Sexual size dimorphism, morphological traits and jump performance in seven species of desert fleas (Siphonaptera)

Metabolic rate and ecological traits of ectoparasites: a case study with seven flea species from the Negev Desert

We studied the relationship between fleas' metabolic rate and their ecological traits, using data on standard metabolic rate (SMR), mean abundance, host specificity, and geographic range size in males and females of seven desert flea species. SMR was measured via mass-specific CO2 emission, whereas host specificity was measured as (a) the mean number of host species used by a flea per region in regions where this flea was recorded; (b) the total number of host species a flea exploited across its geographic range; and (c) the phylogenetic diversity of the flea's hosts. To control for confounding effects of phylogeny when analysing data on multiple species, we applied the Phylogenetic Generalised Least Squares (PGLS) model. We found that the only ecological trait significantly correlating with flea SMR was the phylogenetic diversity of hosts utilized by a flea across its geographic range. The strength of the association between SMR and host phylogenetic diversity was higher in male than in female fleas. We explain the relationship between flea SMR and their host specificity by the necessity of host-opportunistic species to compensate for the high energetic cost of neutralizing multiple defences from multiple hosts by increased SMR.

Fitness consequences of host colonization in two generalist fleas: Context-dependency and the effect of spatial co-occurrence

We studied the fitness consequences of colonizing a novel host by experimental lines of fleas (Synosternus cleopatrae and Xenopsylla ramesis) maintained for 18-22 generations on the principal or novel (sympatric or allopatric) hosts via number, developmental success and size of the offspring of the fleas exploiting these hosts. We asked whether (a) fitness on non-principal hosts increases after prolonged maintenance; (b) the colonization success depends on the spatial co-occurrence of a flea and a host and (c) colonization of a novel host is accompanied by a decreased ability to exploit an original host. The ability of fleas to colonize novel hosts differed between species, with S. cleopatrae, but not X. ramesis, increasing its offspring production on novel hosts. Spatial co-occurrence did not affect colonization success. Maintenance on an alternative host was not accompanied by decreased adaptation to the original host. When fleas returned to the original host, their reproductive output was higher than that of their ancestors. We conclude that the success of colonizing a novel host is (a) context-dependent and varies between flea and host species and (b) not accompanied by the loss of ability to exploit an ancestral host but may lead to an increase in this ability.

Adaptation to a novel host and performance trade-off in host-generalist and host-specific insect ectoparasites

We investigated the performance trade-offs of fleas (Siphonaptera) while adapting to a novel host using two host generalists (Xenopsylla conformis and Xenopsylla ramesis) and one host specialist (Parapulex chephrenis) maintained on their principal hosts (Meriones crassus for Xenopsylla and Acomys cahirinus for P. chephrenis). We asked whether, over generations, (i) a host generalist may become a specialist by evolving the ability to exploit a novel host and losing the ability to exploit an original host and (ii) a host specialist can become a generalist by evolving the ability to exploit a novel host without losing the ability to exploit an original host. We established an experimental line of each species on a novel host (Acomys russatus for Xenopsylla and M. crassus for P. chephrenis) and maintained this line on this host during 23 generations. We compared reproductive performance of progenitors of each line and their descendants when they exploited either original or novel host in terms of egg number and size, hatching success, offspring production, and offspring size. We found changes in performance over generations in female offspring size only. Xenopsylla conformis demonstrated a tendency to become a host specialist (increased performance on the novel host with a concomitant decreased performance on the original host), whereas P. chephrenis demonstrated a tendency to become a host generalist (increased performance on the novel host without a concomitant decreased performance on the original host). We conclude that the probability of generalist to specialist transition, and vice versa, is context-dependent and varies between species.

