Friday, 27 February 2009

Name That Skull

I posted an entry on this blog detailing a walk I had taken through my local woods and fields on a bitterly cold and windy day in early February. As I got home from that walk the sky, which had been threatening snow all day, opened and the first flakes began to drift down. By the morning the whole country was covered to a depth of several inches in the most significant snow event to hit Britain for 20 years. In Durham the snow lasted for two weeks, periodically topped up by fresh falls and hardened by freezing night time temperatures. I can not express how disappointed I was that this period of snow cover coincided with one of my busiest periods at work for several months, making it impossible to take time off and practice snow tracking but when the snow had gone and the weather was slightly warmer I was able to spend a few hours back in the woods so I headed back to the grassy beetle-bank between two fields where I had such a thrilling experience with the hunting weasel. The beetle-bank is perhaps three meters wide and runs from the banks of the river Wear, up the side of the field towards the rear of Cold Stream Farm – an area that the farm has traditionally used as a tipping place for waste material and derelict machinery. The grass on the beetle-bank is yellow and dead, blown flat by the wind and weighted down by snow. On this occasion the weasel was nowhere to be seen but midway up the margin, lying on the grass as if it has just been dropped by a careless passer-by was a small white skull, obviously that of a rodent, and a few meters away the remains of a lower jaw that appeared to have once belonged to the same. Rodents have two incisors in the upper as well as in the lower jaw which grow continuously and must be kept worn down by gnawing; this is the origin of the name, from the Latin rodere, to gnaw, and dens, dentis, tooth. The teeth have enamel on the outside and exposed dentine on the inside causing them self-sharpen during gnawing. Rodents lack canines and have a space, the diastema, between their incisors and premolars. I looked around for any information that would help me identify the skull, or how it had come to be there. I could find no owl or raptor pellets and nowhere such a bird could perch to eat. If there were any weasel runs along the beetle-bank they were covered by the grass and beyond my ken so I placed the skull and jaw in my collecting box and carried on walking. Examining the skull in the warmth and comfort of my home I was able to take measurements. On one side the zygomatic arch, the thin curving bone along the side of the skull below the orbit, had broken away but on the other it remained as a delicate bridge-like structure. The condylobasal length, the length of the skull from the front of the premaxilliary bones to the rear surface of the occipital condyles, was 5cm and the width of the skull across its widest point at the cheekbones, the zygomatic width, was 2.5cm. The skull retained one of its upper orthodont incisors and the gap between this and the three premolars, the diastema, was 1.5cm. The section of lower jaw, which appeared to be from the right side of the animals head, was 3cm long and retained one of the lower pro-odont incisors, which when removed cleanly and smoothly from it’s alveolus measured over 2.5cm in a sweeping scimitar-like curve. I consulted Brown et al's "Animals Tracks, Trails & Signs", which has an extensive reference section on mammal skulls and narrowed my find down to a rodent of the Muridae super-family. Working on the maxim that common things occur commonly the skull, given its size and the location of the find site would most probably belong to a rat, although whether the common or brown rat or the less common black, or ship-rat was still unclear. I was greatly helped in making the final identification by the information contained in the excellent website . Using the easy to navigate species pages I noted that the skull of the brown rat can be distinguished from that of the black rat both by its tendency to be greater than 4.3cm in length and by the prominent temporal ridges that run along the side of the cranium, marking the upper limit of the temporalis muscle. My skull had very clear temporal ridges running almost parallel along the top of the braincase marking it as belonging to Rattus norvegicus, the brown or “Norway” rat. Rats have always been viewed as pests and despite their highly sophisticated social behaviour they are inextricably associated with disease, poverty and squalor. Originally called the “Hanover Rat” by people wishing to link the problems of 18th century Britain to the House of Hannover the origins of this unwelcome invader have been speculated upon by those who wished to shift the blame to others, giving rise to hypotheses that it originated in Ireland or crossed the channel with the Normans. The Brown Rat does not seem to have arrived in Europe until the late middle-ages and is not recorded in Great Britain until 1730 however by the middle of the 19th century naturalists had concluded that John Berkenhout’s 1769 naming of the Brown Rat as Rattus norvegicus, because it has come from Norway in shipments of timber, to be erroneous and an Asian origin for the species was promoted. The Brown Rat is, after human beings, the most successful and ubiquitous mammal on earth and has piggy-backed on humanity, associating itself with their dwellings and travelling along their trade and migration routes until it has colonised every corner of the earth except the poles. It’s opportunistic, omnivorous lifestyle and the habitats afforded it by the industrialisation and urbanisation of the late 18th and 19th centuries have helped the species thrive until it is now estimated that the UK supports a population of 81 million rats – 1.3 per person. I found my brown rat skull along the edge of a field situated between a river and a farm. This is classic rat habitat and affords the species every opportunity to forage and thrive on the waste tips and middens, amongst the feed bins of the stock and along the banks of the river. It is interesting to speculate how this individual met its death. Looking at the only available evidence, the cranium and jaw, it is not possible to identify any signs or marks that would indicate a specific predator but we can speculate as to a likely scenario from the things we know about the local habitat and landscape. I am sceptical that the rat was killed and eaten by a fox. No doubt the local population of foxes do, on occaision, eat rats but my examination of fox scats in the adjacent woods suggests that they favour pheasant and other birds, even in winter, with their winter scats showing a very high volume of quills and feathers. It would also appear that such a clean and well preserved skull, with one zygomatic arch still intact, has not been subject to the crushing and chewing action of a foxes jaw. It may be that the rat was predated by a kestrel or tawny owl, which in the act of eating the body severed and then discarded the head. Whilst it is not impossible to rule this out I am not sure if this behaviour has been previously recorded and the absence of any suitable feeding posts or perches along the treeless field boundary would tend to suggest otherwise; I am sceptical about a kestrel or an owls willingness to devour its prey on the ground in the middle of an open field. Another possible scenario is that the rat was killed by a smaller, ground dwelling carnivore – possibly a weasel or mink. I have witnessed weasel hunting behaviour in this very location and it is certainly possible that such a tenacious predator could kill a rat but I am unclear as to how a small mustelid such as a weasel or mink would set about eating a rat, whether they would discard the head and focus on the body or whether, in hard times such as winter, the whole animal would be consumed? So far I have been unable to find any information on the web to answer this question – how does a weasel eat a rat? Of course it may be that the rat was not predated at all. Perhaps it was decapitated by the ploughing of the field, it’s head flicked onto the beetle-bank by the action of the tractor wheels and the flesh picked from the skull by insect larvae in the autumn long grass. Perhaps the cleaned skull then lay hidden, like a secret jewel, until the winter winds and heavy snows flattened the dead grass and I came along one sunny day in late February and found it?
Top: Rattus norvegicus
Middle: Jaw with low incisor removed
Bottom: View of skull and jaw, showing distinct temporal ridges.
In researching this specimen I found Skullsite to be an invaluable aid. Not only does the site include a very comprehensive collection of photographs detailing the size, shape and variances of animal skulls it also has a comprehensive glossary of zoological, or skullological, terms which will help the interested layman to make sense of the data.

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