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.

Experimental increases in temperature mean and variance alter reproductive behaviours in the dung beetle Phanaeus vindex

Temperature profoundly impacts insect development, but plasticity of reproductive behaviours may mediate the impacts of temperature change on earlier life stages. Few studies have examined the potential for adult behavioural plasticity to buffer offspring from the warmer, more variable temperatures associated with climate change. We used a field manipulation to examine whether the dung beetle Phanaeus vindex alters breeding behaviours in response to temperature changes and whether behavioural shifts protect offspring from temperature changes. Dung beetles lay eggs inside brood balls made of dung that are buried underground. Brood ball depth impacts the temperatures offspring experience with consequences for development. We placed adult females in either control or greenhouse treatments that simultaneously increased temperature mean and variance. We found that females in greenhouse treatments produced more brood balls that were smaller and buried deeper than controls, suggesting brood ball number or burial depth may come at a cost to brood ball size, which can impact offspring nutrition. Despite being buried deeper, brood balls from the greenhouse treatment experienced warmer mean temperatures but similar amplitudes of temperature fluctuation relative to controls. Our findings suggest adult behaviours may partially buffer developing offspring from temperature changes.

Interactions Between Individuals and Sex Rather Than Morphological Traits Drive Intraspecific Dung Removal in Two Dung Beetle Species

Dung beetle functional ecology has traditionally focused on studying the relation between traits and ecosystem functions in multispecies assemblages, often ignoring the contribution of behavioral interactions and trait variability within species. Here we focus on the factors that affect dung removal at an intraspecific level in two horned dung beetle species with dimorphic males (Onthophagus taurus and Onthophagus verticicornis). By setting treatments for each species with single individuals (one female, F; one major male, M; one minor male, m) or with pairs of individuals (MF, mF, MM, mm, FF), we examined the effect on dung removal of morphological traits (head, pronotum, leg, horn), sex, and interactions between individuals. Our results showed that dung removal at an intraspecific level depended more on sex and behavioral interactions than on the underlying morphological traits, whose effects on dung removal were negligible. Single females generally removed more dung than single males, which suggests that females are more effective than males. In both species, pairs with at least one female (MF, mF, FF) showed high dung removal efficiency, but did not perform differently from the sum of single treatments (M + F, m + f, F + F). This suggests an additive effect: males and females (or two females) join their efforts when they are together. The pairs with only males (MM and mm) removed less dung than the sum of the single individuals (M + M and m + m), which indicates a mutual inhibition of males. In both species, male morphs performed similarly as they removed the same amount of dung. Despite our results are limited to two Onthophagus species, we suggest that the intraspecific functional ecology of dung beetles might be more influenced by behavioral interactions and sex rather than by morphological traits.

A Review of Dung Beetle Introductions in the Antipodes and North America: Status, Opportunities, and Challenges

Following the introduction of cattle, exotic dung beetles (Coleoptera: Aphodiidae, Geotrupidae, Scarabaeidae) were imported into the Antipodes (Australia and New Zealand) and North America (primarily the United States) to accelerate the degradation of cattle dung on pastures. The history of dung beetle introductions between the two regions is similar but has not previously been assessed: this is important as new introductions are continuing in the regions. Here, we review these introduction programs, report on their current status, and discuss methodological advances. In doing so, we examine the accidental introduction of exotic (i.e., adventive) species and the contribution of both deliberately introduced and adventive species to endemic dung beetle faunas. Further, we provide a list of pest and parasite species whose populations can be reduced by dung beetle activity. We also identify a combined total of 37 introduced and 47 adventive dung beetle species that have become established in the Antipodes and North America, with exotic species dominating dung beetle assemblages from pasture habitats. Climatic and edaphic matches, the size of founding populations, abiotic and biotic stressors, and the time of year when releases are made are all critical determinants that affect the success of dung beetle introduction programs. Finally, we discuss opportunities, plus the risks and challenges associated with dung beetle introductions. We hope that this review will aid in the success of future introduction programs, either to enhance ecosystem services in areas that they are needed, or potentially to reestablish native species in regions where they have been extirpated.

