Non-Newtonian Harvestmen

Harvestman (though not the Mitostoma chrysomelas of this post)

Photo: Dschwen (Wikimedia Commons) 

One of the most common cases of mistaken identity in the natural world is the colloquial classification of harvestmen as spiders. Fact: harvestmen are not spiders at all; arachnids, yes, but spiders, no. Harvestmen are a group unto themselves (order Opiliones), and are far more closely related to scorpions and pseudoscorpions than to spiders (despite their superficial resemblance). However, this kind of taxonomic milieu is nothing more than pedantic babble if you happen to be a springtail cornered by a harvestmen in the leaf litter. You see, to the average springtail, a harvestmen is death on stilts. Sticky death on silts.

Some interesting research has used high-speed videos to capture the harvestman Mitostoma chrysomelas hunting springtails. Mitostoma ensnare their extremely fast-moving prey using sticky setae (hair-like structures) on their pedipalps. Each setae bears a droplet of super sticky glue that has just the right chemistry and physics for capturing springtails. It has long been suspected that these droplets were used by harvestmen to hunt, but this study is the first to actually document it.

Springtails are one of those groups of hexapods that go more or less unseen. Most species are tiny, like the snowfleas (Hypogastrura nivicola) you may come across on a warm winter's day. Some of the larger southern hemisphere springtails approach a centimetre in length, but they, like their diminutive cousins usually remain out of sight, in the soil and leaf litter. They have some marvellous adaptations, not the least of which is the ability to launch themselves extraordinary distances using their spring-like furcula (the springtails' eponymous jumping organ). Most springtails can also shed liquids like a rubber raincoat, an important adaptation for living in wet soils. Both of these features, it would seem, are just the sort of adaptations that would keep springtails safe from predators like harvestmen, but that's not the case when it comes to Mitostoma.

 

Mitostoma glue is one liquid that springtails can't shed. Rather, the glue can stick to springtails without any trouble at all, establishing a firm hold that becomes ever more firm the longer and harder a springtail struggles. A springtail, with it's power-packed furcula, can put up a good fight, but for all the effort it exerts, the springtail only ensures itself a more certain doom. That's because Mitostoma glue becomes more viscous - it gets thicker, becoming a more effective trap - the faster it is pulled away from the setae by struggling prey. The pressure exerted by the distressed springtail causes the harvestman's glue to, in a sense, solidify. Liquids that posses this property are called non-Newtonian fluids: their viscosity changes when forces are applied to them. Some non-Newtonian fluids become more viscous, like when you stir gravy, while other liquids become less viscous, like when you shake ketchup out of a bottle. In both of these cases, the application of pressure to the fluid changes its viscosity. (Newtonian fluids, by the way, typically don't respond to pressure in these ways; think about splashing your feet in a puddle - there's no change in the water's viscosity.)

The non-Newtonian properties of Mitostoma glue make escape for springtails a highly unlikely outcome, should they be captured in the menacing pedipalps of a harvestman. It's all part of the incredible predator-prey relationships going on at the macro level around us all the time. I have often told students that if they are interested in predator-prey interactions they needn't spend fruitless weeks and months following Polar Bears (Ursus maritimus) on ice flows to see a rare dispatching of a Ringed Seal (Pusa hispida), or trek over endless kilometres of mountain wilderness in hopes of seeing a Cougar (Puma concolor) take a Bighorn Sheep (Ovis canadensis), but rather to begin by watching the countless invertebrate battles for life and death that surround them in their own backyards. Though small, the non-Newtonian harvestmen prove that the macro world is well worth watching.

Bromeliad Boroughs

A Giant Tank Bromeliad (Brocchinia micrantha) grows in front of Kaieteur Falls.

Photo: Philina English

 

At dusk, the humid air above Kaieteur Falls comes alive with swifts. First dozens appear, chattering, swooping, rolling, then hundreds, then thousands, zoom with incredible speed and coordination above the Potaro River Canyon. Three species nest and roost behind the falls: the globally rare White-chinned Swift (Cypseloides cryptus), the highly range-restricted White-tipped Swift (Aeronautes montivagus) and the common but fabulously large White-collared Swift (Streptoprogne zonaris). The flocks, in their energetic maneuvering seem to taunt the local Bat Falcons (Falco rufigularis) which sit near the canyon's rim, erect and stoic, looking to snag a wayward swift. After perhaps an hour of impressive display and ceaseless chatter, the swifts finally go to roost. By the dozens, small flocks break away from the main group, and skirt behind the veils of mist and braids of falling water. They cling to the sandstone cliffs in a most awkward manner, betraying themselves as birds of the air, seldom landing but to nest and sleep (and even then, some may sleep on the wing). So loud is the collective chattering from their roost, it can actually be heard above the din of roaring water!

The sunset, the waterfall and the swifts will offer a spectacular end to a day spent exploring the rainforests and savannahs of Guyana's Kaieteur National Park. But it's just the prelude to another show which is about to begin in the growing darkness. It's a show that is fascinating in the extreme even tough it takes place on a much less grand scale: entirely within the watery recesses of a peculiar plant, the Giant Tank Bromeliad (Brocchinia micrantha).

