SHEDDING LIGHT UPON THE MYSTERY OF LUMINOUS BIRDS - Part 2: ALL AGLOW WITH SUGGESTED SOLUTIONS!

 

Is this what a luminous or glowing owl would look like?

Sceptics notwithstanding, the phenomenon of luminous birds whose lengthy history I surveyed recently on ShukerNature in Part 1 of this two-part review (click here to read Part 1) is assuredly genuine, but how can it be explained? Five principal potential solutions have been suggested by amateur naturalists and professional scientists alike down through the ages, and these are as follows:

 

1) It is due to the bird having made physical external contact with phosphorescent organisms living on decayed wood in tree holes

The idea behind this suggested solution – the most familiar and extensively documented of the five under consideration here – is that such contact would cause phosphorescent bacteria, plants, and fungi growing on the wood to become attached to the bird's feathers, thereby yielding an area of luminescence upon its plumage.

 

How a glowing barn owl with a particularly luminous breast might look

However, whereas the parts of a bird's body most likely to make contact with wood when entering or exiting a tree hole would be its wings and head (brushing against the rim of the hole), the body region actually exhibiting most (or all) of its perceived luminescence in those specimens that have been reported has tended to be the breast, with the wings and head sometimes giving off little (if any) light.

Also, it must be remembered that glowing examples of extremely large birds, such as North America's great blue heron Ardea herodias, standing 45-54 inches tall, have been recorded – and it seems highly unlikely that birds of this stature would (or could) inhabit tree holes.

 

A great blue heron (© Mike Baird/Wikipedia – CC BY 2.0 licence)

In addition, and as its common name suggests, the barn owl, the most popular identity for luminous owls, prefers to roost in barns or deserted out-houses rather than in tree holes (though it will roost in them if need be).

Yet if this option is nonetheless a viable one in relation to certain bird species, a common phosphorescent fungus likely to be involved is the honey fungus Armillaria mellea – a very abundant, widespread, edible species (or species complex, as is nowadays deemed to be the case) that lives on trees and woody shrubs, and sports bioluminescent mycelia yielding an ethereal greenish-blue glow commonly referred to as foxfire.

 

The honey fungus Armillaria mellea (© Stu's Images/Wikipedia – CC BY-SA 3.0 licence)

Indeed, I remember reading long ago a fascinating snippet of information demonstrating just how powerful the foxfire glow of this fungal species can be. Edited by Dilys Breese, and published by the BBC in 1981, the multi-contributor book Wildlife Questions and Answers included the snippet in question, provided by correspondent R. Watling, and which reads as follows:

I have often seen the eerie light of the honey fungus in a tropical rain forest. You see all the leaves and stems and trunks, twenty-five feet tall maybe in an old tree, with this beautiful glow, just like a silver lady among the trees. And these fungi can even take their own photographs! If you set up a camera next to one of them, given enough exposure time, you will get a picture of the fungus all bright and shiny, taken by its own luminescence.

 

Schistostega luminous moss inside a Japanese cave (© Dr TerraKhan/Wikipedia – CC BY-SA 3.0 licence)

Another species likely to play a part in this particular proffered solution is a phosphorescent plant officially called the luminous moss Schistostega pennata, but also known by such charming colloquial names as goblin gold and rabbit's candle. As noted by bryologist Sean Edwards in a letter published by BBC Wildlife Magazine in April 1994, its luminous portions are the first cells produced by germinating spores, which act like thousands of pear-shaped microscopic cat's eyes, collecting and concentrating even the faintest light. It is often found growing inside (and illuminating) rabbit holes, hence its 'rabbit candle' moniker, yielding a greenish-gold glow.

2) It is due to the bird having ingested phosphorescent microbes

As the luminescence of birds is external, and has actually disappeared in some cases following moulting, one would assume this to be a phenomenon associated exclusively with the bird's external covering, i.e. its plumage, rather than due to any digestive or other metabolic process (but see also Solution #4 for some ostensible exceptions to this statement).

Of course, we could speculate that if any phosphorescent microbes were inadvertently ingested with food, they could pass out of the bird's body within its faeces, which might then in some way become smeared upon its plumage, perhaps during preening, rendering it phosphorescent in turn.

Also, it should be borne in mind that not all phosphorescent bioluminescent fungi are harmless. One such species that is poisonous is Omphalotus olearius, the so-called jack-o'-lantern mushroom. This orange-gilled European fungus grows around the bases, stumps, and buried roots of hardwood trees (a related species, O. illudens, occurs in North America). A bird perching upon it may conceivably find itself with fragments of this fungus attached to its plumage, especially upon its breast feathers, rendering them phosphorescent, but if the bird then attempts to remove such fragments by preening, it could inadvertently swallow some of them and thereby become ill from the toxic nature of this fungus.

