I’ve said it before and I’ll say it again: we keep coming back to the subject of flightless bats. Besides fictional future predators and night stalkers, there never have been any flightless bats so far as we know.
Whenever this subject is discussed however, we have to pay appropriate homage to the most strongly terrestrial bats that we know of: the vampires, and the short-tailed bats (or mystacinids). Vampires were done to death here earlier on in the year (go here)… now, at last, it’s the turn of the mystacinids.
New Zealand has – or had – more than its fair share of neat tetrapods, some of which I’ll get round to covering on the blog some day. Moa, kiwis, giant geckos, tuataras, weird rails, big flightless geese, and so on. Of course we always thought that – besides a few cryptids (notably the amphibious or aquatic waitoreke) – Cenozoic New Zealand was devoid of non-volant mammals. We now know that a relict lineage of mammals phylogenetically close to multituberculates (Worthy et al. 2006) inhabited New Zealand until the Miocene. Among the other mammals, there were bats: two of which, the mystacinids, represent a group that was long thought unique to New Zealand. That’s no longer the case, as several species of mystacinid are now known from the Oligocene and Miocene of Australia (Hand et al. 1998, 2005): in fact, based on current evidence it seems that mystacinids originated in Australia before one lineage later spread to New Zealand [image above from here].
Uniquely specialized for fast and agile movement on the ground, mystacinids are primarily insectivorous but have recently been shown to be nectar and pollen feeders as well. They possess particularly tough and leathery wing membranes which they’re able to fold up tighter than any other kind of bat. It used to be thought that it was the absence of small native land mammals (such as mustelids, marsupials and mice) that uniquely enabled mystacinids to exploit the role of terrestrial and arboreal forager. However, read on. Mystacinids are also uniquely adapted for crevice-dwelling and have short, erect, velvety fur that helps them to squeeze through tunnels in wood and soil (Kunz 1982). Tomes (1857) compared it to that of shrews and Daniel (1979) noted that some molossid bats that also hide or shelter in small crevices have similar fur. Mystacinids are active tunnelers in soft tree bark and chew out roosting cavities and tunnels – consequently their teeth are often heavily worn.
Mystacinids have robust hind limbs and feet (see image above: from here): the latter are broad and short with softly padded, deeply wrinkled soles and grooves across the bottoms of the toes, and across the lower surface of the ankles and legs. These wrinkles and grooves probably assist grip in climbing, as do the well developed, needle-pointed claws on the feet and the thumb. The thumb claw is especially remarkable as it has a small, subsidiary talon at its base. Daniel (1979) stated that the foot claws have these subsidiary basal denticles as well.
The most remarkable terrestrial adaptation of Mystacina is the fact that it can tightly curl up its wing membranes beneath the thick leathery membrane that lines the flanks and underside of the thigh, thereby protecting the delicate part of the wings from injury. Its arms now become two straight forelegs that can be used in scurrying on the ground and on trees: the resulting terrestrial agility has often been described as ‘mouse-like’. The second wing digit is notably reduced, consisting only of the metacarpal and a single tiny phalanx; a reduction that may have evolved to reduce the size and bulk of the wing. The third digit has three phalanges and a cartilaginous tip; in the fifth digit, the cartilage tip extends beyond the edge of the patagium. The propatagium is reduced and the plagiopatagium inserts at the level of the ankle.
Mystacinids are reported to be slow and less agile in flight than other microbats and they have been described as having proportionally short wings with rounded tips (Norberg & Rayner 1987). This wing morphology would be well suited for the slow-flying, gleaning habit mystacinids appear to employ. However, Arkins et al. (1999) stated that Mystacina wings are no shorter, nor are the wingtips more rounded, than would be expected in a microbat of their size. Arkins et al. (1999) also noted that the low wing loading and short echolocation calls typical of gleaning bats are not seen in Mystacina. Further work is clearly needed on the ecomorphology of these bats [image above from here].
The mystacinid tail is short (hence the common name of the group) and emerges through the dorsal surface of the uropatagium. When on the ground, the bats can fold up their uropatagium and tuck it away.
It is not only the strong adaptation to terrestriality that makes the mystacinids remarkable: Mystacina has proved to be an exceptionally diverse feeder, and has the broadest diet of any bat. It preys on volant and non-volant insects as well as pollen, berries and other small fruits, and has even been reported scavenging flesh and fat from dead birds (Daniel 1979). Most of the insects eaten are beetles, moths, flies and crickets, and together these prey make up at least 50% of the diet. Spiders and other arthropods are also taken and pieces of moss and bark have been discovered in Mystacina droppings (Arkins et al. 1999). Several anatomical features appear to correlate with this remarkably broad diet. The dentary is robust and the dentary symphysis is fused, the ascending ramus is tall, and the anterior toothrow has a crowded look, despite the fact that two lower incisors are absent. Fossil mystacinids from Australia (they’re united in the genus Icarops) have the same sort of jaw structure and dentition as the modern species from New Zealand, and are highly similar to them in postcrania as well. It therefore seems that the Australian forms were behaviourally and ecologically highly similar to the living species (Hand et al. 2005). To return to a point made earlier, this means that the unusual terrestrial adaptations of Mystacina did not evolve in the isolation of New Zealand. Rather surprisingly, these bats were apparently scurrying around on the ground and folding up their leathery wing membranes in environments where there were already numerous small terrestrial mammals. That seems odd, but it’s what the fossil record indicates [adjacent skull image from digimorph].
