreted in the light of the advantages of early hatching and high fecundity in the northern population, as opposed to large offspring size in the core population. We conclude that the life-history traits studied show enough variation, presumably of an adaptive nature, to cope with environmental challenges at the edge of the species' range.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 87–96.     

SEVERE COSTS OF REPRODUCTION PERSIST IN ANOLIS LIZARDS DESPITE THE EVOLUTION OF A SINGLE‐EGG CLUTCH

A central tenet of life‐history theory is that investment in reproduction compromises survival. We tested for costs of reproduction in wild brown anoles (Anolis sagrei) by eliminating reproductive investment via surgical ovariectomy and/or removal of oviductal eggs. Anoles are unusual among lizards in that females lay single‐egg clutches at frequent intervals throughout a lengthy reproductive season. This evolutionary reduction in clutch size is thought to decrease the physical burden of reproduction, but our results show that even a single egg significantly impairs stamina and sprint speed. Reproductive females also suffered a reduction in growth, suggesting that the cumulative energetic cost of successive clutches constrains the allocation of energy to other important functions. Finally, in each of two separate years, elimination of reproductive investment increased breeding‐season survival by 56%, overwinter survival by 96%, and interannual survival by 200% relative to reproductive controls. This extreme fitness cost of reproduction may reflect a combination of intrinsic (i.e., reduced allocation of energy to maintenance) and extrinsic (i.e., increased susceptibility to predators) sources of mortality. Our results provide clear experimental support for a central tenet of life‐history theory and show that costs of reproduction persist in anoles despite the evolution of a single‐egg clutch.     

Sex‐dependent infection causes nonadditive effects on kissing bug fecundity

The influence of parasites on host reproduction has been widely studied in natural and experimental conditions. Most studies, however, have evaluated the parasite impact on female hosts only, neglecting the contribution of males for host reproduction. This omission is unfortunate as sex‐dependent infection may have important implications for host–parasite associations. Here, we evaluate for the first time the independent and nonindependent effects of gender infection on host reproductive success using the kissing bug Mepraia spinolai and the protozoan Trypanosoma cruzi as model system. We set up four crossing treatments including the following: (1) both genders infected, (2) both genders uninfected, (3) males infected—females uninfected, and (4) males uninfected—females infected, using fecundity measures as response variables. Interactive effects of infection between sexes were prevalent. Uninfected females produced more and heavier eggs when crossed with uninfected than infected males. Uninfected males, in turn, sired more eggs and nymphs when crossed with uninfected than infected females. Unexpectedly, infected males sired more nymphs when crossed with infected than uninfected females. These results can be explained by the effect of parasitism on host body size. As infection reduced size in both genders, infection on one sex only creates body size mismatches and mating constraints that are not present in pairs with the same infection status. Our results indicate the fitness impact of parasitism was contingent on the infection status of genders and mediated by body size. As the fecundity impact of parasitism cannot be estimated independently for each gender, inferences based only on female host infection run the risk of providing biased estimates of parasite‐mediated impact on host reproduction.     

Clutch size in frugivorous insects as a function of host firmness: the case of the tephritid fly Anastrepha ludens

Abstract. 1. Optimal clutch size theory predicts that individuals will oviposit the number of eggs that increases their fitness. In Anastrepha ludens Loew (Diptera: Tephritidae), females oviposit larger clutches in unripe (firm) fruits than in ripe (soft) fruits. The following hypotheses were tested: (1) Using fruit firmness as an indicator of fruit quality, A. ludens females vary the number of eggs per clutch every time they reach an oviposition decision. (2) Maximising offspring survival with respect to either unripe or ripe fruit requires placing large clutches in firm fruit and smaller clutches in soft fruit. 2. Agar spheres were used as artificial hosts. Three agar concentrations resulted in three degrees of firmness. Mango fruits Mangifera indica L. served as natural hosts. Ripe and unripe fruits were used to test soft and firm host conditions respectively. Females laid significantly larger clutches in the firmer artificial hosts than in the softer hosts. They also laid significantly more eggs in artificial hosts without sugar than in hosts with sugar. Firm (unripe) mangoes also received significantly larger clutches than soft (ripe) mangoes. 3. When an individual female was first presented with a firm artificial host, it laid a large clutch. If subsequently offered a soft host, the female laid a significantly smaller clutch. Finally, if again offered a firm host, clutch size was increased significantly. 4. Possible trade‐offs in offspring fitness were explored in ripe and unripe mangoes by measuring offspring egg‐to‐adult survival, pupal weight, mean adult longevity, and fecundity. Despite the fact that larval survival was greater in soft fruit than in firm fruit, parameters such as pupal weight, mean longevity, and fecundity of adults stemming from both fruit types did not differ significantly. 5. A probable trade‐off between high offspring mortality caused by host unsuitability and low offspring and adult mortality caused by parasitism and predation is discussed as the reason for the exploitation of sub‐optimal hosts.     