Nesting of Ceratina nigrolabiata, a biparental bee

Biparental care is very rare in insects, and it was well-documented in only one bee species to this date - Ceratina nigrolabiata. However, biparental care was only recently discovered in this species, and detailed description of natural history of this species is missing. Here, we describe the nesting cycle of C. nigrolabiata. Pairs of C. nigrolabiata are established before female starts offspring provisioning. After provisioning is finished (when youngest offspring reached larval stage), the male abandons the nest. Males which are present in nests where female already finished provisioning brood cells, are probably mainly temporary visitors. The female can perform long-time offspring guarding, but only 22% of completely provisioned nests are guarded by a female. Most nests (54%) are closed and abandoned, when provisioning is completed, and other (24%) are orphaned before provisioning is finished. Guarded nests have statistically higher number of brood cells provisioned than unguarded nests. Generally, C. nigrolabiata is unique among bees due to its biparental behavior, but it has also uncommon traits of nesting biology among Ceratina bees, e.g. fast offspring development in comparison with provisioning rate, and high proportion of nests which are closed and abandoned by mother.

Maternal and larval niche construction interact to shape development, survival, and population divergence in the dung beetle Onthophagus taurus

Through niche construction, organisms modify their environments in ways that can alter how selection acts on themselves and their offspring. However, the role of niche construction in shaping developmental and evolutionary trajectories, and its importance for population divergences and local adaptation, remains largely unclear. In this study, we manipulated both maternal and larval niche construction and measured the effects on fitness-relevant traits in two rapidly diverging populations of the bull-headed dung beetle, Onthophagus taurus. We find that both types of niche construction enhance adult size, peak larval mass, and pupal mass, which when compromised lead to a synergistic decrease in survival. Furthermore, for one measure, duration of larval development, we find that the two populations have diverged in their reliance on niche construction: larval niche construction appears to buffer against compromised maternal niche construction only in beetles from Western Australia, but not in beetles from the Eastern United States. We discuss our results in the context of rapid adaptation to novel conditions and the role of niche construction therein.

Behavioural responses to warming differentially impact survival in introduced and native dung beetles

Anthropogenic changes are often studied in isolation but may interact to affect biodiversity. For example, climate change could exacerbate the impacts of biological invasions if climate change differentially affects invasive and native species. Behavioural plasticity may mitigate some of the impacts of climate change, but species vary in their degree of behavioural plasticity. In particular, invasive species may have greater behavioural plasticity than native species since plasticity helps invasive species establish and spread in new environments. This plasticity could make invasives better able to cope with climate change. Here our goal was to examine whether reproductive behaviours and behavioural plasticity vary between an introduced and a native Onthophagus dung beetle species in response to warming temperatures and how differences in behaviour influence offspring survival. Using a repeated measures design, we exposed small colonies of introduced O. taurus and native O. hecate to three temperature treatments, including a control, low warming and high warming treatment, and then measured reproductive behaviours, including the number, size and burial depth of brood balls. We reared offspring in their brood balls in developmental temperatures that matched those of the brood ball burial depth to quantify survival. We found that the introduced O. taurus produced more brood balls and larger brood balls, and buried brood balls deeper than the native O. hecate in all treatments. However, the two species did not vary in the degree of behavioural plasticity in response to warming. Differences in reproductive behaviours did affect survival such that warming temperatures had a greater effect on survival of offspring of native O. hecate compared to introduced O. taurus. Overall, our results suggest that differences in behaviour between native and introduced species are one mechanism through which climate change may exacerbate negative impacts of biological invasions.

The evolution of paternal care: a role for microbes?

