science

Projects - in progress

The isotopic ecology of African mole-rats informs hypotheses on the evolution of human diet

(2007) Proceedings of the Royal Society B: Biological Sciences. 274 1723-1730 [pdf]

Check out the cover of the July 22 2007 ProcRoySoc journal: [cover]

Here we tested the hypothesis that early hominins, such as Australopithecus africanus and Paranthropus robustus utilized underground storage organs (USOs) from plants. In 1999, the carbon and oxygen isotopic ratios of these species were published by Sponheimer et al., and evinced ratios suggesting that they depended on either C4 plants (tropical grasses and sedges), or animal that ate C4 plants. Later, two researchers, Greg Laden and Richard Wrangham, found an interesting correlation between mole-rat and hominin fossils being at the same sites. Because mole-rats are USO specialists, this correlation suggested that hominins lived in USO-prevalent habitats. To test whether these ancient human relatives may have consumed USOs, we looked at the isotopic ratios of both modern and fossil species of mole-rats. If these mole-rat isotopic ratios and hominin isotopic ratios overlapped, then USOs would have to be considered a plausible food resource. See the below figure of our isotopic data:

Mechanical properties of plant underground storage organs and implications for dietary models of early hominins

(2008) Evolutionary Biology. 35 159-175 [pdf]

To further test the hypothesis that hominins relied on underground storage organs (possibly as a fallback food), we analyzed the mechanical properties (toughness, hardness) of USOs in comparison to other primate foods (chimps and orangutans). This involved going to the field, collecting fresh USOs from across Africa, and measuring the physical properties with a portable mechanical tester. Importantly, in this paper we distinguish between different types of USOs: roots, tubers, rhizomes, bulbs, corms, in order to cess out which categories of USOs were more plausible foods. Our results show a high degree of variation among USOs, and the most plausible for hominin consumption are bulbs and corms. Tubers and rhizomes seem too mechanically taxing for smaller hominins (A. africanus), but may have been consumed by the robust hominins (P. robustus).

Cooperation and individuality among man-eating lions

Panel A: Isotopic results of modern mole-rat species overlaid onto values from hominins, Panel B: Isotopic results of fossil mole-rat species overlaid onto values from the same hominins. Sure enough, the values overlap, suggesting the USOs were plausible foods for hominins.

Projects - published and in press

C. Darimont

Cooperation is the cornerstone of lion social behavior. In a notorious case, a coalition of two adult male lions from Tsavo, southern Kenya cooperatively killed dozens of railway workers in 1898. The ‘man-eaters of Tsavo’ have since become the subject of numerous popular accounts, including three Hollywood films. Yet the full extent of the lions’ man-eating behavior is unknown; estimates range widely from 28 to 135 victims. Here we use stable isotope ratios to quantify increasing dietary specialization on novel prey during a time of food limitation. For one lion, the δ13C and δ15N values of bone collagen and hair keratin (which reflect dietary inputs over years and months, respectively) reveal isotopic changes that are consistent with a progressive dietary specialization on humans. These findings not only support the hypothesis that prey scarcity drives individual dietary specialization, but also demonstrate that sustained dietary individuality can exist within a cooperative framework. The intensity of human predation (up to 30% reliance during the final months of 1898) is also associated with severe craniodental infirmities, which may have further promoted the inclusion of unconventional prey under perturbed environmental conditions. PNAS LINK

Audio InterviewsTsavo_interviews.html
*Newsome SD, *Yeakel JD, Wheatley PV, Tinker MT. Tools for quantifying isotopic niche space and dietary variation at the individual and population level. J. Mammal. In review.
Wheatley PV, Elsey RM, Yeakel JD, Koch PL. Using stable isotope analysis to estimate long- term diets of American alligators in Rockefeller Wildlife Refuge, Louisiana.
Novak M, Wootton JT, Doak DF, Emmerson M, Estes JA, Jacob U, Tinker MT, Yeakel JD. Predicting species responses to perturbations: What is the community matrix? Ecology Letters.

Interaction Sampling Efficiency

Accuracy
Food-Web Size

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Yeakel JD, Stiefs D, Novak M, Gross T. Generalized modeling of ecological population dynamics. Theoretical Ecology. 4 179-194. [pdf]http://www.springerlink.com/content/u20wv8433418u78g/fulltext.pdfshapeimage_15_link_0

Over the past years several works used the approach of generalized modeling to study the dynamics of food chains and food webs. Generalized models come close to the efficiency of random matrix models, while being as directly interpretable as conven- tional differential-equation-based models. Here we present a pedagogical introduction to the approach of generalized modeling. This introduction places more emphasis on the underlying concepts of generalized modeling than previous publications. Moreover, we propose a shortcut that can significantly accelerate the formulation of generalized models. Finally we introduce an iterative procedure that can be used to refine existing generalized models by integrating new biological insights.

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Yeakel JD, Novak M, Guimaraes Jr. PR, Dominy NJ, Koch PL, Ward EJ, Moore JW, Semmens BX. Merging resource availability with isotope mixing models: addressing the role of neutral interactions. PloS One. 6 e22015. [pdf]http://www.pmc.ucsc.edu/~jyeakel/CV_files/Yeakel_2011%20PLoS%20ONE.pdfshapeimage_19_link_0