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Re: So-called Sickle Claws



>I speculated a couple of months ago that the dromaeosaurs might have
>been semi-arboreal, and the sickle claw was a tree-climbing adaptation. 
>Climbing trees, climbing mountains -- might call for some of the same
>adaptations.  Only problem is, one can find features at least vaguely
>like the sickle claw in tree-climbing animals like cats.  But I don't
>know of any mountain-climbing animal that has such a feature.
>
>-- JSW

The only one that comes to mind is a bird, the Wallcreeper Tichodroma
muraria of Eurasia, which has large feet with strong curved claws similar
to those on tree-climbing birds (there are also the terrestrial or
semi-terrestrial Rock Nuthatches and a few woodpeckers, but I am uncertain
as to how much actual rock climbing they do as opposed to flying from perch
to perch.  The adaptations of the Wallcreeper, though, may well have
evolved first for tree climbing as it appears to be related to nuthatches.

Given the hard surfaces of rock faces I am generally inclined to doubt that
dromaeosaur-type claws would last long if they were used for scaling
cliffs.  Perhaps a more typical rock-climbing adaptation is the evolution
of a small foot that can take advantage of tiny projections, etc - the sort
of thing you see in the North American mountain goat or the African
klipspringer.

The idea of dromaeosaurs as climbers does raise one thought.  A large,
forward-directed claw on a climber's hind foot (as opposed to the fore
foot) may not be much use for climbing unless there is a backward-directed
brace to hold the animal's body at least partly free of the bark.  In birds
this may be a large, backwards-directed hallux with a strong claw (as in
Australian treecreepers (Climacteridae)) or,as in woodpeckers, a stiffened
tail.  The tail is even a brace in one mammal family, the scalytails or
African flying "squirrels" (Anomaluridae), which have special scale-like
structures on the underside of the tail that apparently provide traction.

Of course dromaeosaurs had (or at least some had?) remarkably-stiffened
tails.  These I have usually seen interpreted as stabilizers for balance
during sharp turns or other ground-based manouevres - but I am,
hesitatingly, wondering whether they might not have been useful in
tree-climbing as well.  Is there anything about the mechanical structure of
their tail that would make this unlikely?
--
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@inforamp.net