Functional and phylogenetic uniqueness of helminth and flea assemblages of two South African rodents

The loss of a particular species from a community may have different effects on its functioning, depending on the presence or absence of functionally similar or phylogenetically close species in that community (redundancy). Redundancy is thus defined as the fraction of species diversity not expressed by functional or phylogenetic diversity. We assessed functional and phylogenetic alpha- and beta-redundancy in helminth and flea assemblages of two species of South African rodents, Rhabdomys dilectus and Rhabdomys pumilio, using community uniqueness as the inverse indicator of redundancy. We asked whether patterns of functional and phylogenetic alpha- and beta-uniqueness differed between (i) parasite groups (endo- versus ectoparasites), (ii) host species within parasite groups, and (iii) biomes within host species. We found differences between the two hosts in the functional and phylogenetic alpha-uniqueness (but not beta-uniqueness) of flea, but not helminth, assemblages. Significant correlations between the alpha-uniqueness of parasite assemblages and the total parasite prevalence were found only for phylogenetic uniqueness and only in helminths. Pairwise site-by-site dissimilarities in uniqueness (beta-uniqueness) and pairwise dissimilarity in prevalence were significantly associated (positively) in helminths but not in fleas. A between-biome difference in functional (but not phylogenetic) alpha-uniqueness was found in both helminth and flea assemblages harboured by R. pumilio. We conclude that the resilience of parasite assemblages in terms of the effect on hosts depends not only on their transmission strategy but also on traits of host species and environmental factors.

Morphology Reveals the Unexpected Cryptic Diversity in Ceratophyllus gallinae (Schrank, 1803) Infested Cyanistes caeruleus Linnaeus, 1758 Nest Boxes

Purpose

The main aim of our study was to examine morphological differentiation between and within sex of hen fleas—Ceratophyllus gallinae (Schrank, 1803) population collected from Eurasian blue tit (Cyanistes caeruleus Linnaeus, 1758), inhabiting nest boxes and to determine the morphological parameters differentiating this population.

Methods

A total of 296 fleas were collected (148 females and 148 males), determined to species and sex, then the following characters were measured in each of the examined fleas: body length, body width, length of head, width of head, length of comb, height of comb, length of tarsus, length of thorax and length of abdomen.

Results

The comparison of body size showed the presence of two groups among female and male life forms of the hen flea, which mostly differed in length of abdomen, whereas the length of head and tarsus III were less variable.

Conclusion

Till now, the only certain information is the presence of two adult life forms of C. gallinae. The genesis of their creation is still unknown and we are not able to identify the mechanism responsible for the morphological differentiation of fleas collected from the same host. In order to find answer to this question, future research in the field of molecular taxonomy is required.

Intraspecific variation of body size in fleas: effects of host sex and flea phenology

We investigated the effects of host sex and flea phenology (estimated as periods of high versus low abundance) on individual body size in four fleas of small mammals. Amalaraeus penicilliger and Ctenophthalmus uncinatus are ectoparasites of the bank vole Myodes glareolus, whereas Doratopsylla dasycnema and Palaeopsylla soricis are ectoparasites of the common shrew Sorex araneus. We found significant effects of host sex and phenology on the body size of all flea species, although there was no general trend in the directions of these effects. Larger A. penicilliger were found on female hosts, whereas larger P. soricis were found on male hosts. In the remaining species, larger fleas were collected from male hosts during periods of high abundance (male C. uncinatus and female D. dasycnema) and from female hosts during periods of low abundance (male C. uncinatus). Regarding phenology, larger fleas were recorded during periods of either high (A. penicilliger, C. uncinatus, D. dasycnema) or low (C. uncinatus, P. soricis) abundance, but this depended on flea and/or host sex. We conclude that the directions of the host sex and phenology effects varied between flea species. Furthermore, the direction of the host sex effect was mediated by the effect of phenology and vice versa.

Subtle short-term physiological costs of an experimental augmentation of fleas in wild Columbian ground squirrels

Parasites affect many aspects of host physiology and behavior, and thus are generally thought to negatively impact host fitness. However, changes in form of short-term parasite effects on host physiological markers have generally been overlooked in favor of fitness measures. Here, we studied flea (Oropsylla idahoensis and Oropsylla opisocroistis tuberculata) parasitism on a natural population of Columbian ground squirrels (Urocitellus columbianus) in Sheep River Provincial Park, AB, Canada. Fleas were experimentally added to adult female U. columbianus at physiologically demanding times, including birth, lactation and weaning of their young. The body mass of adult females, as well as their oxidative stress and immunity were recorded multiple times over the active season under flea-augmented and control conditions. We also measured the prevalence of an internal parasite (Trypanosoma otospermophili). Doubly labeled water (DLW) was intra-peritoneally injected at peak lactation to examine energy expenditure. Effects of parasites on oxidative stress were only observed after offspring were weaned. There was no direct effect of experimentally heightened flea prevalence on energy use. A short-term 24 h mass loss (-17 g) was detected briefly after parasite addition, likely due to U. columbianus preferentially allocating time for grooming. Our parasite augmentation did not strongly affect hosts and suggested that short-term physiological effects were unlikely to culminate in long-term fitness consequences. Columbian ground squirrels appear to rapidly manage parasite costs, probably through grooming.

