Influences of parasites and thermoregulation on grouping tendencies in marine iguanas

I determined whether grouping behavior influences parasite load and body te mperature of Galapagos marine iguanas, reptiles that rest gregariously. Mob ile (or predatory) Ornithodoros ticks (4.7 mm average body length) approach ed at a ground speed of 65 cm/min and parasitized sleeping marine iguanas f or 3.7 h per night, drawing about 0.1 ml blood. Contagiously transmitted Am blyomma ticks hang on to iguana hosts for days or weeks. Marine iguanas sle eping alone had 2.0 mobile ticks per night, while individuals sleeping in g roups had 0.1 to 1.1 mobile ticks per night. Single iguanas decreased their mobile parasite load to 0.2 ticks per night by sleeping on bushes. Experim ental nightly translocation of iguanas to areas without other sleeping igua nas significantly increased their mobile parasite burden above levels encou ntered by naturally single individuals (n = 4.6 ticks per night). Creating an experimental group of two animals reduced infestation with mobile ticks by 59% compared to levels on single animals. Over the course of weeks, mobi le ectoparasite loads at grouping sites increased to levels found at single sites, at which point marine iguanas changed sleeping sites. Grouping had no effect on the prevalence of contagious ticks. Furthermore, grouping did not help to conserve body temperature in Genovesa iguanas, as measured by r adiotelemetry. I conclude that marine iguanas group during daytime at micro habitats favored for thermoregulation (predation is absent in this populati on). Thermoregulation was not of prime importance for nightly aggregations, which instead served to reduce mobile ectoparasite load. As a minimum cost of infestation, I estimate that individuals sleeping alone would have a 5. 4% lower annual energy budget due to tissue removal, not including potentia l internal infections.