Modification Of Host Behavior Research Articles

Expression of trematode-induced zombie-ant behavior is strongly associated with temperature.

Parasite-induced modification of host behavior increasing transmission to a next host is a common phenomenon. However, field-based studies are rare, and the role of environmental factors in eliciting host behavioral modification is often not considered. We examined the effects of temperature, relative humidity (RH), time of day, date, and an irradiation proxy on behavioral modification of the ant Formica polyctena (Förster, 1850) by the brain-encysting lancet liver fluke Dicrocoelium dendriticum (Rudolphi, 1819). This fluke induces ants to climb and bite to vegetation by the mandibles in a state of temporary tetany. A total of 1264 individual ants expressing the modified behavior were observed over 13 non-consecutive days during one year in the Bidstrup Forests, Denmark. A sub-set of those ants (N = 172) was individually marked to track the attachment and release of infected ants in relation to variation in temperature. Infected ants primarily attached to vegetation early and late in the day, corresponding to low temperature and high RH, presumably coinciding with the grazing activity of potential herbivorous definitive hosts. Temperature was the single most important determinant for the induced phenotypic change. On warm days, infected ants altered between the manipulated and non-manipulated state multiple times, while on cool days, many infected ants remained attached to the vegetation all day. Our results suggest that the temperature sensitivity of the infected ants serves the dual purpose of exposing infected ants to the next host at an opportune time, while protecting them from exposure to high temperatures, which might increase host (and parasite) mortality.

Parasitism in ecosystem engineer species: A key factor controlling marine ecosystem functioning.

Although parasites represent a substantial part of marine communities' biomass and diversity, their influence on ecosystem functioning, especially via the modification of host behaviour, remains largely unknown. Here, we explored the effects of the bopyrid ectoparasite Gyge branchialis on the engineering activities of the thalassinid crustacean Upogebia pusilla and the cascading effects on intertidal ecosystem processes (e.g. sediment bioturbation) and functions (e.g. nutrient regeneration). Laboratory experiments revealed that the overall activity level of parasitized mud shrimp is reduced by a factor 3.3 due to a decrease in time allocated to burrowing and ventilating activities (by factors 1.9 and 2.9, respectively). Decrease in activity level led to strong reductions of bioturbation rates and biogeochemical fluxes at the sediment-water interface. Given the world-wide distribution of mud shrimp and their key role in biogeochemical processes, parasite-mediated alteration of their engineering behaviour has undoubtedly broad ecological impacts on marine coastal systems functioning. Our results illustrate further the need to consider host-parasite interactions (including trait-mediated indirect effects) when assessing the contribution of species to ecosystem properties, functions and services.

The costs of parasite infection: Effects of removing lungworms on performance, growth and survival of free‐ranging cane toads

Abstract Most research on the effects of parasites on their hosts has focused on the parasites of mammals or birds (especially, domesticated taxa) rather than systems in which the hosts are ectothermic wildlife species. We used experimental methods (antihelminthic drugs) to quantify the effects of lungworms (Rhabdias pseudosphaerocephala) on their anuran hosts, the invasive cane toad (Rhinella marina). In captivity, eradicating lungworms enhanced toad activity (measures of boldness and level of spontaneous activity), performance (locomotor speed, climbing ability) and foraging success (feeding rate). In free‐ranging toads (n = 123) at a site in tropical Australia, eradicating lungworm infection increased rates of host survival by 8%, movement by 20%, growth by 28% and elaboration of male secondary sexual characteristics by 30%. The presence of the lungworm thus has a substantial negative effect on fitness‐related traits of the host. Given their long shared evolutionary history and the mild inflammatory and immune response elicited by the parasite in the host, the magnitude of the effects of parasite removal were surprising. Parasites may impose hidden costs, related to modification of host behaviour or metabolism. Experimental removal of parasites can be a useful means of quantifying costs of infection. A plain language summary is available for this article.

A parasite's modification of host behavior reduces predation on its host.

