Novel methods for estimating density of ungulates in tropical forests.

I'm interested in investigating novel ideas and methods for estimating/monitoring densities of shy ungulates in dense forests. Difficulties arise in standard distance sampling techniques due to small sample sizes (the target species are simply too difficult to observe). Capture-recapture techniques using fecal DNA are often too expensive. One could go down the camera trap occupancy route, but I'd love to hear any novel ideas people may have.