Feeding upon and preserving a carcass: the function of prehatch parental care in a burying beetle

Effects of Nutritional Status During Sexual Maturation and Resource Availability on the Resource Allocation of Females in Burying Beetles

Resource availability should have consequences for life-history functions and trade-offs among them because it influences the amounts of resources allocated to different functions. Nutritional status during a key developmental window (sexual maturation) may also have an important impact on life-history functions and such trade-offs. However, less is known about whether and how they interact to influence the resource allocation of individuals. Here, we simultaneously manipulated female nutritional status during sexual maturation and resource availability during breeding in a burying beetle Nicrophorus vespilloides. We then monitored the main and interactive effects of these two factors on somatic maintenance and reproductive performance of burying beetle females. We found that variation in nutritional status during sexual maturation affects the resource allocation of burying beetle females only at the pre-hatching stage. Poor-fed females compensated for the initial differences in energy reserves by feeding from the carcass or engaged in terminal investment strategy and invested heavily at the post-hatching stage. Specifically, poor-fed females allocated more into somatic maintenance (gained more weight) and less into reproduction (provided less pre-hatching care) than well-fed females, whereas they provided a similar amount and duration of post-hatching care. In addition, burying beetles with different nutritional statuses vary in their response to resource availability. Poor-fed females allocated more into both somatic maintenance (gained more weight) and reproduction (provided more pre-hatching care) when bred on large versus small carcasses, whereas well-fed females tend to work near their maximum capacity and thus show no response to resource availability. Finally, our findings suggest that poor-fed females did not suffer a future cost in offspring performance. Meanwhile, a large carcass allowed females to produce more and heavier offspring. These findings enhance our understanding of how important nutritional status during a key developmental window and resource availability during breeding is for the expression of resource allocation.

Competition, cooperation, and parental effects in larval aggregations formed on carrion by communally breeding beetles Necrodes littoralis (Staphylinidae: Silphinae)

Aggregations of juveniles are dominant forms of social life in some insect groups. Larval societies are shaped by competitive and cooperative interactions of the larvae, in parallel with parental effects. Colonies of necrophagous larvae are excellent systems to study these relationships. Necrodes littoralis (Staphylinidae: Silphinae), a carrion beetle that colonizes cadavers of large vertebrates, forms massive juvenile aggregations. By spreading over carrion anal and oral exudates, the beetles form the feeding matrix, in which the heat is produced and by which adults presumably affect the fitness of the larvae. We predict that exploitative competition shapes the behavior of N. littoralis larvae in their aggregations. However, cooperative interactions may also operate in these systems due mainly to the benefits of collective exodigestion. Moreover, indirect parental effects (i.e., formation of the feeding matrix) probably modulate larval interactions within the aggregations. By manipulating parental effects (present/absent) and larval density (0.02-1.9 larvae/g of meat), we found a strong negative group-size effect on fitness components of N. littoralis, in colonies with parental effects over almost the entire density range, and in colonies without parental effects for densities larger than 0.5 larva/g. This was accompanied by positive group-size effects in terms of development time (it shortened with larval density) and thermogenesis (it increased with larval density). A pronounced positive group-size effect on juvenile fitness was found only in colonies without parental effects and only in the low-density range. These results support the hypothesis that larval societies of N. littoralis are shaped by exploitation competition.

The scent of offspring: chemical profiles of larvae change during development and affect parental behavior in a burying beetle

Chemical cues and signals, especially in insects, play a pivotal role in mediating interactions between individuals. Past studies have largely focused on adult semiochemicals and have neglected those of juvenile stages. Especially in the context of parental care, the larval odor might have a profound impact on parenting behavior, guiding parents in how much resources they should allocate to the different developmental stages. However, whether ontogenetic changes occur in subsocial species and whether larval-emitted scents influence parent-offspring interactions is largely unknown. Using 3 different sampling techniques, we analyzed the cuticular and VOC profile of the 3 larval instars of the burying beetle Nicrophorus vespilloides, which is known for its elaborate parental care. We found distinct differences in the cuticular and VOC profiles across the 3 larval stages. Second-instar larvae, which receive more frequent feedings from parents than the other larval stages, released greater amounts of acetophenone, methyl geranate, and octanoic acid isopropyl ester than the first and third instar. Additionally, using a newly developed bioassay with automated video tracking, we found that adding the odor of second-instar larvae to first-instar larvae increased the number of maternal feeding trips. Our results suggest that the odor produced by larvae plays an important role in mediating parent-offspring interactions. Given these findings, burying beetles might emerge as a promising candidate for identifying a potential begging pheromone.

