Social stimuli affect juvenile hormone during breeding in biparental burying beetles (Silphidae: Nicrophorus)

Extended biparental care is rare in insects but provides an excellent opportunity to investigate the interaction between the endocrine system and the physical and social environment in the regulation of this behavior. Burying beetles (Nicrophorus spp.) have facultative biparental care and depend on locating a small vertebrate carcass that they bury and prepare as food for their young. Commonly, both male and female Nicrophorus orbicollis remain in the burial chamber after eggs hatch to feed and guard the larvae. In both sexes, juvenile hormone (JH) rises rapidly in response to the discovery and assessment of the carcass; it returns to near baseline in 24 h; then in females it reaches very high titers at the onset of maternal care. In this paper, we investigate some social (presence of a mate, mating history, larval age) and environmental (carcass size) factors that may affect this endocrine profile. For females, neither the presence of a mate nor mating status (i.e., virginity) affected the initial rise of JH. However, the absence of a mate significantly depressed the JH rise in males. Eighty-seven percent of the single males buried the carcass like paired males but 87% also released pheromones to attract a mate. JH hemolymph titers in females whose broods were replaced every 24 h with newly hatch larvae were significantly higher than those of females rearing aging broods. Lastly, even though larger carcasses took longer to bury and prepare and oviposition was delayed, neither JH titers nor speed of ovarian development was affected by carcass size.     

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.     

The role of juvenile hormone in competition and cooperation by burying beetles

Few studies have addressed the physiological mechanisms that modulate aggression in insects. In some social insects, there is a correlation of JH and aggression in colony defense and in the establishment of dominance, but only a few studies demonstrate a causal relationship. Burying beetles aggressively defend a breeding resource, a carcass, and juvenile hormone (JH) hemolymph titers increase rapidly upon the discovery of a carcass. In this study, I show that treatment with the JH analog, methoprene, in the absence of a carcass increases the probability of injuries from aggressive interactions, but treatment to one member of a pair of competing Nicrophorus orbicollis females does not increase the probability that she will win control of the resource. In addition, higher JH levels are not associated with greater competitive ability in communally breeding Nicrophorus tomentosus females. Treatment of one female N. tomentosus does not increase her share of the communal brood. Methoprene seems to make a less competitive female more persistent and less willing to concede, which, although maintaining her share of reproduction, results in her exclusion from the brood chamber.     

Plasticity in juvenile hormone in male burying beetles during breeding: physiological consequences of the loss of a mate

Burying beetles, Nicrophorus orbicollis, have facultative biparental care. They bury and prepare small vertebrate carcasses that provide food for their young. Here we establish the juvenile hormone (JH) profiles of paired females, paired males and single males and investigate some of the environmental and social factors that may affect these profiles. Before larvae hatch JH profiles of paired males and females were similar. However, after larvae hatch and during brood care, JH titers of females were very high and those of single males were significantly higher than those of paired males. We tested the hypothesis that higher JH was a response to the need for increased parental care by manipulating brood size. Although JH titers of single males caring for small versus large broods were not significantly different, when comparing JH titers and larval growth (a measure of parental effort), a significant positive correlation emerged. In contrast, we found that food quality had no effect on JH levels suggesting that increased feeding by males and females after carcass discovery cannot explain the elevation of JH. The regulation of JH in male burying beetles appears thus to be dependent on the presence of a mate and on critical stimuli from young.     

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.     

Changing dominant–subordinate relationships during carcass preparation between burying beetle species (Nicrophorus: Silphidae: Coleoptera)

I studied the influence of carrion burial on the interaction between Nicrophorus quadripunctatus and Nicrophorus vespilloides. In the preburial phase, N. quadripunctatus, the smaller species, occupied more carcasses than N. vespilloides, the larger species, when both species were allowed to compete for mouse carcasses. However, after carcasses were buried, N. vespilloides was more successful in protecting those it had buried, and more successful in intruding on carcasses buried by N. quadripunctatus. Direct observation supported these findings. These results may suggest that N. vespilloides is cleptoparasitic on N. quadripunctatus for carrion burial. Received: March 25, 1999 / Accepted: August 31, 1999     

A limit on the extent to which increased egg size can compensate for a poor postnatal environment revealed experimentally in the burying beetle,Nicrophorus vespilloides