Paternal care, particularly in cases of uncertain paternity, carries significant costs. Extensive research, both theoretical and experimental, has explored the conditions in which paternal care behaviour would be favoured. Common explanations include an adjustment of care with uncertainty in paternity and limited accuracy in parentage assessment. Here, we propose a new explanation that microbes may play a role in the evolution of paternal care among their hosts. Using computational models, we demonstrate that microbes associated with increased paternal care could be favoured by natural selection. We find that microbe-induced paternal care could evolve under wider conditions than suggested by genetic models. Moreover, we show that microbe-induced paternal care is more likely to evolve when considering paternal care interactions that increase microbial transmission, such as feeding and grooming. Our results imply that factors affecting the composition of host microbiome may also alter paternal behaviour. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Dung beetle species introductions: when an ecosystem service provider transforms into an invasive species

Dung beetle introduction programmes were designed to accelerate exotic livestock dung degradation and to control dung breeding pestiferous flies and livestock parasites. The introduction programmes provided exotic dung beetle species with an opportunity to cross natural barriers and spread beyond their native range. There are no reports that explain what probable adaptation mechanisms enable particular dung beetle species to be the most successful invader. Here we identify the morphological, biological, physiological, ecological and behavioural attributes of the four most widespread and successful dung beetle species in introduced areas on a global scale in relation to the assumption that these species are different from other exotic and native dung beetles. We have recognised Digitonthophagus gazella (Fabricius), Onthophagus taurus (Schreber), Euoniticellus intermedius (Reiche) and Aphodius fimetarius (Linnaeus) as the most successful invaders based on their spread, predominance, distribution range and the reports of invasion. Each of these four species has different natural history traits that increase their fitness making them successful invaders. D. gazella has high fecundity and spreading ability, can instantly locate and colonise fresh and nutritious dung, and has a broad thermal window. O. taurus has morphological plasticity, high fecundity, high brood survival rate due to bi-parenting, and is adapted to extreme thermal and moisture conditions. E. intermedius has remnant-dung feeding abilities, a wide thermal window, functioning best at upper-temperature levels, and successful breeding and survival abilities at extremely low soil moisture conditions. A. fimetarius is small-sized, has high breeding and dispersal abilities, and is adapted to lower thermal and upper moisture extremes and variable soil conditions. Discussed here are perspectives on adaptive attributes of dung beetle species that are important to consider during their selection for redistributions. We have elaborated on the fitness and success characteristics of the four species individually. Further, we recommend a prior-introduction baseline monitoring of native dung beetle assemblages so as to evaluate the future impact of exotic dung beetle introductions on the recipient ecosystem.

Experimental evidence for the role of sexual selection in the evolution of cuticular hydrocarbons in the dung beetle, Onthophagus taurus

A role for sexual selection in the evolution of insect cuticular hydrocarbons (CHCs) is suggested by observations of selection acting on male CHCs during female mate choice. However, evidence that CHCs evolve in response to sexual selection is generally lacking, and there is a need to extend our understanding beyond well-studied taxa. Experimental evolution offers a powerful approach to investigate the effect of sexual selection on the evolution of insect CHCs. We conducted such an experiment using the dung beetle, Onthophagus taurus. After six, 12 and 21 generations of experimental evolution, we measured the CHCs of beetles from three populations subject to sexual selection and three populations within which sexual selection had been removed via enforced monogamy. We found that the male CHC profile responded to the experimental removal of sexual selection. Conversely, the CHC profile of females responded to the presence of sexual selection but not to its removal. These results show that sexual selection can be an important mechanism affecting the evolution of insect CHCs and that male and female CHCs can evolve independently.

Female genitalia can evolve more rapidly and divergently than male genitalia

Male genitalia exhibit patterns of divergent evolution driven by sexual selection. In contrast, for many taxonomic groups, female genitalia are relatively uniform and their patterns of evolution remain largely unexplored. Here we quantify variation in the shape of female genitalia across onthophagine dung beetles, and use new comparative methods to contrast their rates of divergence with those of male genitalia. As expected, male genital shape has diverged more rapidly than a naturally selected trait, the foretibia. Remarkably, female genital shape has diverged nearly three times as fast as male genital shape. Our results dispel the notion that female genitalia do not show the same patterns of divergent evolution as male genitalia, and suggest that female genitalia are under sexual selection through their role in female choice.