Revisiting the ecology and evolution of burying beetle behavior (Staphylinidae: Silphinae)

Investigating fundamental processes in biology requires the ability to ground broad questions in species-specific natural history. This is particularly true in the study of behavior because an organism's experience of the environment will influence the expression of behavior and the opportunity for selection. Here, we provide a review of the natural history and behavior of burying beetles of the genus Nicrophorus to provide the groundwork for comparative work that showcases their remarkable behavioral and ecological diversity. Burying beetles have long fascinated scientists because of their well-developed parenting behavior, exhibiting extended post-hatching care of offspring that varies extensively within and across taxa. Despite the burgeoning success of burying beetles as a model system for the study of behavioral evolution, there has not been a review of their behavior, ecology, and evolution in over 25 years. To address this gap, we leverage a developing community of researchers who have contributed to a detailed knowledge of burying beetles to highlight the utility of Nicrophorus for investigating the causes and consequences of social and behavioral evolution.

The effect of pioneer carrion beetles on the emission of volatile organic compounds and carrion insect community assembly

Mechanisms of carrion insect succession have been interpreted separately from interspecific interactions between early and later colonists or from changes in volatile organic compounds perceived by insects resulting from the progression of decomposition. To link these perspectives, we examined through laboratory and field experiments whether the modification of volatile organic compounds by early colonists could be a mechanism of succession. In the laboratory experiment, we used Necrophila japonica (Coleoptera, Staphylinidae) as an early colonist and examined its effects on the emissions of important volatile attractants for carrion insects, dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS) from carcasses. We collected DMDS and DMTS, using the static headspace method, under the following conditions: (i) rat carcass, (ii) rat carcass with artificial damage to the abdomen, (iii) rat carcass fed on by 10 Ne. japonica individuals, and (iv) 10 Ne. japonica individuals, and analyzed the collected gases using a gas chromatograph-mass spectrometer. After 12 and 30 h, carcasses fed on by Ne. japonica emitted higher concentrations of DMDS and DMTS than in other conditions. In the field experiment, we examined the effects of DMDS and DMTS on the attraction of carrion insects using traps baited with a mixture of DMDS and DMTS, hexane (odors unrelated to carcasses), or an empty microtube. Traps baited with DMDS and DMTS attracted more necrophagous species and individuals than traps not baited with this combination. These results showed that accelerated emissions of DMDS and DMTS from carcasses caused by early colonists may contribute to community assembly during carrion insect succession.

Detection of reproductive trade-offs is influenced by resource availability and maintenance: an experimental study in the burying beetle (Nicrophorus vespilloides)

Abstract

Life-history theory predicts trade-offs between investment in current versus future reproduction. However, many studies find no or even positive correlations among these traits. The absence of the trade-off may result from resource availability, as it influences resource allocation to different traits. In addition, since large amounts of resources require additional effort in processing, resource maintenance may affect the detection of reproductive trade-offs. Here, we carried out two breeding attempts to assess the effects of resource availability and maintenance on reproductive trade-offs for both sexes in the burying beetle (Nicrophorus vespilloides). In the first breeding attempt, we simultaneously manipulated carcass size (small versus large) and carcass preparation (non-prepared versus prepared). In the second breeding attempt, we provided parents with same-sized, non-prepared carcasses. For both breeding attempts, we monitored the main and interactive effects of carcass size and carcass preparation on parental effort and reproductive outcome. In the first breeding attempt, males gained more weight and provided more care as carcass size increased, whereas females gained more weight but did not change their care. In addition, when breeding on non-prepared versus prepared carcasses, both parents provided more care and gained more weight. In the second breeding attempt, with increased investment for the first breeding, parents did not show decreased investment for the second brood, vice versa. In contrast, males breeding on large or non-prepared carcasses gained more weight during the first breeding attempt, then provided more care in subsequent reproduction. There were no differences in subsequent female care among different treatments.

Significance statement

Resource availability and resource maintenance may affect the detection of reproductive trade-offs. Here, we simultaneously manipulated carcass size and carcass preparation to assess these effects. This is the first time that the effects of resource availability and resource maintenance on reproductive trade-offs have been separated in burying beetles. Our findings suggest that despite the increased costs of parental care and resource maintenance, parents breeding on large or non-prepared carcasses gained more benefits in terms of increased body weight by staying longer and feeding more from the carcasses. Such benefits gained during the first breeding attempt offset the costs of current reproduction to some degree and masked the reproductive trade-off between current and future reproduction in terms of parental care. These findings enhance the understanding of the effects of resource availability and maintenance on reproductive trade-offs.

