BERGMANN'S RULE, CLIMATE CHANGE, AND STASIS IN LATE PLEISTOCENE MAMMALS AND BIRDS FROM RANCHO LA BREA

Traditional evolutionary theory has long suggested that species should show a morphological response to dramatic environmental change. Many mammals and birds have larger body sizes in colder climates (Bergmann's rule). The fossil samples of the Page Museum at Rancho La Brea is an excellent place to test this hypothesis, because the La Brea tar pits span an interval from 35 ka to 9 ka (late Pleistocene to early Holocene), with huge samples of many different bones, all recently radiocarbon dated. This age range spans Oxygen Isotope Stage 3 (59-24 ka), the last glacial maximum (24-14 ka), the glacial-interglacial transition (14-10 ka), and the early Holocene (since 10 ka). Based on local plant fossils and pollen from offshore deep-sea cores, the coastal sage/oak chaparral of 35 ka was replaced by a closed-cone juniper-pinon forests with winter snows during the last glacial maximum (LGM), then sage/oak chaparral returned during the glacial-interglacial transition. Such a dramatic vegetational changes should cause an increase in body size during the LGM, but no such trend is observed. Instead, every single common bird and mammal with reasonable sample sizes from well dated pits shows stasis for the entire time interval, with no significant differences between temporally sequential samples, as established by ANOVA.The birds studied include the golden eagle, bald eagle, La Brea condor, the caracara, and the La Brea turkey. Both eagles and the caracara develop larger body sizes in colder climates, yet there is no significant size change at Rancho La Brea. Stasis also prevails among all the common mammals: dire wolves, sabertoothed cats, American jaguars,horses, camels, bison, and ground sloths, even though there is a striking Bergmann's rule trend in living horses, camels and bison. The only exception is a bizarrely dwarfed sample of dire wolves at Pit 13 (16 ka), but this does not appear in any other animal from that pit, nor does it correlate with the peak of cooling at 20 ka. Such stasis in Pleistocene mammals and birds during the glacial-interglacial cycles is actually well known to Pleistocene paleontologists.Perhaps the best explanation is the idea that most Pleistocene bird and mammal species are broadly adapted to a wide range of climates, so they do not respond to local climate changes as much as tradition would predict.