I am fascinated by the myriad of physical forms and behaviors that animals display. In my research My research uses genomic tools to understanding both how this amazing diversity evolves and how to conserve it. Currently most of my work is on zebras. In collaboration with colleagues at UCLA, other institutions in the US and abroad, I utilize the extensive variation in stripe pattern in zebras in an effort to understand the function of stripes and to pinpoint the genetic and developmental mechanisms that create stripe patterns. In more conservation focused work, we have examined the impact of habitat fragmentation on population structure and inbreeding and the potential consequences of inbreeding for the stripe phenotype. In addition, we have recently developed an epigenetic clock based on plains zebras, with which we successfully aged other equids, including two endangered species. The ability to accurately age wild animals is essential to predicting population viability.
Understanding how environmental chemical signals and cues mediate various life-history processes can indicate the forces driving the ecology and evolution of natural systems. Yet, these mechanisms remain largely undescribed. To embrace this challenge, my laboratory is developing new instrumentation and analytical techniques for identifying the structures and concentrations of bioactive molecules, while measuring their distributions over the time and space scales relevant to sensory information processing. Through field and laboratory studies, we are devising new theories about chemical communication systems and their roles in mediating physiological mechanisms and ecological consequences at individual, population and community levels. Investigation is driven by a need to understand the sensory basis for behavioral performance. Armed with this knowledge, we seek to determine how, when, and where in nature such behavior makes a difference. To date, we have established fundamental mechanisms controlling (1) sperm-egg interactions, (2) habitat colonization, (3) predation-prey relationships, and (4) microbe-nutrient dynamics. Each of these processes is seminal in regulating the abundances and distributions of microbes, plants, and animals. Our future research will continue to include interdisciplinary investigations on numerous spatial and temporal scales, emphasizing both laboratory and field work. Incorporation of these findings into larger ecological and evolutionary frameworks will promote understanding of natural physicochemical processes that create and maintain biodiversity.
Cheryl Ann Zimmer
My research focuses on the factors that determine where benthic marine invertebrates live (benthic means living on or in the bottom). Distributions of invertebrate populations, including commercially important clams, snail, worms, and crabs, often reflect initial larval settlement sites. What, then, determines settlement locations? The classical view is that settlement is a two-phase process, first dispersal, then “touch-down,” when larvae contact the seafloor. The touch-down process and cues are less well understood than dispersal. Recently, we discovered that a surface layer of fluffy, aggregated particles, known as “floc,” contain very high concentrations of larvae. Larvae can actively enter or get passively entrained in floc, depending on the strength of water currents near the bottom. What floc provides for the larvae remains unknown, but we are investigating the hypotheses of nutrition and protection from predators. A third hypothesis involves larval transport with floc mats that drift along the sea floor. The itinerant aggregates may expose larvae to more potential settlement sites than the animals would encounter by swimming. The discovery of an association of larvae with floc, and floc’s apparent role in settlement, suggests the existence of an important but previously unrecognized third phase in the settlement process. Greater understanding of floc formation may help explain the success, or failure, of settlement in different benthic environments. Because contaminants attach preferentially to the fine particles that make up floc, pollution may also affect the formation and quality of floc, and thus, settlement distributions.
I am interested in the evolutionary genetics of wild animal populations, particularly of dogs and dog relatives. A new aspect of our work is to identify and study specific genes that influence the form, physiology, and behavior of organisms in nature. This has become possible because the dog genome has now been fully sequenced. Wolves are close relatives of dogs and so much of this new genetic information also applies to them. My colleagues and I took advantage of this new knowledge and identified the gene that causes black coat color common in North American wild wolves. We discovered that the mutation that causes black coat color in wolves came from a hybridization event between Native American dogs and wild wolves that probably occurred thousands of years ago. This dog gene has swept through North American wolves and is the first example of a mutation that appeared in a domestic species that has become common in a wild relative species. This type of discovery brings us closer to the holy grail of evolutionary biology: to understand and record the simultaneous changes in the genome and in the phenotype of organisms under the force of natural selection operating in wild populations. We are accustomed to thinking that such studies can only be done in the laboratory, but the new field of genomics opens the door to doing such studies under natural conditions for the first time. It is a new day in evolutionary biology.