Life history alterations upon oral and hemocoelic bacterial exposure in the butterfly Melitaea cinxia

Life history strategies often shape biological interactions by specifying the parameters for possible encounters, such as the timing, frequency, or way of exposure to parasites. Consequentially, alterations in life‐history strategies are closely intertwined with such interaction processes. Understanding the connection between life‐history alterations and host–parasite interactions can therefore be important to unveil potential links between adaptation to environmental change and changes in interaction processes. Here, we studied how two different host–parasite interaction processes, oral and hemocoelic exposure to bacteria, affect various life histories of the Glanville fritillary butterfly Melitaea cinxia. We either fed or injected adult butterflies with the bacterium Micrococcus luteus and observed for differences in immune defenses, reproductive life histories, and longevity, compared to control exposures. Our results indicate differences in how female butterflies adapt to the two exposure types. Orally infected females showed a reduction in clutch size and an earlier onset of reproduction, whereas a reduction in egg weight was observed for hemocoelically exposed females. Both exposure types also led to shorter intervals between clutches and a reduced life span. These results indicate a relationship between host–parasite interactions and changes in life‐history strategies. This relationship could cast restrictions on the ability to adapt to new environments and consequentially influence the population dynamics of a species in changing environmental conditions.     

Association between haematozoan infections and reproduction in the Pied Flycatcher

1. Parasites may affect breeding success of their host since they compete for the same resources as their hosts. Reproduction may also increase the susceptibility of a host to parasite infections owing to lowered resistance to parasites during breeding.
2. We studied the association between breeding performance and haematozoan parasite infection in the Pied Flycatcher ( Ficedula hypoleuca ) by using both natural data on reproduction and data from clutch size manipulations.
3. The most frequent blood parasites of the Pied Flycatcher in central Finland were Haemoproteus pallidus , Haemoproteus balmorali and Trypanosoma avium complex.
4. We did not find evidence that these haematozoan parasites have any debilitating effects on either reproduction or survival. The variation in reproductive effort did not seem to influence susceptibility to new blood parasite infections.
5. The intensity of Haemoproteus balmorali tended to increase in infected males as the brood size was artificially enlarged. Also, in females intensity of H. pallidus infection tended to increase with the level of clutch size manipulation. Thus, increased reproductive effort seems to debilitate the ability of Pied Flycatcher to control chronic infections.
6. Individuals with enlarged clutches/broods increased their reproductive effort at the expense of defence towards parasites. The cost of current reproduction may then be at least partly mediated by haematozoan infections.     

Maternal investment in reproduction and its consequences in leatherback turtles

Maternal investment in reproduction by oviparous non-avian reptiles is usually limited to pre-ovipositional allocations to the number and size of eggs and clutches, thus making these species good subjects for testing hypotheses of reproductive optimality models. Because leatherback turtles (Dermochelys coriacea) stand out among oviparous amniotes by having the highest clutch frequency and producing the largest mass of eggs per reproductive season, we quantified maternal investment of 146 female leatherbacks over four nesting seasons (2001–2004) and found high inter- and intra-female variation in several reproductive characteristics. Estimated clutch frequency [coefficient of variation (CV) = 31%] and clutch size (CV = 26%) varied more among females than did egg mass (CV = 9%) and hatchling mass (CV = 7%). Moreover, clutch size had an approximately threefold higher effect on clutch mass than did egg mass. These results generally support predictions of reproductive optimality models in which species that lay several, large clutches per reproductive season should exhibit low variation in egg size and instead maximize egg number (clutch frequency and/or size). The number of hatchlings emerging per nest was positively correlated with clutch size, but fraction of eggs in a clutch yielding hatchlings (emergence success) was not correlated with clutch size and varied highly among females. In addition, seasonal fecundity and seasonal hatchling production increased with the frequency and the size of clutches (in order of effect size). Our results demonstrate that female leatherbacks exhibit high phenotypic variation in reproductive traits, possibly in response to environmental variability and/or resulting from genotypic variability within the population. Furthermore, high seasonal and lifetime fecundity of leatherbacks probably reflect compensation for high and unpredictable mortality during early life history stages in this species.     

