If your piggies are inside, they'll be fine! :-) Remember that they have a furry coat on (unless you're talking about a skinny pig which I can't really comment on as I've never had one) so if you're fine with a jumper/fleece on then they'll be fine with a fur coat on! Like MrsM pointed out - if they are all still and hunched up and possible shivering a bit, then yes stick your heating on but if not then don't worry.
Try feeling their ears and pads of their feet - if they're much colder than your fingers then they might be a bit cool. But having said that, my Mums piggies live in a shed up the garden and even though some of their ears are cold, their little bodies are nice and warm so I know they're fine.
If your piggy is acting strangely then it's time to be worried but indoor piggies are usually fine in the cooler weather. Mums pigs got through negative degrees in the shed last year (although we did have lots of insulation, lots of hay etc and a portable heater up there to make sure it wasn't too cold)
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