National Geographic

Giant insects disappeared thanks to falling oxygen levels and agile birds

The largest wings of any living insect belong to the Queen Alexandra birdwing butterfly and the atlas moth. They can span 10 to 12 inches across. But even these giants are puny compared to the insects of prehistory. Meganeura, for example, was a dragonfly that lived 300 million years ago and each of its wings was the length of my arm. Why do such behemoths no longer exist?

The prevailing theory, proposed around a century ago, is that the Earth’s atmosphere used to have much more oxygen—more than 30 per cent in the Permian, compared to just 20 today. This vital gas sets an upper limit on how big animals can be. The seething quantities of past eras allowed flying insects to fuel faster metabolisms and larger bodies.

Matthew Clapham and Jered Karr from the University of California, Santa Cruz have now found some strong evidence to support this idea, after analysing more than 10,500 fossilised insect wings. It took almost 18 months to collect the entire data set, but it clearly showed that the maximum wingspans of flying insects neatly tracked the oxygen in the atmosphere for their first 150 million years of evolution. As the gas reached its peak during the Permian, the insects were at their largest. As levels later fell, the insects shrank.

But this neat correlation stopped between 130 and 140 million years ago, during the early Cretaceous period. Even though oxygen concentrations started climbing from a Jurassic low of 15 per cent, for the first time in their history, the insects didn’t follow suit. If anything, they got smaller. They had finally encountered something that limited their growth more than the oxygen in the air: birds.

During the preceding Jurassic period, small dinosaurs had started to evolve feathers. By the early Cretaceous, the rise of feathered flying dinosaurs –birds—was truly underway. They had also started to evolve features like the alula – a small thumb-like projection at the front of the wing that allows them to manoeuvre at low speeds without crashing.

Smaller insects would still have been too fast and manoeuvrable for them, but larger ones would have been easier prey. Clapham and Karr think that birds exerted a “size-selective pressure” on the insects. By killing the largest individuals, they gave the smallest ones had an advantage, and kept the group as a whole from getting bigger.

“There aren’t a lot of other major environmental or biological events in the Early Cretaceous, so no other factors really jump out as being a possibility,” says Clapham. “Flowering plants evolved around then—a little later, but close—but it’s difficult to see why they would have affected these large, predatory insects.”

Clapham and Karr also found that the pterosaurs – flying reptiles that lived at the same time as the dinosaurs –didn’t have the same effect. There might be a boring reason for this. “In the Late Triassic, pterosaurs evolve about 20 million years before oxygen drops to its Jurassic low, but there are unfortunately very few insect fossils in that time interval,” says Clapham. “We just can’t tell whether their size decrease then was caused by pterosaurs or the reduction in oxygen.”

However, the Jurassic insects were as big as you would expect given the levels of oxygen at the time. This suggests that if the pterosaurs had an effect, it was a small one. Clapham thinks that they just weren’t as manoeuvrable as birds later were.  “I don’t think they would have been as effective aerial predators,” he says.

Reference: Clapham & Karr. 2012. Environmental and biotic controls on the evolutionary history of insect body size. PNAS http://dx.doi.org/10.1073/pnas.1204026109

Image by Hcrepin

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There are 17 Comments. Add Yours.

  1. Stephen
    June 4, 2012

    How far back do bats go?

  2. Ed Yong
    June 4, 2012

    Good question: about 50-60 million years back. The birds came first. By the time the bats arrived, it’s unlikely that they added any significant size-repression on top of what the birds had done.

  3. John
    June 4, 2012

    That PNAS link doesn’t work for me. I’ll find it.

    I would like to at least know of a couple studies that test wing size vs oxygen level (a controlled study with various butterfly and dragonfly species for instance). Im not really convinced by the statement that two trend lines followed each other.

    I don’t know a lot about the bodies of birds an insects. It would be interesting to know what factors are responsible for flying insects have a different size limit then birds.

  4. Kea Giles
    June 4, 2012
  5. amphiox
    June 4, 2012

    It is interesting think about how contingency plays into this. Birds appeared just after insects had shrunk in response to falling O2 levels, and just before an increase in O2 might have prompted the evolution of larger insects. This allowed birds to be predators of the smaller insects and provide selection pressure keeping their sizes down.

