I’ve heard of “carrion beetles” but this is more like a “carry-on beetle”:
i-ea6f90c90d7edadf9d4df4ea49151462-insect_mobbed_by_mites.jpg

Amanda and I were outside the cabin in Cass County, Minnesota last week, cutting pieces of plywood for sub flooring, and we saw this creature among the debris. At first I thought it was some kind of wasp covered with tiny spiders, but on further investigation it turned out to be a beetle covered with mites. When we first saw it, there were many more mites than in this photo, and they were virtually roiling on the insect’s surface. It looked almost as though the insect was foaming.

We captured it for later identification, and Amanda ran it down. This is a carrion beetle. There are several species of carrion beetles, all in the family Silphidae. There are over 20,000 species of beetle in North America. Beetles are incredibly diverse. Therefore, it did not surprise me to learn that there are several dozen species of carrion beetle in Minnesota. This is probably in the genus Nicrophorus.

So, what are the mites doing there?

Carrion beetles face a special problem that we see here and there in the world of animals, and when it occurs there is often an interesting adaptive result. (This is how we know adaptation is a real thing and the diversity we see in nature is often the result of adaptatoin.) Carrion beetles lay their eggs in or on the body of dead mammals or birds. But dead mammals and birds are somewhat rare, especially the relatively undisturbed ones, the ones that are not about to be digested by something. Therefore, a carrion beetle that has a trait that maximizes the use of this rare resource will have a selective advantage over carrion beetles that don’t.

Lots of things lay their eggs in dead critters, or simply find the dead critter’s body and eat it, but many of those species are less discriminating. Think of a simple world in which there is the occasional dead mouse, carrion beetles, house flies, and a large amount of poop. Never mind where the poop comes from, just assume it is there. In this imaginary simplified ecology, house flies will lay their eggs on poop and dead mice, but the carrion beetle, with larva that can only grow on a meat diet, will lay their eggs only on the dead mice. Selection that might lead to the development of a costly trait to monopolize the resource will not be strong for the house fly. If the house fly fails now and then because the carrion beetle out competes it on the mouse carcass, no big deal, there is always poop. But selection that might lead to the development of a costly trait in the beetle to make use of the mouse carcass may be strongly selected for. From the carrion beetle’s perspective, there is a strong possibility that there are already fly larva (maggots) on the carcass eating it when it arrives to lay its eggs, or soon after. This situation is probably made worse by the fact that house flies are quick and seem to be able to travel long distances, while beetles are slower and travel less far than flies in a given period of time. The only way carrion beetles will ever be able to raise their young in this scenario is to do a better job of accessing or using the mouse carcass than the fly does, so any variation that arises in carrion beetles that facilitates this will be strongly selected for.

This is known as the life/lunch dichotomy. In the competition for the use of the meaty carcass of a dead mouse, if the fly loses out it gives up the equivalent of lunch … there are still other opportunities, in this case, poop, for it’s young to eat. The carrion beetle, however, may be giving up its life (or the life of its offspring, really) because mouse carcasses are very rare, so if the one carcass it manages to locate is eaten up by fly maggots, it’s offspring will not survive.

This is where the mites come in.

The carrion beetle pays a huge cost carrying the mites around wherever it goes, because they are heavy and affect its ability to move and fly. But otherwise, the mites do nothing …. they just hang on for the ride, waiting for the beetle to locate a dead mouse. Then, when the beetle does located a dead mouse, the mites do not eat it. Rather, they eat the maggots, the fly eggs, and larva of anything that is not a carrion beetle. They clean the carcass of the potential competitors of the carrion beetle’s larva.

Now, if I can only find a dead mouse somewhere to feed to my new pet and her friends….

More info:

Carrion Beetle Mites
American carrion beetle (Wikipedia)
The Beetles at Cedar Creek Research Station, Minnesota
Carryon

Comments

  1. #1 williewhitehead
    September 30, 2010

    Adaptation. Life always finds a way. I am glad that you took the time to look this information up and share it with us. Thank you.

  2. #2 MadScientist
    October 1, 2010

    Eww. I hope those mites are specific to the beetle.

    Now why is god torturing that animal? Oh, that’s right – the mites are abusing their free will. yeah, I pwn sophisticated theology.

  3. #3 MadScientist
    October 1, 2010

    Oops … bad example I guess since the mites are useful to the beetle.

    Found a dead mouse yet?

  4. #4 Schnäggli
    October 1, 2010

    An impressive example of adaptive behaviour and the complexity of relationships between species. When I first saw the picture, I thought the mites were simply parasiting the beetle, but the truth turns out to be far less simple and more fascinating. Thanks for sharing!

    http://snailsandslugs.wordpress.com/

  5. #5 Jay Converse
    October 1, 2010

    Aww, man, I was hoping for some video. What an awesome animal!

  6. #6 Christina
    October 1, 2010

    Yo dawg, I heard you like carrion beetles…

  7. #7 Albert Hall
    October 1, 2010

    Your carrion beetle must not exceed a certain size, or must be placed beneath the seat in front of you.

  8. #8 Rick Pikul
    October 1, 2010

    @7: Keep that sort of thing up and you’ll get a one way ticket to Blackburn.

  9. #9 Dan
    October 1, 2010

    What stops the mites from eating the beetle’s own larvae?