 

Jack-o'-lantern mushrooms (public domain)

Perhaps this is why some luminous birds that have been physically examined have been found to be in poor health, such as Rolfe's barn owl, and the specimen documented later here that was captured by a Norfolk engineer in his back garden.

Nevertheless, although such a scenario is not impossible, it is certainly not very plausible as an all-embracing solution.

3) It is due to the growth of feather-specific phosphorescent microbes upon the bird's breast plumage

In some ways paralleling the previous two proffered solutions, this third one proposes that phosphorescent bacteria or fungi may grow upon a bird's breast feathers if they have become damp or dirty. Propounded by British zoologist William P. Pycraft (1868-1942) among others during and beyond the Norfolk luminous owl 'flap', it derives support from the fact that breast feathers are often particularly dense (as with those of pigeons, for instance), thereby encouraging microbial proliferation upon them. Also, the breast is a difficult region for many birds, especially short-billed ones, to reach satisfactorily when preening.

 

How an owl with plumage infested with green-glowing phosphorescent fungi may look

Furthermore, in his article, de Sibour noted that avian luminescence is particularly powerful during flight. He sought to explain this occurrence as an effect of superoxygenation, pointing out that if a medium containing phosphorescent particles is agitated, that medium's luminescence does increase.

Consequently, this third suggested solution to the enigma of luminous birds would seem to be the most tenable of the three considered by me here so far. Even so, in view of the comparative rarity of glowing birds while concomitantly bearing in mind that a very great many birds must at some time or another possess damp and/or dirty breast feathers, this solution still falls some way short of providing a wholly satisfactory explanation.

4) It is due to some internal light-generating metabolic process

There are certain especially mystifying cases in the luminous bird files that if accurate seem to indicate that those individual birds' luminosity was directly linked not to any externally-sited phenomenon but instead to their own internal metabolism. For in each case, its external luminosity vanished once the bird itself had died. The earlier-mentioned gamekeeper Fred Rolfe who in 1897 had shot down a luminous sphere in Norfolk and found it to have been a barn owl in very poor condition made no mention of any such occurrence, but it was a notable feature of the two incidents now documented by me here.

The July 1911 issue of The Irish Naturalist contained several reports and reviews by different writers appertaining to luminous birds, especially luminous owls, but the report of especial interest here concerned a luminous specimen of North America's afore-mentioned great blue heron, as I'll be documenting below shortly.

 

How a luminous specimen of a great white heron, the white colour phase of the great blue heron, might look

In his own Irish Naturalist survey of reports, C.B. Moffat referred to a very interesting rural belief apparently prevalent in both Europe and North America that I hadn't previously encountered but which is very pertinent to the luminous great blue heron specimen. Here is what Moffat revealed:

A belief has long prevailed ascribing similar luminosity [to that of owls] to several of the herons and bitterns, which are supposed to be assisted in their nocturnal fishing operations by a phosphorescent light emitted from the "powder-down patches" of the breast-feathers, a light that is thought to serve, perhaps, the double purpose of attracting fish to the vicinity and helping the watchful bird to see them.

Powder-down feathers are specialized down feathers that grow continuously, in specific tracts, but are only produced by four taxonomically-unrelated bird groups (parrots, herons, bustards, and tinamous). In some such species, the tips of these feathers' barbules disintegrate, yielding fine powdery grains resembling dust or talc but composed of keratin; in others, the powder grains originate from cells surrounding the barbules of growing powder-down feathers. When a bird spreads these grains over its body during preening, they assist in protecting, ridding of parasites, and waterproofing the bird's plumage and skin, but well worth noting here is that they also confer upon its feathers a noticeable sheen.

 

James E. Harting (public domain)

Moffat then stated that wildlife observer James E. Harting had presented a resume of the principal evidence relating to this belief within a chapter entitled 'The Fascination of Light' contained in his book Recreations of a Naturalist (1906). In particular, Harting had referred to a detailed account by Philadelphia-based hunter W.J. Worrall of how he had shot a luminous specimen of the great blue heron. According to Worrall, the heron had possessed three phosphorescent spots – "one in front, and one on each side of the hips between the hips and the tail". The description went on to state that as the fatally-wounded bird expired, so too did its luminescence, its lustre "disappearing entirely with death".

