Michelle Pellissier Scott

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.

Changes in biosynthesis and degradation of juvenile hormone during breeding by burying beetles: a reproductive or social role?

Burying beetles, Nicrophorus orbicollis, depend on the location of an unpredictable resource, a small vertebrate carcass, for reproduction. When they discover a carcass, they undergo a correlated rapid rise in titers of juvenile hormone (JH) in the hemolymph and ovarian development. This study investigates the regulation of the changes in JH during breeding in both male and female burying beetles and the role of JH in ovarian development. JH biosynthesis by the corpora allata (CA), measured in vitro, increased in females within an hour of their discovery of a carcass and increased later in males. After returning to low rates as oviposition began, JH biosynthesis rose again 3 days later in females but not in males. Neither the ovaries nor testes synthesized JH. There was a concomitant fall in JH esterase activity within 12 h of discovery of the carcass in both males and females. Although the rise in JH titers and biosynthesis and the fall in JH esterase is correlated with ovarian development, application of methoprene or JH III in the absence of a carcass did not result in vitellogenin uptake by the oocytes. Therefore, we conclude that, in spite of the rapid rise in JH before oviposition, it is not sufficient to regulate vitellogenin synthesis and/or its uptake by the ovaries. We suggest that its role has been preempted to organize social behavior and coordinate parental behavior between mates.

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.

Competition with flies promotes communal breeding in the burying beetle, Nicrophorus tomentosus

Communal breeding through nest-sharing may benefit cooperating individuals indirectly, in increased inclusive fitness, or directly, when environmental constraints reduce the fitness of solitary breeders. Burying beetles provide extensive parental care and can breed either in pairs or in larger groups of unrelated males and females. Parentage of communally-reared broods is usually shared but is skewed in favor of the individuals of each sex that provide longer care. Females provide care longer than males, and two females are more likely to remain together in the brood chamber than two males are. Flies and other burying beetles are the major competitors for carcasses and this study suggests that it is competition with flies that promotes communal breeding in Nicrophorus tomentosus On medium-size carcasses (35–40 g) the presence or absence of oviposition by flies had a significant effect on the size of the brood reared, and on large carcasses (55–60 g) the number of beetles present, two or four, had a significant effect on brood size. On both medium and large carcasses, pairs rearing broods on flyblown carcasses had fewer young than pairs on clean carcasses or foursomes on flyblown carcasses. There was a strong trend for an interaction effect between number of beetles and competition with flies (Table 1). Duration of parental care was not affected by competition with flies except for that of the first male to depart, which provided care longer on flyblown carcasses (Table 2). Pairs and foursomes were equally able to defend the carcass and brood from conspecific intruders and from larger intruding Nicrophorus orbicollis (Table 3).

Reproductive dominance and differential ovicide in the communally breeding burying beetle Nicrophorus tomentosus

Abstract Communal breeding can be characterized by the division of reproduction among cooperating individuals and, if the distribution of reproduction is inequitable, by the mechanisms for achieving skewed reproductive success. The burying beetle (Nicrophorus tomentosus) is a facultative communal breeder. Unrelated adults, especially females, provide extensive parental care to broods of mixed parentage. The frequency and degree of reproductive skew between two females were examined experimentally. On medium-size carcasses, the proportion of eggs attributed to each female was not significantly different from random in 42% of the broods, skewed in 42% and not shared in 16%. Although reproduction was usually skewed in favor of the larger female, the relative sizes of the two females did not predict the degree of skew. On large carcasses, the proportion of eggs attributed to each female was not different from random in 87% of the broods and weakly skewed in 13%. Several mechanisms for biasing reproductive success were investigated. Females increase the proportion of their offspring in the brood by committing differential ovicide. Secondly, burying and preparing a carcass cooperatively stimulates ovarian development of the larger female and slows it for the smaller female, reducing or delaying oviposition by the subordinate. Thirdly, larger females are more likely to be dominant and are more fecund than smaller females.

Sizing Up the Competition: Do Burying Beetles Weigh or Measure Their Opponents?

