Vlêmuis Grot (Swaelskrans), Ceres, Cape Town

Looking up the valley from Swaelskrans. 

INTRODUCTION

One of the streams that runs off the Zuurberg into the Ceres valley disappears into a pile of boulders, and re-emerges about 130m later just before it leaves the mountain side and enters the farmlands.  Whilst underground, the stream flows through Vlêrmuis Grot,  This cave was the target of the Leap Year Grottoes Survey Commemorative Club held every Leap Year on the 29th February.

The cave is a relatively shallow system that runs along the side of a boulder filled valley. The cave was probably a much larger system that has collapsed.  The Swaelskrans entrance is a remnant of this larger system.

As the cave is so close to the surface, and houses such an active stream, it is home to creatures that are  normally found above ground and also to more cave adapted animals.

AQUATIC CREATURES

Cape River Frog.

From the tide marks on the walls of the cave, the stream must become a powerful torrent at peak flow, but during our visit at the end of summer, the water in the cave was restricted very still, cool pools.  At the downstream end of the cave we found a large shallow (30cm deep) pool with Whirligig Beetles on the surface and shrimps in the water. We also found horn shaped casing which probably housed caddisfly larvae.

The largest of the aquatic animals was the Amietia fuscigula, the Cape River Frog.  It was common in many of the larger pools in the cave and varied in size from 20mm head-to-tail, to about 80mm head-to-tail.

We also found large (30mm+) tadpoles in the larger pools.

The frogs were not really concerned by our torch light, and the largest frog (pictured above) seemed quite inquisitive, and swam directly up to a torch placed at the waters edge.

Megaloptera larva.

By far the most aggressive of the creatures was a Megaloptera larva. This fellow did not like being touched and readily attacked a stick when it was brought close to its head. This was the only Dobson fly we found.

Midge walking on water.

Another creature that we found, on (not in) the water,  was a small cream coloured midge.


CAVE CRICKETS

The main reason for us choosing this cave for our special Leap Year outing was to see if we could extend the documented range of the recently described  Speleaiacris monslamiensis.  The original specimens were found, less than 10km away, in two caves on the upper slopes of the Shale Peaks, above Milner Vlakte, on the Heks River Mountain range. The possibility of us finding them in this cave was therefore reasonably good. We were not disappointed.
Rampini, M, C. Di Russo, G. Carchini. 2009 (2010). A second species of cave Macropathinae for Africa (Orthoptera, Rhaphidophoridae)  Subterranean Biology 7: 65-68.

Within 10 minutes of entering the cave we found our first adult cave cricket, and soon found that they were quite common in the cave.  Although the identification has not yet been confirmed, these cave crickets are very probably the Speleaiacris monslamiensis we were looking for.

Spelaeiacris monslamiensis

The crickets are only found in the permanently dark zone of the cave and very sensitive to light. They did not like having their photographs taken, and took cover whenever they were illuminated.  We found one of the more adventurous cave crickets walking across the surface of one of the larger still pools.  As it seemed to quite at home walking on water. The biggest hazard was probably not drowning, but was more likely to be the aforementioned large frog that occupied the same pool.


After the cave crickets, the largest group of invertebrates appeared to be spiders.  We have not managed to identify any of them yet.


BATS

Rhinolophus mother and young.

There were at least 3 species of bats.  The most common was Miniopterus, probably schreibersii.  There were between 800 and 1000 individuals.  In addition to the Miniopterus, there was also a reasonably large population of adult female Rhinolophus and young Rhinolophus.  I suspect that they were R.clivosus.  Many of the Rhinolopohus roost right in the middle of the Miniopterus carpets on the cave ceiling.

The very young Rhinolophus cling to their mothers, top to tail, i.e. the mother hangs head down from the cave ceiling with the babys' tail in her face.  Many of the older juveniles roost together on the fringes of the Miniopterus carpets.  These were light gray in colour, and appeared very interested in our presence.

The very young that were not with their mothers appeared to remain in torpor and did not move when we entered the chamber,  In one instance a mother braved our proximity and returned to her lethargic offspring.  Once the young bat had clambered aboard, the mother flew off deeper into the cave.

After examing the photographs taken during our visit, I found one individual that looks like a Myotis tricolor.


CONSERVATION

Although the visit to the cave was most productive in terms of the animals we found, there were two conservations issues which need highlighting.  The first was that we made a mistake by visiting the cave while the Rhinolophus were using the cave as a nursery.  This was bad timing and the caving club will be advised against visiting the cave from the end of December to the end of March.

Melted black plastic splatters.

The second was that the cave floor near one of the entrances is covered with blobs of melted plastic, and there were burnt ends of black pvc irrigation pipe lying in the cave.  It appears that burning pipe has been used as illumination for exploring the cave, and if done regularly, it could have a serious negative impact on the cave life, especially the bats.