Some science behind the scenes
Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change gives rise to a voltage across the crystal. If the temperature stays constant at its new value, the pyroelectric voltage gradually disappears due to leakage. In a typical demonstration of pyroelectricity, the whole crystal is changed from one temperature to another, and the result is a temporary voltage across the crystal.
The pyroelectric effect is present in both bone and tendon.
We have a tendency to relate the idea of natural heating with infrared - sometimes known as "heat radiation", since many people attribute all radiant heating to infrared light and/or all infrared radiation to heating. This is a widespread misconception, since light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun only accounts for 49% of the heating of the Earth, with the rest being caused by visible light that is absorbed then re-radiated at longer wavelengths. In effect electromagnetic energy in the form of light and infrared radiation can heat the crystals in bones and tendons to produce electrical energy.
So overall we can see bone and tendons have the capability to be heated by the sun’s rays and to then emit electrical charge, albeit in this case a temporary charge.