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The African crested rat’s poisonous hairs studied by attenuated total reflection infrared spectroscopy case study


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Plant toxins are commonly used by many invertebrates, as an effective defence against predation. However, such behaviour has recently been observed in a mammal.1 The African crested rat, Lophiomys imhausi, applies a potent toxin to the fur alongside its flanks. It acquires this toxin from Acokanthera schimperi,1 a shrub abundant in East Africa and dubbed the “poison arrow tree” due to its use by local hunters. The poison itself, a cardenolide called ouabain,2 appears not to affect the rat but causes pain and cardiac dysrhythmia to most vertebrates and even a small amount can kill an elephant. However, in the hands of a skilled medic, this cardiac glycoside is a well know treatment for congestive heart failure.3 To obtain this poison, the African crested rat has been observed gnawing and chewing the bark of the Acokanthera to release ouabain, which is water soluble due to its hydroxyl groups. At the same time, the rat secretes proteins in its saliva that can bind plant polyphenols and precipitate them.4 Once the plant material has been thoroughly chewed, the coarse colloid mixture is applied by the rat onto its two lateral fur lines, extending from behind the ears across its flank. These fur strips consist of highly specialised hairs, normally hidden beneath longer hairs to prevent exposure to rain and sun, which allows the spittle to be rapidly wicked‐up and stored.

When threatened, Lophiomys uses its specialised dermal muscles to erect the hairs covering the poisonous lines, forming an easily identifiable dorsal crest.1 The rat then directs the attention of the predator toward this lateral fur line in a taunting display. Any predator that bites this area exposes itself to the poison, as evidenced by numerous anecdotes of dogs dying from tackling this rat. As part of the defence mechanism of the rodent, we investigated its highly specialised hairs using infrared spectroscopy as well as gravimetry. Methods Lophiomys imhausi lateral hairs were taken from a skin belonging to the National Museums of Kenya (Catalogue number NMK 180396 Coll Sept. 2010 Field No D. 1). The saliva extract from the samples was prepared by washing several hairs with demineralised water and filtered using 0.2 mm pore size membrane under vacuum. The spectra acquisition was performed using a Nicolet 6700 Fourier transform infrared (FT‐IR) spectrometer (Thermo Scientific, Madison, WI, USA) equipped with a liquid‐nitrogen‐cooled mercury‐cadmium‐telluride MCT‐A detector together with a single bounce diamond attenuated total reflection (ATR) accessory (Golden Gate, Specac, UK). This sampling method allowed the selective probing of the first few microns of the hair’s surface. Spectra were obtained from an average of 64 scans at 4 cm‐1 resolution. All spectral operations were performed using Omnic 7.3 (Thermo Scientific, Madison, WI, USA). A simple ATR correction was applied to compensate for the penetration depth dependency on the wavelength. The spectra were offset with the 1900‐1850 cm‐1 region before normalisation with the integral of the 1700‐1300 cm‐1 region. The spectra were recorded from different parts of a rat hair before washing with demineralised water for comparison. The wicking properties of the hair were measured with a Q500 thermal gravimetric analyser (TA Instruments) by immersing the first 1 mm of the tip of the hair in demineralised water. The mass of the suspended hair was recorded as a function of time whilst the water was absorbed by capillarity up the hair.

Results and Discussion
As shown in Figure 1, the specialised hairs are hollow, similar to those of some arctic mammals to aid with insulation but, for the rat, they are also perforated, thus suggesting a totally different function. These hairs consist of a mesh cylinder of 200‐250 μm in diameter, encapsulating many fine strands of around 10 μm. We propose that this open lattice enables capillary uptake and ensures a high storage capacity for the poison payload as well as any future contact resulting in rapid release, especially if pressed flat (i.e. chewed). As illustrated in Figure 1, a straightforward way of monitoring the capillarity effect is to record the increase in mass of the hair whilst its tip is immersed in water. We found that soon after immersion , the mass increased linearly with time at a rate of water uptake of 0.5 nL s‐1 (~30 μm s‐1) before saturating after 15 minutes.

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