One trait that shows interesting evolutionary trends is the size of animals. Body size plays a significant role in the most important interactions between animals: competition (for resources or mates) and predation (for both predator and prey). Body size is also, of course, significant for a variety of physiological reasons. It's no surprise, therefore, that biologists spend a lot of time thinking about body size, and have discovered some intriguing patterns. For instance the so-called island rule in biogeography includes both insular gigantism (relatives of smaller-bodied mainland species tend to be larger in island populations) and insular dwarfism (relatives of larger-bodied mainland species tend to be smaller on islands!) - and both of these patterns are well supported empirically. We also have apparent body size trends in some lineages where successive descendants keep getting bigger and bigger until they reach some apparent limit and go extinct. This might happen because of predator-prey arms races, competition, or runaway sexual selection where female preference drives the exaggeration of a trait (e.g., antlers) in turn selecting for larger body size to support that exaggerated trait.
Our understanding of how large some species can get and still function well is somewhat limited by the fact that many of the largest species ever to have evolved have gone extinct, leaving us to speculate whether their very size led to their extinction. Were the giant Haast's Eagles of New Zealand reduced to scavenging because they evolved too rapidly to become too big to be able to hunt? On the other hand, did Tyrannosaurus rex not become a good predator until it attained a sufficiently large body size? Our hypotheses about body size can thus go in different directions depending upon taxon and ecological circumstances. After all, we can only infer so much from reconstructing the anatomy of some of these giant beasts from their fossilized skeletal remains - but we are getting better at studying the fossils and visualizing their functions, by adapting technologies such as CAT (computed axial tomography) scans. This week some of the media picked up two papers reporting interesting findings about the two taxa mentioned above. The AP and several websites who picked it up from there got all excited about how the Haast's Eagle may have been the fearsome aerial predator of Maori legend as it might have hunted humans - especially (oh the horror) children! The original paper by Scofield and Ashwell while generating that sensational headline, is actually more interesting because of its approach and analysis of skeletal material, and in suggesting greater flexibility in the allometry of body size evolution. The authors used CAT scans to reconstruct and analyze the brain structure of the eagle to show that while the body of this species increased rapidly in size upon their arrival in New Zealand - presumably because of the lack of mammalian predators and the presence of a giant prey, the Moa - their brains did not increase in proportion! They use allometric analysis to show that these birds have much smaller brains for their body size compared to other Falconiformes, and this large size seems to have evolved rapidly: Haast's Eagle became 10-times the size of its putative ancestral lineage within a short 1.8 million years! That's what going after Moas, as seen in the above painting, meant for the evolution of this bird!! But wait, weren't these birds too big to hunt actively, instead scavenging off Moa carcasses like Condors and other large vultures?
The large morphological size of Haast’s eagle has led to competing hypotheses concerning its life style and behavior. Here we analyse neuroanatomical indicators to address questions concerning the behavior of Haast’s eagle: (1) was it an active predator or merely a scavenger of carcasses; (2) was it most likely to inhabit forest or open places; and (3) did its large body weight allow strong active flight?
We predict that if this eagle had vulturine habits it would show a combination of all, or some of, the following: (1) somatic evidence that it made significant use of olfaction; (2) adaptations indicative of an ability to undertake sustained gliding flight; (3) large eyes adapted to locate prey from considerable distances; and (4) a lack of evidence for the ability to attack and kill prey with its legs and talons. Alternately, if these features were not found to be present, we would suggest that Haast’s eagle was most likely an active hunter.
To do this, they look not just at the whole brain, but specific relevant regions of the brain where the size can be inferred from their scans, as well the anatomy of the eye and the optic nerve. They found that the olfactory system of the Haast eagle's brain was proportionally similar to smaller eagles from the Accipitridae and much smaller than in vultures. So these birds probably did not rely so much on smell to find food! Further, the visual system also remained similar to the Eagles, not apparently gaining any further acuity like we find in the vultures.
We suggest this disproportionate growth was only possible in the unique New Zealand environment where Harpagornis was not threatened by mammalian predation or competitors and was able to develop specific adaptations to predating on a particular prey, the moa, by evolving a bigger head and more robust talons. Also we prefer this interpretation rather than the alternative of Harpagornis evolving from a similar sized ancestor and in doing so undertaking a reduction in endocranial capacity, and degradation of the optic and olfactory lobes.