Of interest, the location of this heron's three phosphorescent spots matches the location of some of the paired, dense patches of powder-down feathers in herons, which occur on their breast, flanks, and rump. So, might those phosphorescent spots simply have been extra-powdery (thence unusually pale and shiny) patches of powder-down feathers? Worth noting here is that in a Forest & Stream article written by American naturalist Charles S. Westcott and published in 1874, Westcott stated that he had experimented in a dark room with the powder from the powder-down feathers of least bitterns Botaurus exillis, the New World's smallest species of heron, "and found it to be of the same nature as 'fox-fire'". Moreover, J.P. Giraud, Jr., author of The Birds of Long Island (1844), affirmed that the powder-down of dead herons "gives out a pale glow, not unlike that produced by decayed timber, familiarly termed 'light wood,' or 'fox fire'".

 

A least bittern (public domain)

How, then, can we not only reconcile the above evidence provided independently by Westcott and Giraud that powder-down luminescence is not linked to a bird's life or death with Worrall's contradictory claim that the luminescence of the glowing heron that he had shot faded away once the bird itself died, but also (assuming its veracity) explain his latter claim?

The fundamental biological problem that Worrall's claim poses was highlighted by none other than Charles Fort – America's premier collector and chronicler of newspaper cuttings reporting anomalies across the entire spectrum of "damned" (i.e. scientifically-rejected or ignored) phenomena – when reporting in his book Lo! (1931) a second case in which this same luminescence-themed incongruity featured.

 

Charles Fort (public domain)

Fort referred to a report published on 7 February 1908 in Norwich's Eastern Daily Press newspaper (Norwich being a major city in Norfolk), in which engineer Edward S. Cannell of Lower Hellesdon, Norwich, claimed that on the early morning of 5 February when still dark he had seen something shining on a grass bank in his back garden, and that when it fluttered down a path there he discovered that it was a "bright and luminous" owl. He was able to capture the owl, which seemed to him to be ailing, and took it indoors, where it soon died. According to Cannell: "It was still luminous, but perhaps the glow was not as strong as when I saw it first" – i.e. its luminescence began fading following the owl's death. Moreover, in a sequel report, published by the same newspaper on 8 February, it was revealed that Cannell had taken the dead bird to a Mr Roberts, of Norwich-based taxidermists Roberts & Son, who claimed in an interview: "I have seen nothing luminous about it".

Needless to say, if both Cannell and Roberts were telling the truth, i.e. regarding the former's claim concerning the owl's brighter luminocity when alive than when newly dead and the latter's claim that when he later examined its corpse there was no luminosity at all, this is a most unexpected turn of events. For as Fort astutely pointed out:

Of course a phosphorescence of a bird, whether from decayed wood, or feather fungi, would be independent of life or death of the bird.

Indeed it would. Consequently, the only plausible explanation for any cases like Worrall's heron and Cannell's owl that feature synchronicity between a luminous bird's death and the disappearance of its luminescence would seem to be that the latter characteristic was caused by some intrinsic physiological, bioluminescent process – whereby the living bird was actively generating its luminescence via a specialised metabolic process, which obviously would therefore cease once the bird died.

 

Might a glowing owl's luminescence in reality be bioluminescence?

Yet although bioluminescence is well-documented from a wide range of organisms, it is currently unknown from any birds. (What has been confirmed, meanwhile, is that many bird species possess plumage that glows in the ultraviolet section of the electromagnetic radiation spectrum; but as human eyes cannot detect ultraviolet light, this particular type of plumage glow remains invisible to us.) Nor has this physiological condition been confirmed from any other tetrapod vertebrate (but click here for my investigation of a highly-controversial Trinidad lizard claimed by some researchers to be bioluminescent).

Of significance, furthermore, as revealed in his earlier-cited American Midland Naturalist article from 1947, is that during his researches into glowing birds, McAtee requested fellow American scientist Edwin R. Kalmbach to send him some specimens of the powder-down tracts from American black-crowned night herons Nycticorax nycticorax, a nocturnal species often claimed by eyewitnesses to be luminescent. He duly tested these tract specimens for the presence of luciferin and luciferase, the compounds inducing bioluminescence in known bioluminescent species, but he found no traces of them.

 

Black-crowned night heron (© ramidos/Wikipedia – CC BY-SA 4.9 licence)

Consequently, even if certain bird species are indeed somehow bioluminescent, they nonetheless must also be externally luminescent if like herons they possess powder-downs, judging not only from McAtee's failure to link these feathers to metabolically-induced bioluminescence, but also to the above-reported findings of Westcott and Giraud that samples of these feathers' powder derived from dead birds continue to be luminescent. To my mind, however, this seems a superfluous and therefore impractical, implausible duplication of glowing ability.