Body size and ecological traits in fleas parasitic on small mammals in the Palearctic: larger species attain higher abundance

We studied the relationships between body size and (a) abundance and (b) host specificity in fleas parasitic on small mammals (rodents and shrews) in the Palearctic taking into account the confounding effect of phylogeny. We tested these relationships both across 127 flea species and within separate phylogenetic clades, predicting higher abundance and lower host specificity (in terms of the number or diversity of hosts used by a flea) in smaller species. We also tested for the relationships between body size and abundance separately for species that spend most of their lives on a host's body (the "body" fleas) and species that spend most of their lives in a host's burrow or nest (the "nest" fleas). A significant phylogenetic signal in body size was detected across all fleas, as well as in five of six separate clades. Across all fleas and in majority of phylogenetic clades, mean abundance significantly increased with an increase in body size. The same pattern was found for both the "body" and the "nest" fleas, although the slope of the relationship appeared to be steeper in the former than in the latter. Neither measure of host specificity demonstrated a significant correlation with body size regardless of the subset of flea species analysed. We explain higher abundance attained by larger flea species by higher fecundity and/or competitive advantage upon smaller species at larval stage. We conclude that the macroecological patterns reported to date in parasites are far from being universal.

Parasite performance and host alternation: is there a negative effect in host-specific and host-opportunistic parasites?

Environmental fluctuations are expected to require special adaptations only if they are associated with a decrease in fitness. We compared reproductive performance between fleas fed on alternating (preferred and non-preferred) hosts and fleas fed solely on either a preferred or a non-preferred host to determine whether (1) host alternation incurs an immediate negative effect, and, if yes, then (2) whether this effect is greater in a host specialist (Parapulex chephrenis) than in host generalists (Xenopsylla conformis and Synosternus cleopatrae). We also compared flea performance under alternating host regimes with different host order (initial feeding on either a preferred or a non-preferred host). An immediate negative effect of alternating hosts on reproductive performance was found in P. chephrenis only. These fleas produced 44·3% less eggs that were 3·6% smaller when they fed on alternating hosts as compared with a preferred host. In contrast, X. conformis and S. cleopatrae appeared to be able to adapt their reproductive strategy to host alternation by producing higher quality offspring (on average, 3·1% faster development and 2·1% larger size) without compromising offspring number. However, the former produced eggs that were slightly, albeit significantly, smaller when it fed on alternating hosts as compared with a preferred host. Moreover, host order affected reproductive performance in host generalists (e.g. 2·8% larger eggs when the first feeding was performed on a non-preferred host), but not in a host specialist. We conclude that immediate effects of environmental fluctuation on parasite fitness depend on the degree of host specialization.

Reproductive consequences of female size in haematophagous ectoparasites

To test relationships between maternal size, egg size and size of new offspring, we studied (a) the effect of maternal size on egg size and number, and maternal survival after oviposition and (b) the effect of egg size on the duration of development and new imago size in three flea species (Xenopsylla ramesis, Synosternus cleopatrae, Parapulex chephrenis) with varying host and habitat specificity. In general, the number and size of eggs as well as total egg volume appeared to be independent of maternal body size. There was no trade-off between egg number and size. However, female body size was related to post-oviposition survival, with larger females surviving longer after oviposition than smaller females. In addition, females that produced more eggs died faster after oviposition in X. ramesis but not in the two other species. There were no significant size differences between eggs that developed into new imagoes and eggs that did not survive. Survivorship of male and female eggs did not differ; however, new adult females were significantly larger than new adult males. Female, but not male, new imagoes exhibited a significant positive relationship between egg size and duration of preimaginal development in all three species, with larger eggs developing faster than smaller eggs. In X. ramesis and S. cleopatrae, faster developing eggs also developed into larger new imagoes. We conclude that these patterns were largely consistent among the three flea species, suggesting that they result from the same mechanisms and are weakly affected by the ecological specialization of a given species.

