444 MECHANICAL AND ACOUSTICAL SENSES 



connected to a separate channel. If ungimballed, the transmitter must be 

 mounted level to ensure proper operation of the compass. 



A wholly electronic compass sensor with no moving parts would seem 

 preferable to the previously mentioned units but apparently presents prob- 

 lems in terms of cost, complexity, and power drain. The potential use of 

 fluxgate magnetometers or "thin-film" magnetic detectors is mentioned by 

 Mackay (1970). 



Inclination— The tilt (pitch) of an animal's axis can be telemetered by 

 a fluid-damped sensor in which a weighted pendulum moves a vane between 

 a light source and a photocell. 



Acceleration— Whole-body accelerations can be sensed by a pendu- 

 lum-vane or spring-weight device with the correct kind of damping. Some 

 accelerations may be very brief, e.g., a shark's final rush at prey, which may 

 last only 1 s. In such a case, if a multichannel transmitter is being used, the 

 multiplexer may not be on the appropriate sensor at the instant the event 

 occurs. Thus, the sensor should have sufficient "persistence" to retain an 

 indication of the event for at least one full data frame (approx. 2-3 s). This 

 could be done hydraulically, with the accelerometer mass damped by a fluid 

 dashpot (on the return stroke only). 



The possibility of misinterpreting a brief acceleration as a brief change in 

 tilt and vice versa also must be guarded against. Close examination of the 

 data from both sensors should resolve any such ambiguity. 



Position of Body Parts— Various relative movements of parts of the 

 animal's body can be sensed by devices that measure angles or distances. 

 Thus, a jaw-angle sensor may consist of two rods, one connected to the case 

 and one to the rotor of a miniature one-turn trimpot (Figure 16). With one 

 rod sutured to the upper jaw and the other to the lower jaw, any opening or 

 closing of the mouth will change the angle between the rods, thereby rotat- 

 ing the trimpot wiper. A similar sensor could measure the angle between the 

 pectoral fins of a shark— a postural characteristic important in threat display. 

 The flexing of a shark's clasper might also be sensed, giving data possibly 

 related to sexual activity. 



Body-position sensors would normally be attached at spots some distance 

 from the main transmitter mounting, e.g., jaw sensor on jaw, main UST on 

 back. Thus, an electrical cable must connect the two components unless a 

 wireless method is devised. One such possibility is for the sensor unit itself to 

 contain a tiny UST powerful enough to reach the main unit 1 or 2 m away. 

 Received pulses would then be converted, based on their rate, into a variable 

 resistance, perhaps by driving an LED/photocell vactrol. 



Physiological Variables— Except for body temperature,, most physio- 

 logical parameters are not directly measurable by variable-resistive sensors of 

 the general types described above. Physiological telemetry usually involves 

 sensing or relaying voltage variations of one type or another; such techniques 

 are discussed in detail by Mackay (1970). Although telemetry of internal 

 physiological variables usually involves continuous transmission, certain 



