Stick insects can disperse like plant seeds: in bird poop

June 1, 2018 • 12:45 pm

One of the striking observations about life on oceanic islands—those islands, like Hawaii and the Galapagos, that arose, bereft of life, from volcanic activity below the sea—is the prevalence of native birds, insects, and plants, and the paucity of native reptiles, mammals, and amphibians. (Continental islands, like Great Britain, that were once connected to larger land masses, don’t show this pattern.)

Darwin was the first to make this observation and show that it supported his theory of evolution. Plants, insects, and birds can more easily get to islands, where they evolve in relative isolation into new species, while mammals, reptiles, and amphibians can’t easily cross large expanses of seawater to colonize distant islands. His view could be summed up as biogeographic patterns = dispersal + evolution.

One of the ways that plants get to islands (besides via their seeds floating in seawater) is through bird movement: birds eat fruit and seeds, fly to an island, and the seeds germinate from the bird’s post-migration poop. In fact, I think a lot more plants have arrived on islands this way than by seed flotation, but don’t quote me on that.

But more than plants can get to islands in bird poop. A new short paper in Ecology by S. Kenji et al. (reference below, free pdf here) shows that stick insects (phasmids) produce hard-shelled eggs that can remain viable and hatch after they pass through a bird’s digestive tract. Moreover, since the eggs don’t require fertilization (they’re from “parthenogenesis”), they don’t have to be fertilized right before being laid, as most insect eggs are. They can simply be nommed by the birds right after being laid, or ingested by gobbling a pregnant female.

The eggs of many stick insects are sculptured like seeds and, more important, have a hard layer of calcium oxalate on the outside that is dissolved only by acidic environments like bird stomachs (this layer appears unique to phasmids). You can see some of these tough eggs in part “B” of the figure below, taken from the paper.

The authors fed eggs of three species of phasmids, mixed with an artificial diet, to Japanese brown-eared bulbuls (Hypsipetes amaurotis), which they claim is one of the main predators of stick insects. They then collected fecal pellets of from the birds when they were pooped out within three hours, and measured hatchability of the eggs. Those hatchabilities were 5%, 8.3% and 8.9% (sample sizes between 40 and 60 eggs per species).

The figure below shows bulbuls eating a phasmid, the eggs, and a nymph of one phasmid species:

(From paper): FIG. 1. (A) The parental brown-eared bulbul Hypsipetes amaurotis feeding the stick insect Ramulus irregulariterdentatus to its chicks. (B) Intact Ramulus irregulariterdentatus eggs defecated by the brown-eared bulbul Hypsipetes amaurotis. Bar = 2 mm. (C) First instar nymph of R. regulariterdentatus hatched from the egg defecated by H. amaurotis.

One obvious conclusion is that, given that bulbuls can fly about 40-60 km/hour, they could disperse phasmid eggs over a hundred kilometers (eggs are produced at about the time the Japanese brown eared bulbuls migrate). The authors fed eggs excised from adult phasmids to the birds, which suggests further that the birds could ingest a bunch of eggs at once simply by eating a pregnant female (they didn’t do this test).

The next questions are these:

1.) Did the eggs evolve that hard coat to facilitate dispersal? There are advantage to dispersing your offspring widely, especially if local predation is high or environments uncertain, and many species of animals have evolved elaborate dispersal mechanisms. (Fruits with seeds inside are one of these!). This is possible, but the authors prefer the idea that the tough eggs evolved to reduce parasitism by wasps. But of course the coat could have evolved for several “reasons” (and by that I mean there could have been more than one reproductive advantage to toughening up your eggs).

2.) Have phasmids actually dispersed this way? We don’t know, as the biogeographic studies haven’t been done. As the authors note, this should show up as evidence for wider dispersal of parthenogenetic phasmids than of their sexually-reproducing relatives:

If avian dispersal is important to stick insects, the phylogeographical patterns should reflect occasional long‐distance dispersal events (e.g., Miura et al. 2012). In addition, the patterns of spatial genetic structure will differ among stick insects with parthenogenetic reproductive capability (and hence potential avian dispersal) and non‐parthenogenetic stick insects. The phylogeographical patterns in these stick insects thus deserves further studies.

