Festive greetings readership,
On a gloomy, mist-filled night, the life of a mouse is gently extinguished by disease. Its blood stills, its body cools, and a slight wind carries its almost imperceptible scent through the air. A faint buzzing approaches from the darkness, growing louder and louder; a flash of jet-black and red in the moonlight. Another. Then… silence. Two beetles scamper onto the carcass after their nightly flights. They are burying beetles, a male and a female, and have much to do over the next 2-3 weeks. The pair must hurriedly dig beneath the body, concealing it from the other creatures that would take it for themselves; foxes, badgers, and crows would swallow it in a single gulp, whereas flies would impregnate it with eggs. Whilst digging, the pair remove the fur from the carcass and spread over it an anti-microbial covering that prevents fungal and bacterial growth; this slows decay and reduces the smell of the corpse, making it harder for competitors to find. It may take up to two whole days to complete the covert burial and the corpse may eventually be 60cm underground. The beetles excavate a crypt for the carcass, reinforcing it with the fur removed from the mouse. With the splendid new décor around the crypt, the pair consummate their relationship and the female lays the fertilised eggs in the soil around the body two days later. They must now wait for them to hatch. In the interim, the parents continue to spread the anti-microbial covering over the corpse to keep it as fresh as possible.
When the small, spherical eggs break open, pale, vaguely caterpillar-like young emerge – the larvae. They move into a trough in the body, excavated by their parents. Here, they ‘call’ their mother and father to feed them, who respond attentively by ingesting and regurgitating pieces of the corpse in a liquid form. Both parents are equally involved and have identical roles. They continue to spread the anti-microbial covering, feeding their begging offspring, and protecting their larvae from predators as well as creatures that wish to eat the carcass. These parental jobs go on for the next 2 weeks whilst the larvae grow. By the time they are ready to begin the process of transformation into an adult (pupation), the corpse has been almost completely eaten. These larvae then start digging into the side of the crypt separately to form small, individual chambers. Here, they overwinter and will wait until next spring to pupate, emerging as adults. Meanwhile, as the larvae dig their way out of the crypt, the parents unearth themselves and fly their separate ways.
The life cycle and behaviour exhibited by burying beetles (Silphidae: Nicrophorus) is of considerable importance to ecosystems and people, as well as of substantial interest to evolutionary biologists. Through burying beetles’ concealment of dead bodies, people benefit from a fast undertaker service that eliminates unpleasant odours. Of course, it is in the burying beetles’ interests to make the corpse hard to find. Hinted at above, many other organisms can utilise the corpses of small vertebrates to eat, or to oviposit their eggs upon. Depending on the habitat in which the carcass lays, response times of certain animals may be faster or slower. In open areas, vertebrates, especially birds like hawks and crows will quickly pick up a dead or dying mouse; in closed habitats, flies may more quickly colonise and consume the body.
But what does this have to do with evolution and behaviour? The presence of both parents during larval development is not unheard of in insects and other arthropods, but is certainly unusual, and the burying beetle condition is somewhat more advanced, incorporating many complex behaviours. “But why does the male remain to care for the young after mating instead of deserting and mating with more females?” I hear you ask.
Well, even after the body is hidden from detection, the burying beetles must protect the carcass and their eggs from other interlopers. Certain species of burying beetle (e.g. Nicrophorus defodiens) bury corpses very shallowly, giving necrophagic dipterans additional chances to oviposit eggs on or near the corpse. The presence of the male beetle hugely increases the chances of brood success in this situation, compared to if the brood is only tended by the mother (Trumbo, 1994). This is because the pair can destroy eggs or maggots faster than they can be produced, whilst a lone female cannot in most circumstances. However, the benefits afforded by this parental teamwork don’t seem to extend to other predators. Staphylinids (rove beetles) are broadly carnivorous beetles that would without question feed on the carcass of a vertebrate as well as the growing burying beetle larvae. Interestingly though, a single female burying beetle is usually able to repel all but the largest staphylinid (Pukowski, 1933) and it has been suggested the presence of a male does not seem to greatly influence success of brood defence. However, most of the studies investigating brood defence in burying beetles have used specific predators (e.g. Pukowski, 1933; Bartlett, 1987; Scott, 1990), whilst certain important scavengers, ants for example, remain untested. Of course, male defence may be important in these untested scavenger groups.
