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birds, such as the two species of Lagopus, which abounded in
the steppe-tundra environment, also underwent changes upon
its demise, such as reduced geographic ranges and body size.
Interestingly, the largest subspecies of the genus, L. I. major,
lives today on the steppes of Kazakhstan.
In support of this idea is the fact that all the fossil populations
examined, except that from Rebielice Krolewskie (the
late Pliocene site), come from deposits considered to belong to
cold phases of the Pleistocene. The oldest members of the genus
Lagopus examined after those from Rebielice come from a
cold horizon above the interglacial at Westbury-sub-Mendip in
England, which is early middle Pleistocene (oxygen isotope
stage 12). Unfortunately, the fossils are few in number and are
fragmentary, which makes it difficult to assess to which species
they belong. They do, however, possess relatively robust tarsometatarsi,
so it may be that Lagopus was already adapted to
the steppe-tundra and was larger in relation to today's birds.
The next youngest assemblage examined in this survey is that
from La Fage, which is late middle Pleistocene. Both species
are definitely present, although it may be significant that they
appear to be less divergent from each other in their tarsometatarsal
lengths than are modern birds (Figure 2). This may be
support for Mourer-Chauvire's (1993) suggestion that the species
had diverged not long before.
The greater areas of the crista pectoralis and crista bicipitalis
may indicate that both L. lagopus and L. mutus were larger in
the Pleistocene. If there were a primary selective force for large
body size, so that birds were heavier, they would require greater
muscle bulk to fly, which in turn adds further to body
weight. Alternatively, the birds may have become larger because
of selection for better-developed flight muscles under a
different climatic regime when the birds may have been less
sedentary. This hypothesis would be bolstered by the findings
of Bocheriski (1974, 1985) and Bocheriski and Tomek (1994),
who demonstrated that the distal-wing elements of Lagopus lagopus
and L. mutus during the Pleistocene of Poland and Austria
were relatively longer than in present-day birds, and that
their legs were relatively shorter. This conclusion, however,
was not confirmed by the samples analyzed in the present
study.
Bocheriski (1974) claimed that there was a clear, positive
correlation between temperature and tarsometatarsus length, although
he pointed out that local vegetation type also was influential.
The nature of the variability seen in tarsometatarsal
length over both time and space implies that, unlike the shaft
widths, local factors may have had an influence. This seems
more likely than the variation being a reflection of the other
thermoregulatory biogeographic rule (Allen's Rule), which
would produce more uniform clines across the birds' former
geographic ranges. Therefore, it may be that influences such as
the local terrain are more important than the influence of temperature
because locomotion is generally regarded as important
in determining leg length in mammals (Scott, 1985). Due to the
small magnitude of the differences involved and the small size
SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
of the birds in relation to the ground relief, however, this conjecture
is difficult to test.
Conclusion
Lagopus lagopus and L. mutus differ allometrically between
the Pleistocene and the present, a consistent finding even when
fossils from widely distributed areas and times are compared
with greatly dispersed modern samples from across Europe.
This difference is most readily identified in the tarsometatarsalshaft
width and implies a change that was due to a general or
widespread effect, not a local adaptation similar to that proposed
for the Holocene evolution of the red grouse {L. I. scoticus)
in the British Isles (Voous, 1960; Tyrberg 1991). The
changes in tarsometatarsal length appear to be reactions to variations
in local conditions that remain unknown. The uniformity
of change in tarsometatarsal width across Europe, however,
implies regional rather than local effects. Global events that occurred
at the Pleistocene/Holocene boundary seem to be a likely
explanation; however, two problems arise. First, untangling
cause and effect between climatic and vegetational factors,
which are closely linked, and second, the possibility that a
change occurred in the birds' vagility.
It is perhaps easier to conceive that the two species of Lagopus
changed size due to climate change and its direct effect on
vegetation rather than to changes in the bird's degree of sedentariness.
The birds appear to have reacted in much the same
way as did many mammals that survived the Holocene/
Pleistocene boundary by becoming smaller. It is suggested
herein that the change in seasonal length and vegetation type
was the primary reason for this, and not temperature. The birds
in the northern areas of Europe today, and particularly in mountainous
regions further south in the case of L. mutus, probably
exist at much the same temperatures as they did in the Pleistocene
in southern Europe. This would eliminate temperature
as the primary causal factor of size decrease in the genus over
this period, because birds of comparable size to those of the
Pleistocene are not present in northern Europe today. Dietary
shifts caused by vegetational changes are proposed herein as
the most significant factor leading to size reduction. It therefore
seems reasonable to suggest that the birds in the genus Lagopus
in the Palearctic today are relictual populations that originally
evolved and diversified into Lagopus lagopus and L. mutus on
the steppe-tundras of the late-middle and late Pleistocene.
Bocheriski (1974) and Mourer-Chauvire (1975a) had previously
demonstrated allometric trends in European Pleistocene
Lagopus, but the suggestion of a major reduction in size at the
Pleistocene/Holocene boundary over most of Europe is new, as
is the suggestion that it was due to the vegetational changes described
above.
This work has implications for the taxonomic use of allometric
differences in skeletal elements. Often such differences are
given greater significance than are mere size differences. The
example described above clearly shows that a change in size