5) It is due not to birds at all but features non-living BOLs instead

Investigators of the unexplained will be well aware that all manner of anomalous non-living phenomena involving mysterious glowing balls of light (frequently abbreviated to BOLs or BoLs) have been reported from many parts of the world, and include spooklights, foo fighters, ball lightning, and min-min lights, as well as more familiar, scientifically-resolved examples like the will-o'-the-wisp or ignis fatuus (resulting from the oxidation of phosphine, diphosphane, and methane, compounds produced via organic decay in marshes, bogs, and swamps). So might reports of luminous birds in reality involve BOL phenomena and not feature birds at all? Whereas it is certainly possible that some may have done, examples of such entities being shot down and found to be birds obviously cannot be explained away like this. Moreover, whereas it is true that the Haddiscoe sightings took place in marshes, where will-o'-the-wisp activity would not be surprising, others have occurred far from such terrain.

 

Coloured wood engraving of a will-o'-the-wisp in a marsh, by Charles H Whymper (© wellcomeimages.org/Wikipedia – CC BY 4.0 licence)

Equally problematic for a BOL explanation regarding luminous birds are those examples in which the luminous entities have been observed moving in an evidently conscious, self-aware manner. Relevant here is that in an exact reversal of the above-mentioned suggestion that luminous birds may be BOLs, many investigators of Australia's most famous unexplained BOL phenomenon, the mysterious min-min lights long encountered in Queensland, nowadays deem it more likely that these glowing enigmas are not of any meteorological or chemical-based origin but are actually living creatures, specifically barn owls, precisely because of the ostensible curiosity and inquisitiveness that min-mins demonstrate towards their human observers. Here is a prime example, as documented by me in my book The Unexplained (1996):

In the days of Australia's early European settlers, the Min-Min Hotel was a staging post between Boulia and Winton in western Queensland, whose best-known feature for the people living nearby were the ghostly balls of light that regularly flitted through the air, often white but sometimes changing colour. Still seen today and referred to as min-min lights, these are reminiscent of American spooklights and English will-o'-the-wisps, and display a marked if disconcerting tendency to follow and even taunt their perplexed observers.

For example: You Kids Count Your Shadows, a collection of Wiradjuri aboriginal lore and beliefs from New South Wales compiled by Frank Povah [and published in 1990], contains an account of a sheep drover who was checking his flock on horseback one evening when a blue min-min light appeared over his shoulder, and persistently followed him during his work. In exasperation, he chased after it, still on horseback, but was unable to catch up with it – until he gave up, and began riding home, whereupon the min-min cheekily appeared over his shoulder again!

 

 Man vs Min-Min – envisaging a rider in Australia's outback being trailed one evening by a min-min light (image created by me using Grok)

An alternative 'living entity' explanation for such sightings may be luminous insect swarms, which have also been suggested as explanations for certain UFO reports (click here for my ShukerNature article documenting this possibility), but a curious, inquisitive barn owl, especially if encountered while out hunting at night, could surely explain at least some min-min reports.

 

Reading back through my analysis of the five suggested solutions presented here, I think it most likely that as with so many other mysterious phenomena, luminous birds may not involve just a single solution but instead features a combination of different ones, with some cases resolved by one solution, certain others by a second, and so on. For it is abundantly clear that none of the solutions individually provides a comprehensive explanation for all of the cases documented here.

 

It is sad that such a captivating phenomenon as luminous birds has fallen out of scientific favour in modern times, especially as science is now equipped with so much readily-available sophisticated technology with which to investigate it thoroughly. Of course, this is due in no small way to the equally sad scarcity of reports nowadays. Saddest of all, however, as noted by David Clarke in his Fortean Studies article chronicling the luminous owls 'flap' reported in Norfolk during the early 1900s (referenced by me in Part 1 of this review), is that this scarcity may well be due in turn to how much rarer, as a result of habitat destruction and poisoning by pesticides, the barn owl has become in Britain and elsewhere during the century or more that has passed since the Norfolk 'flap'. Then again, if the numbers of this species, now extensively protected, do eventually re-attain their former level, perhaps this most delightful and whimsical of wildlife anomalies may once again attract the attention of professional and amateur enthusiasts and eyewitnesses all over again, back in fashion at long last.

 

Close encounter of the glowing kind!

