Influences of parasites and thermoregulation on grouping tendencies in marine iguanas

Abstract

I determined whether grouping behavior influences parasite load and body temperature of Galapagos marine iguanas, reptiles that rest gregariously. Mobile (or predatory) Ornithodoros ticks (4.7 mm average body length) approached at a ground speed of 65 cm/min and parasitized sleeping marine iguanas for 3.7 h per night, drawing about 0.1 ml blood. Contagiously transmitted Amblyomma ticks hang on to iguana hosts for days or weeks. Marine iguanas sleeping alone had 2.0 mobile ticks per night, while individuals sleeping in groups 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. Experimental nightly translocation of iguanas to areas without other sleeping iguanas significantly increased their mobile parasite burden above levels encountered 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, mobile 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 radiotelemetry. I conclude that marine iguanas group during daytime at microhabitats favored for thermoregulation (predation is absent in this population). 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 potential internal infections.