Mystacinids do not have a shortened rostrum and their premaxillary bones are not reduced as they are in some other groups of microbats. The tip of the muzzle projects beyond the lower lip and the nostrils are set in a small narial pad which supports numerous short, stiff bristles (Hill & Smith 1984, Koopman 1994). The nostrils are prominent, rectangular and oriented vertically. Their ears are long, fully separated (in some groups of bats the ears are connected by a flap or flaps of skin), slender, and with a long, pointed tragus.
Mystacina also exhibits a number of cranial specialisations for its nectarivorous diet. Its tongue is papillated and somewhat extensible with a small array of brush-like papillae at the tip, and there is a wide gap between the upper incisors. The total number of teeth has been reduced to 28 (a more typical microchiropteran count would be 34). Mystacinids therefore represent a third group of bats containing members specialised for a nectarivorous diet (the other two are pteropodids and phyllostomids).
In an effort to discover more about the ecology of the mysterious parasitic wood rose (Dactylanthus taylorii), a native flowering plant that grows on the roots of small trees, Chris Ecroyd of Landcare Research New Zealand Ltd employed time-lapse video recorders, a method that had previously been successful in revealing predation on Kokako Callaeas cinerea nests. Dactylanthus is peculiar in that it produces copious nectar and has a distinctive musky scent, yet apparently lacks adequate pollinators. Ecroyd filmed various insects, possums and rats visiting the flowers, but he was convinced that none of these represented the true pollinator. Relocating the cameras to film Dactylanthus at various new localities, he was eventually successful and filmed, of all things, Mystacina feeding from the flowers. Mystacina proved to be an important terrestrial pollinator of Dactylanthus, suggesting that the modern rarity of both organisms might be linked (Hunt 1992, Ecroyd 1993, Webb & Kelly 1993, Meyer-Rochow & Stringer 1997). The photo above is one of Ecroyd’s images of a Mystacina with Dactylanthus taylorii (from here).
That Mystacina was nectarivorous had already been determined by Daniel (1976) who had examined stomach contents and droppings, and had also smeared Mystacina chests with vaseline to see what material might stick to the bats while they were foraging. Daniel found that the bats visited a variety of flowers, presumably to feed on both nectar and pollen. Daniel (1979) also reported a captive Mystacina seen ‘excitedly’ crawling over flowers, becoming covered in pollen, and then eating the pollen off its own fur while grooming. This is a unique method of feeding for any bat (though it has not yet been reported from wild mystacinids). The pollen-feeding habit of the mystacinids mean that they join only two other bat groups (pteropodids and phyllostomids) known to feed on pollen.
Throughout this text I’ve consistently referred to mystacinids in the plural. Though mystacinids have a complex and confusing taxonomic history, it is agreed today that there were two species until recently. One survives: Mystacina tuberculata, first described by John Edward Gray in 1843. The second, M. robusta, has not been positively reported alive since 1965. And I’d love to say more, but I have to stop there.
Coming soon: well, actually, I’m going to stop making promises for a while, as the backlog is getting way too long. I will, however, mention more Triassic crurotarsans, more sebecosuchians, proto-narwhals, amazing social lives of green iguanas, pleurodires, and Mesozoic birds.
Refs – –
Arkins, A. M., Winnington, A. P., Anderson, S. & Clout, M. N. 1999. Diet and nectarivorous foraging behaviour of the short-tailed bat (Mystacina tuberculata). Journal of Zoology 247, 183-187.
Daniel, M. J. 1976. Feeding by the short-tailed bat (Mystacina tuberculata) on fruit and possibly nectar. New Zealand Journal of Zoology 3, 391-398.
– . 1979. The New Zealand short-tailed bat, Mystacina tuberculata; a review of present knowledge. New Zealand Journal of Zoology 6, 357-370.
Ecroyd, C. 1993. In search of the woodrose. Forest Bird 1993 (2), 24-28.
Hand, S., Archer, M. & Godthelp, H. 2005. Australian Oligo-Miocene mystacinids (Microchiroptera): upper dentition, new taxa and divergence of New Zealand species. Geobios 38, 339-352.
Hand, S. J., Murray, P., Megirian, D., Archer, M. & Godthelp, H. 1998. Mystacinid bats (Microchiroptera) from the Australian Tertiary. Journal of Paleontology 72, 538-545.
Hill, J. E. & Smith, J. D. 1984. Bats – a Natural History. British Museum (Natural History).
Hunt, R. 1992. Dark secrets revealed by security cameras. BBC Wildlife 10 (9), 12.
Kunz, T. H. 1982. Roosting ecology of bats. In Kunz, T. H. (ed) Ecology of Bats. Plenum Press (New York/London), pp. 1-55.
Koopman, K. F. 1994. Chiroptera: Systematics. Walter de Gruyter (Berlin/New York).
Meyer-Rochow, V. B. & Stringer, I. A. N. 1997. An honorary non-flying mammal pollinator. Trends in Ecology and Evolution 12, 277.
Norberg, U. M. & Rayner, J. M. V. 1987. Ecological morphology and flight in bats (Mammalia: Chiroptera): wing adaptations, flight performance, foraging strategy and echolocation. Philosophical Transactions of the Royal Society of London B 280, 335-427.
Tomes, R. F. 1857. On two species of bats inhabiting New Zealand. Proceedings of the Zoological Society of London 1857, 134-142.
Webb, C. J. & Kelly, D. 1993. The reproductive biology of the New Zealand flora. Trends in Ecology and Evolution 8, 442-447.
Worthy, T. H., Tennyson, A. J. D., Archer, M., Musser, A. M., Hand, S. J., Jones, C., Douglas, B. J., McNamara, J. A. & Beck, R. M. D. 2006. Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific. Proceedings of the National Academy of Sciences 103, 19419-19423.