    What if the timing had been different? What if birds had first appeared at a time when insects had already gotten large body sizes established from rising O2 levels? What implications would a sky full of the likes of Meganeura have had for the evolution of birds?

  6. mfumbesi
    June 5, 2012

    @amphiox, interesting question. Would we have bird eating insects? That’s would be a sight.

  7. Tony Mach
    June 5, 2012

    I recently read that the atmospheric pressure was much higher back than (5 times as high as today) – don’t know if the number is accurate though. Did that have an effect on how large animals could become? Or was it only the oxygen concentration?

    And if large insects were food for large (land-based) dinosaurs, wouldn’t the disappearance of large insects killed off large dinosaurs? So did birds kill the non-avian dinosaurs?

  8. Yotam
    June 5, 2012

    @John
    I don’t know a lot about insect respiration, but from what I remember from my physiology/biochemistry courses, insets do not use hemoglobin to carry oxygen, rather they use diffusion/gas exchange via tubes to deliver it to the tissues. Since diffusion is very distance dependent, the oxygen concentration should set a limit on the effective distance (i.e. size) possible with diffusion alone.
    Birds OTOH, have a circulatory system with hemoglobin that allows them to better deliver oxygen to tissues.

    http://en.wikipedia.org/wiki/Insect#Respiratory_and_circulatory_systems

  9. amphiox
    June 5, 2012

    I recently read that the atmospheric pressure was much higher back than (5 times as high as today) – don’t know if the number is accurate though. Did that have an effect on how large animals could become?

    Well, a higher atmospheric pressure would certainly have an effect on how large flying animals can become.

    Since diffusion is very distance dependent, the oxygen concentration should set a limit on the effective distance (i.e. size) possible with diffusion alone.

    It should be noted that this, of course, doesn’t just apply to flying insects, but to all insects, ground based ones as well. Would predatory pressure from birds have an effect on ground-based insects similar to flying ones? And if not, what accounts for the lack of large ground insects in the latter Cretaceous? (Was there a lack of large ground insects in the latter Cretaceous?)

  10. lkr
    June 5, 2012

    ” insects do not use hemoglobin to carry oxygen, rather they use diffusion/gas exchange via tubes to deliver it to the tissues” — The “diffusion-limit” for insect respiration is something of a textbook canard. All larger insects have a system of tracheal air-sacs, with the tracheae branching from these. Most or all large/largish insects actively ventilate the air sacs [as can be observed at rest in many grasshoppers, moths, beetles, etc..] Further, the thoracic air-sacs invade the wing-muscle compartment in such a way that ventilation is activateded by the flight mechanism. The result is that diffusion need occur over very short distances, and the pumping/diffusion relationship is accessible to evolutionary tinkering…

    My vote for most-inportant limit on size of terrrestrial arthropods is molting — both the mechanics and the vulnerability at that stage.

  11. amphiox
    June 5, 2012

    All larger insects have a system of tracheal air-sacs, with the tracheae branching from these. Most or all large/largish insects actively ventilate the air sacs [as can be observed at rest in many grasshoppers, moths, beetles, etc..] Further, the thoracic air-sacs invade the wing-muscle compartment in such a way that ventilation is activateded by the flight mechanism. The result is that diffusion need occur over very short distances, and the pumping/diffusion relationship is accessible to evolutionary tinkering…

    The size limitation I think comes from the trachea tubes themselves. As the insect’s body size scales up, the trachea have to get wider, and the insect’s leg-joints act as a choke point – the bigger the insect, the larger a proportion of the total diameter of the leg-joint cross section is taken up by trachea, and the less space there is for other important things. But as atmospheric O2 levels go up, the trachea can have a smaller diameter and still deliver the same amount of oxygen.

  12. BioBob
    June 8, 2012

    amphiox, I fail to see what leg size and joints have to do with trachea. Go dissect an insect sometime and see where the various types of muscle mass are versus the “cabling”.

    lkr has it quite right, even though no mention is made of haemolyph transport which ALSO mitigate gas exchange. You also fail to understand that insects are quite able to quickly evolve whatever gas exchange diffusion system they need; they have invented EVERY biological concept I have ever seen (admittedly I have many more limitations than millions of species of insects) many millions of years before mammals were even a gleam in some tetrapods eye.

