hi, you can use external markers such as chromium oxide or titanium dioxide. find this article it will assist with the procedure to determinme the concentration of the marker in the feces,you will be able to establish the rate of recovery of the marker...you will be able to estimate for digestibility.

Short, F.J., Gorton, P., Wiseman, J., Boorman, K.N., 1996. Determination of titanium dioxide added as an inert marker in chicken digestibility studies. Anim. Feed Sci. Technol. 59, 215-221.

An alternative method could be trying to look for the presence of indigestible components of plant material such as cellulose or lignin in the faecal matter

To add to Lawrence's statement you could do an NDF acid wash, dissolving of all organic material except for structural carbohydrates - weigh faecal samples prior to acid wash, way NDF (neutral detergent fibre) after wash, difference is structural cellulose (plant lignin, etc). Will not allow you to measure non-structural plant material though.

With Zibani's suggestion, you need to add the markers to the plant material, collect the faeces, dry, grind, dissolve, and determine faecal marker concentrations via ICP analysis (remembering that very small amounts of marker elements such as Cr and Ti are present in all feed). In a controlled experiment with controlled feeding, this would be possible but not in the wild. Also, controlled feeding changes animal behaviour if it is unfamiliar to handling.

So many things to consider when doing what seems like a simple experiment!

I agree with Mr. Mark Barnett

I think that one could analyse chlorophyll and pheophytin (a degradation product of chlorophyll) both can be easily be estimated an as far as I know both are not absorbed through the intestine. By measuring both can be an standard marker for digestibility. Therefore I think it can be used to detect any vegetable material in faeces.

In most animals that consume plants, some plant material passes through the GI tract because unless microorganism capable of producing cellulase are present, the cell walls often stay intact. Two staining protocols useful for staining cell walls are 0.1% Safranin O or 0.1% Toluidine blue (stains lignans). Preparation protocols are available online. You will have to dilute your fecal samples and stain them directly with a drop or two of stain after applying them to a glass slide. Simply add a coverslip and examine at 10X with a microscope.

Great! Thank you all for your help!

Hello Raquel,

"I'm trying to detect if reptiles eat plants looking to their drops, but it is really difficult to see plant material using magnifying lens if they are digested."

You have revealed few details of your study but it sounds as if you were looking for signs of a diet complementation in predominantly carnivorous reptiles. Am I right? One thing is to determine whether plants were ingested and another thing is to say whether the presence of plant remains (or biomarkers suggesting a herbivorous diet) in a fecal sample truely implies an omnivorous diet of its producer. The first task is the simplest one, the second one may be quite problematic.

Plant elements are generally hard in digestion, especially those rich in lignin or silica, but cuticles are also abundantly present in feces. Reptiles are characterized by a longer food retention time in the gastrointestinal tract i.e. longer digestion than mammals, but due to the lack of chewing in extant reptiles, their feces contain plant elements of some bigger dimensions than feces produced by mammals of comparable body sizes (Fritz et al., 2010). I believe you should be able to detect plant elements under a stereoscope microscope after dissolution of the samples if they were indeed ingested (rather than expect them to have been completely digested), but complementary methods as lipid biomarkers (see Linseele et al., 2013) and even isotopic analyses (see Bajdek et al., 2014) to distinguish between carnivorous/omnivorous/herbivorous diets are interesting too. Because the first point (remains and markers) have been already explained above by others, let me comment briefly on the second one.

The problem is that carnivores consume herbivores' intestines. Thus, if you examine feces produced a reptile which had consumed e.g. a cow or a mouse, you will likely found some plant elements (perhaps not too abundant, but detectable). For example, plant matter is known from stomach contents of crocodiles (Pauwels et al., 2007) and alligators (Keenan et al., 2013), but ingested rather with prey intestines or incidentally. Moreover, several papers document and discuss the presence of plant remains in carnivore coprolites (fossil feces) (e.g. Hollocher et al., 2010), and even biomarkers (Zatoń et al., 2014). I mean, you may find some plant remains in feces of carnivorous reptiles, but the results may be misleading if interpreted too literally. I hope this is helpful to you... :-)

Just as an anecdote, some herbivorous animals as e.g. various tortoise species (Esque and Peters, 1994) are also known to ingest bones, perhaps for supplementation of minerals, but they are not necessarrily omnivorous animals as such, as one could deduce from such a dung ;-).

Cited literature:

Bajdek, P., Owocki, K., Niedźwiedzki, G. 2014. Putative dicynodont coprolites from the Upper Triassic of Poland. Palaeogeography, Palaeoclimatology, Palaeoecology 411: 1–17.

Esque, T.C., Peters, E.L. 1994. Ingestion of Bones, Stones, and Soil by Desert Tortoises. Fish and Wildelife Research 13: 105-111.

Fritz, J., Hummel, J., Kienzle, E., Streich, W.J., Clauss, M. 2010. To chew or not to chew: fecal particle size in herbivorous reptiles and mammals. J. Exp. Zool. A Ecol. Genet. Physiol. 313A (9): 579–586.

Hollocher, K.T., Hollocher, T.C., Rigby Jr., J.K. 2010. A phosphatic coprolite lacking diagenetic permineralization from the Upper Cretaceous Hell Creek Formation, northeastern Montana: importance of dietary calcium phosphate in preservation. Palaios 25: 132–140.

Keenan, S.W., Engel, A.S., and Elsey, R.M., 2013. The alligator gut microbiome and implications for archosaur symbioses. Scientific Reports 3: 2877. http://dx.doi.org/10.1038/srep02877

Linseele, V., Riemer, H., Baeten, J., De Vos, D., Marinova, E., Ottoni, C. 2013. Species identification of archaeological dung remains: A critical review of potential methods. Journal of Environmental Archaeology 18(1): 5-17.

Pauwels, O.S.G., Barr, B., Sanchez, M.L., Burger, M. 2007. Diet records for the dwarf crocodile, Osteolaemus tetraspis tetraspis in Rabi Oil Fields and Loango National Park, southwestern Gabon. Hamadryad 31(2): 258–264.

Zaton, M.P., Marynowski, L., Niedzwiedzki, G., Benzerara, K., Pott, C., Cosmidis, J., Filipiak, P. 2014. Integrative studies of Late Triassic vertebrate coprolites from Poland. 2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014), Geological Society of America Abstracts with Programs 46(6), p.76.