Finally: worth noting here is that phosphorescent bacteria were declared the official answer to the anomaly of a leg of lamb that glowed in the dark and which had recently been purchased in the Worcestershire town of Kidderminster, England, during spring 1988. As reported by the Sandwell Express & Star newspaper on 12 March 1988, when the discovery was first announced there were fears of Chernobyl-derived radioactive fall-out from its nuclear power station's explosion two years earlier. However, Hereford-Worcester's county analyst and scientific advisor Geoffrey Keen rightly rejected this melodramatic notion in favour of phosphorescent bacteria being responsible, thereby solving with Sherlockian skills of deduction the curious case of the luminous leg of lamb.

If you haven't already done so, be sure to check out Part 1 of my luminous birds review article here on ShukerNature.

NB – All images of luminous owls included here were created by me using Grok X1.

 

More close encounters of the glowing kind!

 

 

THE LIGHTBULB LIZARD OF BENJAMIN SHREVE - ILLUMINATING A HERPETOLOGICAL CONTROVERSY FROM TRINIDAD

Artistic representation of the Trinidad luminous lizard's possible appearance when glowing, as based upon Ivan T. Sanderson's claim (© Philippa Foster)

Among the fishes and several different taxonomic groups of invertebrate (including comb jellies, cnidarians, molluscs, insects, centipedes, millipedes, crustaceans, and annelid worms) are many bioluminescent species. That is, living creatures which actively carry out chemical processes to produce and emit light.

Officially, however, there are no bioluminescent species among the terrestrial vertebrates - but claims have been made that there may in fact be a notable exception of the reptilian kind.

I first learnt about, and then duly investigated, this fascinating yet surprisingly little-known case back in the mid-1990s, and here is what I uncovered at that time, followed by the extraordinary revelations that have occurred since then – yielding in this present ShukerNature blog article of mine the most comprehensive account ever published online.

A selection of fully-confirmed bioluminescent creatures depicted in a vintage illustration from 1890 (public domain)

In March 1937, during an animal collecting trip to the West Indies, American zoologist and cryptozoologist Ivan T. Sanderson visited Mount Aripo (aka El Cerro del Aripo), at 3,084 ft high the loftiest peak in Trinidad and part of this island's Northern Range. He had been capturing some freshwater crabs in a series of dark subterranean pools there when he suddenly spied a faint light in a crevice beneath a ledge. The light promptly went out, but Sanderson was curious to discover its source, so he flashed his torch into the crevice - and was most surprised to find a small lizard.

Attempting to coax it into his net, Sanderson gently tickled the lizard, but instead of running out it turned its head away - and as it did so, Sanderson was very startled to see both of its flanks momentarily lighting up "...like the portholes on a ship". When he finally succeeded in capturing it, this remarkable reptile lit up again, glowing brightly in his hand with a pale greenish hue that Sanderson subsequently likened to the glow produced by the hands and figures of a luminous watch.

As zoologists were previously unaware of any bioluminescent lizards, Sanderson was very thrilled by his discovery, which he documented in his book Caribbean Treasure (1939). Ironically, however, apart from its unique glowing ability the lizard, which was a male, seemed relatively nondescript in general appearance - with a long tail but short legs, a sharply-pointed muzzle, dark brown upperparts, and rosy salmon-pink underparts (turning yellow under its head) surfaced with large rectangular scales of plate-like form.

Ivan T. Sanderson's book Caribbean Treasure (© Viking Press, reproduced here on a strictly non-commercial Fair Use basis only)

Its only distinctive features were its body's lateral eyespots or 'portholes', constituting a series of large circular black blots running from the neck to the groin on both flanks, because each of these blots contained a vivid white bead-like spot. And it was these spots that were the source of the lizard's apparent luminescence, as determined by Sanderson during some basic experiments:

We made it [the lizard] hot and cold, and moist and dry alternately; we blew a loud whistle in its ear, we tickled it, and we subjected it to flashes of bright light...This creature seemed to produce its light in response to sudden emotional disturbance, rather than through actual physical reactions...The loud whistle, sudden winds, and flashes of light greatly agitated our lizard, causing it to switch on its 'portholes'. We noticed that this light was much brighter the first time it was switched on after the animal had been quiescent for a period, and more especially after it had previously been subjected to intense illumination.

Eventually, Sanderson shipped off his amazing little lizard to the British Museum (Natural History) in London, where it was studied in detail by fellow zoologist H.W. Parker. It was found to belong to a species already known to science (indeed, Parker himself had formally named and described it in 1935), but only just. An exceedingly rare member of the tejid (aka tegu) family Teiidae, and normally measuring 11-15 cm long, it was called Proctoporus (=Oreosaurus) shrevei (in honour of the very gifted American amateur herpetologist Benjamin Shreve), and had hitherto been represented in scientific collections only by a single preserved juvenile and one preserved adult female. Sanderson's specimen was therefore the first male of this species to have been brought to scientific attention, and until now no-one had suspected that it may be bioluminescent when alive.