    Oxygen concentration is almost certain to have some effects on some species but would be irrelevant to others. Do try to get your head around the concept of MILLIONS of species and MILLIONS of years and even a cockroach typing “war and peace” becomes feasible, LOL.

  13. BioBob
    June 8, 2012

    Concerning the relationship of oxygen concentration to wing length:

    How many time do we NEED to repeat that “correlation is NOT causation”?

    It IS interesting that a correlation has been observed but a bit presumptuous to call it conclusive ignoring all other possible causation. However, that seems to be the meme in too much of today’s pseudoscience science: let’s ignore and minimize the details that do not fit, the statistical noise (or valid sampling/statistics at all), the large number of unknowns and move immediately to the sexy conclusions so we can bring in the grant money !

  14. Kim
    June 11, 2012

    I believe that as the diversity of birds increased so did their impact. They were willingly hunt and eat insects during any stage of development. Many different kinds of birds eat the same insects, plus their favorites. In addition they eat bugs in the air, in water, on plants, in plants, in trees, on the ground and in the ground. Most early stages of insect development contain dangerous periods of time with little mobility, soft bodies, and a lack of protective measures. Insects generally reproduce by laying many eggs in one spot without parental protection. The mobile birds also developed great ears, great eyes and could have been eating insects at their most vulnerable points as masses of eggs, as slow moving larva, as stationary pupas greatly reducing the number that made it to maturity and decreasing the likely hood of adaptation. My vote is… for the birds…

  15. B. Boudinot
    June 12, 2012

    This paints a beautiful picture in my mind of paleo selection pressures and the diversification of the modern insect fauna. I state this with as yet a speculative perspective, but the evolution of birds – as a retrospectively obvious pressure – never came to me, a neontologist.

    Continuing speculation: massive flying insects, relative to the modern fauna, may have been a driver for Archosaur flight. Consider the potential relative calorie intake of a terricolous insect specialist versus an early clade experimenting with flight and specialization on flying insects, tapping the resource of giant flying insects. I haven’t studied the paleofauna (of anything), but should large flying Cretaceous insects have very strong cuticles (relative, of course) the possession of teeth by early flying stem-group Archosaurs may have been an asset. This is to say, based on my speculations, I hypothesize that the loss of teeth by stem-Archosaurs may have been caused (at least correlated) with reduction in flying insect size/cuticle strength.

  16. I.C. Math
    September 3, 2012

    The weight of an insect’s body increases with the square of it’s length, as it gets larger (so an insect doubling in length quadruples in weight). However, so does the surface area of the insect (so an insect doubling in length quadruples its surface area) . The surface area should be proportional to the amount of ogygen that it should be able to extract from the air. It should also be proportional to to its “lift” ability in flight (ability to fly), since both the weight of the insect and its wing as well as breathing organ surface area are quadrupled as the insect doubles in length.

    This puzzled me. I couldn’t figure out why a larger insect would need more oxygen. Then I realized why this is: The volume of an insect’s body increases with it’s length to the third power, as it gets larger (so an insect doubling in length obtains a body volume eight times as large). At first I thought, “the volume should be irrelevant; it’s the weight that matters”, but then I realized that the volume also matters since each body cell taking up volume needs oxygen. Thus, the amount of oxygen required increases faster with the length of the insect than the surface area available to provide that oxygen increases with the length of the insect.

    This is the answer people, as far as I can see, based purely on math. There comes a point where the surface area to breathe with cannot support the insect’s volume, cannot give the cells within enough oxygene – unless the amount of oxygene in the air is increased.

    You don’t need to know squat about insects to figure this out. In fact, you don’t need to have any kind of knowledge about biology or evolution whatsover to figure this out. For example, this would hold true even if insects had lungs (where it still would be the surface area inside the lungs that would matter).

    Math rules. Someone knowing only math can solve problems in biology or history, but someone knowing only biology and history can never solve a problem in math. :-)

  17. I.C. Math
    September 3, 2012

    OK, OK, I guess one has to know that cells require oxygen. Sorry, basic biological knowledge IS required!

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