Balázs Gerics

An unexpected advantage of whiteness in horses: the most horsefly-proof horse has a depolarizing white coat

White horses frequently suffer from malign skin cancer and visual deficiencies owing to their high sensitivity to the ultraviolet solar radiation. Furthermore, in the wild, white horses suffer a larger predation risk than dark individuals because they can more easily be detected. In spite of their greater vulnerability, white horses have been highly appreciated for centuries owing to their natural rarity. Here, we show that blood-sucking tabanid flies, known to transmit disease agents to mammals, are less attracted to white than dark horses. We also demonstrate that tabanids use reflected polarized light from the coat as a signal to find a host. The attraction of tabanids to mainly black and brown fur coats is explained by positive polarotaxis. As the hosts colour determines its attractiveness to tabanids, this parameter has a strong influence on the parasite load of the host. Although we have studied only the tabanid–horse interaction, our results can probably be extrapolated to other host animals of polarotactic tabanids, as the reflection–polarization characteristics of the hosts body surface are physically the same, and thus not species-dependent.

Glial fibrillary acidic protein immunoreactivity in the rat suprachiasmatic nucleus: circadian changes and their seasonal dependence.

The pacemaker of the biological clock, the suprachiasmatic nucleus (SCN) of the hypothalamus, was studied in intact male rats to determine its immunoreactivity to glial fibrillary acidic protein (GFAP), a specific marker of astrocytes. Animals were kept under 12‐h light–dark cycles in synchrony with day–night periods. Immunohistochemical reactions were carried out at midday and late at night in both winter (January) and summer (July). In winter, GFAP immunoreactivity was found to be low during the day and high at night. The findings were reversed in summer, when GFAP immunoreactivity was high during the day and low at night. Increased GFAP immunoreactivity appeared in the form of an abundance of thick immunopositive fibres rather than of cell bodies. This was interpreted as a hypertrophy of pre‐existing astrocytes due to alternating photic stimulation conveyed by retinofugal fibres to the SCN.

Erroneous quadruped walking depictions in natural history museums

Summary Since the work of the photographer Eadweard Muybridge in the 1880s [1,2], experts know well how quadruped animals walk. All walking tetrapods advance their legs in the same sequence, and only the timing of supporting feet may differ [3–6]. Given the long time since Muybridges work, one would assume that this knowledge should be reflected in the depictions of walking quadrupeds made by work of painters, taxidermists, anatomists and toy designers. The postures of legs of walking horses, however, are frequently erroneously illustrated in the fine arts [7]. To see if this also applies to museums, veterinary books and toy shops, we collected hundreds of walking depictions and tested whether or not they correctly display limb positions. We found that almost half of the depictions are wrong. This high error rate in walking illustrations in natural history museums and veterinary anatomy books is particularly unexpected in a time where high-speed cameras and the internet offer ideal possibilities to obtain reliable quantitative information about tetrapod walking.

How does the relative wall thickness of human femora follow the biomechanical optima? An experimental study on mummies

SUMMARY We studied how the ratio (K) of the internal:external diameter of human femora follows the biomechanical optima derived earlier by other researchers for marrow-filled tubular bones with circular cross section and minimum mass designed to withstand yield and fatigue, or stiffness, or bending fracture, or impact strengths. With evaluation of radiographs of 107 femora from 57 human mummies the values of K were measured. We found that Kposterior=0.498±0.085 for the posterior radiographic view, and Kmedial=0.589±0.070 for the medial view with Kmin=0.345 and Kmax=0.783. The theoretical optima for K depend on the ratio (Q) of the marrow:bone density. Accepting the assumption of earlier authors that Q=0.50, our data show that human femora are optimised to withstand bending fracture, or yield and fatigue strengths. There were no sex-, age- and length-specific differences in K, and the means of K of the right and left femora of individuals were statistically not significantly different. The biomechanical optimization for K of human femora is not finely tuned. Compared with fox femora, K of human femora follows the biomechanical optimum to a much lesser extent. Although the relative wall thickness W=1–K of human femora are optimised, the very low relative mass increment due to deviation of K from the optimum and the considerable intraspecific variance of K make it probable that an accurate optimization of the relative wall thickness is irrelevant in humans.

Vasoactive intestinal polypeptide in neuroglia? Immunoelectron microscopic localization in astrocytes of the rat mesencephalon

The dorsal region of the rat interpeduncular nucleus (IPN) was found highly immunoreactive for vasoactive intestinal polypeptide (VIP). This area appeared as a cap‐like structure at the midcaudal level of the nucleus. Unlike other brain areas, however, VIP immunoreactivity within the “cap” appeared vaguely punctuate with no light microscopically identifiable cell structures. Ultrastructurally, a dense meshwork of VIP‐immunopositive bouton‐laden axons was revealed. Labeled neuronal perikarya were not encountered. Some lightly immunoreactive dendrites were observed. In addition, immunopositive glial profiles were frequently seen. Perikarya and numerous fine processes, occasionally perivascular, identified as astroglia by established ultrastructural criteria, exhibited VIP immunoreactivity. Constant feature of the peptide immunolocalization was the predilection for the intermediate filament bundles of astrocytic perikarya and processes. This was usually accompanied by a thick coating of the inner aspect of glial plasmalemma and, in perikarya, by highly reactive vesicular profiles. Glial immunopositive elements were never seen beyond the boundaries of the diffuse “cap.” In view of the repertoire of metabolic, neurotrophic, and neuroprotective mechanisms in which VIP neurons are involved in conjunction with astroglia, the presence of VIP‐immunoreactive astrocytes in a circumscribed area, confirms the heterogeneity of astrocyte populations. GLIA 34:229–233, 2001.