Blaire Van Valkenburgh
I am a paleontologist interested in deducing the ecology and behavior of extinct animals. For example, how can we determine if extinct species were social? At Rancho La Brea (the famous tar pits in Los Angeles), large Pleistocene carnivores were lured to their death by the sounds and smells of herbivores trapped in sticky asphalt. Studies of living African carnivores reveal that social species, such as lions, hyenas, and jackals, are much more likely to come to the sounds of dying antelope than solitary species such as leopards and cheetahs. Apparently, it is better to arrive at a carcass in a group of others in case you have to fight for possession. At La Brea, the dominant carnivore species were dire wolves, sabertooth cats, and coyotes. Like modern lions, the sabertooths even brought their cubs occasionally. Based on the African studies, these findings strongly suggest that all three of these species were social, and provides some of the best evidence for sociality in a sabertooth cat thus far.
I have worked on many interesting and diverse research problems in my laboratory, including machine intelligence. However, I am currently pursuing my other passion, which is addressing major world health problems. In particular, I have been working on how to tackle the worldwide resurgence of malaria, which is one of the deadliest diseases in the world, killing more than a million people every year. We are working, along with many other labs, toward malaria control by genetically modifying the local mosquito populations. Before trials can be conducted we will need to have a good understanding of the population genetics and ecology of the local mosquito populations so that the progress and evolutionary consequences of the modification can be evaluated. We have also been analyzing things that might go wrong with mosquito control programs.
I study how tree populations exist and persist. To do this, I integrate theories and methods from ecology, evolutionary biology, and population genetics. Because genetic diversity is a key ingredient to the ability of populations to respond to environmental change, one element of my research explores how genes move within and among landscapes through pollen dispersal. In addition, I am curious about how seeds move because seed dispersal is key to the regeneration of populations. Both pollen and seeds can be dispersed by animals, which mean that we must understand how birds and bees and bats and other mammals (to name a few) move pollen and seeds. If animal pollinators and seed dispersal agents disappear, many tree species are jeopardized. I often say that my research goes from the DNA sequence to the landscape because I use genetic tools to understand the landscape processes. Our research group conducts studies in Central America, Mexico, Africa, South America, North America, and especially California. In California, I am studying the impact of landscape change on oak populations and using this information to develop priorities for conservation that will enhance the long-term survival of oaks in the face of climate change. By studying tree species, I am always focusing on the plants that define the ecosystem. Thus, my research generates information on the ecological and evolutionary concepts that shape natural populations, and provides criteria to manage their long term survival.
I am interested in improving the scientific basis for conservation programs to preserve biological diversity. Many approaches in conservation are not sufficiently evidence- and science-based, often relying on personal experience or conventional wisdom with little or no connection to theoretical or empirical work on the evolutionary ecology of the organisms to be conserved. For example, many criteria for prioritizing regions for conservation are based on relatively simple indices such as the number of species in the area or degree of perceived threat, so-called “hotspot” approaches. These approaches often fail to capture essential evolutionary and ecological processes that promote and sustain biodiversity over time. Moreover, the hotspot approaches are static and do not acknowledge the tremendous challenge that global climate change poses for predicting where species will need to relocate, and how quickly. My group is engaged in a relatively new field called “niche modeling,” which takes data on the ecology of organisms targeted for conservation efforts, defines the biotic and climatic envelopes characterizing their current geographic distribution, and then projects the likely changes in species ranges under various climate change scenarios. If these are done for a multitude of species with overlapping requirement a picture emerges of what to expect for entire communities of plants and animals. This is particularly important but difficult in the tropics, which are expected to face novel climates in this century that they have not experienced in at least the last 10 million years. We have developed niche modeling methods that we hope will allow conservation decision makers to map not just the patterns of biodiversity, but also the processes that will produce and sustain it under changing climate regimes predicted for the near future.
