I came across a dead mole as I was walking along a logging tract in a pine forest in Galicia, northern Spain. It looked as though it had only just died, as the first wave of colonisation – by blow flies (Diptera: Calliphoridae) – was only just in progress.
Green bottle flies (probably Lucilia sericata) were already inspecting an opening in the corpse and presumably laying their eggs inside the mole on the flesh.
It did not take long for the second wave of colonists to arrive: sarcophagid flies (Diptera: Sarcophagidae) – such as Sarcophaga carnaria or the common flesh fly (below) – which together with calliphorid and muscid flies, all deposit eggs or live larvae on fresh corpses.
I carried on with my walk, and returned to the carcass about 30 minutes later. By this stage a number of burying or carrion beetles were in attendance around the carcass. Burying beetles like Nicrophorus vespillo and Nicrophorus vespilloides (Silphidae: Nicrophorinae) are attracted by volatile chemicals emitted by the decomposing carcass; the ‘irresistible bouquet of death‘ as one paper (1) charmingly calls it! I would love to know how far the beetles traveled to reach the dead mole. One never sees such beetles in the absence of a carcass, but they quickly arrive to exploit the resource, so they must be lurking everywhere, waiting for something to die! The burying beetles (or Sexton beetles) appeared to be Nicrophorus vespilloides, because the clubs of their antennae were black (rather than orange or reddish as in other species).
The large phoretic (hitch-hiking) mites on the front of the beetle’s thorax are a Poecilochirus species, probably Poecilochirus carabi. Individual beetles can sometimes carry as many as 40 mites at one time; quite a burden it would seem. Poecilochirus mites have a symbiotic relationship with silphid burying beetles which transport them from one food source to another in exchange, it is thought, for protecting the beetle’s larva, or its food supply (the carcass), from competing fly larvae.(2) Only one particular life stage of the mite – sexually immature ‘deutonymphs’ – hitch a ride on the burying beetles, where they get off, moult into adult mites, and mate. The adult P. carabi females are ‘oophagous’ and feed on the eggs of the fly larvae, thus creating more of an opportunity for the beetle larvae to feed; but like many things in biology, it is complicated, because they have also been observed to consume Nicrophoros eggs and larvae, their supposed allies or symbionts! (3, 4) It seems that whether the relationship is antagonistic, neutral or mutualistic, depends on the life stages involved: ‘some members of the family gain more from interacting with mites than others’! (4)
Most Sexton beetles bury the carcass, but before they do so they must win ownership of it and this frequently involves fights between members of the same species – particularly females – as well as other insects. The larger beetles invariably win! (5) If the carcass is not too heavily infested with fly eggs, the beetles roll it into a ball, removing the fur and applying antimicrobial secretions to the surface (6).
The female Sexton beetle then constructs a brood chamber above the carrion ball in which she deposits between 10-50 eggs. The parents both regurgitate food in this chamber for their larvae. The eggs hatch and the larvae make their way towards the carcass; they can either feed directly from the carcass or they can beg and be fed by the parents.
Another carrion beetle investigating the scene was the Red-breasted Carrion Beetle, Oiceoptoma thoracica (below). Apparently, this beetle does not feed directly on the carcass, but instead predates on other species which are attracted to the remains.
At this early stage of decomposition, it is presumably a dynamic situation with different species – as well as different individuals of the same species – competing for the “bonanza” resource. There were other beetles – Thanatophilus species – competing with the green bottles for access to the carcass via the hole in the skin (below).
This little encounter stimulated my interest in the biology of decomposition and the role of nature’s undertakers in recycling dead organisms. It is a fascinating subject and one full of complex interactions and life cycles. What most impressed me was the way in which all of these organisms appear, as if by magic, summoned by some mysterious chemical signal to come and carry out their business. The clean-up squad, waiting in the background, ready to go to work at a moments notice! It really is an impressive ecological service and barely noticed most of the time.
- Kalinova, B., Podskalska, H., Růžička, J., & Hoskovec, M. (2009). Irresistible bouquet of death—how are burying beetles (Coleoptera: Silphidae: Nicrophorus) attracted by carcasses. Naturwissenschaften, 96(8), 889-899.
- Horst H. Schwarz, & Josef K. Müller. (1992). The Dispersal Behaviour of the Phoretic Mite Poecilochirus carabi (Mesostigmata, Parasitidae): Adaptation to the Breeding Biology of Its Carrier Necrophorus vespilloides (Coleoptera, Silphidae). Oecologia, 89(4), 487-493
- Beninger, C. W. (1993). Egg predation by Poecilochirus carabi (Mesostigmata: Parasitidae) and its effect on reproduction of Nicrophorus vespilloides (Coleoptera: Silphidae). Environmental entomology, 22(4), 766-769.
- De Gasperin, O., & Kilner, R. M. (2015). Friend or foe: inter‐specific interactions and conflicts of interest within the family. Ecological entomology, 40(6), 787-795.
- Scott, M. P. (1998). The ecology and behavior of burying beetles. Annual review of entomology, 43(1), 595-618.
- Eggert, A. K., Otte, T., & Müller, J. K. (2008). Starving the competition: a proximate cause of reproductive skew in burying beetles (Nicrophorus vespilloides). Proceedings of the Royal Society of London B: Biological Sciences, 275(1650), 2521-2528.