Re: Sifaka and Chukar Behavior: Trees Down or Ground Up?

[Ronald Orenstein <ornstn@rogers.com>]

At 12:47 AM 23/01/03 -0800, Jaime A. Headden wrote:
> What this appears to be, rather, is that one group of small, predatory
> theropods in the trees became adapted to gliding independantly of all
> other  avian-style theropods, and that in no way do these finds support
> either the trees down or the ground up hypotheses. Merely an interesting
> sideline. The tetrapteryx of Beebe and the feathered legs of Feduccia et
> al. are interesting predictors of four-winged animals, but again do not
> show how flapping flight would have evolved.

In fact I wonder, given the range of new discoveries, whether ANY single 
fossil discovery can now shed light on this question.  What we seem to have 
is a radiation of feathered or proto-feathered dinosaurs that may well have 
lived a variety of lifestyles in a variety of habitats, with their feathers 
possibly serving a variety of functions.  It may be that the adaptive 
shifts involved occurred rapidly and, evolutionarily speaking, fairly 
easily.  Therefore a finding that one species (or another) seems to have 
been arboreal (or not) may do little more than attest to the variety of 
lifestyles and behaviours taken up by feathered dinosaurs.

Certainly within living birds there are numerous examples of 
closely-related species with quite different lifestyles (I have mentioned 
the flickers (Colaptes) before, which include purely terrestrial species 
confined to treeless areas and highly-arboreal forest-dwellers, with 
intermediates).

The assumption of this whole debate seems to be that there was a very clear 
dichotomy between terrestrial and arboreal modes of life, and that only one 
of these led to the evolution of flight in living birds.  I do not see why 
this has to be correct - especially if flight or near-flight was being 
achieved repeatedly in a variety of forms.  What, for example, does it mean 
to be arboreal?  There are many birds today that are both arboreal and 
terrestrial; birds (like herons) that are primarily terrestrial but nest in 
trees; others (like some wood warblers) that forage in trees but nest on 
the ground; others (like some grouse) that feed and nest on the ground but 
will fly into trees to escape predators or to roost.

Why can we not speculate (especially considering Dial's work) that 
pre-flight adaptations may have evolved neither for a purely arboreal or a 
purely terrestrial animal?  Is it possible that their chief advantage may 
have been to allow their bearers the versatility to make use of both trees 
and the ground to varying degrees, for a variety of purposes, rather than 
to enhance one way of life or the other?


--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@rogers.com