Acoustic communication of rare and threatened crocodilians and its use for population monitoring
Abstract
Freshwater animal populations can be more vulnerable to human impact than those in terrestrial and marine ecosystems, but may receive less conservation investment, often due to limited availability of information. In this thesis I explore strategies for the conservation monitoring of crocodilians, an iconic group of apex predators that play a key role in tropical freshwater ecosystem regulation, but are unfortunately vulnerable to human activities. Population assessments, crucial in developing conservation programs, require robust methodologies that take advantage of our knowledge of organismal biology and ecology. Traditional crocodile survey techniques that rely on spotlight or aerial counts are well established for the more conspicuous species, but can provide limited information when applied to species that are shy or difficult to access. These hard-to-survey species are also often the ones that are most vulnerable to habitat modification, and are consequently of greater conservation concern. Crocodiles are the most vocal of reptiles, which opens up the potential for novel methods of surveying. Here I provide baseline information on general ecology and acoustic communication in three threatened crocodilian genera in Africa and Southeast Asia—Mecistops, Osteolaemus, and Tomistoma—and then go on to test how the crocodile vocalisations can be exploited in a monitoring and survey context. I find that (i) sympatric African crocodiles are highly partitioned in their habitat preferences, and so monitoring methods need to be tailored to individual species ecology, even when species are found in close proximity; (ii) West African slender-snouted crocodiles, Mecistops cataphractus, of all size classes produce distress calls and will respond to pre-recorded calls of their own species, but while the calls produced by small individuals attract conspecifics of all size classes, calls emitted by adults tend to repel them; (iii) spotlight surveys incorporating playback of Mecistops distress calls show greater detection rates compared to spotlight-only surveys; (iv) spotlight-only surveys detect a greater number of Mecistops than passive acoustic monitoring; (v) adult Central African dwarf crocodiles, Osteolaemus tetraspis, produce four distinct vocalisation types previously unreported in crocodylids, and are readily detected during passive acoustic monitoring; (vi) adult Sunda gharials, Tomistoma schlegelii, produce a range of previously-unreported underwater acoustic signals, but these appear restricted only to direct mating activities, therefore limiting utility of acoustic monitoring for their population assessment. This research provides an insight into the diversity of crocodilian acoustic repertoires, offers potential for acoustic-based survey methodologies in conservation, and opens up exciting new directions in reptile behavioural ecology.