Proctoporus shrevei (copyright holder presently unknown to me despite my having made considerable efforts to discover this; reproduced here on a strictly non-commercial Fair Use basis only)

During his visit to Trinidad, Sanderson collected seven more individuals of this species, and as preserved specimens these too were examined by Parker. In a paper published by the zoological journal of London's Linnaean Society in 1939, Parker revealed that it was sexually dimorphic, with only the males sporting the distinctive 'porthole' markings (a further reason why no bioluminescence had been reported from the specimens procured prior to Sanderson's), and that in every porthole the epidermis of the white bead at the centre was less than half the thickness of the epidermis of the black ring surrounding it. In addition, the white bead's epidermis was transparent, lacking any form of pigment. In other words, each porthole literally constituted a black-edged circular window.

How the portholes functioned, however, remained a mystery, because Parker found no associated nerve endings or an increased blood supply, thereby eliminating any likelihood that they were directly connected with the sensory or circulatory systems. Nor did he find any ducts connecting them with the exterior, or any complex lenses or reflecting structures.

Whatever they were, therefore, these portholes were clearly very simple in structure, and Parker offered three possible explanations for their luminosity. In life, the portholes may contain some substance that either glows when it breaks down (the principle of bioluminescence in various fishes), or glows when exposed to light (as with the paint used in luminous watches). The third option is that the transparent central beads of the portholes are underlain with reflective tissue. (A fourth possibility, that the portholes contain glowing bacteria which create their luminosity, can be rejected, because Parker did not report the presence of any bacteria within them.)

Ivan T. Sanderson as a young man (copyright owner presently unknown to me despite considerable searches made; reproduced here on a strictly non-commercial Fair Use basis only)

Inevitably, the prospect of a luminous lizard duly attracted attention from several other zoologists, who studied specimens of P. shrevei and various related tejids to find out whether any of them really did glow - but none of them did! And so in 1960, reporting at some length in the journal Breviora their own negative findings with P. achlyens from Venezuela and Neusticurus [now Potamites] ecpleopus ocellatus from Peru (both of which possess porthole markings resembling those of P. shrevei), American biologists Drs Willard Roth and Carl Gans rejected Sanderson's claims regarding P. shrevei's bioluminescence.

Yet Sanderson was an extremely experienced field zoologist, and Parker's histological studies convinced him that the portholes were genuine luminous organs. So who was correct? If P. shrevei were the only bioluminescent species, this would of course render worthless any comparative studies with related species. Moreover, at the time of my own initial examination of this case, only one zoologist other than Sanderson had actually investigated luminosity with living P. shrevei specimens, and he may simply not have stimulated them sufficiently for them to light up. (I subsequently found out that this latter zoologist was Prof. Julian S. Kenny – see later.)

My above account presents the situation concerning Trinidad's intriguing 'glowing lizard' that I had uncovered during my mid-1990s investigations. Since then, however, much additional information has come to light (pun intended!), and, as I discovered after unearthing it, this extra data includes some very significant new insights into P. shrevei and its alleged bioluminescent capabilities.

Map of Trinidad, highlighting its Northern Range, where Mt Aripo is (public domain)

First of all, it is nowadays deemed not to be a true tejid, so it is housed within a separate taxonomic family, Gymnophthalmidae, which contains many species. These are sometimes referred to as microtejids, because they are smaller than true tejids. Also, they tend to be quite skink-like in appearance, with certain species possessing reduced limbs.

I was pleased to learn that following further field studies, P. shrevei is no longer considered to be as rare as previously claimed. Indeed, the IUCN officially categorises it as being of Least Concern, and the IUCN Red List website states: "...although the distribution [of this species] is limited (with an extent of occurrence of 210 km²), the population trend appears to be stable, there are no current threats, and it occurs in at least two protected areas".

In addition, the IUCN assigns this species to the genus Riama, although quite a few other authoritative sources checked by me retain it within Proctoporus (so I shall do the same here for text consistency purposes), and refers to it via a very memorable common name that ties in with its supposed abilities – Shreve's lightbulb lizard. But is this name warranted?