Further, stick insects themselves should in general show dispersal different from non-stick insects (I don’t mean Teflon ones!), since some of the former have the ability to get their eggs dispersed hundreds of kilometers. But all this awaits further study, as there was no reason to investigate those patterns before this new paper appeared.

h/t: Dom


Kenji, S., F. Shoichi, T. Asuka, I. Katsura, and Y. Takeshi. 2018 Potential role of bird predation in the dispersal of otherwise flightless stick insects. Ecology. doi: 10.1002/ecy.2230

30 thoughts on “Stick insects can disperse like plant seeds: in bird poop

  1. I do envy insects and their eggs. I wish I could lay a bunch of eggs and poof! a bunch of Diana’s the world can’t handle. And now, they can even move around through bird nomming!

  2. Wish I had something smart to say about this, but I don’t. And I’m desisting in posting one of my usual peurilisms about “poop.” But I don’t want to let a science post go by with one measly comment, so you’re getting this.

      1. Sorry, I meant the number of, not the content of your most trenchant comment. Deepest apologies!

  3. Sounds like your poop-migration vs seed flotation theory could be studied by looking at ratios of these things on islands vs continents. I suppose that this would only work if we can restrict the study to species that disperse via poop-migration and not seed flotation and vice-versa. Perhaps seeds of all species can disperse via flotation. That would sink my idea.

  4. Remarkable — I always wondered why phasmid eggs were so over-engineered! The anti-parasitism idea never seemed adequate — you’d expect the wasps would keep up in an arms race.

    And flightless phasmids are over-represented on small oceanic islands like Lord Howe.

    1. Lord Howe Island Stick Insects were what immediately occurred to me. How does this relate to them? Wouldn’t they or descendants be expected to have dispersed beyond LHI then?

  5. For those without the direct experience, calcium oxalate is the most common type of kidney stone. Nice to see it being put to use for something beneficial.

  6. I wonder if brown-eared bubuls have some type of adaptation that allows them to find stick-bugs better than other birds. After all, stick-bugs are very cryptic. So maybe really good eyesight or perhaps it’s a learned behavior.

    Either way, thanks for this interesting post.

  7. “phasmid” is an awesome word…

    … alright, it’s Friday, so I’ll say it : there’s a Star Wars character, a stormtrooper leader, named Captain Phasma.
    And now I’ve fully embarrassed myself.

    Thank you.

    1. well, if someone finds a new Phasmid genus or species, perhaps they’ll take that into consideration when deciding on a name. Gwendolinidae, perhaps? Although they could mix fandoms and go for Tarthenensis.

      My appologies to Linnaeus, Gwendoline Christie, and Latin teachers everywhere.

  8. It’s a pretty lousy deal for the momma stick insect though, isn’t it? And I thought male praying mantises had it bad.

    My question is, does this only work for short ranges (like 100 miles or so?) Presumably if there are stick insects on islands in the middle of the Pacific (something I don’t know at all) then they must have got there some other way as a bird wouldn’t be expected to ‘hold it’ for that long?


    1. If carried by strong forceful winds, a hurricane, cyclone say (reasonably common in the South Pacific) it’s possible the eggs could be rapidly carried by birds, to your point, even if the bird does not survive the ordeal (no pooping going on) decays, exposing the eggs, thus, reach one island and “hop” from one to the other by the same process, or not.

  9. I agree, I don’t see this as a means of dispersal per se, I mean, if Phasmids are tasty and prone to being eaten (and I guess they are, or why else would they go to such great lengths to be cryptic) then dispersal would be a benefit secondary to the eggs being able to survive being eaten along with the parent. Surviving being crapped out matters much more than how far away you get crapped out, at least in my book. My question would be, what kind of evironment do these eggs have to contend with, even without being eaten? Could not an environment (outside the avian intestines and cloaca) hostile to dessication, wind, or heavy rain, as well as possible predation by any number of insects be the primary driver of the hardiness of the egg, which by chance also creates a situation in which they can pass through the belly of the beast that ate they’re mother?

    Again, I always feel the need to say I’m a history major, so I may be talkin’ out me arse here…

  10. Very interesting! I am curious whether this particular dispersal mechanism evolved for passage through bulbul gut or more generically to survive through any predator that may fancy stick insects as their noms?

  11. “I don’t mean Teflon ones!”

    Now *that* would be an interesting organism – one which had evolved to contain hydrofluoric acid or have a non-stick surface for “internal stir-frying” 😉

    1. I thought you were suggesting that insects evolve a Teflon coating in order to slip the grasp of potential predators, thereby making them the “greased pigs” of the insect world.

    2. Some sealife incorporates bromine, I think some seaweeds

      fluorine, I think is so close a diameter to hydrogen, and is the most electronegative element (-4.0?), by a lot to the next-most (gotta look this up), I don’t think they’d incorporate it by accident … I can’t recall any organisms incorporating fluorine deliberately…

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