The former couple - who are in the process of divorcing after just over a year of marriage - further fuelled speculation they have reunited as they locked lips at the Santa Barbara Zoo.
Heidi was also wearing her wedding ring.
The pair recently returned from a trip to Costa Rica, where they were arrested for carrying illegal weapons.
Alluding to the incident, Heidi tweeted on Tuesday (14.09.10): "First few days back In America! I dont think I will ever leave again, at least I am never letting @SpencerPratt pack again!!!! Praise Jesus! (sic)"
Though they haven't directly confirmed their reunion, Heidi - who turned 24 today (16.09.10) - has hinted she wants to halt divorce proceedings.
She said: "As far as I'm concerned the last thing in the world after everything we have just been through is to go through with this divorce. But we are just taking life second by second."
Spencer, 27, also claimed the trip had brought them closer together.
He said: "It was clear we're soulmates. I don't see any reason we can't get back on track."