I am interested in the proximate mechanisms controlling animal behavior, particularly in how hormones are involved. You might have thought that hormones are always slow-acting, but this is not true. For example, we have, for the first time, measured rapid changes in the amounts of the sex-steroid estradiol in the auditory system of birds as they interact with other birds or hear the sounds of other birds. Our study species is the zebra finch, a model species for most laboratory studies of birds because they are very easy to maintain in the lab. Rapid synthesis of estrogen occurs in the brains of zebra finches, and the estrogen quickly increases the strength of nervous system responses to songs of other zebra finches. We have discovered that estradiol functions almost like a rapid-acting neurotransmitter, modulating core functions such as song learning and song expression in birds. Similar changes appear to occur in the hippocampus of scrub jays as they form and retrieve memories of sites where they store food. Birds are exceptional model organisms for understanding hormones and complex cognitive behaviors.
I am a plant ecologist interested in the great diversity of morphological and physiological features and functions that plants have evolved to live in their environments. One of the critical physiological functions of plants is the regulation of their water use. There are enormous differences among plants and among entire plant communities such as forests in how efficiently they use water. These differences in turn have practical implications for the retention and supply of clean water by different forest ecosystems. We recently discovered that in Hawaiian timber plantations, alien tree species that were introduced into Hawaii can use more than twice the amount of water to grow as native forests. This finding indicated that preserving native tropical forest has value beyond the conservation of the native trees, because water is expensive. Consequently, land management decisions can place ecosystems-and the people who depend on them-at increased risk of water shortages. We are conducting further research to understand why plants use different amounts of water that should eventually allow construction of water budgets over entire landscapes with complex land use mosaics, and from small-scale gardens to forests to large parks. In other projects, we are investigating the adaptation of plants to their environment, with a special focus on the evolution of plant groups that include endangered species. While our research is centered on pure science and discovery, this work has major environmental and economic implications, including applications toward the conservation of rare species and native forests.
I am interested in the physiological ecology of plants, and in particular how plants respond to their complex microclimatic environments. In tropical forests, where I do much of my research, these microclimates are especially complex, and the microclimates for individual trees change radically as they grow from seedling, through sapling stages, and eventually reach full sun in the canopy as an adult tree. Howe do tropical trees cope with the large changes in microclimate they experience as they pass though these very different developmental stages? To find out, my engineering colleagues at the Center for Embedded Networked Sensing at UCLA and I have been instrumenting a tropical forest at La Selva Biological Station in the lowland tropical rainforest of northeastern Costa Rica. We have constructed a system of three walk-up canopy towers, two radio towers, and a 25-m canopy walkway and are instrumenting them with multiple nodes of microclimate sensors above and within the canopy, along with a series of digital cameras with pan-tilt-zoom capacity to monitor plant phenology and animal movements. The data gathered and camera imaging will be available over the Internet anywhere in the world. It is a very exciting time to be in ecology because ecological research is undergoing a major technological revolution as interfaces develop between the environmental sciences and engineering and information technology. As a result, many problems that once were seemingly impossible to address are becoming tractable. These advances have been spurred by decreasing cost, size and weight, and improved reliability, of both environmental sensing hardware and new, more efficient software.
I study sound production and hearing in frogs. Frogs call to attract mates. Several years ago Dr. Albert Feng from the University of Illinois and I discovered a Chinese frog that has an unusual ear morphology-it has an ear canal, much like mammals, unlike any of the other 6,000 species of amphibians, except one other. What was this ear morphology for? We discovered that this canal enabled this frog to communicate with ultrasound-previously unheard of, if you get my drift. We knew of a second frog from Borneo with similar ear morphology, and we wanted to see if it also communicated with ultrasound. So in 2007 my graduate student, Tori Arch, Dr. Ulmar Grafe from Brunei, and I went to Sarawak, Borneo, where we found the frog in Gunung Mulu National Park. Sure enough, this frog not only produces calls with ultrasonic components, it also produces calls that are completely in the ultrasonic range, inaudible to humans. They responded to ultrasound in playback experiments done in the animal’s natural environment. Back at UCLA we recorded from the midbrain of these frogs and showed that they could detect sounds of up 38 kHz, the highest frequency known to be perceived by any amphibian. Humans can hear about 20 kHz; anything beyond that we call “ultrasound.” But why do these frogs make ultrasonic calls? The answer became clear when we examined their habitat acoustics. They live in rushing streams with loud sounds of falling water, but these sounds are in the audible range. So the frogs call in ultrasound so they can hear each other at frequencies where the rest of the environment is quiet. Now we need to rewrite the textbooks on hearing ranges of vertebrates.