Parasite modification of host behavior is common, and the literature is dominated by demonstrations of enhanced predation on parasitized prey resulting in transmission of parasites to their next host. We present a case in which predation on parasitized prey is reduced. Despite theoretical modeling suggesting that this phenomenon should be common, it has been reported in only a few host–parasite–predator systems. Using a system of gregarine endosymbionts in host mosquitoes, we designed experiments to compare the vulnerability of parasitized and unparasitized mosquito larvae to predation by obligate predatory mosquito larvae and then compared behavioral features known to change in the presence of predatory cues. We exposed Aedes triseriatus larvae to the parasite Ascogregarina barretti and the predator Toxohrynchites rutilus and assessed larval mortality rate under each treatment condition. Further, we assessed behavioral differences in larvae due to infection and predation stimuli by recording larvae and scoring behaviors and positions within microcosms. Infection with gregarines reduced cohort mortality in the presence of the predator, but the parasite did not affect mortality alone. Further, infection by parasites altered behavior such that infected hosts thrashed less frequently than uninfected hosts and were found more frequently on or in a refuge within the microcosm. By reducing predation on their host, gregarines may be acting as mutualists in the presence of predation on their hosts. These results illustrate a higher‐order interaction, in which a relationship between a species pair (host–endosymbiont or predator–prey) is altered by the presence of a third species.

Competitive growth, energy allocation, and host modification in the acanthocephalan Acanthocephalus dirus: field data.

The acanthocephalan Acanthocephalus dirus is a trophically transmitted parasite that modifies both the physiology and behavior of its intermediate host (isopod) prior to transmission to its definitive host (fish). Infected isopods often contain multiple A. dirus individuals and we examined the relationships between host sharing, body size, energy content, and host modification to determine if host sharing was costly and if these costs could influence the modification of host behavior (mating behavior). Using field-based measures of parasite energy content (glycogen, lipid) and parasite body size (volume), we showed that host sharing was costly in terms of energy content but not in terms of body size. Analysis of the predictors of host behavior revealed that energy content, and body size, were not predictors of host behavior. Of the variables examined, parasite intensity was the only predictor of host behavior. Hosts that contained more parasites were less likely to be modified (i.e., less likely to undergo mating suppression). We suggest that intraspecific competition influenced parasite energy content and that the costs associated with competition are likely to shape the strategy of growth and energy allocation adopted by the parasites. These costs did not appear to have a direct effect on the modification of host mating behavior.

A survey of innovation through duplication in the reduced genomes of twelve parasites.

We characterize the prevalence, distribution, divergence, and putative functions of detectable two-copy paralogs and segmental duplications in the Apicomplexa, a phylum of parasitic protists. Apicomplexans are mostly obligate intracellular parasites responsible for human and animal diseases (e.g. malaria and toxoplasmosis). Gene loss is a major force in the phylum. Genomes are small and protein-encoding gene repertoires are reduced. Despite this genomic streamlining, duplications and gene family amplifications are present. The potential for innovation introduced by duplications is of particular interest. We compared genomes of twelve apicomplexans across four lineages and used orthology and genome cartography to map distributions of duplications against genome architectures. Segmental duplications appear limited to five species. Where present, they correspond to regions enriched for multi-copy and species-specific genes, pointing toward roles in adaptation and innovation. We found a phylum-wide association of duplications with dynamic chromosome regions and syntenic breakpoints. Trends in the distribution of duplicated genes indicate that recent, species-specific duplicates are often tandem while most others have been dispersed by genome rearrangements. These trends show a relationship between genome architecture and gene duplication. Functional analysis reveals: proteases, which are vital to a parasitic lifecycle, to be prominent in putative recent duplications; a pair of paralogous genes in Toxoplasma gondii previously shown to produce the rate-limiting step in dopamine synthesis in mammalian cells, a possible link to the modification of host behavior; and phylum-wide differences in expression and subcellular localization, indicative of modes of divergence. We have uncovered trends in multiple modes of duplicate divergence including sequence, intron content, expression, subcellular localization, and functions of putative recent duplicates that highlight the role of duplications in the continuum of forces that have shaped these genomes.

Naturally occurring Toll-like receptor 11 (TLR11) and Toll-like receptor 12 (TLR12) polymorphisms are not associated with Toxoplasma gondii infection in wild wood mice

