Someone told me that the extra charge was for having to heat the hold, which they wouldn't normally do if they didn't have livestock on board. I know air/con burns fuel, but didn't realise heating the hold cost fuel too?
A quick search brings the following information:
* Hold temperature varies according to ambient conditions at time of flight
* Typically one hold is pressurised and with temperature control
* With no livestock or perishables, typical temperature appears to be around 10 degrees C
* For perishables, the hold is kept below 10 degrees C
* For livestock, temperature is kept between 18-24 degrees C