I am a geographer and paleoclimatologist interested in the lessons of the past for understanding our present-day climate change predicament, especially here in the North American Southwest. This region has experienced arid conditions since the start of the 21st century. In Southern California this has resulted in “Perfect Drought” conditions, with negative impacts on local water resources, supplies from northern California, and water from the Colorado basin. Allocations from the Colorado River were based on optimistic estimates in the 1930s of long-term average discharge. This over-allocation underscores the problems with relying upon short temporal data sets to set long-term resource policy. An important question is whether the Southwest is experiencing a temporary drought or a transition to a longer-term, more arid climate. One great unknown is how the Pacific Ocean and the El Nino – Southern Oscillation (ENSO) system will respond to increasing global temperatures. Empirical data from the 20th century and climate model predictions are inconsistent. I have been approaching this question by examining how previous prolonged episodes of climate warming have impacted the ocean and the hydroclimatology of the Southwest. Past warming periods include the Medieval Warm Period (~800-1300 AD) an Holocene Thermal Maximum (~8000 to 3000 years ago). Climate and ocean measurements are not available from these periods, but we can use natural records such as tree-ring chronologies and fossil diatom assemblages from lake sediments to provide proxy-based records of past hydrology and past ocean temperatures. Although such reconstructions are imperfect, the consistency of resu among different methods indicates that the eastern eastern Pacific has typically cooled during past warm periods and California has been particularly prone to increased aridity.
I am interested in the ecology and evolution of infectious diseases, and I build mathematical models of disease spread and test them against epidemiological data to better understand disease dynamics. From HIV to pandemic influenza, most infectious diseases of humans originate in other animal species, and many (like rabies or West Nile virus) cross the animal-human boundary all the time. These shared animal-human pathogens, known as zoonoses, impose heavy health burdens worldwide and are difficult to study and control because of their complex ecology. For example, they often have multiple host species interwoven with complex environmental influences. Mathematical models are a vital tool to tackle this complexity. In a recent study I led a team of scientists in an effort to identify shared patterns among zoonotic infections and set priorities for future research. We found that models have helped build basic understanding and health policy for infections like SARS, pandemic flu, and “mad cow” disease. However, we also found that research efforts have been heavily skewed to these particular diseases, and that little or no work has been done on many other globally significant diseases. There are also important gaps in modeling of crucial processes such as animal-to-human disease transmiss and the rapid evolution of pathogens once they’re circulating among humans. We have proposed steps for the research community to address these shortcomings and propel the global fight against zoonoses – and I am pursuing these aims in my lab at UCLA, where I work on zoonose such as SARS, human monkeypox and leptospirosis.
My research combines paleontological studies and geology with molecular approaches to understand the early evolution of animal life. We are pursuing this work as part of the Advent of Complex Life -NASA Astrobiology group, combining studies of the geologic and geochemical context of this early evolution with studies of the phylogeny, genomics and metabolic processes of relevant modern organisms. My student David Gold and I were just in Newfoundland looking at the wonderfully preserved enigmatic Ediacaran fauna that precede the Cambrian Radiation, the rapid appearance of most animal body plans a little over 500 million years ago. In our lab we use molecular approaches to examine the development and evolution of sensory and neural organization of early branching animals such as jellyfish to better understand this early evolution of animal complexity. Another research theme in my lab explores the differentiation and speciation of marine organisms along the west coast in the context of their geologic history. Our work shows that the fauna of the California coast-perhaps the most diverse temperate coastal marine fauna in the world-is a relict of a burst of speciation that took place 12 to 5 million years ago when upwelling of nutrient rich waters and biotic productivity was far higher than today. Our work has conservation implications. Understanding the processes that form coastal habitats such as estuaries, and the ecology of the many endangered species that evolved and live in them, is essential for making informed management decision as we confront the dual challenges of development and climate change along the California and Gulf of California coastlines.
