Page 156 - Introduction to Paleobiology and The Fossil Record

FOSSIL FORM AND FUNCTION 143

Box 6.2 The Irish deer: too big to survive?

The Irish deer Megaloceros, formerly called the Irish elk, is one of the most evocative of the Ice Age

mammals (Fig. 6.5a), and one of the most misunderstood. When the first fossils were dug out of the
Irish bog, Thomas Molyneux wrote of them in 1697, “Should we compare the fairest buck with the
symmetry of this mighty beast, it must certainly fall as much short of its proportions as the smallest
young fawn, compared to the largest over-grown buck.”
This was a large deer, some 2.1 m tall at the shoulders, and it famously had massive antlers, the
largest spanning 3.6 m. The old story was that this deer simply died out because its antlers became
too large. Paleontologists understood that the antlers were subject to sexual selection and, as in
modern deer, the male with the largest antlers and the scariest display probably gathered the largest
harem of females and so passed on his genes most successfully. But can a species really be driven to
extinction by sexual selection?
In a classic paper, the young Steve Gould (1974) showed that this was clearly nonsense. He measured
the body lengths and antler dimensions of dozens of specimens and showed that they fell precisely on
an allometric curve, and that the allometric curve was the same as for other relatives such as the smaller,
living red deer and wapiti (Fig. 6.5b). It is clear that sexual selection and natural selection were at odds
in this case, as often happens, but the balance was maintained and indeed the Irish deer was successful
throughout Europe, existing until 11,000 years ago in Ireland and 8000 years ago in Siberia. It proba-
bly died out because of climate change at the end of the Pleistocene and hunting by early humans, rather
than by collapsing beneath the weight of its overgrown antlers.
It is worth reading Gould’s (1974) classic study of positive allometry in the Irish deer, and a
broader review of positive allometry in sexually-selected traits by Kodric-Brown et al. (2006). Read
more and see color illustrations at http://www.blackwellpublishing.com/paleobiology/.

100 M
A
80
60 A
Maximum length of antler (cm) 40 D

20

20 40 60 80
Height of shoulder (cm)
(a) (b)

Figure 6.5 Positive allometry in the antlers of the giant Irish deer Megaloceros. (a) A famous
photograph of an Irish deer skeleton mounted in Dublin in Victorian times. (b) Positive allometry
in the antlers of modern deer, showing that Megaloceros (M) falls precisely on the expected trend
of its closest living relatives. Note that the fallow deer (D) plots above the slope (i.e. antlers are
larger than expected from its height), and the European and American moose (A) plot below the
line (i.e. antlers are smaller than expected from their height). Two regression lines, the reduced
major axis (steeper) and least squares regression, are shown. The allometric equation is antler
length = 0.463 (shoulder height) 1.74 . (Based on information in Gould 1974.)