It is often assumed that there is a positive relationship between egg size and offspring fitness. However, recent studies have suggested that egg size has a greater effect on offspring fitness in low‐quality environments than in high‐quality environments. Such observations suggest that mothers may compensate for poor posthatching environments by increasing egg size. In this paper we test whether there is a limit on the extent to which increased egg size can compensate for the removal of posthatching parental care in the burying beetle, Nicrophorus vespilloides. Previous experiments with N. vespilloides suggest that an increased egg size can compensate for a relatively poor environment after hatching. Here, we phenotypically engineered female N. vespilloides to produce large or small eggs by varying the amount of time they were allowed to feed on the carcass as larvae. We then tested whether differences between these groups in egg size translated into differences in larval performance in a harsh postnatal environment that excluded parental care. We found that females engineered to produce large eggs did not have higher breeding success, and nor did they produce larger larvae than females engineered to produce small eggs. These results suggest that there is a limit on the extent to which increased maternal investment in egg size can compensate for a poor posthatching environment. We discuss the implication of our results for a recent study showing that experimental N. vespilloides populations can adapt rapidly to the absence of posthatching parental care.     

Resource defense and juvenile hormone: the "challenge hypothesis" extended to insects

As predicted for vertebrates by the challenge hypothesis, the endocrine system of invertebrates can respond to social stimuli to modulate aggression. Testosterone (T) is generally considered to moderate aggression associated with reproduction, i.e. the establishment of breeding territories, mate guarding and offspring defense; juvenile hormone (JH) serves an analogous function in burying beetles. Hemolymph titers of JH increase significantly in Nicrophorus orbicollis, a species with facultative biparental care, when challenged by an intruder to defend their resource. During the first 12 h after the discovery of a carcass, the necessary breeding resource, competition is intrasexual, and JH of males responds only to a challenge by males, and JH of females responds only to a challenge by female intruders. After this period, competition is intersexual, and JH increases significantly in both males and females challenged by an intruder of either sex. In contrast, JH titers in a nonparental species are much higher throughout the breeding season, and neither males nor females respond hormonally to an intruder. These findings support the challenge hypothesis and suggest that mating systems and breeding strategies can promote plastic responses in insect, as well as vertebrate, endocrine systems.     

Parental care masks a density‐dependent shift from cooperation to competition among burying beetle larvae

Studies of siblings have focused mainly on their competitive interactions and to a lesser extent on their cooperation. However, competition and cooperation are at opposite ends on a continuum of possible interactions and the nature of these interactions may be flexible with ecological factors tipping the balance toward competition in some environments and cooperation in others. Here we show that the presence of parental care and the density of larvae on the breeding carcass change the outcome of sibling interactions in burying beetle broods. With full parental care there was a strong negative relationship between larval density and larval mass, consistent with sibling competition for resources. In the absence of care, initial increases in larval density had beneficial effects on larval mass but further increases in larval density reduced larval mass. This likely reflects a density‐dependent shift between cooperation and competition. In a second experiment, we manipulated larval density and removed parental care. We found that the ability of larvae to penetrate the breeding carcass increased with larval density and that feeding within the carcass resulted in heavier larvae than feeding outside the carcass. However, larval density did not influence carcass decay.     

The cockroach Leucophaea maderae needs more than juvenile hormone, vitellogenin and reserves to make a yolky egg

For the cockroach Leucophaea maderae the developmental profile of lipophorin (Lp) concentrations in the hemolymph was determined through the entire vitellogenic period. At mid-vitellogenesis the concentrations of Lp had risen to 6 times the level at emergence and then declined to 2/3 of such high values at ovulation. The racemic 10R,10S-JH-III bound to Lp with an affinity of K(d) = 5.76 nM and the natural enantiomer 10R-JH-III with a K(d) = 1.60 nM. Injections of anti-Lp into mated females caused a significantly reduced rate of oocyte growth and a substantial degree of oosorption. Injections of gamma-globulin did not significantly reduce oocyte growth and caused only a small number of oocytes to resorb. Starvation after mating had similar effects as treatment with anti-Lp. Because of the high affinity of JH to Lp and since Lp occurs in micromolar concentrations during vitellogenesis one can assume that practically all JH is bound and not available for hydrolysis by the JH esterases. Lp appears to function as an inhibitor of JH metabolism by the JHEs through substrate depletion. One may conclude that a normal rate of egg growth is only achieved when titers of Lp exceed those of JH and remove major portions of this substrate from degradation by the JHEs.     

