Influences of parasites and thermoregulation on grouping tendencies in marine iguanas

I determined whether grouping behavior influences parasite loadand bodytemperature of Galápagos marine iguanas,reptilesthat rest gregariously. Mobile (or predatory) Ornithodorosticks(4.7 mm average body length) approached at a ground speed of65 cm/minand parasitized sleeping marine iguanas for 3.7 hper night, drawing about 0.1ml blood. Contagiously transmittedAmblyomma ticks hang on to iguanahosts for days or weeks. Marineiguanas sleeping alone had 2.0 mobile ticksper night, whileindividuals sleeping in groups had 0.1 to 1.1 mobile tickspernight. Single iguanas decreased their mobile parasite load to0.2 ticksper night by sleeping on bushes. Experimental nightlytranslocation of iguanasto areas without other sleeping iguanassignificantly increased their mobileparasite burden above levelsencountered by naturally single individuals(n = 4.6 ticks pernight). Creating an experimental group of twoanimals reducedinfestation with mobile ticks by 59% compared to levelson singleanimals. Over the course of weeks, mobile ectoparasite loadsatgrouping sites increased to levels found at single sites,at which pointmarine iguanas changed sleeping sites. Groupinghad no effect on theprevalence of contagious ticks. Furthermore,grouping did not help to conservebody temperature in Genovesaiguanas, as measured by radiotelemetry. Iconclude that marineiguanas group during daytime at microhabitats favored forthermoregulation(predation is absent in this population). Thermoregulationwasnot of prime importance for nightly aggregations, which insteadserved toreduce mobile ectoparasite load. As a minimum costof infestation, I estimatethat individuals sleeping alone wouldhave a 5.4% lower annual energybudget due to tissue removal,not including potential internal infections.