Parasites may impair host behavior in ways that reduce host fitness, especially when access to territories or mates becomes disrupted. Western fence lizards (Sceloporus occidentalis) are a key host to western black-legged ticks (Ixodes pacificus). Males are highly aggressive during the mating season, competing with rivals through displays of color badges, pushups, and other behaviors. We hypothesized that experimental tick infestation diminishes the performance of male western fence lizards in intrasexual contests, via either blood loss, damage to sensory structures or both. We infested males with larval ticks and staged contests between infested and quality-matched control males, and measured their behavior in enclosure arenas. Infested lizards were less aggressive and exhibited decreased hematocrit, compared to non-infested animals. We found no relationships between aggression and either body size or blue ventral badge color traits. There was also no effect of tick attachment location and hemoparasite infection on host contest behavior. This is the first demonstration of the impact of I. pacificus parasitism on intraspecific interactions of western fence lizards, and suggests that tick infestation has substantial impacts on lizard fitness. Because I. pacificus rely heavily on these lizards for blood meals and dispersal, these impacts could also influence the abundance of ticks and the pathogens they vector.  

Behavior data collected using BORIS with video playbacks of encounters between male Sceloporus occidentalis in enclosures. Data processed in R 4.0.0. 

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