Rock hyrax midden

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. 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

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