Although male genital shape is known to evolve rapidly in response to sexual selection, relatively little is known about the evolution of female genital shape. Here, the authors show that across onthophagine dung beetles, female genital shape has diverged much more rapidly than male genital shape.

How individual and relative size affect participation in territorial defense and cortisol levels in a social fish

For many species, behaviors such as territory defense and parental care are energetically costly, but are nonetheless can provide substantial fitness gains. In systems in which both parents provide parental care, each of the parents benefits from exhibiting (or having their partner exhibit) these behaviors. However, in many cases, costs and benefits differ between parents due to factors such as size or sex. Different intruder types may also impose different costs on parents. Predatory intruders might consume offspring, whereas conspecifics might threaten the social status of a parent, or provide benefits as a potential group joiner or mate. Responses to these intrusions may also be associated with variation in individual stress responses. We investigated associations among male and female sizes, and the interaction between these, with defense against conspecific and heterospecific territorial intruders by members of successfully breeding pairs in the cichlid fish Neolamprologus pulcher. We also investigated whether cortisol levels were associated with size or participation in territory defense because each may be a cause or consequence of individual variation in the stress response. We found that females paired with large males performed fewer defensive behaviors than females paired with smaller males. Males paired with relatively large females had higher baseline cortisol levels than those paired with smaller females. Collectively, individual characteristics such as size have consequences for each individual's behavior, and also influence the behavior, and endocrine state of social partners.

Delayed dispersal and prolonged brood care in a family-living beetle

Delayed juvenile dispersal is an important prerequisite for the evolution of family-based social systems, such as cooperative breeding and eusociality. In general, young adults forego dispersal if there are substantial benefits to remaining in the natal nest and/or the likelihood of dispersing and breeding successfully is low. We investigate some general factors thought to drive delayed juvenile dispersal in the horned passalus beetle, a family-living beetle in which young adults remain with their families in their natal nest for several months before dispersing. Fine-scale population genetic structure indicated high gene flow between nest sites, suggesting that constraints on mobility are unlikely to explain philopatry. Young adults do not breed in their natal log and likely disperse before reaching breeding age, suggesting that they do not gain direct reproductive benefits from delayed dispersal. We also examined several ways in which parents might incentivize delayed dispersal by providing prolonged care to adult offspring. Although adult beetles inhibit fungal growth in the colony by manipulating both the nest site and deceased conspecifics, this is unlikely to be a major explanation for family living as both parents and adult offspring seem capable of controlling fungal growth. Adult offspring that stayed with their family groups also neither gained more mass nor experienced faster exoskeleton development than those experimentally removed from their families. The results of these experiments suggest that our current understanding of the factors underlying prolonged family living may be insufficient to explain delayed dispersal in at least some taxa, particularly insects.

Maternal effects on male weaponry: female dung beetles produce major sons with longer horns when they perceive higher population density

BACKGROUND

Maternal effects are environmental influences on the phenotype of one individual that are due to the expression of genes in its mother, and are expected to evolve whenever females are better capable of assessing the environmental conditions that their offspring will experience than the offspring themselves. In the dung beetle Onthophagus taurus, conditional male dimorphism is associated with alternative reproductive tactics: majors fight and guard females whereas minors sneak copulations. Furthermore, variation in dung beetle population density has different fitness consequences for each male morph, and theory predicts that higher population density might select for a higher frequency of minors and/or greater expenditure on weaponry in majors. Because adult dung beetles provide offspring with all the nutritional resources for their development, maternal effects strongly influence male phenotype.

