great sushi,
You raise several separate issues, each requires a response.
1. These monitoring systems are examples that do not require the animal to transmit anything, thus no electric power is necessary. Many animals, both large and small, plus many insects are now being tracked by scientists studying their behaviors. Some schemes use simple passive reflectors, similar to bar codes, on honeybees. When they pass in and out of the hive they must pass through a “reader”.
http://www.dailymail.co.uk/sciencet...-forcing-dance-streetlights-falling-dead.html
Kin-selection theory underlies our basic understanding of social evolution. Nest drifting in eusocial insects (where workers move between nests) help is greatest. By using a novel monitoring technique, radio frequency identification (RFID) tagging,
http://www.ncbi.nlm.nih.gov/pubmed/17240339
2. Other tracking devices do transmit data, so these require the animal to carry the battery to power the transmitter. Biotrack is a company of dedicated biologists with over 30 years of experience in radio tracking and radio tag manufacture. They supply tracking devices for amphibians, fish, birds, invertebrates, mammals, and reptiles. See their website:
http://www.biotrack.co.uk/
Be sure to check out their smallest and lightest radio transmitter:
http://www.biotrack.co.uk/small-beeper-transmitters.php#s3
In your post you mentioned “Charging a capacitor using a piezoelectric crystal and the animals’ motion”. While this is possible, be advised: the amount of electrical power available it extremely small. Yes, there may be some instances where the tiny batteries already in use would not suit the experimenter’s needs, so there can be a use for “piezoelectric energy harvesters” mounted on animals and elsewhere. And, there are some special applications where the mechanical pressure waves in the local ambient environment are being harvested to recharge a human heart pacemaker battery, for example. Here on Physics Forums there have been several threads discussing this topic thoroughly. For instance, see:
https://www.physicsforums.com/showthread.php?t=691529
And, in particular, see this website from our PF member “old jim hardy”: http://www.designnews.com/author.asp...dfpLayout=blog
You may gain new information by researching the basics of energy harvesting using piezoelectric materials. Notice I did not use your term “crystal”. That’s because there are many materials that are piezoelectric, including very flexible thin films. I suggest you start with Wikipedia.
3. All piezoelectric materials are “two-way” as you mentioned. Consider them transducers: they convert one form of energy to another. A microphone is an acoustic transducer, converting mechanical sound wave energy into electrical energy. But we would not apply a high powered amplified signal to a microphone to play our music. A speaker is designed for that. In sonar, for instance, an image of a baby in the womb is created using ultrasonic frequencies. A single piezoelectric crystal-like ceramic is used for both transmission and reception. A high voltage pulse of AC electrical energy at the resonant frequency of the crystal is applied to the crystal. The crystal vibrates and emits ultrasound. The pulse passes into the body, is reflected by various materials, and the echo returns to that same crystal. Now, the mechanical acoustical pressure waves cause the crystal to vibrate (because it’s resonant at that frequency) and thus it generates a small output voltage. This voltage is then processed by the receiver to generate an image of the baby. Try to Google “sonar” or “ultrasonic imaging”.
Now, one might build a piezoelectric transducer which converts the mechanical energy from the wingbeats, say, of a bumblebee at 120 Hertz into electrical energy, then harvest that energy to store in a capacitor or battery. But when you wanted to transmit some data back to your receiver/data collection point, your piezoelectric transducer is resonant to 120 Hertz! Normally radio frequencies are used for this purpose, so that piezoelectric could not act as a “two-way” transducer.