Sun Loving City Dwellers
We aren’t the only ones who have enjoyed the lovely warm summer days this year. Much of our wildlife is reliant upon the weather for certain life history events to take place. Plants coming into flowers, birds laying their eggs and butterflies emerging are just some of the wildlife events which rely on the weather, temperature, and day length to tell them when they should occur.
Some of the animals for whom weather has the most significance are those whose body temperature is dictated by the ambient environment, we call them cold blooded. Us humans, and all mammals and birds, are warm blooded. We can function even when the external environment is cold because we retain a constant core temperature.
Reptiles and amphibians are familiar examples of cold blooded animals. They heat up in warm conditions and get cold when the temperature drops low. There is a much larger group of ‘cold blooded’ animals living among us, and I bet you see at least one every day; the insects.
To say that reptiles, amphibians and insects (amongst others) have no control over their body temperature would be inaccurate. Rather they have only limited control, which is normally behaviour based. We are all familiar with the idea of snakes choosing to bask on warm sunny rocks to heat up in the morning, or with frogs seeking out cool damp undergrowth to shelter from the heat of a July day. What is perhaps less well known is that insects also use behavioural means to manage their body temperature.
A couple of weeks ago I was walking through one of London’s green spaces, when a vibrant strike of blue sped past my head. I couldn’t believe my luck when the dragonfly came to rest a short distance from me. It was in clear view on the outer branches of a low willow tree. It had even positioned itself at a great orientation for photography, with the sun shining directly onto its body and wings, highlighting all the wonderful details of its markings. I set up my camera and excitedly clicked away. At that moment I was completely absorbed in photography and it didn’t occur to me that this set up wasn’t entirely for my benefit.
Knowledge of the natural world is the key to explaining many of the wonderful wild spectacles we are lucky enough to witness. Yes, even in London! The dragonfly’s position perched on the edge of the willow tree wasn’t to avoid stray twigs getting in the way of my picture, but to give the dragonfly a clear exit flight path should danger threaten. Its orientation to the sun wasn’t to reveal all the beauty of its markings for my camera, but to allow the animal to absorb as much of the sun’s heat as possible. Within the dragonflies wings are numerous tiny veins which provide support within the delicate wing, like scaffolding. These veins also allow the wing to function as a solar panel, collecting up the heat of the sun and transporting it to the body through fluid flowing in the veins. The dragonfly which I was keenly photographing was here due to its desire to sunbath.
The species I was looking at was a male Migrant Hawker (Aeshna mixta), a large dragonfly seen on the wing in late summer and autumn. At this time of year, when the heat of the sun begins to fade, it is important to be able to make the most of the warmth that is available and sun bathing is a great way to achieve this.
Sometimes dragonflies are faced with the opposite problem; they get overheated on a very warm day. A shady perch is a good remedy to this problem, but if shade is not available then the behavioural solution to this problem is both simple and ingenious. An overheated dragonfly will perch with the tip of its abdomen (bottom) pointing directly at the sun. Imagine us pointing straight at the sun with a finger; our fingertip will be directly heated by the suns rays, but the rest of our finger will be slightly shaded behind our fingertip. If we had our finger laid out flat beneath the sun, we could risk burning the entire length of the finger. By pointing the tip of its abdomen at the sun the dragonfly is minimising the surface area directly exposed to the sun rays, and thus absorbing as little of the sun’s heat as possible, gradually cooling down.
This Migrant Hawker is a highly mobile species, if conditions such as temperature don’t suit it in one place it can fly to another. Spectacular migrations of Migrant Hawkers are recorded across Europe, hence the name! Regular migrations to the UK from the continent increase numbers of this species here in late summer each year.
The Migrant Hawker is just one example of the different ways certain animal behaviours can be traced back to temperature, whether it be basking on a cool day with wings like solar panels, pointing one’s bottom at the sun in attempt to cool down or setting off on an epic migration in search of a suitable climate. As the weather in London starts to turn chilly and we don coats and scarves, spare a thought for all the residents of the animal kingdom. They will be making their own adjustments.