WASER LAB - PROJECTS
The Banner-Tailed Kangaroo Rat | Primates | Local Small Mammals | Carnivores
THE BANNER-TAILED KANGAROO RAT...
...is a hamster-sized, nocturnal, seed-eating rodent, locally common in parts of the southwestern US and northern Mexico. It is not a rat at all, but rather one of the largest members of the new world rodent family Heteromyidae. Kangaroo rats are well known for their spectacular abilities to conserve water (they never need to drink) and as model species for studies of interspecific competition. We chose this species, however, because it builds and lives for generations in conspicuous mounds, which makes it easy for us to locate and monitor every animal in a population, from birth to death, and therefore (among other things) to follow dispersal movements.
Image: an adult female with an eartag, showing off her bipedal stance and her spectacular tail (hence the Latin name, Dipodomys spectabilis)
OUR STUDY SITE...
...is in extreme SE Arizona, near the town of Portal, the Chiricahua Mountains, and the Southwestern Research Station. Since 1979, we've studied banner-tailed kangaroo rats on two sites. Our Portal site, in Chihuahuan Desertscrub, is the location of our initial studies of within-population dispersal. Our Rucker site, about 25 miles away in Desert Grassland, is the location of our current studies of between-population dispersal. Here, we've mapped and followed 8 contiguous populations in a 2 x 3 km area. While some of our work involves spool-and-line tracking or behavioral observations, most is based on twice-yearly mark-recapture censuses and associated genetic sampling. We give every animal a numbered ear tag, in exchange for a pencil-point-sized piece of ear tissue from which we extract DNA.
Studies Involving the Banner-Tailed Kangaroo Rat
DISPERSAL...
.. lets animals avoid inbreeding and find new habitat, recolonize areas in which the species has become extinct, and maintain genetic variation. On the other hand, when animals don't disperse, neighbors are related and kin selection facilitates the evolution of social cooperation. Dispersal tends to follow a comparable course in all species, as shown by the similar relationship between numbers of dispersers and distance moved in banner-tailed kangaroo rats (distances between natal and breeding sites for 787 animals) and Purdue graduate students (distances between Purdue and first jobs for 83 PhD students). But the frequency and scale of dispersal differ enormously between species and often between the sexes. We are using banner-tailed kangaroo rats as a model system to ask questions like: what are the survival and reproductive consequences of dispersal? Is dispersal a mechanism for avoiding inbreeding, or avoiding competition, or both? What are the costs to parents of tolerating undispersed young, and is such tolerance a form of parental investment? In wild populations characterized by philopatry, how serious a problem is inbreeding depression? What are the consequences of philopatry for social evolution?
MOLECULAR GENETIC TECHNIQUES..
..are increasingly valuable in population biology. Microsatellite genotypes can be used to estimate dispersal rates between populations, to determine parentage, to estimate effective population sizes and other standard population genetics parameters, to estimate dispersal distances within populations, and even as census tools. In collaboration with Dr. Curtis Strobeck at the University of Alberta, Canada, we've developed the first microsatellite primers for kangaroo rats, and we're currently developing more with Dr. Andrew DeWoody at Purdue's Department of Forestry and Natural Resources. We're not a molecular genetics lab, this work is all done collaboratively either with Dr. DeWoody or with Dr. Dennis Minchella and Purdue's High-throughput Genomics Center.
Image: Two-locus microsatellite genotypes for 10 banner-tailed kangaroo rats (the red bands are internal size standards), using an ABI 377 automated sequencer.
MATING SYSTEMS AND REPRODUCTIVE SUCCESS
The raw material for natural selection is gene copies, and microsatellite genotypes now let us accurately determine how behaviors influence the numbers of gene copies an individual passes on to the next generation. Current students Molly Steinwald and Jon Winters are using the banner-tailed kangaroo rat to determine how female mate choice and male-male competition interact with dispersal to determine the genetic relatedness of mates, and how this influences reproductive success.
EFFECTIVE POPULATION SIZES AND GENETIC BOTTLENECKS
Effective population size is a crucial parameter in conservation biology, because it predicts how fast a small population will lose genetic variation. Former PhD student Brad Swanson exploited our long-term records of kangaroo rat demography, in combination with microsatellite genotypes, to test how well traditional estimators of effective population size predicted the actual changes in allele frequency through time. Swanson also investigated the utility of microsatellite-based techniques for detecting recent genetic bottlenecks.
