Field use of maximal sprint speed by collared lizards (Crotaphytus collaris): compensation and sexual selection. 
To understand how selection acts on performance capacity, the ecological role of the performance trait being measured must be determined.  Knowing if and when an animal uses maximal performance capacity may give insight into what specific selective pressures may be acting on performance, because individuals are expected to use close to maximal capacity only in contexts important to survival or reproductive success.  Furthermore, if an ecological context is important, poor performers are expected to compensate behaviorally.  To understand the relative roles of natural and sexual selection on maximal sprint speed capacity we measured maximal sprint speed of collared lizards (Crotaphytus collaris) in the laboratory and field-realized sprint speed for the same individuals in three different contexts (foraging, escaping a predator, and responding to a rival intruder).  Females used closer to maximal speed while escaping predators than in the other contexts.  Adult males, on the other hand, used closer to maximal speed while responding to an unfamiliar male intruder tethered within their territory.  Sprint speeds during foraging attempts were far below maximal capacity for all lizards.  Yearlings appeared to compensate for having lower absolute maximal capacity by using a greater percentage of their maximal capacity while foraging and escaping predators than did adults of either sex.  We also found evidence for compensation within age and sex classes, where slower individuals used a greater percentage of their maximal capacity than faster individuals.  However, this was true only while foraging and escaping predators and not while responding to a rival.  Collared lizards appeared to choose microhabitats near refugia such that maximal speed was not necessary to escape predators.  Although natural selection for predator avoidance cannot be ruled out as a selective force acting on locomotor performance in collared lizards, intrasexual selection for territory maintenance may be more important for territorial males.