Tracking collar release mechanism?

We have a client who tracks animals in the wild. These animals are darted, measurements are taken, and a collar put on them. The animal
wakes up and go on its way. After some time, the client would like the collars to drop off so that they could be recovered and the data stored on them downloaded.
Does anyone know of a mechanism that would serve as a triggerable detach link? Clearly low energy, small size and weight, and reliability in a physically challenging environment are key. And of course the device must never cause injury to the animal. Low cost is always good, too. Not an easy set of requirements!
We've thought about a variety of solenoid-operated mechanisms, but aren't satisfied with any of them. Fusible or chemically "loaded" links seem a possibility, but may pose safety concerns.
Any good ideas? Thanks!
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