Why do horseflies need polarization vision for host detection? Polarization helps tabanid flies to select sunlit dark host animals from the dark patches of the visual environment

Horseflies (Tabanidae) are polarotactic, being attracted to linearly polarized light when searching for water or host animals. Although it is well known that horseflies prefer sunlit dark and strongly polarizing hosts, the reason for this preference is unknown. According to our hypothesis, horseflies use their polarization sensitivity to look for targets with higher degrees of polarization in their optical environment, which as a result facilitates detection of sunlit dark host animals. In this work, we tested this hypothesis. Using imaging polarimetry, we measured the reflection–polarization patterns of a dark host model and a living black cow under various illumination conditions and with different vegetation backgrounds. We focused on the intensity and degree of polarization of light originating from dark patches of vegetation and the dark model/cow. We compared the chances of successful host selection based on either intensity or degree of polarization of the target and the combination of these two parameters. We show that the use of polarization information considerably increases the effectiveness of visual detection of dark host animals even in front of sunny–shady–patchy vegetation. Differentiation between a weakly polarizing, shady (dark) vegetation region and a sunlit, highly polarizing dark host animal increases the efficiency of host search by horseflies.

Diurnal variation of the melanin-concentrating hormone level in the hypothalamus

Melanin-concentrating hormone (MCH), the neuropeptide produced mainly in the hypothalamus, plays an operative role in regulating food intake and the sleep/wake cycle. Considering that these physiological functions pursue diurnal variations, we checked whether the total hypothalamic MCH level depends on the time of the day. The aggregated MCH peptide content of the whole MCH neuron population was significantly higher at the end of the sleeping period (lights on), than at the end of the active period (lights off). This result, together with earlier observations, indicates that in contrast to the MCH gene expression, the level of MCH peptide is object of circadian variation in the hypothalamus.

Distribution and synaptology of vasoactive intestinal polypeptide (VIP) immunoreactive structures in the rat periaqueductal grey

Light microscopic analysis of the rat midbrain periaqueductal grey (PAG) showed vasoactive intestinal polypeptide immunoreactive (VIP-ir) neurons localized at the lateral and ventral walls of the aqueduct. Some varicose VIP-ir elements were detected closely associated with the ependyma. While several VIP-ir elements were encountered immediately under the ependyma, in a few cases, VIP-ir cell bodies were seen on the luminal surface of the ependymal cells lining the aqueduct. Electron microscopy revealed that most of these cells possessed the characteristics of a local circuit neuron. All VIP-ir cells had indented nuclei. Two types were distinguished: one with rounded cell body receiving numerous axo-somatic synapses established by VIP-negative axons. The other cell type was fusiform and its surface was almost fully isolated from axonal contacts by a glial sheath. The VIP-ir processes were interconnected with other periaqueductal cells by a variety of synaptic contacts. VIP-ir axon terminals formed asymmetric synapses with immunonegative dendritic shafts often in glomerulus-like assemblies. The postsynaptic immunonegative dendrites were of the aspinous, beaded type. We suggest that VIP-ir cells and processes in the midbrain PAG establish connections between the longitudinal functional columns of this region. On the basis of their morphology, VIP-ir cells in the PAG appear to be excitatory, terminating on inhibitory interneurons. Thus, a VIP-stimulated inhibition may be instrumental in the coordination of responses evoked by the stimulation of PAG columns.

Experimental evidence that stripes do not cool zebras

There are as many as 18 theories for the possible functions of the stripes of zebras, one of which is to cool the animal. We performed field experiments and thermographic measurements to investigate whether thermoregulation might work for zebra-striped bodies. A zebra body was modelled by water-filled metal barrels covered with horse, cattle and zebra hides and with various black, white, grey and striped patterns. The barrels were installed in the open air for four months while their core temperature was measured continuously. Using thermography, the temperature distributions of the barrel surfaces were compared to those of living zebras. The sunlit zebra-striped barrels reproduced well the surface temperature characteristics of sunlit zebras. We found that there were no significant core temperature differences between the striped and grey barrels, even on many hot days, independent of the air temperature and wind speed. The average core temperature of the barrels increased as follows: white cattle, grey cattle, real zebra, artificial zebra, grey horse, black cattle. Consequently, we demonstrate that zebra-striped coats do not keep the body cooler than grey coats challenging the hypothesis of a thermoregulatory role of zebra stripes.