Biology can be a real pain sometimes – things are just not standard. A carbon atom is a carbon atom, and while it might change slightly with varying numbers of neutrons or electrons, these occur in a limited number of way and with standardised nomenclature. On the other hand, does anyone want to try and define a species? Thought not.
Biology is all about continuums from gene pools, cross-breeding, polyploidy and horizontal gene transfer, to populations, individual variation, biogeography, generic and species definitions and most noticeable of all – behaviours. There are ‘carnivores’ that eat plants and ‘herbivores’ that kill and eat animals, arboreal animals that live on the ground, and terrestrial ones that rarely leave the water. No matter what an animal is ‘supposed’ to do, you will always find an exception sooner or later. Not just a species that has broken the mould of its relatives, but even individual animals.
While morphology obviously gives us a great deal of information about the behaviour of an animal, it can of course only point us in the right direction. Even a highly specialised tool can be used for all manner of other tasks, even if it is poorly suited to them, if the selection pressure is high enough to retain a specialised shape. Eagles still build nests with hooked beaks, cats and sloths can swim and giraffe graze at ground level when each is clearly poorly suited to the job. These are still all no great surprise really (most birds build nests, most animals can swim, most herbivores will take whatever they can from time to time) but there are behaviours exhibited by both individuals and species that are very well wide of what you might expect.
My favourite example of this is climbing. Arboreality, or even just climbing into and out of trees (as opposed to spending a long time in them) is considered quite a speciality. Animals have evolved all manner of adaptations to this problem like reversible wrists and ankles, strong and curved claws, gripping digits, unusually long penultimate phalanges, sticky secretions and more to aid the basic up and down activity. So it is worth remembering just how good goats are at climbing trees. Very, very good in fact. And you will often find small crocodiles far higher in trees that they have any right to be. Hedgehogs are excellent climbers (of walls, if not trees) and wolverines happily shin up trees like oversized martens. Even lions and tigers are pretty good at getting up into trees, despite their traditionally being considered solely the home of the leopard in Africa and Asia. In other words, just because it doesn’t look like it was built to climb, doesn’t meant it can’t.
These animals are hardly at home in the branches, leopards are certainly far better adapted to climb than lions, but they can at least get well off the ground. However the important point here is that they can do it and they do do it. Even if this behaviour is not common or even essential to their survival to consider a lion terrestrial in the same way that one would a rhinoceros is clearly incorrect. The importance here is to remember this when considering palaeo species and their lifestyle and ecology – just because they look like they could not climb trees, swim, dig etc. does not mean that they could not or did not, and in some cases this might actually formed an important part of their repertoire of behaviour. While clearly we are unlikely to be able to be able to trace many of these behaviours except in exceptional circumstances, they should not be ruled out of ecological considerations. Yes, it’s hard to account for every possible behaviour of every single species when considering palaeoecology, but some of the more common ones often get short shrift.
Arboreality (or at least strong climbing abilities) is rampant in a great many apparently non-specialised species and I do not see why it might not have been common in other (apparently) unspecialised species in the past that lived in tree-rich environments. Trees could have offered the usual benefits of safety, new hunting grounds, additional food, storage and an increased sightline and would have thus been exploited by any animal that could make a decent fist of climbing them. The obvious group to cite in this circumstance is the derived maniraptoran theropods which are at the centre of the bird evolution debate with reference to flight starting from the ‘ground-up’ or ‘trees-down’. A lack of obvious arboreality-related characters are cited as evidence against the ‘trees-down’ model but this is exactly my point – why do they have to have them to be a) able to climb trees and b) make this an important part of their biology. True a maniraptoran with reversible wrists and ankles would really help the claim (and yes, I do know about Epidendrosaurus), but for me it need not be discovered to make me think that a significant number of small theropods were quite capable, and indeed able, to climb and even move around in the trees. The niches / ecospace was there and they had the capability to exploit it, so I would be very surprised if they did not.
That’s about it really, many things can climb that look like they can’t, so just because something doesn’t look especially suited to climbing does not mean that it could not, and indeed could have been quite suited to the job. This should be remembered in palaeoecological analyses and suppositions, and really should include more than just arboreality – there are a fair few of these examples that are worth considering.
(For more on climbing theopods, see Darren’s recent post over at Tet Zoo here. As it happens we wrote these posts simultaneously as I contacts Darren about some details only to discover he was digging up examples of his own for his post. Great minds and all that I suppose, though obviously he beat me to the punch by a few weeks, but the focuses of the posts are rather different, and I think they compliment each other rather well).