Benefits of communal breeding in burying beetles: a field experiment

1. The ultimate causes of communal breeding and joint parental care in various species of Nicrophorus burying beetles have not been resolved satisfactorily. One hypothesis suggests that females remain on the carcass for extended periods of time because joint defence affords them improved probabilities of retaining the carcass successfully in the face of intense competition from intra‐generic competitors. 2. In a field experiment designed to test this hypothesis in N. defodiens (Mannerheim), breeding associations of two females and a male were no more successful at retaining their carcass than were monogamous pairs, lending no support to the hypothesis. 3. Intra‐generic intruders that usurped already‐buried carcasses were typically much larger than the original residents. 4. The body size of original residents affected both the burial depth and the probability of a takeover. Larger beetles buried the carcass deeper and were more likely to retain possession of the carcass. Group composition also did not affect the depth at which carcasses were buried. 5. Severe and even fatal injuries incurred by some residents indicated the occurrence of violent and damaging fights between competitors over carcasses in the field.     

High water-loss rates and rapid dehydration in the burying beetle, Nicrophorus marginatus

Abstract.  During the summer months, there is a high mortality of burying beetle (Coleoptera: Silphidae) species in pitfall traps containing dry soil. The present study investigated the possibility that the burying beetle Nicrophorus marginatus is highly susceptible to death from desiccation. In the laboratory, adult beetles lose 1–5% body mass per hour in low humidity conditions (25–30% relative humidity), depending on temperature. This rate of water-loss results in a 50% mortality within 7–16 h at temperatures between 16 and 28 °C. Nicrophorus marginatus produces oral and anal defensive secretions when disturbed but these secretions do not significantly contribute to the high rate of water loss. Beetles readily drink and thus beetles with access to water or in high humidity conditions suffer near zero mortality. For comparison, the similarly sized mesic bess beetle (Coleoptera: Passalidae), Odontotaenius disjunctus , and a large nocturnal tiger beetle (Coleoptera: Cicindelidae), Amblychila cylindriformis , are exposed to dehydrating conditions. Nicrophorus marginatus have higher rates of water-loss (by 26% and 79%, respectively). It is predicted that the diurnal N. marginatus uses behavioural mechanisms to avoid lethal water loss. In the field, N. marginatus displays a strong bimodal activity pattern, avoiding the hottest hours of the day. The significance of these findings is that N. marginatus defies normal predictions of association between water-loss rates and habitat type, and has extended its range into apparently unfavourable habitats despite high water-loss rates. In addition, the results suggest that researchers should provide moisture when studying burying beetles, including the endangered American burying beetle ( Nicrophorus americanus ).     

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.     

Host shift by the burying beetle, Nicrophorus pustulatus, a parasitoid of snake eggs

Recent work [Ecoscience (2000) vol. 7, 395-397] suggests that the burying beetle Nicrophorus pustulatus may have undergone a remarkable host shift, exploiting snake eggs rather than carrion as resources for breeding. We conducted behavioural and physiological experiments to examine the hypothesis of a host shift and to formulate hypotheses on its origin. Two congeners of N. pustulatus, Nicrophorus orbicollis and Nicrophorus defodiens did not respond to snake eggs with typical breeding behaviour. When N. pustulatus male-female pairs (n = 14) were presented with clutches of snake eggs, the number of offspring but not the mean size of offspring varied with snake egg mass, indicating effective regulation of brood size. When breeding on turtle eggs, N. pustulatus had a more variable response than when exploiting snake eggs, suggesting that turtle eggs are not a primary resource for breeding. Nicrophorus pustulatus presented with both snake eggs and a mouse carcass combined and exploited the two resources within the same nest (10 of 12 trials). Mouse carcasses and snake eggs were treated differently. Carcasses were moved, buried and stripped of hair in a manner characteristic of burying beetles, whereas snake eggs were not moved or buried. Females that discovered a mouse carcass also had a significantly greater juvenile hormone increase than did females discovering snake eggs. Some responses to the two resources, however, were similar. Female N. pustulatus oviposited rapidly in response to either a mouse carcass or snake eggs, and males elevated sex pheromone emission in response to either resource. The efficient use of snake eggs, the ability to regulate brood size and the different responses to snake eggs and carrion suggest that N. pustulatus is well adapted to exploiting snake eggs for breeding. The use of snake eggs by N. pustulatus has potential implications for conservation of oviparous reptiles.