The body size of an organism reflects complex tradeoffs among numerous processes. Nevertheless, certain size-dependent relationships are repeatedly observed for mammals and other taxa. For example, the distribution of mammalian body sizes (i.e., minimum, maximum, and modal size) is remarkably similar across continents, despite little speices overlap. Moreover, distributions appear to have been similar for the past 50 million years. Do patterns arise because of common ancestry, because organisms exist in similar environments, or because they face similar design or life history constraints? The broad goal of this project is to assess the generality of body size patterns and investigate general underlying processes. The project assembles an international and distinguished team of scientists with expertise spanning the full spectrum of time, space, and various disciplines (e.g., paleontology, marine and terrestrial ecology, evolutionary biology, genetics). Anticipated results include the development of a comprehensive global database on life history, body size, geography, and phylogenetic relatedness for mammals as well as the development of novel analytical and statistical tools.
The intellectual merit of the project stems from investigation of the influence of various intrinsic and extrinsic factors in generating 'invariant' body size patterns across vastly different scales of space and time. Since the largest mammals are often critically endangered (e.g., African elephant, blue whale) and small mammals are comparatively understudied, results will provide important insights into conservation challenges posed by species at extremes of size. Broader impacts include: 1) development of much needed analytic tools and methodologies of use to scientists working in many emerging fields, 2) a macroecological database freely available to the public and other scientists, 3) development of an integrated network of scientists including international collaborators and both minority and female scientists, 4) training and mentoring of graduate and undergraduate students, and 5) outreach efforts targeted especially to underrepresented students including development of a course on macroecology.
What controls what the largest species on the planet are? We used the evolutionary history of mammals to understand this question. When dinosaurs existed, the largest species of mammal was about the size of a skunk. After the dinosaurs went extinct, however, increasingly larger species of mammal evolved. We collected data on the weight of species in different lineages of mammals (for example: carnivores, rodents, primates) across 4 continents over the last 65 million years. We found that after the dinosaurs went extinct, every lineage of mammal started producing larger and larger species (Figure 1). For most lineages, this continued for ~20 million years before reaching a maximum size that remained relatively stable until relatively recently or until that lineage went extinct. The only exception to this trend is the whales, which show no indications that they have reached the largest body size that lineage can produce. When we looked at the tree of life for mammals (i.e., phylogeny), we found that the evolutionary signal in more recent geological ages is different from the signal that emerged immediately after the dinosaurs went extinct. Our results suggest that the extinction of the dinosaurs created evolutionary opportunities for mammals to reach much larger sizes, but that it took millions of years for mammals to fully exploit this opportunity. Not only did it take millions of years for most mammal lineages to reach their largest possible size, but some lineages were able to do this faster and/or reach larger body sizes than others. The largest whale, for example, is much larger than the largest rodent or primate. We asked what processes could control the rate a lineage can evolve and how big it could reach. One possibility is that as the size of a species gets larger, its generation time decreases. Generation time is the time between when an individual is born and when it is sexually mature enough to produce the next generation of individuals. A house mouse is sexually mature at around 6 weeks, while for an African Elephant this takes around 11 years. Because evolution reflects changes from one generation to the next, evolution can only move as fast as a new generation of offspring is produced. While the size of an organism influences generation time, so does how many resources it can devote to producing offspring. Producing offspring more quickly shortens generation time. It is also important for determining whether or not a species can persist. No species can persists if it is not producing new offspring fast enough to balance the death of individuals. If a species hits a size where it produces new individuals too slowly to compensate for individuals dying, then it will go extinct. We developed a theory for how generation time and productivity could interact to determine the maximum size of a lineage and the rate it took to evolve to it. Data collected on sizes of lineages through time were well fit by our theory, suggesting that both the rate at which new offspring are produced and the increases in generation time as a species get larger, combine to influence the patterns of size evolution that we see in earth’s history. Using catastrophic events from Earth’s history provides opportunities for understanding how life on this planet may respond to the large number of changes humans are currently enacting on nature. However, using the past to inform the future requires a different approach to asking and addressing scientific questions. To help strengthen the ability of scientists to engage in this type of research, we invested in a number of educational activities, including training of young scientists, creation of new courses at several universities, and publishing three books to be used by junior and senior scientists to learn about non-experimental approaches to ecology and evolution and the insights they can provide.