Mark O'Shea lecturing at West Midlands Safari Park, England (© Ghaly-Wikipedia – CC BY-SA 3.0 licence)

During my original investigations of Shreve's lightbulb lizard in the mid-1990s, I communicated with the West Midlands Safari Park's internationally-renowned herpetological expert Mark O'Shea, famed not only for his numerous scholarly publications but also for his fascinating TV show O'Shea's Big Adventure in which he travelled the world seeking rare or unusual reptiles and amphibians. Mark was very interested in this mystifying lizard species, and I was delighted when he subsequently visited Trinidad to look for it. His search featured in 'Exotic Island', the tenth episode of his show's first series, screened in 1999.

After arriving in Trinidad, Mark and his camera crew teamed up with Caesar, a local guide, and with Dr Victor Quesnel (named as Quinnel in some reports), a retired Trinidad-based economic botanist who was also a very knowledgeable all-round naturalist (he died in 2014). But before they set off on their arduous trek in the hope of emulating Sanderson's original success in encountering this island's luminous enigma in 1937, they were able to chat with Javrien Capriata (aka Capriata Dickson), who had been Sanderson's guide back then, and was now over 80 years old (Sanderson himself had died in 1973). Happily, Mark's search proved successful too, as the team found two specimens, a male and a female. (Moreover, during a much later expedition in 2008, Dr Quesnel actually rediscovered the specific cave where Sanderson had captured his lizard in 1937 but which had not been found since then; it is now known as Sanderson's Cave.)

These two lizards were duly videoed in a dark room by Mark's cameraman while they were being illuminated artificially and for a time after the artificial illumination had been turned off. The video was then viewed closely to see whether there were any signs of luminescence from them. Not surprisingly, the female lizard did not glow, as it lacked the all-important porthole markings. Conversely, the male did indeed appear to glow for a short time after the illumination had been turned off. Unfortunately, however, it was not possible to determine whether this constituted bona fide glowing from the lizard, or whether it was merely a trick of the light caused by filming and the camera adjusting to the darkness after the illumination had been extinguished.

Ivan T. Sanderson in later years (© Dr Bernard Heuvelmans)

During the first half of the year 2000, I exchanged a series of letters with herpetological specialist Hans E.A. Boos from Port of Spain, Trinidad's capital, who is extremely knowledgeable concerning the reptilian fauna of this island. Needless to say, therefore, one of the subjects that we discussed was P. shrevei and its alleged bioluminescence. Hans was very sceptical about this, and even more so concerning the reliability of Sanderson's eyewitness testimony (it has to be said here that Sanderson was well known for exaggerating claims at times, although this behaviour may have been caused by a brain tumour that developed over time and apparently contributed to his relatively early death, aged just 62).

In one of his letters to me, dated 22 January 2000, Hans revealed that he had kept specimens of this lizard species in captivity for a considerable time but had never seen them light up. He also noted that both Dr Quesnel and Trinidad-based zoologist/newspaper columnist Prof. Julian S. Kenny had attempted to repeat the conditions reported by Sanderson but again had failed to achieve any success in stimulating the lizards to illuminate. In a subsequent letter, dated 14 April 2000, Hans mentioned to me that during the previous evening he'd had dinner with Dr Quesnel and had discussed fully with him the subject of P. shrevei. Quesnel had announced that he planned to try to collect a couple more specimens and this time arrange for high-quality histological sections to be prepared, with the tissues of the portholes properly fixed, in the hope of deducing something new regarding their supposed luminosity.

On 3 October 2004, the Trinidad Express newspaper published a short article written by Prof. Kenny that expanded upon Hans's comment concerning his investigations of Shreve's perplexing little lightbulb lizard. After referring to Sanderson's capture and claims regarding this species, Kenny revealed that American zoologist Prof. E. Newton Harvey (died 1959), a leading authority on bioluminescence, had once asked him to conduct an experiment to confirm his belief that Sanderson's claims were unfounded. The Harvey/Kenny experiment involved injecting some living specimens of P. shrevei with 1:10000, and 1:1000 doses of adrenalin. This treatment had already been shown to trigger light production in bioluminescent fishes, but it did not induce any reaction in the lizards.

Prof. E. Newton Harvey (public domain)

Also in 2004, what is acknowledged to be the defining scientific paper dealing with this contentious species' reputed glowing behaviour was published in the Caribbean Journal of Science. One of its three authors was Dr Quesnel, who revealed that, in fulfilment of his hopes expressed to Hans Boos in 2000, he had indeed succeeded in conducting further field investigations of P. shrevei, in May 2001 and again in May 2002.