Flea fitness is reduced by high fractional concentrations of CO₂ that simulate levels found in their hosts' burrows

Nidicolous ectoparasites such as fleas and gamasid mites that feed on small and medium-sized mammals spend much of their time in their hosts' burrows, which provide an environment for living, and often feeding, to their pre-imaginal and/or adult stages. Thus, these ectoparasites should be adapted to environmental conditions in burrows, including high fractional concentrations of CO2 (F(CO2)). We examined how a high F(CO2) (0.04) affected survival and reproductive success of a hematophagous ectoparasite of burrowing rodents using fleas Xenopsylla ramesis and Sundevall's jirds Meriones crassus. In the first experiment, fleas fed on hosts housed in high-CO2 (F(CO2) =0.04) or atmospheric-CO2 (F(CO2) ≈0.0004) air, and were allowed to breed. In a second experiment, fleas were maintained in high CO2 or CO2-free air with no hosts to determine how CO2 levels affect survival and activity levels. We found that at high F(CO2) fleas laid fewer eggs, reducing reproductive success. In addition, at high F(CO2), activity levels and survival of fleas were reduced. Our results indicate that fleas do not perform well in the F(CO2) used in this experiment. Previous research indicated that the type and intensity of the effects of CO2 concentration on the fitness of an insect depend on the F(CO2) used, so we advise caution when generalizing inferences drawn to insects exposed to other F(CO2). If, however, F(CO2) found in natural mammal burrows brings about reduced fitness in fleas in general, then burrowing hosts may benefit from reduced parasite infestation if burrow air F(CO2) is high.

The life history of whole-organism performance

For almost 40 years, studies of whole-organism performance have formed a cornerstone of evolutionary physiology. Although its utility as a heuristic guide is beyond question, and we have learned much about morphological evolution from its application, the ecomorphological paradigm has frequently been applied to performance evolution in ways that range from unsatisfactory to inappropriate. More importantly, the standard ecomorphological paradigm does not account for tradeoffs among performance and other traits, nor between performance traits that are mediated by resource allocation. A revised paradigm that includes such tradeoffs, and the possible ways that performance and fitness-enhancing traits might affect each other, could potentially revivify the study of phenotypic evolution and make important inroads into understanding the relationships between morphology and performance and between performance and Darwinian fitness. We describe such a paradigm, and discuss the various ways that performance and key life-history traits might interact with and affect each other. We emphasize both the proximate mechanisms potentially linking such traits, and the likely ultimate factors driving those linkages, as well as the evolutionary implications for the overall, multivariate phenotype. Finally, we highlight several research directions that will shed light on the evolution and ecology of whole-organism performance and related life-history traits.

Host reproductive status and reproductive performance of a parasite: offspring quality and trade-offs in a flea parasitic on a rodent

We investigated offspring quality in fleas (Xenopsylla ramesis) feeding on non-reproducing, pregnant or lactating rodents (Meriones crassus) and asked whether (a) quality of flea offspring differs dependent on host reproductive status; (b) fleas trade off offspring quantity for quality; and (c) quality variables are inter-correlated. Emergence success was highest when parents exploited pregnant hosts, while development time was longest when parents exploited lactating hosts. Male offspring from fleas fed on non-reproductive and pregnant hosts were larger than those from lactating hosts whereas female offspring from fleas fed on pregnant hosts were larger than those from both lactating and non-reproductive hosts. Male offspring survived under starvation the longest when their parents exploited lactating hosts and the shortest when their parents exploited pregnant hosts. Female offspring of parents that exploited lactating hosts survived under starvation longer than those that exploited non-reproductive and pregnant hosts. Emergence success and development time decreased as mean number of eggs laid by mothers increased. Fleas that were larger and took longer to develop lived significantly longer under starvation. These results indicate the presence of a trade-off between offspring quantity and quality in fleas exploiting female Sundevall's jird in varying reproductive condition but this trade-off depended on the quality trait considered.