I study very species-rich plant communities-tropical rain forests. I particularly want to know the answer to a classic question in ecology: how can so many tree species coexist in tropical forests? To try to find out, in 1980 a colleague, Robin Foster, and I set up a permanent forest plot on Barro Colorado Island (BCI) in the Panama Canal. Our plot was huge by 1980 standards, measuring 1 kilometer by half a kilometer (roughly the size of the UCLA campus). We tagged, mapped, measured, and identified >240,000 trees and saplings. Every five years we repeat this census to record growth, survival and new recruitment. Tropical forests truly do have more tree species: the BCI plot has >300 tree species, half the number in all of North America north of Mexico-an area 44 million times larger! My collaborators and I now have a network of 24 large forest plots set up on the BCI model in 16 tropical countries. These plots contain >6,000 tree species, about 10% of the world’s known tree species. So, what have we found out so far? Two examples: (1) Most tropical tree species are extremely rare. In the global network, >50% of all tree species together make up <1% of all individuals. This is worrisome because rare species are more vulnerable to extinction under likely climate change scenarios. (2) Another worrisome finding is that tropical tree growth rates are slowing down worldwide. The best explanation is rising nighttime temperatures which increase nighttime respiration, reducing net carbon gain from daytime photosynthesis. We are also getting some still tentative but surprising answers to the question of why tropical forests have so many tree species.
As a student of animal behavior, I am curious why some species are aggressive to other species, and I want to understand the evolutionary consequences of such aggression. The Nobel Prize winning ethologist, Konrad Lorenz, suggested in 1962 that interspecific aggression might help explain the spectacularly diverse coloration of coral reef fishes. Lorenz’s aggression-based mechanism for generating species differences has always been overshadowed by Brown and Wilson’s (1956) reproductive character displacement hypothesis, which says that species evolve divergent color patterns for species recognition to avoid mating with the wrong species. Previous attempts to test these two hypotheses have not cleanly separated the predictions, which predict the same geographic patterns in signals (less overlap in signals when species occur together than when they are geographically separated). We got around this problem by directly testing for shifts in what animals recognize as competitors. Our study animals are damselflies (related to dragonflies, but damselflies can fold their wings over their back when resting, and dragonflies cannot). We discovered that territorial male damselflies pay attention to species differences in wing coloration to tell intruders of their own species apart from intruders of other species, but only in places where the two species naturally occur together. Our experiment is one of the clearest demonstrations yet of an evolutionary process that is probably very common in nature but which has largely been overlooked.
I study sex. Don’t get too excited: I study sex in Drosophila fruit-flies and in bluebirds, and sometimes in mallards and mice-most often in the field and sometimes in the lab. I am interested in the reproductive decisions that males and females make, i.e., when and with whom to mate and reproduce, and what factors influence or constrain these decisions. I also study mathematical models of mating decisions in individuals and in populations. I am testing my hypothesis that freely made mating decisions in both sexes are about having healthy kids. My colleagues and I recently did experiments to test whether preferring one’s mating partner mattered to offspring viability. It does: the offspring of both males and females who were mated with a partner they preferred rather than with one they did not prefer had higher viability in all the species we studied. In another experiment with Drosophila we found that polyandrous females (females who mate with more than one male, in our case a new male every day) have more offspring that survive to adulthood than monogamous females who only mate with one male over their adult lives-even though the polyandrous females don’t lay any more eggs than the monogamous females. This fitness difference was not due to differences in lifespan: the lifespans of polyandrous and monogamous females were the same. If this result holds in species besides fruit flies, the occurrence of polyandry-which has been found in most species so far studied-may reflect natural selection due to the offspring viability benefits of polyandry.
