## 1 Introduction
### Fundamentals of odor detection  
To accurately discuss the ability of dogs to detect a target (in our study a tortoise) by olfaction there is a necessary sequence of events that must occur. First the odor molecule(s) that dogs recognize as \"tortoise\" must evaporate. Our experience training dogs to find tortoises using gauze wiped over the tortoise neck and front legs suggests that whatever the dogs are using to identify the tortoise can be transferred to the gauze from these parts of the body [20]. An alternate source for tortoise odor could be from the breath, but given that dogs trained on skin transfer items can identify a live tortoise we feel the skin is the most likely source for the scent dogs recognize as 'tortoise'. Once the odor molecule(s) evaporates it must diffuse through the boundary layer next to the tortoise's skin and then move via diffusion and bulk air movement to the dog's nose. Once there, the dog must recognize the odor and determine its source direction, which enables the dog to move towards the source based on increased concentration. The concentration of the tortoise odor at the dog's nose will be a function of both the source strength (evaporation rate) and its movement and dissipation in the air. Source strength is a function of the vapor pressure of the compound, the ambient temperature, and is influenced by any binding or affinity the compounds have for the substrate they are on. We presume here that the compounds comprising tortoise odor are semi-volatile and in low concentration, thus the evaporation rate is effectively independent of the size of the tortoise since the larger surface area is not significant considering the low vapor pressure and low concentration of these compounds on the tortoise surfaces. This presumption is based on the fact that the dogs were trained on residual scent captured in gauze pads. If the tortoise odor molecules were not semi-volatile, the dogs would have been trained to find gauze and would not have recognized live tortoises. The binding or affinity between the skin or other surface, such as vegetation or soil, and the odor compound will be affected by ambient humidity since water can displace odor at binding sites. This effect has been quantified in the interaction between soil and applied pesticides [21] and we suspect a similar effect occurs with tortoises.  
Transport of the evaporated odor compounds in the air will be a function of the wind speed and direction and the roughness of the terrain. Scent movement is often described by experienced dog handlers as \"pooling\" or getting \"trapped\" in areas. This is a qualitative way of describing the impact of wind and terrain on movement of any chemical in the air under the influence of these effects. At higher wind speeds, scent is transported rapidly down wind, but it also moves vertically and perpendicular to wind direction due to turbulent air movements, thus the resulting scent concentration pattern is described as either a plume or cone of scent with the highest concentration at the mid-line.  
The shape of the plume is influenced strongly by wind speed and roughness of the terrain. At the extreme of no wind, all movement of the compound will be by molecular diffusion and a \"pool\" of scent will form with concentric rings of decreasing concentration. Concentration is critical since it is clear that at some low concentration the dog is unable to detect the scent which represents a detection limit for the sensor.