A rock hyrax midden is a stratified accumulation of fecal pellets and a brown amber-like urinary product known as hyraceum excreted by the rock hyrax and closely related species.
Hyrax middens form very slowly (ranging from ~5 years to >1000 years for 1 mm of hyraceum accumulation), over long periods of time, with many spanning tens of thousands of years and some dating as far back as ~70,000 years. Hyrax middens contain a diverse range of paleoenvironmental proxies, including fossil pollen and stable carbon, nitrogen and hydrogen isotopes. Combined with the antiquity of hyrax middens, and the often-continuous nature of their deposition, hyrax middens have become a valuable means of reconstructing past environmental and climate change.
Rock hyraxes are known to use communal latrines. These sites are often found in sheltered locations, where the threat of predation is limited, and middens form when they are protected from the elements. At well-protected sites, it may accumulate in deposits in excess of a meter thick and several meters across. The thickness of hyrax middens depends on the nature of the shelter and the regional climate history and geology. Hyraceum shows hygroscopic properties and periods of increased precipitation or elevated ambient humidity will destroy existing middens, while more arid periods allow their development/preservation. Thicker formations tend to occur in shallow shelters that during more arid periods, presumably provided sufficient shelter from rainfall for substantial midden accumulations, but under wetter conditions no longer provide adequate protection, resulting in the removal of the more soluble components of the midden. At poorly protected sites in arid regions hyrax urine leaves a white, calcium carbonate precipitate on the rocks. Varying degrees of protection result in varying degrees of midden preservation. Small overhangs, vertical fractures in cap rocks, and groundwater flow along weakness in the shelter's architecture may lead to midden degradation if rainfall exceeds a certain amount and/or intensity. The thickest middens have been found at sites composed of massive, horizontally bedded rock such as granite and quartzites with between ~30 and 480 mm of annual rainfall. In more humid environments (>800 mm mean annual rainfall), there is little to no evidence of hyraceum accumulation, and middens typically resemble piles of compost, as the masticated plant material in the pellets rapidly decomposes. Hyraceum-rich middens do not typically form in coastal situations, despite the presence of hyraxes, and it is considered that the ambient humidity of the air and the occurrence of coastal fogs preclude midden development
Comparisons with fossilised herbivore middens
Studies of other herbivore midden remains have been very effective in palaeoenvironmental studies in dryland regions on several continents. In the southwestern United States pack rat middens have provided an unprecedented record of environmental changes over the last 40,000 years. As a result of this work, the vegetation dynamics of this area are some of the best understood for any of the world's drylands at this timescale, and the critical data provided have dramatically helped define the range of regional climate variability. This work has also led to important perspectives on ecological theory, which have impacted on management strategies by allowing a distinction to be made between anthropogenic environmental impacts and natural processes.
Midden studies have also been undertaken in Australia and South America. This work has highlighted a fundamental difference between middens from these regions and hyrax middens. American and Australian middens are essentially nests composed of sticks and other macrobotanical remains. These middens are generally reported have no clear stratigraphy, and researchers have thus adopted the methodology of processing them as single samples that provide a palaeoenvironmental snapshot.Spaulding, W.G., Betancourt, J.L., Croft, L.K., Cole, K.L., 1990. Packrat middens: their composition and methods of analysis, in: Betancourt, J.L., Devender, T.R.V., Martin, P.S. (Eds.), Packrat Middens: The Last 40,000 Years of Biotic Change. University of Arizona Press, pp. 59–84 Hyrax middens, on the other hand, are primarily urino-fecal deposits, and are deposited progressively as a series of layers. This diachroneity is one of the fundamental advantages of hyrax middens over nest middens, which are only secondarily preserved as the animals urinate in their shelters.
Examinations of the internal and external structure of hyrax middens suggest flow/deposition dynamics similar to speleothems (cave deposits, e.g. stalactites), with the fresh urine flowing across the surface of the midden, then drying and crystallising, preserving the stratigraphic integrity of the midden. The general morphology of middens is often characterised by (1) lobate forms, (2) undulating weathering features on exposed midden faces, and (3) in some cases the formation of thin (1–3 mm in diameter) stalactites on the underside of some middens. As a result, questions over the potential for post-depositional remobilisation of hyraceum may be raised. The examination of over 150 middens, however, has confirmed the visible stratigraphic integrity of the middens, and while some surficial alteration of exposed surfaces can occur, consistently coherent age-depth models, and the nearly vertical exposed external faces of the middens indicate that, once dry, hyraceum is not prone to significant remobilization.