Effects of Bartonella spp. on flea feeding and reproductive performance

Numerous pathogens are transmitted from one host to another by hematophagous insect vectors. The interactions between a vector-borne organism and its vector vary in many ways, most of which are yet to be explored and identified. These interactions may play a role in the dynamics of the infection cycle. One way to evaluate these interactions is by studying the effects of the tested organism on the vector. In this study, we tested the effects of infection with Bartonella species on fitness-related variables of fleas by using Bartonella sp. strain OE 1-1, Xenopsylla ramesis fleas, and Meriones crassus jirds as a model system. Feeding parameters, including blood meal size and metabolic rate during digestion, as well as reproductive parameters, including fecundity, fertility, and life span, were compared between fleas experimentally infected with Bartonella and uninfected fleas. In addition, the developmental time, sex ratio, and body size of F1 offspring fleas were compared between the two groups. Most tested parameters did not differ between infected and uninfected fleas. However, F1 males produced by Bartonella-positive females were significantly smaller than F1 males produced by Bartonella-negative female fleas. The findings in this study suggest that bartonellae are well adapted to their flea vectors, and by minimally affecting their fitness they have evolved to better spread themselves in the natural environment.

Reproductive consequences of host age in a desert flea

We tested for the effect of age of a rodent host (Meriones crassus) on reproductive performance of fleas in terms of number and quality of offspring and predicted that fleas would perform better on juvenile and old than on subadult and adult hosts. The number of flea offspring was evaluated via egg and new imago production, while their quality was estimated via duration of development, resistance to starvation and body size. Although fleas produced more eggs when they exploited adults than when they exploited juvenile, subadult and old hosts, significantly more new imago emerged from fleas fed on juvenile and old hosts than on subadult and adult hosts. Fleas performed better when they fed on juvenile and/or old hosts than on subadult and adult hosts in 2 of 3 measures of offspring quality (duration of development and body size). Nevertheless, when offspring quality was estimated via resistance to starvation of a new imago, fleas demonstrated good performance in young (juvenile and subadult) hosts, while they performed poorly in old hosts. Thus, general reproductive performance of fleas was better when they exploited young and old hosts than when they exploited median age cohorts. However, the effect of host age on flea reproductive performance was manifested somewhat differently between (a) male and female hosts and (b) male and female flea offspring.

Trade-offs between burst performance and maximal exertion capacity in a wild amphibian, Xenopus tropicalis

Trade-offs are thought to impose barriers to phenotypic diversification and may limit the evolutionary responses of organisms to environmental changes. In particular, locomotor trade-offs between endurance or maximal exertion capacity and burst performance capacity have been observed in some species and may constrain the ability of organisms to disperse. Here, we tested for the presence of locomotor trade-offs between maximal exertion and burst performance capacity in an aquatic frog, the tropical clawed frog (Xenopus tropicalis). Given the importance of overland dispersal for this species, we focused on terrestrial exertion capacity (time and distance jumped until exhaustion) and tested whether it trades-off with aquatic burst performance capacity (maximum instantaneous velocity and acceleration), which is likely to be relevant in the context of predator escape and prey capture. Our data show that in both sexes, individuals with longer hindlimbs display higher endurance. Additionally, in females forelimb length was positively correlated with aquatic burst performance capacity and negatively correlated with terrestrial exertion. Trade-offs between endurance and burst performance capacity were detected, but were significant in males only. Finally, males and females differ in morphology and performance. Our data suggest that trade-offs are not universal and may be driven by sex-dependent selection on locomotor capacity. Moreover, our results suggest that locomotor trade-offs may result in sex-biased dispersal under selection for improved endurance capacity as is expected under habitat fragmentation scenarios.