Geographic and host‐mediated population genetic structure in a cestode parasite of the three‐spined stickleback

Comparative studies of genetic diversity and population structure can shed light on the ecological and evolutionary factors that influence host–parasite interactions. Here we examined whether geography, time and genetic variation in Alaskan three‐spined stickleback (Gasterosteus aculeatus Linneaus) hosts shape the population genetic structure of the diphyllobothridean cestode parasite Schistocephalus solidus (Müller, 1776). Host lineages and haplotypes were identified by sequencing the mitochondrial cytochrome b gene, and host population structure was assessed by Bayesian clustering analysis of allelic variation at 11 microsatellite loci. Parasite population structure was characterized according to allelic variation at eight microsatellite loci. Mantel tests and canonical redundancy analysis were conducted to evaluate the proportion of parasite genetic variation attributable to time and geography vs. host lineage, haplotype, and genotypic cluster. Host and parasite population structure were largely discordant across the study area, probably reflecting differences in gene flow, environmental influences external to the host, and genomic admixture among host lineages. We found that geography explained the greatest proportion of parasite genetic variation, but that variation also reflects time, host lineage, and host haplotype. Associations with host haplotypes suggest that one parasite genotypic cluster exhibits a narrower host range, predominantly infecting the most common host haplotypes, whereas the other parasite cluster infects all haplotypes equally, including rare haplotypes. Although experimental infection trials might prove otherwise, distributional differences in hosts preferentially infected by S. solidus could underlie the observed pattern of population structure.     

When parasites disagree: Evidence for parasite‐induced sabotage of host manipulation

Host manipulation is a common parasite strategy to alter host behavior in a manner to enhance parasite fitness usually by increasing the parasite's transmission to the next host. In nature, hosts often harbor multiple parasites with agreeing or conflicting interests over host manipulation. Natural selection might drive such parasites to cooperation, compromise, or sabotage. Sabotage would occur if one parasite suppresses the manipulation of another. Experimental studies on the effect of multi‐parasite interactions on host manipulation are scarce, clear experimental evidence for sabotage is elusive. We tested the effect of multiple infections on host manipulation using laboratory‐bred copepods experimentally infected with the trophically transmitted tapeworm Schistocephalus solidus. This parasite is known to manipulate its host depending on its own developmental stage. Coinfecting parasites with the same aim enhance each other's manipulation but only after reaching infectivity. If the coinfecting parasites disagree over host manipulation, the infective parasite wins this conflict: the noninfective one has no effect. The winning (i.e., infective) parasite suppresses the manipulation of its noninfective competitor. This presents conclusive experimental evidence for both cooperation in and sabotage of host manipulation and hence a proof of principal that one parasite can alter and even neutralize manipulation by another.     

Life history and virulence are linked in the ectoparasitic salmon louse Lepeophtheirus salmonis

Models of virulence evolution for horizontally transmitted parasites often assume that transmission rate (the probability that an infected host infects a susceptible host) and virulence (the increase in host mortality due to infection) are positively correlated, because higher rates of production of propagules may cause more damages to the host. However, empirical support for this assumption is scant and limited to microparasites. To fill this gap, we explored the relationships between parasite life history and virulence in the salmon louse, Lepeophtheirus salmonis, a horizontally transmitted copepod ectoparasite on Atlantic salmon Salmo salar. In the laboratory, we infected juvenile salmon hosts with equal doses of infective L. salmonis larvae and monitored parasite age at first reproduction, parasite fecundity, area of damage caused on the skin of the host, and host weight and length gain. We found that earlier onset of parasite reproduction was associated with higher parasite fecundity. Moreover, higher parasite fecundity (a proxy for transmission rate, as infection probability increases with higher numbers of parasite larvae released to the water) was associated with lower host weight gain (correlated with lower survival in juvenile salmon), supporting the presence of a virulence–transmission trade‐off. Our results are relevant in the context of increasing intensive farming, where frequent anti‐parasite drug use and increased host density may have selected for faster production of parasite transmission stages, via earlier reproduction and increased early fecundity. Our study highlights that salmon lice, therefore, are a good model for studying how human activity may affect the evolution of parasite virulence.     

