Bigger Is Better, Sometimes: The Interaction between Body Size and Carcass Size Determines Fitness, Reproductive Strategies, and Senescence in Two Species of Burying Beetles

The cost of reproduction hypothesis suggests that allocation to current reproduction constrains future reproduction. How organisms accrue reproductive costs and allocate energy across their lifetime may differ among species adapted to different resource types. We test this by comparing lifetime reproductive output, patterns of reproductive allocation, and senescence between two species of burying beetles, Nicrophorus marginatus and N. guttula, that differ in body size, across a range of carcass sizes. These two species of burying beetles maximized lifetime reproductive output on somewhat different–sized resources. The larger N. marginatus did better on large and medium carcasses while the smaller N. guttula did best on small and medium carcasses. For both species, reproduction is costly and reproduction on larger carcasses reduced lifespan more than reproduction on smaller carcasses. Carcass size also affected lifetime reproductive strategies. Each species’ parental investment patterns were consistent with terminal investment on carcasses on which they performed best (optimal carcass sizes). However, they exhibited reproductive restraint on carcass sizes on which they did not perform as well. Reproductive senescence occurred largely in response to carcass size. For both species, reproduction on larger carcasses resulted in more rapid senescence. These data suggest that whether organisms exhibit terminal investment or reproductive restraint may depend on type and amount of resources for reproduction.

Evolutionary change in the construction of the nursery environment when parents are prevented from caring for their young directly

Parental care can be partitioned into traits that involve direct engagement with offspring and traits that are expressed as an extended phenotype and influence the developmental environment, such as constructing a nursery. Here, we use experimental evolution to test whether parents can evolve modifications in nursery construction when they are experimentally prevented from supplying care directly to offspring. We exposed replicate experimental populations of burying beetles (Nicrophorus vespilloides) to different regimes of posthatching care by allowing larvae to develop in the presence (Full Care) or absence of parents (No Care). After only 13 generations of experimental evolution, we found an adaptive evolutionary increase in the pace at which parents in the No Care populations converted a dead body into a carrion nest for larvae. Cross-fostering experiments further revealed that No Care larvae performed better on a carrion nest prepared by No Care parents than did Full Care larvae. We conclude that parents construct the nursery environment in relation to their effectiveness at supplying care directly, after offspring are born. When direct care is prevented entirely, they evolve to make compensatory adjustments to the nursery in which their young will develop. The rapid evolutionary change observed in our experiments suggests there is considerable standing genetic variation for parental care traits in natural burying beetle populations—for reasons that remain unclear.

Does Communal Breeding Promote an Increase in Social Immunity in Burying Beetles? A Test Case with Nicrophorus Defodiens

Communal breeding is a reproductive system in which more than a single pair of individuals share parental care duties. Burying beetles (genus Nicrophorus) breed on small vertebrate carcasses, which is used as a food source for their young. On larger carcasses, burying beetles will breed communally, forming multiple male-female associations. A significant and costly component of parental investment by burying beetles is the preservation of the carcass with secretions containing immune molecules. Because this immune investment is for the benefit of the offspring, the behavior is a form of social immunity. We test the hypothesis that communal breeding in burying beetles evolved as a mechanism to increase the social immune investment on larger carcasses, which are more difficult to preserve. We used N. defodiens, a communal breeding burying beetle species to test the hypothesis. There were two experimental treatments wherein, the females either bred communally or non-communally. Our results show that the combined immune activity in the secretions were higher in communally breeding pairs than in the immune contribution of single male-female pairs. However, subordinate females were rarely observed on the carcass, and the level of social immune activity of dominant females was lower than females breeding singly. These data suggest that communal breeding in N. defodiens decreases the level of investment in social immunity. Our results demonstrate that the presence of multiple females, which is common under natural conditions, can greatly complicate patterns of social immunity investment in burying beetles.

Behavioral dominance interactions between Nicrophorus orbicollis and N. tomentosus burying beetles (Coleoptera: Silphidae)

Asymmetric interference competition, where one species is behaviorally dominant over another, appears widespread in nature with the potential to structure ecological communities through trade-offs between competitive dominance and environmental tolerance. The details of how species interact and the factors that contribute to behavioral dominance, however, are poorly known for most species, yet such details are important for understanding when and why trade-offs occur. Here, we examine behavioral interactions between two species of burying beetles (Coleoptera: Silphidae) that compete for limited breeding resources (i.e., small vertebrate carcasses) in nature, to identify behaviors involved in interference competition and to test if large body size, species identity, or time of arrival best predict behavioral dominance among species. To test these ideas, we placed same-sex individuals of Nicrophorus orbicollis (early to mid-summer breeder) and N. tomentosus (late summer to fall breeder) into an enclosure together with a 25–30 g mouse carcass (Mus musculus). We then video-recorded all behaviors, including neutral and aggressive interactions, for 13 h per trial (N = 14 trials). For each interaction, we assigned a winner based on which beetle retained its position instead of fleeing or retained possession of the carcass; the overall behavioral dominant was determined as the individual that won the most interactions over the length of the trial. We found that large body size was the best predictor of behavioral dominance. In most interactions, N. orbicollis was larger and dominant over N. tomentosus; however, when N. tomentosus was larger they outcompeted smaller N. orbicollis, illustrating the importance of body size in aggressive contests. The order of arrival to the carcass (priority effects) did not predict behavioral dominance. The larger size and abundance of N. orbicollis in nature suggest a competitive asymmetry between the species, supporting the idea that N. orbicollis constrains the ability of N. tomentosus to breed earlier in the summer.

Habitat use of co-occurring burying beetles (genus Nicrophorus) in southeastern Ontario, Canada

The coexistence of closely related species plays an important role in shaping local diversity. However, competition for shared resources can limit the ability of species to coexist. Many species avoid the costs of coexistence by diverging in habitat use, known as habitat partitioning. We examine patterns of habitat use in seven co-occurring species of burying beetles (genus Nicrophorus Fabricius, 1775), testing the hypothesis that Nicrophorus species partition resources by occupying distinct habitats. We surveyed Nicrophorus abundance and 54 habitat characteristics at 100 random sites spanning an environmentally diverse region of southeastern Ontario, Canada. We found that three species occupied distinct habitat types consistent with habitat partitioning. Specifically, Nicrophorus pustulatus Herschel, 1807, Nicrophorus hebes Kirby, 1837, and Nicrophorus marginatus Fabricius, 1801 appear to be specialists for forest canopy, wetlands, and open fields, respectively. In contrast, Nicrophorus orbicollis Say, 1825, Nicrophorus sayi Laporte, 1840, and Nicrophorus tomentosus Weber, 1801 appear to be generalists with wide breadths of habitat use. We were unable to identify the habitat associations of Nicrophorus defodiens Mannerheim, 1846. Our findings are consistent with habitat acting as an important resource axis along which some Nicrophorus species partition; however, divergence along other resource axes (e.g., temporal partitioning) also appears important for Nicrophorus coexistence.