I study the biomechanics and hydrodynamics of how fishes and other aquatic animals swim. This involves a surprisingly complex set of issues. Actively swimming fishes, whales, etc., are mostly streamlined, but streamlining alone does not explain the great variety of body shapes and sizes in relation to swimming speeds and the efficiencies with which these animals convert energy into motion. Swimming animals confront all sorts of challenges, including how to navigate in and move stably through turbulent waters. We have been studying these and other questions using anatomically exact models of specific fishes (boxfishes). We also film real fishes swimming in raceways to check our models. From the models we discovered a series of passive mechanisms arising from the shapes of boxfishes that stabilize their swimming trajectories. However, I am very excited about our latest discovery: boxfish have a set of important, but previously unrecognized, hydrodynamic mechanisms that stabilize their swimming even in very turbulent waters. It appears likely that similar mechanisms, also previously unrecognized, are used by many other kinds of fishes and aquatic animals as well. Our work on boxfish swimming has had some unexpected engineering spinoffs. The US Navy has developed a new generation of small autonomous underwater vehicles that incorporate important features of boxfish hydrodynamics that we discovered. The automobile industry, led initially by Mercedes Benz in Europe, has also applied boxfish design principles to reduce aerodynamic drag of some newer models of cars. This is a case of engineering bioinspired design and “biomimicry.”
Caulerpa is a common tropical green large alga (macroalga) that has been dubbed the killer alga by the media, not because it kills humans, but because of massive blooms of this species that blanket and smother coral, killing whole sections of reefs. But how does Caulerpa persist between blooms, when it can become rare? We became interested in the question of how patches of Caulerpa are maintained in low abundance in the eastern Pacific but then can episodically bloom and spread over entire reefs. We discovered that blooms only happen when a combination of factors is just so. These factors involve interactions between herbivores, nutrients. and another alga that grows on Caulerpa and acts as a defense against herbivores. Together, they can create the perfect storm of interactions, triggering harmful blooms. The lesson of our study is that to understand and be able to predict the complex behavior of coral reef communities, it may be necessary to consider more than a single potential causal factor, but rather, the interaction of several factors, some of which might be quite unexpected. Our new understanding of the complex set of interactions leading to bloom events will contribute to the protection and conservation of coral reef habitats around the world.
I am interested in the evolutionary history of fishes, and in the genetic tools used in deducing this evolutionary history. DNA sequence analysis-that is, analyzing differences among species and higher groups of organisms in their sequences of base pairs in the DNA molecule in particular regions-has become the paradigm for phylogenetic studies of fishes, amphibians, and reptiles. There are always controversies in science, and I happen to be involved in one of them, having to do with how to use DNA in phylogenetic studies. Phylogeneticists have used both mitochondrial DNA (mtDNA) and nuclear DNA for these studies. However, mtDNA and nuclear DNA have quite different properties, and I have been arguing in the literature that some of the properties of mtDNA violate prerequisites for using them in phylogenetic studies. In a nutshell, the problem is that mitochondria are genetically reduced, once-upon-a-time bacterial intracellular symbionts whose genetic material is extrinsic to the host organism’s genome. This makes mtDNA a questionable proxy for the genome of the host. mtDNA is inherited as a unit and can be subject to intact lateral transfer from one species to another via hybridization, yielding results in conflict with studies basing the phylogeny of the hosts on nuclear DNA. There are valid uses of mtDNA, for example, in the study of the direction of hybridization among species. However, mtDNA should not be used as a primary database in phylogenetic studies. All previous phylogenetic applications of mtDNA should be checked with nuclear DNA sequences and/or other intrinsic data.