Loading effects on jump performance in green anole lizards, Anolis carolinensis

Locomotor performance is a crucial determinant of organismal fitness but is often impaired in certain circumstances, such as increased mass (loading) resulting from feeding or gravidity. Although the effects of loading have been studied extensively for striding locomotion, its effects on jumping are poorly understood. Jumping is a mode of locomotion that is widely used across animal taxa. It demands large amounts of power over a short time interval and, consequently, may be affected by loading to a greater extent than other modes of locomotion. We placed artificial loads equal to 30% body mass on individuals of the species Anolis carolinensis to simulate the mass gain following the consumption of a large meal. We investigated the effects of loading on jump performance (maximum jump distance and accuracy), kinematics and power output. Loading caused a significant 18% decline in maximum jump distance and a significant 10% decline in takeoff speed. In other words, the presence of the load caused the lizards to take shorter and slower jumps, whereas takeoff angle and takeoff duration were not affected. By contrast, jump accuracy was unaffected by loading, although accuracy declined when lizards jumped to farther perches. Finally, mass-specific power output did not increase significantly when lizards jumped with loads, suggesting that the ability to produce mechanical power may be a key limiting factor for maximum jump performance. Our results suggest that mass gain after a large meal can pose a significant locomotor challenge and also imply a tradeoff between fulfilling energy requirement and moving efficiently in the environment.

Biomechanics of jumping in the flea

It has long been established that fleas jump by storing and releasing energy in a cuticular spring, but it is not known how forces from that spring are transmitted to the ground. One hypothesis is that the recoil of the spring pushes the trochanter onto the ground, thereby generating the jump. A second hypothesis is that the recoil of the spring acts through a lever system to push the tibia and tarsus onto the ground. To decide which of these two hypotheses is correct, we built a kinetic model to simulate the different possible velocities and accelerations produced by each proposed process and compared those simulations with the kinematics measured from high-speed images of natural jumping. The in vivo velocity and acceleration kinematics are consistent with the model that directs ground forces through the tibia and tarsus. Moreover, in some natural jumps there was no contact between the trochanter and the ground. There were also no observable differences between the kinematics of jumps that began with the trochanter on the ground and jumps that did not. Scanning electron microscopy showed that the tibia and tarsus have spines appropriate for applying forces to the ground, whereas no such structures were seen on the trochanter. Based on these observations, we discount the hypothesis that fleas use their trochantera to apply forces to the ground and conclude that fleas jump by applying forces to the ground through the end of the tibiae.

Host gender and offspring quality in a flea parasitic on a rodent

The quality of offspring produced by parent fleas (Xenopsylla ramesis) fed on either male or female rodent hosts (Meriones crassus) was studied. The emergence success, duration of development, resistance to starvation upon emergence and body size of the flea offspring were measured. It was predicted that offspring of fleas produced by parents that fed on male hosts (i) will survive better as pre-imago, (ii) will develop faster, (iii) will live longer under starvation after emergence and (iv) will be larger than offspring of fleas fed on female hosts. The emergence success of pre-imaginal fleas was relatively high, ranging from 46.9% to 100.0% and averaging 78.4±3.0%, and was not affected by host gender. The duration of development of pre-imaginal fleas depended on the gender of the host of parents and differed between male and female offspring, with female fleas developing faster. Furthermore, male fleas developed faster if their parents fed on female rather than on male hosts, whereas no difference in the duration of development between host genders was found in female fleas. The time to death under starvation did not depend on the gender of either the flea or the host. A newly emerged flea, on average, lived 31.9±1.0 days without access to food. The relationship between host gender and body size of male flea offspring was the only effect that supported the predictions. An increase in body size in male fleas could increase their mating success and, ultimately, their fitness.

Sex ratio in flea infrapopulations: number of fleas, host gender and host age do not have an effect

This study set out to determine whether the sex ratio of fleas collected from host bodies is a reliable indicator of sex ratio in the entire flea population. To answer this question, previously published data on 18 flea species was used and it was tested to see whether a correlation exists between the sex ratio of fleas collected from host bodies and the sex ratio of fleas collected from host burrows. Across species, the female:male ratio of fleas on hosts correlated strongly with the female:male ratio of fleas in their burrows, with the slope of the regression overlapping 1. Controlling for flea phylogeny by independent contrasts produced similar results. It was also ascertained whether a host individual is a proportional random sampler of male and female fleas and whether the sex ratio in flea infrapopulations depends on the size of infrapopulations and on the gender and age of a host. Using field data, the sex ratio in infrapopulations of 7 flea species parasitic on 4 rodent species was analysed. Populations of 3 species (Nosopsyllus iranus, Parapulex chephrenis and Xenopsylla conformis) were significantly female-biased, whereas male bias was found in 1 species (Synosternus cleopatrae). In general, the sex ratio of fleas collected from an individual rodent did not differ significantly from the sex ratio in the entire flea population. Neither host gender, and age nor number of fleas co-occurring on a host affected (a) the sex ratio in flea infrapopulations and (b) the probability of an infrapopulation to be either female- or male-biased.