Parent-offspring conflict and its outcome under uni-and biparental care

Conflicts over parental investment are predicted to be common among family members, especially between parents and their offspring. Parent-offspring conflict has been studied in many brood-caring organisms, but whether its outcome is closer to the parental or offspring optimum is usually unknown, as is whether the presence of a second parent, a caring male partner, can affect the outcome. Here, we manipulated the initial brood size of single and paired female burying beetles to examine how many offspring are necessary to maintain parental care in the current brood. We found that mothers continued to invest in small broods even if their reproductive output would have been higher if they had discontinued their care and produced a second brood instead. Consequently, our data suggests that the offspring have the upper hand in the conflict. However, our results further show that paired females laid a second egg clutch more often and produced more offspring than single females, suggesting that the presence of a male partner shifts the conflict outcome towards the parental optimum. This latter result not only is a novel aspect of parent-offspring theory, but also represents an additional factor that might explain the evolution of biparental care.

Parental Care System and Brood Size Drive Sex Difference in Reproductive Allocation: An Experimental Study on Burying Beetles

Life-history theory predicts that increased resource allocation in current reproduction comes at the cost of survival and future reproductive fitness. In taxa with biparental care, each parent can adjust investment on current reproduction according to changes in their partner’s effort, but these adjustments may be different for males and females as they may have different reproductive strategies. Numerous theoretical and empirical studies have proposed the mechanism underlying such adjustments. In addition, the value of the brood or litter (brood size) has also been suggested to affect the amount of care through manipulation of brood size. While the two conditions have been studied independently, the impact of their interplay on potential sex-dependent future reproductive performance remains largely unknown. In this study, we simultaneously manipulated both care system (removal of either parent vs. no removal) and brood size in a burying beetle (Nicrophorus vespilloides) to understand their joint effect on reproductive allocation and trade-off between current and future reproduction. Our results show that males compensated for mate loss by significantly increasing the level of care regardless of brood size, while females exhibited such compensation only for small brood size. Additionally, with an increase in allocation to current reproduction, males showed decreased parental investment during the subsequent breeding event as a pair. These findings imply a dual influence of parental care system and brood size on allocation in current reproduction. Moreover, the impact of such adjustments on sex-dependent differences in future reproduction (parental care, larvae number, and average larval mass at dispersal) is also demonstrated. Our findings enhance the understanding of sex roles in parental investment and highlight their importance as drivers of reproductive allocation.

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.

Fitness costs of phoretic nematodes in the burying beetle, Nicrophorus vespilloides

Nicrophorus vespilloides is a social beetle that rears its offspring on decomposing carrion. Wild beetles are frequently associated with two types of macrobial symbionts, mites, and nematodes. Although these organisms are believed to be phoretic commensals that harmlessly use beetles as a means of transfer between carcasses, the role of these symbionts on N. vespilloides fitness is poorly understood. Here, we show that nematodes have significant negative effects on beetle fitness across a range of worm densities and also quantify the density-dependent transmission of worms between mating individuals and from parents to offspring. Using field-caught beetles, we provide the first report of a new nematode symbiont in N. vespilloides, most closely related to Rhabditoides regina, and show that worm densities are highly variable across individuals isolated from nature but do not differ between males and females. Next, by inoculating mating females with increasing densities of nematodes, we show that worm infections significantly reduce brood size, larval survival, and larval mass, and also eliminate the trade-off between brood size and larval mass. Finally, we show that nematodes are efficiently transmitted between mating individuals and from mothers to larvae, directly and indirectly via the carcass, and that worms persist through pupation. These results show that the phoretic nematode R. regina can be highly parasitic to burying beetles but can nevertheless persist because of efficient mechanisms of intersexual and intergenerational transmission. Phoretic species are exceptionally common and may cause significant harm to their hosts, even though they rely on these larger species for transmission to new resources. However, this harm may be inevitable and unavoidable if transmission of phoretic symbionts requires nematode proliferation. It will be important to determine the generality of our results for other phoretic associates of animals. It will equally be important to assess the fitness effects of phoretic species under changing resource conditions and in the field where diverse interspecific interactions may exacerbate or reduce the negative effects of phoresy.

Pre-ovipositional maternal care alleviates food stress of offspring in the flower beetle Dicronocephalus wallichii

Unlike most other flower beetles, females of Dicronocephalus wallichii exhibit nesting behaviour. The female constructs a burrow in the soil, cuts dead plant leaves into small pieces to provision the nest, and then lays one egg inside the nest. Hatched larvae have been thought to feed on the nest materials prepared by their mothers, but the effects of pre-ovipositional care on larval performance have not been tested. The hatched larvae were found to stay in the nest for 15–30 days until they consumed the nest materials. We examined whether the presence of provisioned nests enhanced larval performance under both benign and food-stress conditions. With high-nutrient soil, larval survival rate and growth speed were not affected by the presence of provisioned nests. By contrast, with low-nutrient soil, mortality of the larvae was much higher in the absence than in the presence of provisioned nests. The growth speed of larvae with nests located in low-nutrient soil was as high as those reared in high-nutrient soil. These results indicate that females alleviate the food stress of larvae during their initial developmental stage by constructing provisioned nests.