MAMMAL ORIGINS | Halloween Special:

Bats!

Bats are indelibly associated with Halloween, a festival with Irish roots

Halloween's association with Ireland goes back to the very beginning, as its origins are thought to lie in the great fires set ablaze on the Hill of Ward in Co. Meath in the heartlands of the island for the ancient festival of Samhain. This was to mark the end of the harvest season and the onset of winter, the dark part of the year. Over the centuries, of course, this festival has morphed into something new, and yet it retains its fundamental association with the dark, not least in its preoccupation with the creatures of the night, chief among them those masters of darkness that are one of the most iconic features of any Halloween – the bats.

This month, then, is a perfect opportunity for us to delve into the evolutionary origins of these little mammals that haunt the night air in Ireland, and especially to see how they evolved those wonderful wings that allow them to sail through black skies and flirt with the moon.

Our search for the origins of the bats plunges us deep into the mists of time, to a world which was only just beginning to recover from a great calamity when a rogue heavenly body crashed into the Yucatán Peninsula in Mexico around 66 million years ago. The impact of this celestial invader wiped the non-avian dinosaurs from the Earth and opened the way for what would become the Age of Mammals.

But the takeover by the mammals of the Earth was not a victory won in an instant, and, in the time immediately following the demise of the dinosaurs, most were still small, shrew-like creatures scurrying around on the forest floor, regardless of whether they were the ancestors of a horse, a human, or even a whale.

The ancestors of the bats too would have been small and shrew-like, but as with a number of other lineages, including the ancestors of the primates, they must have become increasingly adapted to life in the trees as time wore on.

But just how the ancestors of the bats evolved from these supposed tree-dwelling forms to the aerial maestros of today is impossible to say, as the first recognisable bats in the fossil record already display a more or less modern form.

The first true bat, Icaronycteris, which lived in Wyoming in the U.S. around 51 million years ago, looked much the same as today's bats, and not long after this bats turn up in the fossil record all over the world. Indeed, one form, Eppsinycteris, is known to have inhabited England, so we can be almost certain that little bats like these also ruled the night skies of Ireland's ancient forests.

Life reconstruction of the early bat Palaeochiropteryx, which lived 48 million years ago in Messel, Germany, although we now know that it would have been redder in colour than depicted here

It is quite surprising, to say the least, that one of the most specialised mammals inhabiting Ireland today should have been among the first to assume its modern shape, and exactly how this occurred has given rise to much speculation.

Some studies have suggested that the ancestors of the bats first evolved into gliders, with a skin membrane connecting their fore and hind feet, allowing them to glide from branch to branch. This is a very reasonable proposition as gliding has evolved many times independently in different mammal lineages, including today's colugos ('flying lemurs') and certain marsupials and rodents, as well as some very ancient groups that are now extinct.

In fact, the relatively recent find of the glider Volaticotherium antiquus ('ancient gliding beast') in China could possibly date from as far back as 164 million years ago, meaning that mammals may have been sailing through the air millions of years before the birds!

In any case, these ancient proto-bats may also have used the webbed fingers of their 'hands' to steer while gliding, and it is here that we may find the origins of the wing. Over time, these fingers may have become ever longer and the skin membrane they supported ever larger to the point that they achieved true powered flight, which involves the flapping of wings rather than the more passive negotiation of the air in gliding.

This was a landmark achievement, the bats being the only mammals ever to develop the ability to truly fly, joining the birds and those ancient flying reptiles the pterosaurs in a very select group of vertebrates to manage this feat.

47-million-year-old bat skeleton from Messel, Germany displaying those very elongated fingers

Whatever the truth about the evolution of this true wing, though, it allowed the bats to colonise almost every part of the planet, resulting in them giving rise to many different species. Over 1,100 inhabit the Earth today, and together the bats account for around 20 per cent of all living mammals.

Indeed, in Ireland this figure is closer to 40 per cent, with nine of the 24 living native species of mammal on the island being bats. Of course, to some degree this reflects the difference in how much easier it has been for a flying mammal to colonise Ireland since it became an island at the end of the last Ice Age , with the sea constituting an impassable barrier to many terrestrial forms, although it is also true that some bat species have arrived only in very recent times.

Ireland's nine species of bats, then, could rightly be hailed as the masters among its mammals today. In more than one sense they soar above their mammalian compatriots, not a horror but a wonder, minnows in mass but titans in design, living their lives on the wing as no other mammal can.