  10. #10 Greg Laden
    October 1, 2010

    Dan: The ultimate answer to that is evolution … this is a mutualism of a sort. Mites that ate the eggs of their host species had offspring that had fewer host species, etc. At the proximate level, presumably some sort of detection/avoidance mechanism.

  11. #11 Roland
    October 1, 2010

    Just give that beetle a hunk of meat. Put beetle & meat in a glass jar with a ventilated lid. Hours of fun for the whole family (until it starts to stink).

  12. #12 daedalus2u
    October 1, 2010

    Use cat food.

    You should expose the bait outside so that flies can deposit eggs so there is something for the mites.

  13. #13 Joshua Zelinsky
    October 1, 2010

    Some stores sell mice to give as prey for snakes. You could get a mouse from there although then you’d have to kill it.

    Also echoing request for video.

  14. #14 daedalus2u
    October 1, 2010

    I remember reading that one of the reasons for the demise of carrion beatles in NA was the extinction of the passenger pigeon.

    Passenger pigeons had the mobility to follow the mast events of oaks and other nut bearing trees so that tree biomass ended up as animal biomass instead of bacterial biomass.

    There is some thought that this might be a mast year for oaks in the Northeast.

  15. #15 Joe Fogey
    October 1, 2010

    Lovely, life, just bloody lovely.

  16. #16 Joe Fogey
    October 1, 2010

    I realise that my last post was somewhat incoherent, but then it’s Friday night and Messrs Brahms and Lizst are paying a visit. What I mean is, wouldn’t you rather live in a world where carrion beetles foster mites that eat maggots than one where “he” made them all, low and mighty, the rich man in his castle, all things “bright and beautiful” and ordered their rotten estates?

    What poverty of imagination the other lot must suffer from!

  17. #17 Darren Naish
    October 2, 2010

    As I learnt just two days ago (from a carrion beetle that was also crawling with mites), they don’t just eat dead mice and such but are also very fond of cat food.

  18. #18 Chelydra
    October 4, 2010

    If you really want to try a mouse, most pet stores, including chains, sell frozen mice. Get an adult and let it thaw at room temperature or in warm water. If you want to see a wider range of behavior, put enough soil in with the beetle that she can bury the carcass (if it’s a male I believe he’ll simply wait for a female rather than burying it himself).

    @ #14 daedalus2u

    Specifically it is the largest native species, American burying beetle (Nicrophorus americanus), that is now extirpated throughout most of eastern North America. They prefer chipmunk-sized carcasses, and the elimination of passenger pigeons as an abundant food source is one of several hypotheses proposed to explain their decline.

  19. #19 Derek Sikes
    October 7, 2010

    The mites on Nicrophorus are not always beneficial: Blackman, S. W. 1997. Experimental evidence that the mite Poecilochirus davydovae (Mesostigmata: Parasitidae) eats the eggs of its beetle host. Journal of Zoology 242(1): 63-67.

    The classic paper that showed mutualism is: Springett, B. P. 1968. Aspects of the relationship between burying beetles, Necrophorus spp., and the mite, Poecilochirus necrophori Vitz. Journal of Animal Ecology 37: 417-424.

    but far more extensive work done in the field by D. S. Wilson showed the relationship was much more complex, sometimes mutualistic, sometimes only mutualistic for 2nd generation offspring, sometimes commensalistic (no benefit, no harm) and sometimes parasitic – see:
    Wilson, D. S. 1983. The effect of population structure on the evolution of mutualism: a field test involving burying beetles and their phoretic mites. The American Naturalist 121(6): 851-870.
    and
    Wilson, D. S. & W. G. Knollenberg. 1987. Adaptive indirect effects: The fitness of burying beetles with and without their phoretic mites. Evolutionary Ecology 1(2): 139-159.
    Nice post!

  20. #20 Greg Laden
    October 7, 2010

    Derek: Thanks for the excellent annotated bibliography!

  21. #21 Emily
    October 10, 2010

    If you requested a dead mouse from a local bakery they’d likely have one in a trap somewhere… though whether they’d own to it is another matter. Also, they’d have all kinds of larder beetles. Stored grains = so many nifty pests.

  22. #22 James
    October 18, 2010

    My favorite spotting of one of these beetles was floating in a swimming pool. The beetle was not moving anymore and the mites were huddled away from the slowly encroaching water. It was “Titanic” in miniature.

    Derek, any suggestion that the beetles/larvae eat the mites?

    I just want to add to the more general audience that these types of interactions, traditionally termed mutualism, almost always vary all over the place in terms of proximate effects. They can range from mutualistic to competitive to parasitic.

  23. #23 Margarethe Brummermann
    October 18, 2010

    While the relationship between beetle and mites is not as black and white as described it is interesting to see evolutionary principles applied to this detail of Nicrophorus behavior. Of course, the beetle’s efforts to secure the rare resource for its offspring go much further – when the pair prepares the carcass for burial they reshape it and may well get rid of or destroy other eggs/larvae. They also stay underground and feed their young larvae – overall a level of involvement that’s rare in beetles and understandable under the premises that you describe in your third paragraph.

  24. #24 macroinstantes
    October 19, 2010

    A very interesting post. The complexity of Nature is amazing! Best Regards.

  25. #25 Kurt
    October 20, 2010

    Wow, this is absolutely fascinating! I used to feel sorry for beetles with mites. Interesting to know that those mites are actually friends rather than foes.