From Tissues to Landscapes: how thermal physiology, water use, and climate influence patterns of landscape use in elephants
The Asian (Elephas maximus) and African (Loxodonta africana) elephant are the largest extant terrestrial vertebrates. At a maximum, the male African elephant is six orders of magnitude larger than the smallest terrestrial mammal, the Etruscan shrew (Suncus etruscus). Across this extreme range of body masses, the fundamentals of mammalian physiology remain the same, yet a simple but key relationship, the surface to volume ratio, dramatically changes. As an animal increases in size, surface area increases to the square while volume increases to the cube. Thus, large animals have a much smaller surface to volume ratio relative to small animals. Because the surface of the animal is the primary site of biophysical exchange (e.g. heat and water) with the environment, this straightforward physical relationship has a cascade of effects. Most important for this work is the impact that a decreased surface to volume ratio has on the thermal and water balance of an elephant and this idea is a central theme of my work. A second major theme is the value of physiological data and methodology in predicting landscape level effects which stem from interactions between whole animal physiological and biophysical processes and the abiotic and biotic environment.
Elephants present a complex management challenge. Although officially listed as vulnerable by the IUCN, historical management practices of African elephants for example, have led to the sequestration of elephant populations into numerous small to medium sized reserves or into otherwise fragmented landscapes. The result is that in many of these areas, high local elephant densities and their resulting impacts are detrimental to biodiversity and lead to increased incidence of human-elephant conflict (Owen-Smith, Kerley, Page et al. 2006). The recognition that elephant distribution is significantly influenced by surface water availability has led to support for surface water management, a more ethically appealing and sustainable form of population regulation relative to culling or translocation. Although surface water management has gained support, its implementation has been challenged by an inability to adequately predict the outcomes and likely success of surface water management plans for particular populations. With this context in mind the overarching objective of my research was to identify and measure the physiological basis for the elephant’s dependence on surface water and then use these data to develop a quantitative and predictive framework with which to examine the influence of surface water and interacting factors on the elephant’s use of landscape.
ADAPTATIONS OF ELEPHANT SKIN FOR NON-EVAPORATIVE AND EVAPORATIVE HEAT LOSS
Collaborators: Drs. Ann Pabst, Heather Koopman, and Richard Dillaman, UNCW
The primary organ system responsible for the biophysical exchange of heat and water is the integument, which lies at the interface between the animal’s internal environment and the outside world. Despite lacking sweat glands, elephants have among the highest rates of cutaneous water loss (CWL) of a variety of arid dwelling herbivores. Our results indicate that elephant integument conducts heat up to 11 times better than mammals with arctic or sub-arctic pelage and loses water at rates that are comparable to some amphibians, allowing elephants to maximize both non-evaporative and evaporative heat loss.
CLIMATE INFLUENCES THERMAL BALANCE AND WATER USE IN AFRICAN AND ASIAN ELEPHANTS: PHYSIOLOGY CAN PREDICT DRIVERS OF ELEPHANT DISTRIBUTION
Collaborators: Dinah Wilson (Wildlife Safari), Nicolas Way (Six Flags Marine World), Kari Johnson (Have Trunk Will Travel), Terrie M. Williams (UCSC)
To predict how surface water and ambient temperature drive elephant movement patterns, I quantified thermal and water budgets of African and Asian elephants across 30oC range of temperatures. This work specifically addresses the whole animal level mechanism of the elephant’s water dependence by quantifying the exchange of heat and water in relation to an environmental variable, thus determining the physiological demand for water at an ecologically relevant temporal scale. Simulated thermal and water budgets using climate data from Port Elizabeth, South Africa and Okaukuejo, Namibia suggested that the 24-hr evaporative cooling water debt incurred in warm climates can be more than 4.5 times that incurred in mesic climates. This study confirms elephants are obligate evaporative coolers but suggests classification of elephants as water dependent is insufficient given the importance of climate in determining the magnitude of this dependence. These data highlight the potential for a physiological modeling approach to predicting the utility of surface water management for specific populations.
A PHYSIOLOGICAL BASED MODEL OF LANDSCAPE USE FOR ELEPHANTS: INTERACTIONS BETWEEN THERMAL CONSTRAINTS, WATER USE, AND ENERGY DEMAND
Collaborators: Dr. Tim Tinker (USGS) and Dr. Terrie Williams (UCSC)
Finally, in chapter 3, the relationships between thermal and water balance and ambient temperature are used to develop a biophysical model coupled with a stochastic dynamic programming model. This modeling framework was used to investigate the separate and combined effects of climate, thermal and water balance, and food availability in determining landscape level patterns of habitat use and habitat impact.
Have Trunk Will Travel, Inc.
Dunkin, R. C., Wilson, D., Way, N., Johnson, K. and Williams, T. M. (2013). Climate influences thermal balance and water use in African and Asian elephants: physiology can predict drivers of elephant distribution. J Exp Biol 216, 2939-2952. Cover article.
Dunkin, R. C. From Tissues to Landscapes: How Thermal Physiology, Water Use, and Climate Influence Patterns of Landscape Use in Elephants. Ph.D., University of California at Santa Cruz. Proquest/UMI, 2012. (Publication No. ).