ESTIMATING DISPERSAL DISTANCE
Population biologists have long sought accurate ways to estimate dispersal rates and distances from animal movement. An early example is the method for estimating within-population dispersal distance developed by Francois Rousset at the University of Montpellier, France, illustrated with data from our study population. More recently, it's been suggested that microsatellites could be used to infer things about dispersal between populations. Since microsatellite genotypes differ so much among individuals, they act like built-in individual tags and have been used to assign individuals of unknown origin to their population of birth. It's been suggested that such assignment techniques could also be used to identify immigrants and therefore to estimate rates of movement between populations. We're using our long-term data on dispersal and our ability to genotype virtually everyone in our populations to assess the accuracy of these methods.
COMPUTER SIMULATIONS..
..can explore complex issues in population demography and genetics where field data are not yet available. For example, we've explored how well genetic assignment tests estimate dispersal rates under various genetic and demographic assumptions using a simulation, GENETIX, written by Andrew Waser. In collaboration with Drs. Nicolas Perrin and Jerome Goudet at the University of Lausanne, Switzerland, we've used this and related simulations to investigate the possibility of using microsatellites to detect sex differences in dispersal.
Image: Simulation results show that the proportion of animals misassigned using a genetic assignment test closely tracks the interpopulation dispersal rate over a wide variety of population sizes.
MARK-RECAPTURE STATISTICS..
..confirm that we catch very close to 100% of the individuals in our study population, and allow us to estimate rates of survival, recruitment, and dispersal. We've begun a collaboration with Dr. Jim Nichols at the USGS Patuxent Wildlife Research Center to extend and evaluate current mark-recapture techniques to estimate dispersal rates and test hypotheses about the factors that influence them. Undergrad honors students Jamie Skvarla and Chad Cross have done much of this work. Future research in this area will focus on techniques for distinguishing death from emigration in census data, and on possible density-dependence and source-sink dynamics in our populations.
PRIMATES..
A major lab project that recently reached its conclusion involved the dispersal of gray-cheeked mangabeys Lophocebus albigena, arboreal primates related to baboons, in the Kibale Forest, Uganda. PhD student William Olupot pioneered darting techniques for these animals, attaching radiocollars to nearly all the males in a 5 x 5km block of forest and following their movements over a 2-year period. Not only was his study among the first to document rates and distances of movement for any African forest monkey, but it also documented long-term adverse consequences of logging on these animals. Radiotelemetry data is still being analyzed in collaboration with Dr. Olupot, now at the Institute for Tropical Forest Conservation in Kabale, Uganda, and involved in gorilla research and conservation at Uganda's Bwindi/Impenetrable Forest National Park.
New PhD student Jenny Cooper, coadvised by Dr. Melissa Remis in Purdue's Department of Sociology and Anthropology, plans to pursue a combination of molecular genetic and field analyses of movement patterns and social behavior in lowland gorillas.
Images: A gray-cheeked mangabey shows off his fashionable new radiocollar; William Olupot and field assistant prepare for darting in Kibale Forest, Uganda
LOCAL SMALL MAMMALS..
Recent PhD student Cathy Mossman used microsatellite genotyping to investigate the role of habitat and distance in genetic differentiation of white-footed mice, Peromyscus leucopus, in Indiana woodlots. She showed that this approach can indeed detect differentiation between mouse populations separated by as little as 500m. She also demonstrated the ability of microsatellite genotyping to detect a male bias in dispersal in this species. Dr. Mossman is now at the University of Wisconsin, Parkside.
Local forest habitat is highly fragmented by agriculture, and there is great potential for further projects using similar techniques to investigate the ability of other local mammals to disperse through fragmented habitat. Such projects would potentially fit into collaborative work with Dr. Robert Swihart of Purdue's Dept. of Forestry and Natural Resources, examining the impact of local land use practices on wildlife populations.
CARNIVORES:
In a now-completed project, former student Scott Creel and collaborators Brian Keane and Lee Elliott used field observations in the Serengeti National Park, in combination with lab analyses of endocrine status and DNA fingerprinting, to analyze the evolution of phenomena ranging from reproductive suppression to lactation by nonpregnant females to dispersal to inbreeding avoidance, in the dwarf mongoose Helogale parvula.
Currently, PhD student Maureen McColgin is investigating inbreeding avoidance in a social carnivore in which males are thought not to disperse, the coati Nasua narica.