Toxoplasma gondii is a highly successful parasite with a worldwide prevalence. Small rodents are the main intermediate hosts, and there is growing evidence that T. gondii modifies their behaviour. Chronically infected rodents show impaired learning capacity, enhanced activity, and, most importantly, a reduction of the innate fear towards cat odour. This modification of host behaviour ensures a successful transmission of T. gondii from rodents to felids, the definitive hosts of the parasite. Given the negative fitness consequences of this behavioural manipulation, as well as an increased mortality during the acute phase of infection, we expect rodents to evolve potent resistance mechanisms that prevent or control infection. Indeed, studies in laboratory mice have identified candidate genes for T. gondii resistance. Of particular importance appear to be the innate immune receptors Toll-like receptor 11 (TLR11) and Toll-like receptor 12 (TLR12), which recognise T. gondii profilin and initiate immune responses against the parasite.Here we analyse the genetic diversity of TLR11 and TLR12 in a natural population of wood mice (Apodemus sylvaticus), and test for associations between TLR11 and TLR12 polymorphisms and T. gondii infection, as well as for epistatic interactions between TLR11 and TLR12 on infection status. We found that both TLR11 and TLR12 were polymorphic in wood mice, with four and nine amino acid haplotypes, respectively. However, we found no evidence that TLR11 or TLR12 genotypes or haplotypes were significantly associated with Toxoplasma infection. Despite the importance of TLR11 and TLR12 in T. gondii recognition and immune defence initiation, naturally occurring polymorphisms at TLR11 and TLR12 thus appear to play a minor role in mediating qualitative resistance to T. gondii in natural host populations of A. sylvaticus. This highlights the importance of assessing the role of candidate genes for parasite resistance identified in a laboratory setting in an ecologically meaningful context to quantify their role in mediating host–parasite interactions in the wild.

Microbial Endocrinology in the Microbiome-Gut-Brain Axis: How Bacterial Production and Utilization of Neurochemicals Influence Behavior

The ability of bacterial pathogens to influence behavior has been recognized for decades, most notably bacteria that directly invade the nervous system. However, increasing evidence is mounting that microorganisms may directly interact with elements of the host's neurophysiological system in a noninvasive manner that ultimately results in modification of host behavior. This ability of microorganisms contained within the microbiome to influence behavior through a noninfectious and possibly non-immune-mediated route may be due to their ability to produce and recognize neurochemicals that are exactly analogous in structure to those produced by the host nervous system. This form of interkingdom signaling, which is based on bidirectional neurochemical interactions between the host's neurophysiological system and the microbiome, was introduced two decades ago and has been termed microbial endocrinology [1]. Many of the neuroendocrine hormone biosynthetic pathways that are more commonly associated with eukaryotic cells are found in prokaryotic cells, and the acquisition of such neurochemical-based synthesis pathways by eukaryotic systems is believed to be due to lateral gene transfer from bacteria. Approaching the microbiome from a microbial endocrinology-based vantage point may provide an understanding of the specific pathways by which microorganisms may influence behavior and thereby lead to new approaches to the treatment of specific mental illness based on modulation of the microbiome-gut-brain axis.

Ecto- and endoparasite induce similar chemical and brain neurogenomic responses in the honey bee (Apis mellifera)

BackgroundExclusion from a social group is an effective way to avoid parasite transmission. This type of social removal has also been proposed as a form of collective defense, or social immunity, in eusocial insect groups. If parasitic modification of host behavior is widespread in social insects, the underlying physiological and neuronal mechanisms remain to be investigated. We studied this phenomenon in honey bees parasitized by the mite Varroa destructor or microsporidia Nosema ceranae, which make bees leave the hive precociously. We characterized the chemical, behavioral and neurogenomic changes in parasitized bees, and compared the effects of both parasites.ResultsAnalysis of cuticular hydrocarbon (CHC) profiles by gas chromatography coupled with mass spectrophotometry (GC-MS) showed changes in honey bees parasitized by either Nosema ceranae or Varroa destructor after 5 days of infestation. Levels of 10-HDA, an antiseptic important for social immunity, did not change in response to parasitism. Behavioral analysis of N. ceranae- or V. destructor- parasitized bees revealed no significant differences in their behavioral acts or social interactions with nestmates. Digital gene expression (DGE) analysis of parasitized honey bee brains demonstrated that, despite the difference in developmental stage at which the bee is parasitized, Nosema and Varroa-infested bees shared more gene changes with each other than with honey bee brain expression gene sets for forager or nurse castes.ConclusionsParasitism by Nosema or Varroa induces changes to both the CHC profiles on the surface of the bee and transcriptomic profiles in the brain, but within the social context of the hive, does not result in observable effects on her behavior or behavior towards her. While parasitized bees are reported to leave the hive as foragers, their brain transcription profiles suggest that their behavior is not driven by the same molecular pathways that induce foraging behavior.

