Nature 2013 October

The curious incidence of the bark in the night-time and the colours in the autumn

Some animals have names which describe exactly what it says on the tin. Harvestmen usually appear in the late summer and later on often finds a home in your home. They are sometimes mistaken for spiders having a pill-shaped body suspended on eight very long legs. Sometimes they are called ‘daddy-long-legs’ which is a really a collective name for any spindly-legged invertebrate which cause distress as they bumble along and includes six-legged craneflies, and in the US, cave spiders which are the only dangerous ones as they bite.

Harvestmen differ from spiders in having an undivided body (spiders have a head and abdomen) no fangs and they do not make webs. Their main defence are their stink glands. If picked up they have two defence mechanisms. They can produce an unpleasant odour. Funny though, as their main predator are birds which do not have well-developed olfactory organs, it must also cause a chemical reaction in the mouth of birds making them speedily reject them from their beaks. The other escape trick is to detach a limb or two which are all too easily lost if grabbed. Not exactly an ideal solution as it must impact on their mobility but it’s a trade-off for having otherwise superior abilities to track across uneven ground. Losing their second pair of legs though would be fatal as housed inside these minute slit-like legs is an olfactory centre, the equivalent of a nose, tongue, ears and eyes all in one. They are packed with nerves, attached to thousands of sense organs which they use to track down their food. Their tastes are truly catholic, and as the ‘tin says’, they ‘harvest’ their food from all sorts of places; ranging from smaller bugs, fungi and bird droppings, all the way up to scavenging the carcasses of large animals and birds.

Autumn also signals a change in behaviour for many mammals. With a hoard of new adolescents kicking around the patch since the previous year, adults both old and new have to re-establish their territories. Because Muntjac breed all year-round and are unlike their much larger relatives typically inhabiting the Highlands, Islands and Parks which can muster large harems, our local deer do not have a rutting season. Instead they need to scent mark-out sufficient habitat to provide adequate food supply and protection. Once established or re-established the male emits those all too familiar barks in the night to warn other males to keep away and to draw one or more females into their locality. If a challenger emerges though the male muntjac has short antlers it will use its downward pointing canine teeth to attempt to rebuff the challenger. Meanwhile an equally frequent and persistent bark in the night-time is proffered by the dog-fox which has an even tougher job to protect its domain. Badgers can also assert their authority by barking out short, sharp admonishments. Even grey squirrels express themselves with aggressive bursts of corpuscular barking.

Despite their delicate appearance lacewings, those green translucent-winged flies are hardy beasts. This is a valuable insect for the gardener as they predate aphids and small larvae. These insects share an autumn habit with deciduous trees. Some have suggested physiological changes in the insect are an adaptive behaviour providing camouflage at the same time when autumn colours are emerging. In fact the processes going on in leaves and these insects are very similar. The drivers behind these changes shortening day length and falling temperatures. The final generation of this season’s flies will seek out a safe haven to hibernate, such as the crevices beneath bark or, in my experience, the gaps between wooden windows and frames in sheds. Over a matter of days they change colour to a dull pink. In effect they lose the green colouration which exposes the background blood pigments. It is likely that they are conserving vital body foods and elements by breaking down the complex molecules that generate the green colouration. A similar process is afoot within the leaf as was in the wings of the insect. The trees need to recoup the valuable minerals and chemicals in the leaves over winter. As the cell contents empty this reveals the burnt oranges and russet red colours we all admire. Winter colour-change is also a feature of many animals and birds that live on high ground in the North; hares, foxes, grouse. Away from the snowy conditions such colouration is unusual. One example that breaks this rule is the stoat which takes on an almost white colouration in winter months. Despite the absence of frequent significant snowfall in southern England this ermine mantle has persisted. However, this habit is changing. If you are lucky enough to see one it is likely to be half white and brown.

Some of you may have noticed the play-on-words in the choice of the title. “The curious incident of the dog in the night-time” was a line in a Sherlock Holmes story ‘Silver Blaze’.


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