WHY STUDY TIGERS?
A Disappearing Species
Endangered, critically endangered, or viably extinct – all of these classifications describe the remaining subspecies of Panthera tigris left in the wild. Since the early 1990s, tiger populations of all subspecies have plummeted by over 50% throughout their shrunken ranges currently only occupying 7% of their historic range (Seidensticker et al. 2001). Primarily responsible for this rapid decline is the growing illegal wildlife trade in various body parts from this species sold as health tonics and economic charms (Dinerstein et al. 2007). Habitat degradation and the increasing rate of prey depletion are also to blame. Functioning as a keystone species, all tiger subspecies indirectly manage forest ecosystems through prey control (Mills et al. 1993). Not only is the rapid disappearance of this keystone species creating a measurable impact on the ecosystems they support but it is also leaving behind many unanswered questions about their biology including communication systems.
Little is known outside of basic vocalizations about how tigers communicate and locate each other over vast distances. A fundamental study analyzed the low frequency and infrasonic calls of tigers, as previous studies had only focused on calls within the human hearing range (Muggenthaler et al 2000). This research further supported the suggestions that, like other inhabitants of dense forests such as primates and birds, tigers rely primarily on vocal communication due to limited visibility – with some calls found to transmit several kilometers (Muggenthaler et al. 2000).
Morphological studies of the anatomical apparatus for tiger vocalizations indicated complex vocal folds adapted for low frequency (pitch) calls (Kelmuk et al. 2011). Additionally, through brain-stem response from play-back of long-calls, it was determined tigers are especially sensitive to the lower components of each vocalization (Walsh et al. 2008). A question therefore arises -if they are highly sensitive to certain acoustic frequencies, can or do they discriminate individuals based on a unique call pattern?
Current Efforts to Monitor Wild Tiger Populations
Currently, most censusing involves humans actively tracking tigers on foot, looking for pug marks (commonly known as pawprints) (Sharma et al. 2003). These outdated tracking methods may cause more harm than good due to habitat invasion and disturbance and low efficiency (Karanth et al. 2003). Automatic cameras triggered by animal movement (a.k.a camera-traps) can provide more accurate information about tiger density, but here too there are inherent problems (e.g. tigers frequently cross a trap’s path without a sufficient photo taken). Improved census techniques are essential and important for habitat protection and anti-poaching enforcement
Combining Scientific Inquiry with Method Development
Acoustic monitoring of wildlife is a new field, being pioneered by bird call research (Farnsworth and Russell 2007). However, identification of individual animals by vocalizations has already been shown to be plausible within the field of primate research (Chapman and Weary 1990; Lillehei and Snowdon 1978). In addition, Spotted Hyena vocalization studies have revealed sex discrimination possibility (Theis et al. 2007).
Determining if Panthera tigris individuals do have unique vocalizations perhaps correlated with sex, age, or other individual-related attributes could lead to new methods of remote acoustic monitoring. This could allow more efficient as well as minimally disruptive census of critical populations where dense jungle restricts visual confirmation.
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