Patterns of parasitism and body size in red squirrels (Tamiasciurus hudsonicus)

Parasites use their hosts for nutrition, shelter, and even dispersal; the latter can result in sex-biased parasite distribution. Host sex-biased parasitism has been well documented in vertebrates, including mammals, and males are often more parasitized than females. Male-biased parasitism is often attributed to sexual size dimorphism, with larger animals being more parasitized. Here, we used a natural population of red squirrels ( Tamiasciurus hudsonicus (Erxleben, 1777)), a species without sexual size dimorphism, to test for sex-biased parasitism in ectoparasites and intestinal helminth parasites. We also tested for size-dependent parasitism to determine the importance of body size on parasitism. We predicted that males would be more parasitized and that larger individuals would be more parasitized. As well, we predicted a male-biased flea distribution on male squirrels. Parasitism fluctuated over the course of 4 months, with flea infection peaking in August and helminth infection peaking in June. We found evidence of male-biased parasitism in helminth and ectoparasite infections. Flea infection was weakly correlated with body size in females but not in males, while no correlation was found between body size and helminth infection. Lastly, fleas had a female-biased population; however, male fleas were more likely to be found on male squirrels, and this could be to maximize dispersal to avoid inbreeding.

Sex differences in metabolic rates in field crickets and their dipteran parasitoids

Sex differences in metabolic rate (MR) can result from dimorphism in the performance of energetically demanding activities. Male crickets (Teleogryllus oceanicus) engage in costly calling and aggressive activity not performed by females. Consistent with this difference, we found higher maximal MR, factorial scope, and fat content in males than females. T. oceanicus song is also costly because it attracts the parasitoid fly Ormia ochracea. Parasitized crickets had reduced maximal MR consistent with a metabolic cost to harboring larvae. This cost was greater for females, either because females invest more heavily into reproduction at the expense of metabolic capacity, or because males are under stronger selection to respond to infection. Little is known about O. ochracea outside of its auditory system and parasitic lifestyle. We observed greater resting MR in male flies, possibly reflecting a sex difference in the requirement for metabolic power output, because male flies perform potentially costly mating behavior not seen in females. We found a positive relationship between larval density within a cricket and pupal resting MR, suggesting that crickets in good condition are able to both harbor more larvae and produce larvae with higher resting MR. These results reveal a complex interplay between the metabolism of crickets and their fly parasitoids.

Metabolic rate and jump performance in seven species of desert fleas

We hypothesized that sexual and interspecific differences in jumping performance of fleas found in our previous study are correlated with differences in resting metabolic rate (RMR) between sexes and among species. To test this hypothesis, we measured RMR of seven flea species (Xenopsylla conformis mycerini, Xenopsylla ramesis, Xenopsylla dipodilli, Parapulex chephrenis, Synosternus cleopatrae pyramidis, Nosopsyllus iranus theodori and Stenoponia tripectinata medialis). We compared RMR between sexes and among species and examined whether there is intra- and interspecific correlation between RMR and jumping ability. Both mass-specific and mass-independent RMR were the highest in female S. t. medialis, whereas mass-specific RMR was the lowest in male X. dipodilli and mass-independent RMR was the lowest in three Xenopsylla species and P. chephrenis. Mass-specific and mass-independent RMR were significantly higher in females than in males in all fleas except S. t. medialis. Differences in jumping ability between males and females were found to be correlated with sexual differences in mass-specific or mass-independent RMR. Interspecific comparison showed that the length of jump in both male and female fleas was strongly affected by their mass-specific and mass-independent RMR.