RESULTS

Here we tested whether female O. taurus are capable of perceiving population density, and responding by changing the phenotype of their offspring. We found that mothers who were reared with other conspecifics in their pre-mating period produced major offspring that had longer horns across a wider range of body sizes than the major offspring of females that were reared in isolation in their pre-mating period. Moreover, our results indicate that this maternal effect on male weaponry does not operate through the amount of dung provided by females to their offspring, but is rather transmitted through egg or brood mass composition. Finally, although theory predicts that females experiencing higher density might produce more minor males, we found no support for this, rather the best fitting models were equivocal as to whether fewer or the same proportions of minors were produced.

CONCLUSIONS

Our study describes a new type of maternal effect in dung beetles, which probably allows females to respond to population density adaptively, preparing at least their major offspring for the sexual competition they will face in the future. This new type of maternal effect in dung beetles represents a novel transgenerational response of alternative reproductive tactics to population density.

A game theoretic model of kleptoparasitism with strategic arrivals and departures of beetles at dung pats

Dung beetles Onthophagus taurus lay their eggs in brood balls within dung pats. The dung that is used must be sufficiently fresh, and so beetles must keep moving from pat to pat to find fresh dung. If another beetle finds a brood ball it will usually eat the egg inside and lay its own egg in the brood ball instead of constructing its own ball. Thus, beetles will often stay near their eggs to guard them. We model a population of beetles where the times of arrival and departure from pats are strategic choices, and investigate optimal strategies depending upon environmental conditions, which can be reduced to two key parameters, the cost of brood ball construction and the ease of finding balls to parasitise. We predict that beetles should follow one of three distinct behaviors; stay in patches for only short periods, arrive late and be purely parasitic, remain in pats for longer periods in order to guard their brood balls. Under different conditions populations can consist of the first of these types only, a combination of the first and second types, or a combination of all three types.

Reproductive competition promotes the evolution of female weaponry

Secondary sexual traits in females are a relatively rare phenomenon. Empirical studies have focused on the role of male mate choice in their evolution; however, recently it has been suggested that secondary sexual traits in females are more likely to be under selection via reproductive competition. We investigated female competition and the influence of female phenotype on fitness in Onthophagus sagittarius, a species of dung beetle that exhibits female-specific horns. We compared reproductive fitness when females were breeding in competition versus breeding alone and found that competition for breeding resources reduced fitness for all females, but that smaller individuals suffered a greater fitness reduction than larger individuals. When females were matched for body size, those with the longest horns gained higher reproductive fitness. The fitness function was positive and linear, favouring increased horn expression. Thus, we present evidence that female body size and horn size in O. sagittarius are under directional selection via competition for reproductive resources. Our study is a rare example of female contest competition selecting for female weaponry.

Effects of the insect growth regulator, methoprene, on Onthophagus taurus (Coleoptera: Scarabaeidae)

A bioassay was conducted to determine the impact of methoprene, an insect growth regulator (IGR), on fecundity, larval survival, and size of progeny for Onthophagus taurus Schreber. Adult O. taurus dung beetles were offered methoprene-treated manure in three to five replications each at concentrations of 0.08, 0.45, and 4.5 ppm, respectively. An additional group of adult beetles was immersed in a methoprene-water solution and allowed to reproduce in containers with untreated manure. Data from all treatment groups were compared with untreated control groups. Methoprene did not seem to hinder brood production at 0.45 ppm. Survival of O. taurus was not affected by methoprene-treated manure at 0.08 ppm or when parent beetles were immersed in methoprene-water solution. However, progeny survival was significantly reduced on manure treated with methoprene at 4.5 ppm. Mean pronotal width of O. taurus progeny was significantly smaller in beetles fed methoprene-treated manure (4.5 ppm). The low dose of 0.08 ppm did not affect pronotal widths nor did topical application of methoprene to adults affect pronotal widths in resulting offspring. Although some adverse effects of methoprene were observed at higher concentrations, use of methoprene at concentrations of 0.08 ppm as part of a horn fly control program likely would not greatly affect populations of O. taurus, the most common paracoprid dung beetle in North Carolina.