Two male specimens were captured in rock crevices near to a cave entrance at the summit of Mount Aripo – i.e. the same general locality as Sanderson's own discovery. After examining them in the field, Quesnel took them to a field station for further investigation, where they were studied under light and dark conditions at different times of the day. Yet no observations, either in the field or at the field station, revealed any light emission from the portholes. The same was true with a third specimen that had been captured and studied previously by Quesnel. Clearly, therefore, they did not appear capable of bona fide bioluminescence, i.e. the active generation of light by living organisms via chemical means.

But what about the prospect that the portholes were highly reflective, or perhaps even phosphorescent? (That is, reflecting incident invisible light as visible light but over a longer time period than in fluorescence and without heat.)

Vintage illustration from 1904 depicting a further selection of known bioluminescent creatures (public domain)

To test this possibility, Quesnel directed high-intensity light from a xenon lamp at the lizards from varying angles. No light was emitted by the lizards, thereby demonstrating that they were not phosphorescent. However, light was readily reflected by their porthole (ocellar) scales. As Quesnel et al. explained in their paper:

...if P. shrevei is observed along the same plane from which light is directed, the normally obvious white ocelli cannot be seen against the reflection from all other scales. But, when viewed from an angle oblique to the light source, the ocelli appear brighter, while surrounding scales show no reflection. By varying the angle of reflected light, an illusion is created that the ocellar scales are intermittently emitting light, thus providing an explanation of Sanderson's original account of the lizard "switch[ing] on its portholes." The illusion produced by the reflective scales also explains recent accounts, as well as Sanderson's description of the white ocelli "remain[ing] plainly discernable in a darkened box when the rest of the animal was invisible." The ocellar scales reflect and intensify ambient light while the darker ground coloration renders the rest of the lizard invisible in a dimly lit environment.

It was also noted that the illusory effect of the reflective porthole scales was enhanced by a varying in intensity of the black pigment surrounding these scales, and that this varying of the black pigment's intensity appeared in turn to be dependent upon the lizards' stress levels - because it became darker when the lizards were first handled, but faded somewhat after several minutes. The black pigment surrounding the porthole scales heightened their reflective effect, making them look a brighter white:

When viewed immediately after handling the lizards, the ocelli appear to pulse or fluctuate in brightness as the surrounding pigment changes intensity. After a quiescent period, the ocelli are still reflective but do not appear as bright as when the surrounding skin pigmentation is darker. Again, this could explain Sanderson's description that light from the lizard "was much brighter the first time it was switched on after the animal had been quiescent for a period of time," and "after one brilliant display... it refused to shine with full brightness." The darker dermal pigmentation, presumably associated with higher stress levels during handling, heightens the reflective appearance of the white ocellar scales. Decreased pigmentation during inactive periods gives the illusion that the lizard is not producing light at full intensity.

In short, these studies appear to have comprehensively refuted Sanderson's claims that Shreve's lightbulb lizard is bioluminescent. Instead:

...the lizard's unique scales act like small parabolic mirrors, reflecting light at oblique angles. The intensity of this reflectivity is, in turn, influenced by the intensity of surrounding dermal pigmentation and by the angle at which a lizard is oriented relative to a light source. Thus, ocellar reflection produces an illusion that light is emitted by P. shrevei at varying intensities, a phenomenon which obviously has confused a number of persons.

Partial view of the Northern Range, Trinidad, whose Mount Aripo is home to Shreve's still-mystifying microtejid (image cropped) (© Sanjiva Persad/Wikipedia – CC BY 2.0 licence)

Even so, one major light-related mystery concerning Shreve's very surprising microtejid still remains unsolved. Namely, why has so remarkable a morphological feature as this lizard's parabolic mirror scales evolved in the first place, and why only in male specimens?

These are questions that the study of Quesnel and his co-workers did not seek to answer, although, as they did point out, P. shrevei is a reclusive nocturnal species that inhabits dark localities and whose behaviour in the wild is unknown – all of which make any attempt at speculation fraught with difficulty. Nevertheless, it occurs to me that in view of their sex-specific and also age-specific occurrence, perhaps these scales' light-reflecting abilities function as a means of visual communication by which adult males attract adult females for mating purposes. An alternative option is that this light-reflection ability is used as a defence mechanism, to startle or ward off potential predators, but if this were true, why do only males possess the necessary scales?

Clearly it is high time that some comprehensive field studies were conducted in relation to this small yet very thought-provoking lizard, neglected by science for far too long, in the hope of finally shedding some much-needed light (in every sense!) upon the currently cryptic purpose(s) of its unique parabolic portholes.

The present ShukerNature blog article is a greatly expanded and fully-updated version of a short account that appears in my book Mysteries of Planet Earth (1999).