Resource concealment and the evolution of parental care in burying beetles

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 competitive exclusion-tolerance rule explains habitat partitioning among co-occurring species of burying beetles

Habitat partitioning among co-occurring, ecologically similar species is widespread in nature and thought to be an important mechanism for coexistence. The factors that cause habitat partitioning, however, are unknown for most species. We experimentally tested among three alternative hypotheses to explain habitat partitioning among two species of co-occurring burying beetle (Nicrophorus) that occupy forest (Nicrophorus orbicollis) and wetland (Nicrophorus hebes) habitats. Captive experiments revealed that the larger N. orbicollis (forest) was consistently dominant to N. hebes (wetland) in competitive interactions for carcasses that they require for reproduction. Transplant enclosure experiments in nature revealed that N. hebes had poor reproductive success whenever the dominant N. orbicollis was present. In the absence of N. orbicollis, N. hebes performed as well, or better, in forest versus its typical wetland habitat. In contrast, N. orbicollis performed poorly in wetlands regardless of the presence of N. hebes. These results support the competitive exclusion-tolerance rule where the competitively dominant N. orbicollis excludes the subordinate N. hebes from otherwise suitable or preferable forest habitat, while the subordinate N. hebes is uniquely able to tolerate the challenges of breeding in wetlands. Transplant experiments further showed that carcass burial depth-an important trait thought to enhance the competitive ability of the dominant N. orbicollis-is costly in wetland habitats. In the presence of N. hebes, N. orbicollis buried carcasses deeper; deeper burial is thought to provide a competitive advantage in forests but further compromised the reproductive success of N. orbicollis in wetlands. Overall, results provide evidence that the competitive exclusion-tolerance rule underlies habitat partitioning among ecologically similar species and that the traits important for competitive dominance in relatively benign environments are costly in more challenging environments, consistent with a trade-off.

Spatiotemporal variation in the competitive environment, with implications for how climate change may affect a species with parental care

Burying beetles of the genus Nicrophorus have become a model for studying the evolution of complex parental care in laboratory studies. Nicrophorus species depend on small vertebrate carcasses to breed, which they process and provision to their begging offspring. However, vertebrate carcasses are highly sought after by a wide variety of species and so competition is expected to be critical to the evolution of parental care. Despite this, the competitive environment for Nicrophorus is rarely characterized in the wild and remains a missing factor in laboratory studies. Here, we performed a systematic sampling of Nicrophorus orbicollis living near the southern extent of their range at Whitehall Forest in Clarke County, Georgia, USA. We determined the density of N. orbicollis and other necrophilous species that may affect the availability of this breeding resource through interference or exploitation competition. In addition, we characterize body size, a key trait involved in competitive ability, for all Nicrophorus species at Whitehall Forest throughout the season. Finally, we compare our findings to other published natural history data for Nicrophorines. We document a significantly longer active season than was observed 20 years previously at Whitehall Forest for both N. orbicollis and Nicrophorus tomentosus, potentially due to climate change. As expected, the adult body size of N. orbicollis was larger than N. tomentosus, the only other Nicrophorus species that was captured in 2022 at Whitehall Forest. The other most prevalent insects captured included species in the families Staphylinidae, Histeridae, Scarabaeidae, and Elateridae, which may act as competitors or predators of Nicrophorus young. Together, our results indicate significant variation in intra- and interspecific competition relative to populations within the N. orbicollis range. These findings suggest that the competitive environment shows extensive spatiotemporal variation, providing the basis to make predictions for how ecology may influence parenting in this species.

Why do males stay in biparental burying beetles?

Biparental associations are rare in most taxa. A second parent should stay with its current brood only to enhance brood fitness or to increase its own future breeding opportunities. I compared the breeding performance of Nicrophorus defodiens pairs and single females while confronting a conspecific male intruder. Pairs fared better than single females in defending a brood, and age had a strong effect on contest outcomes even though defenders and intruders were of the same age. Pairs also produced larger broods than single females, but only on larger carcasses. The results and a review of prior studies suggest that the benefits for a male that stays with a partner during the post-hatch period, whether gains from offspring success or protecting mating opportunities, are observed primarily in the context of intruder pressure, and that paternal care such as regurgitations to young and nest maintenance would not have evolved in a rival-free world.

Differences in sibling cooperation in presence and absence of parental care in a genus with interspecific variation in offspring dependence

The widely spread evolutionary strategy of parental care is considered an important driver of social evolution. Although offspring were long thought to primarily interact competitively, recent studies revealed the potential importance of sibling cooperation. Theories suggest that the degree of cooperation in offspring interactions depends on the degree of offspring dependence on parental care: offspring unable to forage on their own should compete more, whereas more independent juveniles may increase the degree of cooperation. In this study, we tested the occurrence of sibling cooperation in the absence of posthatching care in several burying beetle species exhibiting varying degrees of offspring dependence. To this end, we measured larval growth rate and survival in the presence and absence of prehatching care using different brood sizes. We found that sibling cooperation cannot be exclusively explained by offspring dependence on parental care. Although only species with more independent larvae cooperated when receiving prehatching care, larval cooperation occurred across species in the absence of care. The latter result suggests that sibling cooperation was already present in an early ancestor of the genus Nicrophorus. Overall, these findings give important insights into the transition from facultative to obligate family life.

Competition of insect decomposers over large vertebrate carrion: Necrodes beetles (Silphidae) vs. blow flies (Calliphoridae)

Large carrion is inhabited by highly variable and interactive communities of insects. Positive interactions in carrion insect communities have been recently the focus in carrion ecology. In contrast, competition between carrion insects is rather undervalued. Here we provide evidence that blow flies (Calliphoridae) and Necrodes beetles (Silphidae), dominant decomposers of large carcasses in terrestrial habitats, compete over carrion. By reanalyzing the results from 90 pig carcasses, we demonstrated that the contribution of the flies and the beetles to the decay was negatively related. The greater part of the large carrion pool was monopolized by blow flies, whereas Necrodes beetles abundantly colonized carcasses, on which blow flies were less effective as decomposers. In behavioral assays, we found that adult beetles killed 4 times more frequently feeding than postfeeding third instar larvae of the flies, with the large decrease in the killing frequency after the larvae reached the age of early third instar. Therefore, adult Necrodes beetles preferentially killed the larvae that were before or in their peak feeding. The study provides evidence that the interaction between blow flies and Necrodes beetles is a combination of indirect exploitative effects of the flies and direct interference effects of the beetles (the mixed competition).