Effects of parasitism on diel vertical migration, phototaxis/geotaxis, and swimming speed of the bloom-forming dinoflagellate Akashiwo sanguinea

Eukaryotic parasites are believed to play important roles in bloom dynamics of red-tide dinoflagellates; however, little is known about their impact on host behavior. To address this issue, we examined the influence of the parasitic dinoflagellate Amoebophrya sp. on diel vertical migration (DVM), phototaxis/geotaxis, and swimming speed of the bloom-forming dinoflagellate Akashiwo sanguinea. Uninfected A. sanguinea exhibited strong DVM, yielding subsurface aggregations during the day and evenly dispersed populations at night. DVM of parasitized A. sanguinea was compara- ble to that of uninfected host for much of the infection cycle; however, subsurface accumulations of host cells were dramatically reduced or eliminated as parasites matured. The absence of DVM in the latter part of the infection cycle appeared to be associated with changes in phototaxic response and swimming speed of parasitized cells. While uninfected A. sanguinea exhibited strong positive photo- taxis, hosts with late-stage infections exhibited strong negative phototaxis. By contrast, infected and uninfected A. sanguinea did not differ in geotaxis or sinking. Swimming speed of infected A. san- guinea decreased with parasite age, differing significantly from that of uninfected hosts 24 h after ini- tiation of infections. A. sanguinea with late-stage infections exhibited a 37.3% lower swimming speed than uninfected hosts. Results indicate that parasitism by Amoebophrya sp. disrupts 'internal' factors that control DVM in dinoflagellates. Data also suggest that modification of host behavior by para- sitism may contribute to vertical variations in phytoplankton species-composition and -abundance, particularly during epidemics when late-stage infections are prevalent.

Male-like mudballing behavior of some female fiddler crabs ( Uca tangeri )

At each low tide, male and female Uca tangeri remove mudballs from inside their burrows and place them on the surface. Previous studies have shown clear inter- sexual differences in mudball arrangements. However, we noticed that some females placed their mudballs in an ar- rangement similar to that of males. In this study, we inves- tigated several factors that may have been responsible for this change in female mudballing behavior. We found no significant effect of the lunar cycle, female size and repro- ductive state, or burrow features. We briefly discuss the avoidance of sexual coercion or parasite modification of host behavior as possible factors. Our study shows that intersexual differences in mudballing behavior are more complex than previously thought.

Wipfelkrankheit : modification of host behaviour during baculoviral infection

Infection with an endoparasite frequently alters host behaviour. This study provides the first quantification of larval behaviour in a baculovirus/ Lepidoptera system, and attempts to assess the ecological consequences of behavioural modification during infection. Larvae of the moth Mamestra brassicae (Lepidoptera: Noctuidae) exhibited higher rates of dispersal in the laboratory and field when infected with Mamestra brassicae nuclear polyhedrosis virus (MbNPV) than did uninfected larvae. They adopted positions at death which were not characteristic of healthy larvae, climbing higher on the foodplant and onto the top and edge of leaves. The horizontal and vertical distribution of virus following larval lysis and the effects of rainfall on this distribution were assessed for comparison with the distributions of healthy and infected larvae. Exposure to rainfall increased the infectivity of vegetation in bioassays. Alternative explanations for the evolutionary origins of behavioural modification are considered. I suggest that the behavioural changes observed are most likely to benefit the virus. In particular, climbing prior to death is likely to result in contamination of more foliage with virus particles than would otherwise occur by increasing exposure of cadavers to rainfall. Thus it may profoundly influence horizontal transmission and the dynamics of the host-virus interaction.

Parasitic infection impairs spatial learning in mice

Although parasite modification of host behaviour is well established, little is known about the effects of parasites on the spatial learning abilities of the host. The effects of an acute, sub-clinical infection with the naturally occurring, single host, enteric, protozoan parasite, Eimeria vermiformis, on spatial water-maze learning by male laboratory mice, Mus musculus, were examined. Individual mice had to learn the spatial location of a submerged hidden platform using extra-maze visual cues. Determinations of spatial acquisition were made with different groups of mice on days 4 (pre-nfective), 7 (onset of infectivity or patency) and 10 (infective) post-infection. Task retention was examined in a probe trial 1 day later. The infected mice displayed significantly poorer acquisition and retention of the water-maze task over 1 day of testing (six blocks of four trials) than either sham-infected or control mice, with mice at days 4 and 7 post-infection displaying significantly poorer spatial performance than mice on day 10 post-infection. This attenuation of spatial learning occurred in the absence of either any symptoms of malaise or illness, or any evident motor, visual and motivational impairments. These parasitic-infection-induced decreases in spatial learning are likely to arise as side-effects of the immunological and neuromodulatory responses of the host and may be considered as a fitness cost associated with response to infection.