Not putting all their eggs in one basket: bet‐hedging despite extraordinary annual reproductive output of desert tortoises

Bet‐hedging theory makes the counter‐intuitive prediction that, if juvenile survival is low and unpredictable, organisms should consistently reduce short‐term reproductive output to minimize the risk of reproductive failure in the long‐term. We investigated the long‐term reproductive output of an Agassiz's desert tortoise (Gopherus agassizii) population and conformance to a bet‐hedging strategy of reproduction in an unpredictable but comparatively productive environment. Most females reproduced every year, even during periods of low precipitation and poor germination of food plants, and the mean percentage of reproducing females did not differ significantly on an annual basis. Although mean annual egg production (clutch size × clutch frequency) differed significantly among years, mean clutch size and mean clutch frequency remained relatively constant. During an El Niño year, mean annual egg production and mean annual clutch frequency were the highest ever reported for this species. Annual egg production was positively influenced by maternal body size but clutch size and clutch frequency were not. Our long‐term results confirm earlier conclusions based on short‐term research that desert tortoises have a bet‐hedging strategy of producing small clutches almost every year. The risk of long‐term reproductive failure is minimized in unpredictable environments, both through time by annually producing multiple small clutches over a long reproductive lifespan, even in years of low resource availability, and through space by depositing multiple annual clutches in different locations. The extraordinary annual reproductive output of this population appears to be the result of a typically high but unpredictable biomass of annual food plants at the site relative to tortoise habitat in dryer regions. Under the comparatively productive but unpredictable conditions, tortoises conform to predictions of a bet‐hedging strategy of reproduction with relatively small but consistent clutch sizes. Published 2015. This article is a U.S. Government work and is in the public domain in the USA, Biological Journal of the Linnean Society, 2015, 115 , 399–410.     

Changing costs of reproduction: age-based differences in reproductive allocation and escape performance in a livebearing fish

The reproductive value hypothesis predicts that if residual reproductive value declines as a female ages, then young females should allocate less of available energy to current fecundity and more to future reproduction; whereas, older females should allocate more of available energy to current fecundity and less to future reproduction (i.e. survival). We test the prediction that older female Gambusia affinis exhibit higher levels of allocation to reproduction (i.e. fecundity) and consequently experience greater decline in escape performance (survival cost) during pregnancy compared to young females. Old females had relatively larger clutch wet masses and clutch wet mass increased more during pregnancy compared to young females. Correspondingly, old females exhibit a significant decline in escape velocity over the course of pregnancy; whereas young females show no change in escape velocity throughout pregnancy. Old females have higher escape velocities early in pregnancy and their performance only declines to about the level of performance of young females by the end of pregnancy. Thus, although old females exhibit a greater decline in performance they are better able to ameliorate the cost of decreased performance.     

Some (worms) like it hot: fish parasites grow faster in warmer water,and alter host thermal preferences

Elevated environmental temperatures associated with anthropogenic warming have the potential to impact host‐parasite interactions, with consequences for population health and ecosystem functioning. One way that elevated temperatures might influence parasite prevalence and intensity is by increasing life cycle completion rates. Here, we investigate how elevated temperatures impact a critical phase of the life cycle of the bird tapeworm Schistocephalus solidus – the growth of plerocercoid larvae in host fish (three‐spined sticklebacks Gasterosteus aculeatus). By 8 weeks post‐infection, plerocercoids recovered from experimentally infected sticklebacks held at 20 °C weighed on average 104.9 mg, with all exceeding 50 mg, the mass considered consistently infective to definitive hosts. In contrast, plerocercoids from sticklebacks held at 15 °C weighed on average 26.5 mg, with none exceeding 50 mg. As small increases in plerocercoid mass affect adult fecundity disproportionately in this species, enhanced plerocercoid growth at higher temperatures predicts dramatically increased output of infective parasite stages. Subsequent screening of thermal preferences of sticklebacks from a population with endemic S. solidus infection demonstrated that fish harbouring infective plerocercoids show significant preferences for warmer temperatures. Our results therefore indicate that parasite transmission might be affected in at least two ways under anthropogenic warming; by enhancing rates of parasite growth and development, and by increasing the likelihood of hosts being able to seek out proliferating warmer microhabitats. Furthermore, our results suggest the potential for positive feedback between parasite growth and host thermal preferences, which could dramatically increase the effects of even small temperature increases. We discuss the possible mechanisms underpinning our results, their likely ecological consequences and highlight key areas for further research.