Friday 27 January 2023


The Nightstalker is, as most readers will know, one of Dougal Dixon’s creations presented in his 1981 book 'After Man', a book that proved fundamental for speculative biology. In that book, he presented completely novel themes, such as penguin whales and terrestrial cephalopods or bats; for more on the 1981 setting, see an earlier post here.

Click to enlarge; copyright Dougal Dixon

The image of the nightstalker in the 1981 version of After Man was later changed by Dixon, who did not like the original one very much. The new version is shown above and is published in recent editions of 'After Man', such as the '40th anniversary edition' (which has new information too!).  

The nightstalker descended from bats that were among the first animals to arrive on the newly emerged volcanic Batavian Islands in the Pacific. Facing no serious terrestrial competition, the bats lost the ability to fly, became fully terrestrial and diversified. Once other mammals arrived as well, a bat species started to hunt them and evolved into the nightstalker, a formidable bipedal predator of one meter and a half in height, or as tall as an 11-year old child.

Nightstalkers are blind and use echolocation to find their prey in the night, ‘screaching and screaming through the Batavian forest’. This may mean that the nightstalker uses echolocation at sound frequencies we can hear too, although the text does not literally say so. The screeches might also be used for communication within the pack, leaving ultrasonic sound for echolocation. In either case I wondered whether its prey can hear the echolocation sounds too, which would make life more difficult for the nightstalker.  I have compared the relative merits of vision and echolocation in three posts (one, two, and three). It turned out that echolocation is like someone shouting at the top of their voice ‘WHERE ARE YOU!?’. Provided the prey can hear the sounds used in echolocation, echolocation is the opposite of stealth.

The nightstalker is bipedal, with the interesting twist of walking on its front legs. That makes sense in that the wings of bats are much larger and stronger than their hind legs. The animal uses claws on its hind legs to help overcome it prey, to which end the hind legs pass the front legs on the outside. In my 'review with hindsight' of After Man, posted in 2018, I wondered whether it would make more sense if the hind legs moved forwards between the front legs. I asked Dougal at the recent 2022 TetZooCon if he would mind me writing a blog post about this particular revision of the nightstalker. He did not, so here it is. I could not help myself thinking some more about terrestrial bats. I do not doubt that bats could evolve to walk efficiently again, as there are bats alive today that not only walk, but run too. 

Researchers managed to get vampire bats to run on a treadmill, and the animals obliged by using a unique hopping run. That is the video above. That odd gait must be due to the extreme difference in size between front and hind legs, which poses an unusual problem. During walking, legs that are on the ground at the same time must all propel the body over the same distance in the same time, or else the shoulder would walk faster or slower than the hip. From this it follows that the shorter leg will be on the ground for a shorter period than the longer leg, so the shorter leg only supports the body for a short time. That may be impractical, which suggests three different evolutionary solutions.

The first and weirdest solution is to have the hind legs move twice in the time the front legs move once. That is definitely possible, at least in theory. I know that because I was once requested to program such a gait to help visualise a terrestrial shark, posted here. The videos above show the result. This solution does not seem the most likely one though...

Click to enlarge; copyright Marc Boulay / Jean-S├ębastien Steyer

 The second adaptation would involve quick enlargement of the hind legs, which appears altogether sensible and straightforward. the result would be very similar to the Steyer/Boulay terrestrial bat shown in ‘Demain. Les animaux du futur’ (and discussed on this blog here and here).
The third possibility means the animal no longer uses its hind legs for locomotion, so they can be used for something else, such as being weapons. If front limbs are liberated from their walking role, I would call that ‘centaurism’ (see here for the first mention of the principle). But the nightstalker freed its hind legs, so we probably need another name than centaurism; 'reverse centaurism'?

Click to enlarge; copyright Gert van Dijk

Of the three, the second option seemed the most straightforward one, and I considered stopping alternate evolution right there. Then again, the result wouldn’t be a proper nightstalker! I suppose a bipedal animal can still evolve from the enlarged hind limb version, so there you are: a bipedal terrestrial erstwhile bat with reverse centaurism.

What else did I change?

  • I made the ears smaller than in the original. When animals species increase in size, organs do not necessarily scale linearly with body size. Eyes, for instance, are relatively small in large animals. Beyond a certain size an organ's function may not improve noticeably, so there is no point in making the organ larger than necessary to do its job. I am not certain this also holds for echolocating ears but assumed this to be the case.
  • The skull and face are less bat-like than the original, because I assumed that the larger size would require a sturdier build. Bats have many pointy needle-like teeth, useful to catch insects. But an animal the size of a large dog would need teeth that can handle larger stresses.   
  • I kept the leaf-based nose because it is part of the basic package of vampire bats. However, it seems very vulnerable.
  • The eyes are still there because eyes seemed much too useful to abolish altogether. They are still small though, but useful for unforeseen circumstances.
  • There are no fingers, just thumbs. Bats fold their fingers, that support a large part of the wing membrane, out of the way when roosting and walking. They use their big thumbs to hang from. What will happen to the fingers if the wing atrophies during evolution? I foresee the fingers disappearing completely, and not coming back as toes. The thumb has grown and now extends towards the midline to support the body underneath the centre of gravity. Normally animals place their feet close to the midline for that purpose, but the nightstalker needs room under the body for the hind legs. The thumb could help support the body directly under the centre of gravity by extending towards the midline. The two stubs you see on each hand do not have nails or claws, because they are not fingers! They are pseudo-fingers, supported by former wrist bones.                

So here we are: an alternate nightstalker with its hind legs between the front legs. When I look at the result, it looks much less like a bat then the original, which may not be good from a didactic point of view. The image serves to illustrate an evolved bat, so people who see it should immediately associate it with bats. My revised version probably does that less well than the original. Mind you, my first version had smaller ears, no leaf nose, a longer snout and sturdier teeth, so it looked even less like a bat than the one you see now. The version shown above was 'batified' on purpose, but it still doesn't shout 'bat'. That raises the interesting question of balance between  presumed biological underpinning and what the image is supposed to evoke. It is fun to play with both aspects, and adds another layer of speculation to speculative biology.


The above was all seen by Dougal. His response to reading the text was this:

"I claim it is an example of speciation in the Batavian archipelago! A new species on one of the newer volcanic islands in the "hot-spot" conveyor belt island chain. Shared ancestor with Manambulus perhorridus rafted across from the Big Island at a time of its early appearance along with its potential prey species."

And so it shall be; the new species deserves a new name though, and I think the differences are too large to use the same genus.  I therefore present Condylovador terriloquus! (from condylus: knuckle; vadere; to go or to walk;  terriloquus: uttering frightening words)