Burying Beetle Parents Adaptively Manipulate Information Broadcast from a Microbial Community.

AbstractMicrobial volatiles provide essential information for animals, which compete to detect, respond to, and perhaps control this information. Burying beetle parents have the opportunity to influence microbially derived semiochemicals, because they monopolize a small carcass for their family, repairing feeding holes and applying exudates that alter the microbial community. To study adaptive manipulation of microbial cues, we integrated mechanistic and functional approaches. We contrasted gas chromatography-mass spectrometry (GC-MS) volatile profiles from carcasses that were or were not prepared by a resident pair of Nicrophorus orbicollis. Methyl thiocyanate (MeSCN), the primary attractant for burying beetles seeking a fresh carcass, was reduced 20-fold by carcass preparation, while dimethyl trisulfide (DMTS), which deters breeding beetles, was increased 20-fold. These results suggest that parental care serves to make previously public information more private (crypsis, MeSCN) and to disinform rivals with a deterrent (DMTS). Functional tests in the field demonstrated that carcass preparation reduced discovery and use by congeners (threefold) as well as by dipteran rivals. Because microbes and their chemicals influence nearly every aspect of animal ecology, animal manipulation of microbial cues may be as widespread as manipulation of their own signals.

Juvenile hormone and parental care in subsocial insects: implications for the role of juvenile hormone in the evolution of sociality.

Juvenile hormone (JH) has both gonadotropic and non-gonadotropic roles in eusocial insects. There is controversy over whether the non-gonadotropic role is novel, related specifically to social evolution, or is a second conserved role. Study of subsocial insects suggests that when JH is non-gonadotropic, the specific associations of high JH, low vitellogenin, suppressed ovarian development and elevated metabolism may parallel those in workers of eusocial insects. This suggests that a more fundamental understanding of JH in insect biology is required.

Offspring dependence on parental care and the role of parental transfer of oral fluids in burying beetles.

BACKGROUND: Immature stages of many animals can forage and feed on their own, whereas others depend on their parents' assistance to obtain or process food. But how does such dependency evolve, and which offspring and parental traits are involved? Burying beetles (Nicrophorus) provide extensive biparental care, including food provisioning to their offspring. Interestingly, there is substantial variation in the reliance of offspring on post-hatching care among species. Here, we examine the proximate mechanisms underlying offspring dependence, focusing on the larvae of N. orbicollis, which are not able to survive in the absence of parents. We specifically asked whether the high offspring dependence is caused by (1) a low starvation tolerance, (2) a low ability to self-feed or (3) the need to obtain parental oral fluids. Finally, we determined how much care (i.e. duration of care) they require to be able to survive. RESULTS: We demonstrate that N. orbicollis larvae are not characterized by a lower starvation tolerance than larvae of the more independent species. Hatchlings of N. orbicollis are generally able to self-feed, but the efficiency depends on the kind of food presented and differs from the more independent species. Further, we show that even when providing highly dependent N. orbicollis larvae with easy ingestible liquefied mice carrion, only few of them survived to pupation. However, adding parental oral fluids significantly increased their survival rate. Finally, we demonstrate that survival and growth of dependent N. orbicollis larvae is increased greatly by only a few hours of parental care. CONCLUSIONS: Considering the fact that larvae of other burying beetle species are able to survive in the absence of care, the high dependence of N. orbicollis larvae is puzzling. Even though they have not lost the ability to self-feed, an easily digestible, liquefied carrion meal is not sufficient to ensure their survival. However, our results indicate that the transfer of parental oral fluids is an essential component of care. In the majority of mammals, offspring rely on the exchange of fluids (i.e. milk) to survive, and our findings suggest that even in subsocial insects, such as burying beetles, parental fluids can significantly affect offspring survival.

Carcass Age and Reproductive Costs for Nicrophorus orbicollis (Coleoptera: Silphidae).

The life history costs of living in microbially dense environments can be high. To assess the importance of these costs for natural selection, it is necessary to know how costs vary with environmental conditions. Adult burying beetles prepare vertebrate carcasses of varying age and states of deterioration as a resource for their young. In the present study, Nicrophorus orbicollis Say females were presented with either fresh or aged (80 h) large carcasses (30-39 g) to examine effects on their lifetime reproductive success (four reproductive attempts) and survival. Reproductive success and survival were also measured for female offspring that developed on fresh and aged carcasses. Carcass age had no measurable effect on lifetime reproductive output or life span of either parents or their adult offspring. In a separate experiment using medium-sized carcasses (22-24 g), the reproductive performance of females in a single reproductive attempt was negatively affected by carcass age (13.9% less brood mass). The results suggest that the costs of using a single older carcass are relatively minor when measured over a lifetime. Alternative costs of using older carcasses and explanations for the complex antimicrobial traits of burying beetles are discussed.

Molecular systematics and biogeography of Nicrophorus in part--the investigator species group (Coleoptera: Silphidae) using mixture model MCMC.