I am interested in phylogeography-putting the evolutionary tree of life (phylogeny) into a geographic context. I am decoding (from sequencing the DNA of genes) the history of the origination and dispersal of marine organisms, particularly among the complex archipelagoes of the Indopacific. I study both invertebrates and vertebrates (e.g., fish). The story of the giant clam is an interesting example. It bores into tropical coral reefs and can’t move as an adult. However, the larval stages are planktonic and drift passively on ocean currents. One might think that these currents would mix the larvae of giant clams everywhere, but our genetic studies show that this hypothesis is not true; many populations of giant clams are very isolated. One indication of isolation, we discovered, is that these populations have evolved their own unique communities of symbiotic algae in response to different water temperatures. There are several potential practical benefits of our research. The genetic structure of giant clam populations provides useful data for planning networks of marine reserves for thes threatened ecosystems. Also corals and giant clams share the same algal species (from which clams and corals derive sugars and other vital substances). Corals lose these algae when sea temperatures rise, so-called coral bleaching, but clams do not. Thus, clams may provide a local reservoir of algae to help corals recover following bleaching episodes.
Populations of animals and the ecological communities they inhabit are spread out across the earth, and dispersal ties these populations and communities together. I am trying to understand how dispersal affects the composition and spatial dynamics of animal communities, particularly how dispersal affects the trophic relationships (who eats whom) in communities across a landscape. We know a lot about other processes in animal communities, but dispersal is hard to observe “in the act” of happening, and we know much less about it. So I study the spatial dynamics of model animal communities to make predictions that I can test with experiments and field observations. One of our important discoveries is that the dispersal of some species will have much larger effects on the species richness and composition of animal communities, and on their stability, than others. I call these species “keystone dispersers.” These keystone disperser species may impact not only natural communities, but also agricultural ecosystems as well. For example, dispersing species that predators don’t like to eat but that are good competitors may disrupt food webs and cause local extinction of beneficial predators that control crop pests. We expect that our findings will be very useful, for example, in designing strategies to control invasive species, a big problem in agriculture and conservation.
I am interested in understanding why different groups of vertebrates-animals with backbones-
have very unequal diversity-vastly different numbers of species. For example, there are about
10,000 bird species but only 23 crocodilians. This is an old question, but it is very exciting that
with modern tools allowing us to sequence the base pairs in the DNA of individual genes, we can
get some answers. These DNA sequences evolve, changing at a slow steady rate, so we can use
the cumulative change as a molecular “clock” to estimate the age of different vertebrate groups.
With these tools we tested the hypothesis that groups having the most species today were just the
ones that have been around the longest and have had the most time to diversify. We recently
showed that this hypothesis is false. The evolutionary history of the vertebrates is dominated by
the explosive diversification of relatively young groups, including some (but not all) subgroups of
mammals, birds, and fish. Many classical explanations for species richness don’t hold up. For
example, one hypothesis is that bird diversity is linked to the evolution of flight or feathers.
However, we found that the real increase in the rate of bird diversification happened long after
these traits evolved. It is a very exciting time to be in evolutionary biology because at last we can
answer many of our most profound questions about the history of life on earth-questions that
seemed intractable just a few years ago.
I am fascinated by animal behavior, and I have been studying behavior in a population of yellow- bellied marmots at Rocky Mountain Biological Station for many years. One long-standing question we have been attempting to answer in marmots is: how is the size of marmot groups regulated? These are social animals that live in extended family groups, but not all the kids can remain at home. There aren’t enough resources and presumably there is value in dispersing to avoid inbreeding. But not all the kids leave home; generally the sons disperse, but not all of the daughters. Recently, my students and I asked, what is different about daughters that leave versus those that stay home? We tested the “social cohesion hypothesis,” which says basically that “animals that play together stay together.” Remarkably, this idea has never been properly tested in the 30 years since it was proposed. We found that daughters that stayed home had more social interactions with more of their extended family members than did daughters that dispersed. Interestingly, it did not matter much whether the interactions were positive or negative, so long as there were more of them. We don’t yet know the reasons why females differ in their social interactivity. This question is interesting in relation to offspring dispersal in traditional human societies. Typically it is daughters that disperse more than sons, but it would be interesting to know whether there is variation among offspring that disperse or stay at home in terms of how socially interactive they are with their extended families.