But there’s a sudden plot twist: the single most important threat to successful breeding is the discovery and usurpation by other burying beetles! Duels by members of the same species will only commence between individuals of the same sex, and the winner is almost always the largest beetle (Bartlett, 1988). During these bouts, the other sex does not intervene. However, after the carcass is buried, the female mated, and eggs laid, the male and female appear to assist one another against interloping beetles of the same species (Scott, 1990). This makes evolutionary sense, as it is in the interest of the parents to maximise the survival of the eggs which have already been lain, instead of the potential eggs which might be lain if the new beetle deposes its same-sex competitor. Because the larger beetle usually wins, species of greater size can more easily usurp carcasses found by smaller species, but these interactions do not happen often (Scott, 1994). This draws us back to the question of habitat.
Burying beetle species are usually restricted somewhat to a certain habitat type. Small and medium sized species (e.g. N. vespilloides and N. defodiens) are better able to dig in damp soil rich in organic material, typical of coniferous forests, whilst the larger species (e.g. N. germanicus and N. americanus) manage in dry, harder substrate, typical of hardwood forest or meadows (Scott, 1998). Ergo, the size of burying beetle species is likely dependent on the substrate in which they must dig to bury the carcasses they find. The number of carcasses usurped by larger Nicrophorus species are therefore limited because of the increased difficulty of digging in non-native habitats.
Essentially, biparental care in burying beetles stems from both the attractiveness of small vertebrate carcasses as a food source for other scavengers (including other Nicrophorus species!) and the need for burying beetles to maximise the chances of survival in their current brood, because of the unpredictability of corpses appearing (Jenkins et al., 2000). The presence of the male and the female together suggests that without them carcasses are purloined or larvae killed often, with the male conferring significant benefits to defence and larval rearing that greatly improve larvae survival rates compared to a lone female.
These glorious beetles are only distributed in temperate regions – most tropical habitats are too warm (except for on tropical mountains). Because decay is much faster in the tropics, the fungi and bacteria on a corpse release chemicals too quickly for the beetles to be able to find and bury it in time; flies and vertebrates are better able to find the carcasses here. So, we in the UK should count ourselves lucky to have such an interesting, charming, and flesh-eating insect living among us, true to its other name – the sexton beetle, clearing up dead bodies left, right, and centre.
Until next time
Article written by Maximillian Tercel. Twitter: @MaximumInsect; Email: firstname.lastname@example.org
Image credits and sources
Featured photo: Nicrophorus sp. probably vespilloides. Photographed by the magnificent and all-round 100% long lost brother, George Hicks, in the Lake District. Twitter: @GeorgeHicksEnto
Nicrophorus vespillo photo: http://www.arkive.org/burying-beetle/nicrophorus-vespillo/
Nicrophorus life cycle: wikimedia user Dakuhippo, downloaded from https://commons.wikimedia.org/wiki/File:Nicrophorus_life_cycle.jpg
Bartlett, J. (1987) Filial cannibalism in burying beetles. Behavioral Ecology and Sociobiology, 21, pp. 179–183
Bartlett, J. and Ashworth, C. M. (1988) Brood size and fitness in Nicrophorus vespilloides (Coleoptera: Silphidae). Behavioral Ecology and Sociobiology, 22, pp. 429–434.
Jenkins, E. V., Morris, C. and Blackman, S. (2000) Delayed benefits of paternal care in the burying beetle Nicrophorus vespilloides. Animal Behaviour, 60, pp. 443-451.
Pukowski, E. (1933) Okoloische untersuchungen an Necrophorus F. Zeitschrift für Morphologie und Ökologie der Tiere, 27, pp. 518–586.
Scott, M. P. (1990) Brood guarding and the evolution of male parental care in burying beetles. Behavioral Ecology and Sociobiology, 26, pp. 31–39.
Scott, M. P. (1994) The benefit of paternal assistance in intra- and interspecific competition for the burying beetle, Nicrophorus defodiens. Ethology, Ecology and Evolution, 6, pp. 537–543
Scott, M. P. (1998) The ecology and behavior of burying beetles. Annual Review of Entomology, 43, pp. 595-618.
Trumbo, S. T. (1994) Interspecific competition, brood parasitism, and the evolution of biparental cooperation in burying beetles. Oikos, 69, pp. 241-249.