They then examined flight morphology to conclude that here the bird was more like vultures in being better adapted to soaring (despite its relatively short wings) than the fine control exhibited by its forest dwelling cousins. It may have dwelt more in open grassland habitats where it could swoop down upon its prey from cliffside perches. And here they draw upon corroborative evidence from Maori legend - for this fearsome beast went extinct from NZ skies only about 500 years ago!
Although no European scientist ever observed Haast’s eagle hunting, Maori oral tradition provides some evidence to support our behavioral reconstruction. One description given to Sir George Grey in 1872 (Grey, 1873, pg 435) states: “This bird, the Hokioi, was seen by our ancestors. We (of the present day) have not seen it. That bird has disappeared now-a-days. The statement of our ancestor was that it was a powerful bird, a very powerful bird. It was a very large hawk. Its resting place was on the top of the mountains; it did not rest on the plains. On the days in which it was on the wing our ancestors saw it; it was not seen every day as its abiding place was in the mountains. Its colour was red and black and white. It was a bird of (black) feathers, tinged with yellow and green; it had a bunch of red feathers on the top of its head. It was a large bird, as large as the moa”. Another description reported by the Reverend Stack (Stack, 1878, pg 63) said: “A Pouakai had built its nest on a spur of Mount Tawera, and darting down from thence it seized and carried off men, women, and children, as food for itself and its young. For, though its wings made a loud noise as it flew through the air, it rushed with such rapidity upon its prey that none could escape from its talons”. The carrying off of men and women is undoubtedly an exaggeration, but the description of its presence over open ground and mountainous terrain is consistent with our deductions.
So that is the sensational tidbit that the media can run with - this was a terrifying eagle that carried off children!! But the main thrust of the paper itself is in demonstrating that Haast's eagle was an active predator, with an oversized skull (hence disproportionately smaller brain) that attacked moas from the air, striking them with its strong, sensitive talons in the lower back over the kidneys and at the base of the skull. In the bigger evolutionary picture, this is a fascinating case study showing that neurological (brain) and somatic (body) expansion can be mismatched even in cases of island gigantism - there appears to be considerable flexibility in how natural selection affects different parts of the brain and body under different ecological conditions (such as the lack of predators or presence of new bigger prey).
Meanwhile, the other paper making the news this week goes the other way, describing a small Tyrannosaurid precursor of T. rex from
China that appears capable of hunting on its own too - because that lineage had already evolved skeletal features for the predatory lifestyle made famous by their much larger descendants! Large size was thus not a prerequisite for that lifestyle to evolve. Here's the abstract from Sereno et al in Science:
Tyrannosaurid dinosaurs comprised nearly all large-bodied predators (>2.5 tons) on northern continents during the Late Cretaceous. We show that their most conspicuous functional specializations—a proportionately large skull, incisiform premaxillary teeth, expanded jaw-closing musculature, diminutive forelimb, and a hindlimb with cursorial proportions—were present in a new small-bodied, basal tyrannosauroid from Lower Cretaceous rocks in northeastern China. These specializations, scaled up in Late Cretaceous tyrannosaurids with body masses approaching 100 times greater, drove the most dominant radiation of macropredators of the Mesozoic.
Better yet, thanks to Futurity, the new science portal I wrote about recently, we can get the story straight from Dr. Sereno's in this video:
Cool as a small Tyrannosaur sounds, this dude was still over 9 feet tall, so I'd still avoid it! And this latest among an already large set of fossil finds from China makes me wonder if the Jurassic Park franchise might head eastwards next!
- G. Grey (1873). Description of the extinct gigantic bird of prey, Hokioi, by a Maori. Transactions of the New Zealand Institute, 5: 435
- Scofield, R., & Ashwell, K. (2009). Rapid Somatic Expansion Causes the Brain to Lag Behind: The Case of the Brain and Behavior of New Zealand's Haast's Eagle (Harpagornis moorei)Journal of Vertebrate Paleontology, 29 (3), 637-649 DOI: 10.1671/039.029.0325
- Sereno, P., Tan, L., Brusatte, S., Kriegstein, H., Zhao, X., & Cloward, K. (2009). Tyrannosaurid Skeletal Design First Evolved at Small Body Size Science DOI: 10.1126/science.1177428
- J. W. Stack (1878). Sketch of the traditional history of the South Island Maoris. Transactions and Proceedings of the New Zealand , 10, 57-92