Burying beetles (Silphidae: Nicrophorus) are well-known for their biparental care and monopolization of small vertebrate carcasses in subterranean crypts. They have been the focus of intense behavioral ecological research since the 1980s yet no thorough phylogenetic estimate for the group exists. The relationships among the species, and the validity of some species, are poorly understood. Here, we infer the relationships and examine species boundaries among 50 individuals representing 15 species, primarily of the investigator species group, using a mixture-model Bayesian analysis. Two mitochondrial genes, COI and COII, were used, providing 2129 aligned nucleotides (567 parsimony-informative). The Akaike Information Criterion and Bayes Factors were used to select the best fitting model, in addition to Reversible Jump MCMC, which accommodated model uncertainty. A 21 parameter, three-partition GTR+G was the final model chosen. Despite a presumed Old World origin for the genus itself, the basal lineages and immediate outgroups of the investigator species group are New World species. Bayesian methods reconstruct the common ancestor of the investigator species group as New World and imply one later transition to the Old World with two return transitions to the New World. Prior hypotheses concerning the questionable validity of four species names, Nicrophorus praedator, Nicrophorus confusus, Nicrophorus encaustus and Nicrophorus mexicanus were tested. No evidence was found for the validity of the Nicrophorus investigator synonym N. praedator. We found evidence rejecting the species status of N. confusus (NEW SYNONYM of Nicrophorus sepultor). Weak evidence was found for the species status of N. encaustus and N. mexicanus, which are tentatively retained as valid. Our results strongly reject a recently published hypothesis that Nicrophorus interruptus (NEW STATUS as valid species) is a subspecies of N. investigator.

Social and nonsocial stimuli and juvenile hormone titer in a male burying beetle, Nicrophorus orbicollis.

We investigated the interaction of social and nonsocial stimuli on juvenile hormone (JH) titer in male burying beetles (Nicrophorus orbicollis). The initial JH response to discovery of a carcass was substantial (10-15-fold increase over controls) and rapid (<1h), and occurred whether or not a female was present. By 3h after discovery, JH titers were declining, the decline being more pronounced when a female was not present. We also tested the effect of larval stimulation on JH titer in care-giving males by removing a male's brood and replacing it with a brood of first or third instar larvae. Males initially providing care for begging first instar larvae continued to maintain high titers of JH when the replacement broods were first but not third instars. Males caring for third instar larvae (normally low JH titers) maintained low levels of JH regardless of the developmental stage of the replacement brood. This suggests that once males begin to care for nutritionally independent third instar larvae, JH titers remain low regardless of subsequent larval stimulation. Burying beetles are socially and hormonally complex organisms in which stimuli from a breeding resource, mating partners, rivals and young interact to alter the JH profile of breeding adults.

Can the "challenge hypothesis" be applied to insects?

The challenge hypothesis, conceived for testosterone and vertebrates, has recently been applied to juvenile hormone (JH) and insects. Scott [Scott, M.P., 2006a. Resource defense and juvenile hormone: the "challenge hypothesis" extended to insects. Horm. Behav. 49, 276-281] found that JH in the burying beetle Nicrophorus orbicollis increased in response to a social challenge in the presence of a breeding resource, while there was no such JH response in the purported brood parasite Nicrophorus pustulatus. Two important implications of the challenge hypothesis that need to be tested are whether JH affects dominance and why there are interspecific differences in the JH response to a social challenge. The effect of JH on dominance in burying beetles was examined by topical application of JH III to one of two competing females at 24 h and again at 1 h prior to presentation of a breeding resource (mouse carcass). JH supplementation had no effect on dominance in intraspecific interactions in N. orbicollis, as measured by possession of the carcass on Day 1, 3 or 7. Similarly, JH did not alter carcass ownership during competition between N. orbicollis and N. pustulatus nor did it affect reproductive success. Irrespective of JH supplementation, N. pustulatus became increasingly dominant as the trials progressed, rarely occupying the carcass on Day 1, but excluding N. orbicollis in nearly half the trials by Day 7. These findings, and a brief review of the burying beetle literature, suggest that the challenge hypothesis, as applied to testosterone and vertebrates, does not yet have an analogous model for JH and insects.

Nutrition, hormones and life history in burying beetles.

Nutrition, hormones and the allocation of physiological resources are intricately related. To investigate these inter-relationships in female burying beetles (Nicrophorus spp.), we examined the effect of diet quality on juvenile hormone (JH) levels and reproduction, and the effect of JH supplementation on reproduction and resistance to starvation. Nicrophorus orbicollis adult females fed a less preferred mealworm larvae diet gained less body mass, had smaller ovaries and had lower titers of JH in their hemolymph than females fed a preferred blowfly diet. When presented a carcass for breeding, females on a less preferred diet oviposited 33% fewer eggs, and eggs were of 18% less mass. Females on the less preferred diet also took longer to begin oviposition as indicated indirectly by the time when their eggs hatched. To investigate the effects of JH, independent of nutrition, JH was topically applied to single and paired females of Nicrophorus tomentosus. When presented a carcass, JH-treated paired females oviposited more eggs (28%-year 1, 44%-year 2) than control females, and also showed a trend toward faster oviposition. JH supplementation had a greater effect on single females. JH treatment increased the proportion of single females attempting reproduction (at least one viable larva), increased the number of eggs (69%-year 1, 123%-year 2), and increased the proportion of females ovipositing early. In separate experiments, treatment with JH or a JH analog negatively affected resistance to starvation in three species. Treatment with JH reduced starvation survival by 10.3% days in N. tomentosus females. Treatment with the JH analog methoprene reduced starvation survival 17.8% in N. orbicollis females and by 18% in Ptomascopus morio females. These results suggest that JH has positive and negative effects on different components of life history.