MIRED AND RETIRED: AGE-SELECTIVE COLUMBIAN MAMMOTH (MAMMUTHUS COLUMBI) MORTALITY FROM RANCHO LA BREA, CALIFORNIA AND ITS IMPLICATIONS FOR EVALUATING TOP-DOWN FORCING IN LATE PLEISTOCENE TROPHIC SYSTEMS

Rancho La Brea (RLB), California has produced an unrivaled assemblage of late Pleistocene carnivores in North America, but also a sizable assemblage of Columbian mammoth (Mammuthus columbi) bones and teeth from several asphalt pits. We conducted this study as an opportunity to supplement RLB mammoth assemblage age profile data with an unparalleled assemblage of Pleistocene large carnivores to test the top-down trophic control hypothesis by Ripple and Van Valkenburgh (2010). This hypothesis is supported by multiple datasets from the fossil record, which suggest that North American Pleistocene carnivores exerted top-down ecological controls on megaherbivore populations that kept them under carrying capacity. Thus, if Pleistocene carnivores engaged in prey-switching, their prey preference would change based on regional population sizes of megaherbivores; any small change in megaherbivore population sizes due to climate change or human hunting could potentially tip the scales and lead to ecological collapse.

Fossil mammoth molariform teeth were selected from La Brea Tar Pits and Museum for this study to determine the tooth assignment and percentage of wear. Tooth assignments were determined by measuring the length, width, height, and tooth plate count measurements, referencing an extensive literature review of proboscidean dental progression and wear.

We observe selective mortality within the mature-adult age class, with a background “Type A” pattern in other age classes. This mortality pattern could be reflective of carnivore activity and mortality during the warmest months of the year and warmer overall years between 40-14ka. In particular, this mortality was likely additive, considering presumably healthy, mature adults became mired within the asphalt, but not likely contributing to population instability. Our data, corroborated by other studies, do not suggest mammoth population instability or decline at RLB due to resource stress during the interval of 40-14 ka, allowing us to refute our alternative hypothesis of bottom-up control. Although we cannot support the top-down trophic control hypothesis directly with our dataset, data from numerous studies of large carnivores at RLB paint a picture of a diverse and competitive predator guild during the late Pleistocene.