Reduced spatial learning in mice infected with the nematode, Heligmosomoides polygyrus

Parasite modification of host behaviour influences a number of critical responses, but little is known about the effects on host spatial abilities. This study examined the effects of infection with the intestinal trichostrongylid nematode, Heligmosomoides polygyrus, on spatial water maze learning by male laboratory mice, Mus musculus. In this task individual mice had to learn the spatial location of a submerged hidden platform using extramaze visual cues. Determinations of spatial performance were made on day 19 post-infection with mice that had been administered either 50 or 200 infective larvae of H. polygyrus. The infected mice displayed over 1 day of testing (6 blocks of 4 trials) significantly poorer acquisition and retention of the water maze task than either sham-infected or control mice, with mice that had received 200 infective larvae displaying significantly poorer spatial performance than individuals receiving 50 larvae. The decrease in spatial learning occurred in the absence of either any symptoms of illness and malaise, or any evident motor, visual and motivational impairments. It is suggested that in this single host system the parasitic infection-induced decrease in spatial learning arises as a side-effect of the host's immunological and neuromodulatory responses and represents a fitness cost of response to infection.

Multiple opioid system involvement in the mediation of parasitic-infection induced analgesia

Although parasite modification of host behaviour is well established, little is known about the mechanisms underlying such effects. The present study examined the relationships between subclinical infection with the enteric sporozon parasite, Eimeria vermiformis, nociceptive responses and endogenous opioid systems in male mice. Infected mice displayed significant analgesia which increased through the prepatent period [oocyst formation (pre-infective); days 1–7 post-infection (PI)], reached a maximum with the onset of patency (onset oocyst shedding and infectivity; days 7–8 PI) and declined during patency (oocyst shedding), with response latencies declining to basal levels with the cessation of oocyst production and infectivity (day 15 PI). The increasing nociception during the prepatent period (day 4 PI) was associated with k opioid mechanisms, being reduced by the k antagonist, nor-binaltorphimine, and insensitive to either the σ antagonist, ICI 174,864, or the general, predominately μ antagonist, naloxone. Maximum analgesia (day 7 PI) associated with the onset of patency (infectivity) was sensitive to both the k and μ antagonists, but insensitive to the σ antagonist, while the declining analgesia during patency (day 10 PI) was reduced by the μ and σ antagonists, but was insensitive to the k antagonist. These results indicate that μ, σ and k opioid systems are involved in the mediation of su subclinical parasitic infection-induced analgesia and likely other associated parasite-induced modifications of host behaviour.

Evidence for Involvement of Endogenous Opioid Peptides in Altered Nociceptive Responses of Mice Infected with Eimeria vermiformis

Parasite modification of host behavior is a well established phenomenon; however, little is known about the modulatory mechanisms regulating such effects. This study examined the relationship between Eimeria vermiformis infection, nociceptive responses, and endogenous opioid peptide activity in male RML mice. Infected mice displayed increases in centrally mediated antinociceptive responses (i.e., analgesia, measured as the latency of a foot-lifting response to a 50 C surface) throughout the prepatent period. The level of analgesia declined following onset of patency on day 8 postinfection (PI). Opioids were implicated in the increased antinociceptive response of the infected mice as the response was blocked by administration of a prototypic opiate antagonist, naloxone (1.0 mg/kg), on day 7 PI when maximum levels of analgesia were noted. This indicates that the analgesia evident in the parasitized mice was associated with increased opioid activity. Analgesia is one of a variety of behaviors influenced by changes in opioid activity, thus these observations provide further support for the contention that other parasite-induced alterations in host behavior may, in part, be the result of alterations in the activity of opioid modulatory systems.

Host behaviour modification by the endoparasitoid Aphidius nigripes: a strategy to reduce hyperparasitism

Abstract. 1. One possible component of successful parasitism by insect parasitoids is the reduction of predation and hyperparasitism through the modification of host behaviour.2. Just prior to death, potato aphids, Macrosiphum euphorbiae (Thomas), containing diapausing larvae of the parasitoid Aphidius nigripes Ashmead have been shown to leave the host plant to mummify in concealed sites, while those parasitized by non‐diapausing individuals generally leave the aphid colony to mummify on the upper leaf surfaces.3. The present study examines the within plant distribution of aphids and mummies containing non‐diapausing A.nigripes, and compares parasitoid survival among microhabitats under field conditions.4. On potato, Solunum tuberosum L., a significant proportion of aphids were found on lower leaf surfaces, whereas mummies were usually on the upper surfaces, particularly in the apical stratum of the plant canopy.5. In both field surveys and experimental manipulations, parasitoid survival was higher on the upper surface of apical leaves than in other microhabitats, due to lower levels of hyperparasitism and, to a lesser extent, lower predation. This suggests that the pressure exerted by natural enemies has influenced the evolution of host behaviour modification.

