Different rates of body size evolution across dinosaurs (including birds). Benson et al (2014)

Body mass is probably the most important physiological features for all animals. It corresponds strongly with a range of life features, including metabolic and growth rates, population density, diet and dietary strategy, locomotion style and mechanics, and mode of reproduction.

It comes as perhaps no surprise then that body mass is one of the most widely explored features of extinct organisms by palaeontologists. Last year, a slew of papers explored the evolution of body size in dinosaurs, including birds (e.g., this one in PLOS Biology). Most of these found that rapid changes in maniraptoran theropods, the dinosaurian lineage leading to modern birds, occurred from the Middle Jurassic (about 160 million years ago) and onwards.

Importantly, this means that, in terms of body size, birds were constantly and rapidly innovating and changing, which might have set the scene for the origins of the great bird radiation. It's weird to think about, but with 10,000 living species of bird, we are still technically in the 'reign of the dinosaurs", and it might be due to an early ability to rapidly evolve body size and adapt to changing conditions.

In birds, body mass has one additional and unique factor in that it correlates with the amount of lift an animal can generate, and therefore influences whether or not they can fly! Therefore, being able to accurately estimate body mass in extinct birds has important implications for our understanding of the origins of flight.

Previous studies, including those mentioned above, have had to rely on proxies to estimate body mass. It's ridiculously unlikely that we'll ever find a complete dinosaur, and we only have their skeletons to go off. One way of estimating body mass has been to use the circumference of the femur, which correlates strongly with body mass in a range of living organisms – known as an 'allometric' relationship. Estimates of body mass in birds have also been applied to pterosaurs, a group of now extinct flying reptiles related to dinosaurs. But the question remains, how accurate are our estimations of body mass in the fossil record?

A new study, led by Liz Martin-Silverstone at the University of Southampton in the UK, set out to divine the relationships between skeletal mass and complete or total body mass in birds (i.e., involving all the fleshy parts).

Linear scale association between total body mass and skeletal mass in birds.

What they found, using a range of analyses and datasets, was a strong positive association between body mass and skeletal mass, as we might expect – as the skeleton of an animal gets bigger, so does its overall mass. This is important, as it means that for living neornithine birds (at least), estimates of skeletal mass accurately reflect total body mass, and therefore skeletal mass can be used as a proxy to estimate the life traits mentioned at the beginning of this post.

Despite overall good correlations, the authors found quite a lot of natural variation within species, based on an extensive new dataset compiled from the collections at the Royal British Colombia Museum (Victoria, Canada). This is simply due to the fact that we have different animals of different sizes within species – take a look at humans, for just one obvious example of this. An example from birds is the rhinoceros auklet, which has a total body mass ranging from 258-616.2 grams!

The reason for such variation can also be due to age – it's a pretty well established phenomena that animals get bigger as they grow up. This has drastically important implications for estimating skeletal mass across animals in the fossil record. For each animal, they would have to be shown to be the same growth stage, or ontogenetic age, so that their body masses could be directly comparable. There's not really much point comparing the body mass of a juvenile of one species to that for a fully grown individual of another! Birds also grow ridiculously fast (when was the last time you saw a baby pigeon?), so it can be very difficult to accurately tell what their ages are without detailed examination.

The authors also identified a range of confounding factors that influence estimates of body mass. For example, when female birds are ready to lay eggs, they accumulate and deposit more calcium within their bones to save it for egg production. So we might expect the skeletal mass to vary between males and females of the same species, depending on sexual maturity. However, there were no significant differences between the sexes, despite this possible variation. As well as this, migratory birds have very different weights before and during migrations, although this is relatively slight at just a few percent difference, but whether or not this affected the results is unknown.

What about flight mode? Does this affect estimates of body mass, as we might expect flight capable birds to have bigger muscles for flapping their wings, or perhaps be lighter in order to generate more lift for flight. Martin-Silverstone and colleagues found, however, that there was again no statistical difference in the relationship between body mass and skeletal mass across different flight modes. This is great, as in fossil birds, it suggests that even if we don't know their flight style, as it's notoriously difficult to infer in extinct animals, we can still accurately estimate their body mass. The authors are careful to note though that their analyses did not cover all birds, and seems to have excluded a whole range including penguins, ratites (kiwis, emus, and ostriches), cormorants and a whole load of other avian weirdos.

Logarithmic scale association between total body mass and skeletal mass in birds.

Importantly, this scalar relationship between skeletal mass and body mass changes when evolutionary relationships are accounted for. When analysing the evolution of 'traits' such as body mass, a portion of similarity between species will simply be due to the fact that we expect more closely related organisms to adopt similar morphologies. This suggests that when estimating body mass, or using raw body mass estimates to make big macroevolutionary statements, that we should make sure that phylogeny (the evolutionary relationships of organisms) is well accounted for.

What does this mean overall for estimating body sizes in extinct organisms? Well, the raw scalar relationship between skeletal mass and body mass is clear for birds, that is, the clade known as Neornithes. However, in different but closely related groups, such as extinct birds like Enantiornithes, Hesperornithes, as well as pterosaurs and non-avian dinosaurs, it is likely that this scalar relationship will be invariably different. This is due to the simple fact that each of these groups of animals are distinct from modern birds – that's what makes them different groups! The authors suggests that there might be better ways of estimating the body mass in organisms like dinosaurs, such as using allometric relationships (such as the femur circumference one mentioned above), or estimates of whole body volume by using scanning methods! Both of these have been widely used, but often produce quite different results.

Sexual variation between total body mass vs. skeletal mass relationships in birds.

For pterosaurs, the close cousins of dinosaurs, the scaling relationships between skeletal mass and body mass have been used before to predict the body masses of a range of species. However, this comparison might not have been appropriate, as pterosaurs are vastly different animals to birds, and have completely different wing anatomy, as well as individual bone masses. This means that previous estimates of body mass in pterosaurs might not have been too accurate, and probably need refining in light of the relationship between skeletal mass and body mass, as well as a deeper understanding of the morphology and pneumatisation (how much air a bone contains) of different pterosaur species.

So, the tl;dr version of this would be: body mass is really difficult to estimate in extinct organisms, should be cross-checked using extant organisms where possible, and confounding factors such as phylogeny, mode of life, sex, and ontogeny must be accounted for!

Explore further New algorithm for estimating body mass of extinct quadrupedal mammals

More information: Elizabeth Martin-Silverstone et al. Exploring the Relationship between Skeletal Mass and Total Body Mass in Birds, PLOS ONE (2015). Elizabeth Martin-Silverstone et al. Exploring the Relationship between Skeletal Mass and Total Body Mass in Birds,(2015). DOI: 10.1371/journal.pone.0141794 Roger B. J. Benson et al. Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage, PLoS Biology (2014). DOI: 10.1371/journal.pbio.1001853 Journal information: PLoS Biology , PLoS ONE

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