Hyrax midden structure, accumulation rates and age
Hyrax midden structures and accumulation rates can vary considerably based on the relative proportion of their two primary components, pellets and hyraceum, which is determined by the architecture of the site itself. Depending on the shape and irregularities of the floor of the site in question, pellets are likely to either accumulate (in concave structures) or roll away (in convex or inclined structures). Whereas hyrax urine will deposit only a very thin film of hyraceum after evaporation, pellets are usually 0.5–1 cm in diameter, and thereby accumulate much more quickly., with deep piles accumulating perhaps within just a few years, or even months. Compared to this, we have observed that middens composed primarily of hyraceum accumulate much more slowly; generally between ~5 and >1000 years/mm. The rate of hyraceum accumulation depends on the morphology of the midden, the architecture of the site, as well as presumably the size of the hyrax colony, and as such net rates can be highly variable
Hyrax midden ages Radiocarbon ages from hyraceum are not subject to reservoir effects or the inclusion of new carbon.Kaufman, A., Broecker, W., 1965. Comparison of Th230 and C14 Ages for Carbonate Materials from Lakes Lahontan and Bonneville. J. Geophys. Res. 70, 4039–4054.Grimm, E.C., Maher Jr, L.J., Nelson, D.M., 2009. The magnitude of error in conventional bulk-sediment radiocarbon dates from central North America. Quaternary Research 72, 301–308. This is primarily a function of middens being isolated systems, and that through respiration the hyraceum is brought into equilibrium with atmospheric 14C at the time of deposition. Published data show that hyrax middens can be of considerable antiquity, and middens from the Groenfontein site in the Cederberg Mountains of South Africa are considered to have begun accumulating ~70,000 years ago
It has been commonly observed that many middens are no longer actively accumulating. Often this is controlled by the shelters in which they are found, with accumulation ceasing when the middens grow to such an extent that the hyraxes can no longer physically enter the shelters. Until recently, field sampling was limited to the collection of middens that were most accessible and easiest to sample. In many cases this meant that the individual sampled middens were relatively thin (<5 cm) with aggregate records subsequently constructed from fragments of as many as 25 separate middens. (Scott and Woodborne, 2007a, b). With recent developments in sampling tools and techniques, larger, more stratigraphically coherent middens are more regularly sampled, which better represent the full period of accumulation at a given site
Composition of hyrax middens The very nature of hyrax middens implies that they comprise a mixture of materials, which include animal metabolic products, undigested food, and any allochtonous material blown into the middens or deposited via feet or fur. In terms of organic matter, the existence of such potentially distinct sources (i.e. extraneous organic matter and animal metabolites) implies that a range of information concerning inter alia: animal diet, animal behaviour, metabolic responses to environmental stress, changing behaviour, as well the wider palaeoecological setting of the site may all be preserved within hyraceum.
Chemical composition
Hyraceum essentially comprises a mix of organic compounds, soluble salts, calcium carbonate and the mineral sylvite. More recent data from Raman Spectroscopy and Fourier Transform Infrared (FTIR) Spectroscopy demonstrate the presence of a number of CaCO3 polymorphs, the abundance of sylvite (KCl) and an organic componentPrinsloo, L.C., 2007. Rock hyraces: a cause of San rock art deterioration? Journal of Raman Spectroscopy 38, 496–503.
The organic components within hyraceum have been investigated using pyrolysis-GC/MS (py-GC/MS) and GC/MS analysis of solvent-extractable lipids. Py-GC/MS is commonly applied to elucidate macromolecular organic matter structure and composition. Py-GC/MS measurements on samples from two distant sites, Spitzkoppe, Namibia and Truitjes Kraal, Western Cape Province, South Africa produced remarkably similar suites of pyrolysis products, despite their contrasting environmental settings. The pyrolysis products were dominated by aromatic compounds; notably the nitrogenous compounds benzonitrile and benzamide. Pyrolysis in the presence of a methylating agent tetramethylammonium hydroxide (TMAH) implied that benzamide is a monomer of a larger polymeric structure, the major organic component of the hyraceum OM. This is further supported by the ubiquity of benzamide within solvent extracts and it is probable that it is derived from hippuric or benzoic acid, which are common metabolites in ruminants.Bristow, A.W., Whitehead, D.C., Cockburn, J.E., 1992. Nitrogenous constituents in the urine of cattle, sheep and goats. Journal of the Science of Food and Agriculture 59, 387-394. Given its abundance, the metabolite (or metabolite product) benzamide is likely the major source of organic nitrogen and carbon measured in bulk stable isotope analyses, and can therefore provide insights into animal diet and its isotopic signature. Interestingly, common plant-derived pyrolysis products, such as lignin were not detected using py-GC/MS, although low molecular weight polysaccharide pyrolysis products (e.g. acetyl furan, furaldehyde, dimethyl furan) were found in trace amounts