Seasonal Microhabitat Selection by an Endoparasitoid Through Adaptive Modification of Host Behavior

Differences in the distribution of parasitized and unparasitized hosts has been used to infer modification of host behavior by insect parasitoids. Data are presented showing that not only do parasitized hosts behave differently from unparasitized ones, but that the behavior of parasitized hosts varies in function of the physiological state of the parasitoid. Aphids containing nondiapausing parasitoids leave the aphid colony and mummify on the upper surface of the leaves, whereas those containing diapausing parasitoids leave the host plant and mummify in concealed sites. Modification of host behavior by diapausing parasitoids results in the selection of a suitable microhabitat that reduces the incidence of hyperparasitism and should decrease the action of adverse climatic conditions during the lengthy dormant period.

Vertical distribution of an estuarine snail altered by a parasite.

Estuarine snails Ilyanassa obsoleta bearing larvae of the trematode Gynaecotyla adunca behave singularly in comparison with conspecifics lacking this parasite. Following high tides, and especially at night, infected snails were found stranded high on beaches and sandbars. Semiterrestrial crustaceans living well up on the shore serve as the next host, and the modified (induced) snail behavior is apparently a parasite adaptation facilitating cercarial transmission to these crustaceans. The altered behavior is unusual because of its apparent enhancement of host-to-host transmission by cercariae rather than predation, the process commonly recognized as being enhanced by parasitic modification of host behavior.

Behavioral Interactions between Parasites and Hosts: Host Suicide and the Evolution of Complex Life Cycles

Parasites and their hosts show behavioral as well as physiological interactions. Parasites are able to alter the behavior of their hosts, and hosts in turn may employ behavioral retaliation in response to parasitic infection. The four types of endoparasites discussed in the paper are parasitoids (group 1), parasitic castrators (group 2), single-host true parasites (group 3) and multihost true parasites (group 4). For each of these four types of parasites the role of parasite induced modifications of host behavior is analyzed with respect to (1) dispersal of the parasite propagules to new hosts, (2) modification of the host's energy budget to provide energy for the parasite's growth and maturation, and (3) keeping the host alive until the parasite has completed its life cycle. It is found that modification of the host's energy budget is profitable for parasitoids and castrators, but not for group 3 parasites. During some stages of its life cycle the group 4 parasite can employ modification of the host's energy budget, but only if the larval parasite resembles parasitoids or castrators in certain ways. Modification of host behavior to facilitate dispersal of propagules to new hosts, or to the next host in a series of hosts, should be employed most often by the intermediate larval stages of group 4 parasites. The reproductive fitness of the host is reduced by its parasitic load. A parasitoid or castrator may reduce the host's reproductive fitness to zero, while a single- or multiple-host true parasitic infection may scarcely impair it. A host may defend itself by preventing infection, or, once infected it can protect its kin by attempting to prevent their infection. Host suicide is the term coined to describe aberrant behavior on the part of an infected host which leads to increased probability of death by predation. Host suicide will increase the inclusive fitness of the host if (1) the suicide prevents the maturation of the parasite; (2) the mature parasite is more likely to infect the host's kin than nonkin; and (3) the benefit to the host, in terms of the increased fitness of the kin, is greater than the cost of the suicide, measured in terms of the loss of the host's own reproductive fitness. Host suicide is most profitable when the host is infected by a parasitoid. The parasitoid reduces the host's reproductive fitness to zero, so that the cost of the suicide is also zero. Suicide is of variable value when the host is infected by a castrator, the value depending on the degree to which the parasitoid reduces the host's reproductive fitness. Host suicide is not expected when the host is infected with group 3 or 4 parasites. Mature parasitoids may be more likely to infect the host's kin than nonkin in social or colonial animals, or in species with small, isolated, inbred populations. The similarity between host suicide and the passage of group 4 parasites from intermediate hosts to definitive hosts is noted, and the possible role of host suicide in the evolution of such complex parasitic life cycles is examined.