it required another blog post. I won't over blog it as it is open access and apparently far clearer than some of my other papers to my parents (my metric for how overly complicated I've made things). This one is about the importance of validating finite element analyses (see FEA for "dummies"), but will also touch on the joys of trying to publish negative results (i.e. when experiments don't match computer models). A quick background for those who don't want to read the previous post, finite element analysis (FEA) is a method for analysis how complex structures deform under loads, by simplifying them to a series of finite interconnected units (be it bricks, tetrahedra or any triangles: the elements) that have been given material properties appropriate for the structure (e.g. if it is a steel beam, the elements are given steel structural properties). It is known the method works incredibly well on man-made objects, and it is indeed the engineering tool used for everything from designing cars (and crashing them virtually) or planes, to bridges and buildings. Basically anything that an engineer might build, there is probably a finite element model out there somewhere.You may see where I am going with this then, the method works with varying degrees of success on biological structures for replicating strain magnitudes and orientations. Most recent work on mammals (monkeys, pigs) and reptiles (particularly alligators) manages to get very close replication of strain patterns across the models, but to date few studies have looked at birds. Birds are important as they have very mobile skulls (they have loads of extra little joints in the skull compared to most mammal and reptile skulls) are in a palaeontological context are important as the nearest living relatives to dinosaurs (being descended from them). Many studies have looked at how dinosaur skulls perform under feeding loads, but what does that really mean if we don't know how accurate models are on even their living relatives?
From Cuff et al., 2015. Ex-vivo experimental set up. (A) Experimental testing of ostrich with gauges attached, under loading of the artificial tendons. (B) Schematic of experimental rig showing load and constraints.
|From Cuff 2014. Ostrich cortical bone, and muscle model showing strain patterns.|
These results are particularly interesting as similar methods have worked on mammals and alligators producing models that closely match those of the experiments. As for why the results are so far off in our models is unknown, and something that needs further investigating. It may come down to how we modelled the materials of the cranium, because joints in the skull are far more difficult to model than we have, because our new tendons were worse than before, or a myriad of other factors that I've not discussed here or in the paper. However, the data in the paper are all interesting and this is the first full attempted cranium validation of a bird ever. As a spin off issue from the paper, it showed me how difficult it is to publish negative results. Negative results are where the results of a study show no match between models and the experiments or in the case of medical science, where the medicine are no better than a placebo. However, these results are really poorly represented in publishing as they don't make sexy stories. This leads to the potential for replication of experiments that don't work repeatedly through time:
Cuff AR, 2014. Functional mechanics of ornithomimosaurs. Thesis. University of Bristol.