Limited resources, like food, space, and mates, set the stage for intraspecific competition in many species. These conflicts are usually settled through the assessment of various asymmetries, or differences, between the competitors. In conflicts there are three basic asymmetries: (1) resource holding potential (RHP), determined by the fighting ability; (2) pay-off, or the cost–benefit ratio for each individual; and (3) an uncorrelated asymmetry, such as ownership, which is not affected by the other two (Maynard Smith and Parker, 1976; Hammerstein, 1981). However, the importance of each asymmetry differs among species. For example, in the grass snake, Natrix natrix, RHP is the determining factor (Luiselli, 1996), but in the speckled wood butterfly, Pararge aegeria, ownership is crucial (Davies, 1978). Burying beetles (Nicrophorus spp.) compete fiercely for possession of a small carcass, a valuable and unpredictable resource. A mating pair buries the carcass, mates, and raises the young using the carcass as a food source. Reproductive success is based on finding and possessing a carcass, thus setting the stage for intrasexual competition. Same-sex burying beetles compete when they meet at a carcass (the limited

The benefit of paternal assistance in intra- and interspecific competition for the burying beetle, Nicrophorus defodiens

Burying beetles, Nicrophorus spp., bury small carcasses as food for their young. Both males and females provide extensive parental care and the major advantage of paternal assistance is to reduce the probability that the carcass will be usurped by a conspecific competitor and the brood killed. Whereas male assistance in N. defodiens significantly reduces the probability of usurpation by a conspecific, it is ineffective against larger congeneric intruders. Male N. defodiens provide shorter parental care than do males of the larger, N. orbicollis, whose assistance is effective against all burying beetles in preventing usurpation. I compare the duration of paternal care in this New Hampshire population of N. defodiens and one in Michigan and argue that this behavior is shaped by the competitive environment.

Facultative Adjustment of the Duration of Parental Care by Burying Beetles

Organisms face a trade-off of investing in current versus future reproduction. In species for which offspring benefit from parental care, parents continue to provide care at the expense of departing to search for additional breeding opportunities (Trivers, 1972). Many factors may influence the decision to remain or leave: the probability that the mate will remain, the effect on offspring survival and growth, the probability of finding another breeding opportunity, the costs of searching, the effect on fecundity, and the level of intrasexual competition for mates or breeding opportunities (Pianka and Parker, 1975; Maynard Smith, 1977; Zeh and Smith, 1985; Alcock, 1994). Thus relatively available breeding opportunities and low search costs, relatively little intrasexual competition, and good competitive ability may all select for an earlier departure. As the probability of future reproduction decreases, individuals may be selected to provide longer parental care. Because male fitness is more likely to be limited by mating opportunities than female fitness (Trivers, 1972), males are expected to invest less in current offspring than females and perhaps to be more responsive to social and environmental factors that affect the duration of care. Male and female burying beetles (Nicrophorus orbicollis) provide extended parental care (Pukowski, 1933). They bury and prepare small vertebrate carcasses as food for their young. Eggs are laid in the soil nearby 24-60 h after the carcass is discovered. These hatch 3-4 days later into altricial larvae that are fed by both parents. Although the young of some species, including N. orbicollis, fail to develop without some parental feeding (Trumbo, 1992),

The frequency and fitness consequences of communal breeding in a natural population of burying beetles: a test of reproductive skew

Abstract 1. Reproductive cooperation occurs in diverse taxa and a defining characteristic of these social systems is how reproduction is shared. Both male and female burying beetles (Nicrophorus spp.) facultatively form associations to bury a carcass and rear a single brood, making burying beetles a model system for testing skew theory.

Molecular measures of insect fitness

Many questions concerning social behavior and reproductive ecology require an accurate measure of direct and/or indirect fitness. Recently developed molecular techniques that measure parentage and relatedness have made these basic questions more tractable. In this paper, we provide a summary of the methods and analyses of the most commonly used techniques for measuring fitness that have been applied to insect systems and discuss the strengths and limitations of each. The use of highly polymorphic microsatellite loci as genetic markers has been steadily increasing in the past few years, and we focus on their usefulness in the study of natural populations.