Giant Tank Bromeliads are enormous succulents, resembling a 2-3 m tall Pineapple (Ananas comosus) top. They are among the dominant plants in Kaieteur's savannah, lording over Yellow-eyed Grasses (Xyris spp.), tiny carnivorous Red Sundews (Drosera kaieteurensis) and a great diversity of fruiting Clusia shrubs. The so-called bromeliad savannahs are alive with shimmering orchid bees, enormous red and black grasshoppers, microcosmic armies of ants, and a pantheon of birds, such as the Guianan Cock-of-the-Rock (Rupicola rupicola) which display their fabulous orange costumes with absurd extravagance. But for me at least, the most interesting species is the little Golden Rocket Frog (Anomaloglossus beebei), whose association with Giant Tank Bromeliads represents a case of extreme specialization. Not only will you find Golden Rocket Frogs only on Giant Tank Bromeliads, but you'll also find them only in the vicinity of Kaieteur Falls.



Golden Rocket Frog (Anomaloglossus beebei), yellow morph.

Photo: Philina English

The entire world-wide range of the Golden Rocket Frog encompasses less than 20 square km and is centred right on Kaieteur Falls itself. It's no coincidence that these frogs have gathered around the falls like worshipers around an idol. Kaieteur Falls spills half a million litres of water over a precipitous 225 m drop every second (during its peak flow). This fantastic volume, making it one of the world's most powerful waterfalls, creates a permanent mist that blankets the rugged sandstone gorge below the falls, and periodically shrouds the cliff tops and savannah above it. The microclimate caused by Kaieteur's humidity is what allows Golden Rocket Frogs to spend their entire lives entirely within those over-sized bromeliads.

Again, recall the Pineapple top-like form of the tank bromeliad, but unlike a Pineapple the leaves are not stiff and armoured with spines, but are instead smooth and often gracefully arching. The broad bromeliad leaves radiate out from a central stalk, forming loose fractal-like whorls. In the axis of each leaf, right where it attaches to the bromeliad's central stalk, a small reservoir of stagnant water forms. Filled up by rain and the condensed mists of Kaieteur Falls, each of these eponymous tanks (or phytotelmata) is a world unto itself, supporting a wide assortment of organisms. The shady, slightly acidic waters are a favourite breeding site for mosquitos and midges. Bromeliad crabs, dragonfly and damselfly larvae hunt other bromeligenous inhabitants. The world's largest bladderwort, the purple flowered Humboldt's Bladderwort (Utricularia humboldtii), sucks up tiny invertebrate prey. And of course there are the frogs.

Like most other dart poison frogs (family Dendrobatidae), Golden Rocket Frogs seem to explode with colour; or rather, about half of them do. Anomaloglossus beebei is a polymorphic species, with yellow frogs found among a bromeliad's fresh green leaves, and brown frogs found among the dead and dying ones. Since each morph is strongly associated with a specific leaf colour it stands to reason that they derive some kind of advantage, likely camouflage, in their respective habitats. Juveniles actually change colour, appearing green during the night and yellow-green during the day, again presumably as a form of camouflage.

Golden Rocket Frog (Anomaloglossus beebei), brown morph.

Photo: Philina English

 

Regardless of colour, both males and females set up and defend territories around bromeliad tanks. With few exceptions, such as when females wander to find mates, they'll spend their whole lives within these territories. Although adult frogs may use the numerous phytotelmata in their territories for foraging or cover, the primary function of the tanks is to serve as nurseries. After mating, females deposit eggs on the underside of a leaf that overhangs a tank. When the egg hatches, the tadpole falls into the water where if everything goes according to plan, it will find a well appointed menu of detritus, algae and small invertebrates to dine upon. Kaieteur's generous mists normally keep water levels topped up and humidity high, but during times of low humidity, and in locations more distant from the falls, further parental care may be needed to combat desiccation. Male frogs will water their eggs by squirting them with liquid from their cloaca. When food is scarce, females will lay non-fertilized trophic eggs for their tadpoles to eat. Generally speaking though, the small yellowish pollywogs subsist on their own.

Finding these small yellow and brown frogs  among the whorls of bromeliad leaves can at first be difficult, but their calls, which resonate through the evening air (just as the swifts go to roost behind the falls), reveal their abundance. If there's one aspect of the Golden Rocket Frog's life history that has been studied (and can be easily observed) it's their calling behaviour.

In Golden Rocket Frogs, calls play two roles. First, males sing an advertisement song to attract mates. That's chiefly what you'll hear as you wend your way across the darkening savannah, watching your every step for Bushmasters (Lachesis muta) and other venomous surprises coiled beneath the bromeliads. Females seek out calling males, looking particularly for big boys with faster call rates than their neighbours. Like in many other dart poison frogs, courtship is somewhat elaborate, involving a little snuggling (tactile interactions, in the dry language of the herpetologist), a slow guided tour of the male's territory and finally the actual mating.

The second call type is one that territorial males sing to dissuade rival males from encroaching on their turf. Golden Rocket Frogs are highly territorial but their approach to it is nuanced. Males seem to exhibit a curious form of territoriality sometimes known as "dear enemy". In essence, a male gets to know the individual calls of all his immediate neighbours. After a period of sorting out territories, the neighbourhood finds itself in a state of more or less peaceful equilibrium, with each male generally respecting each other's claims. But when an unfamiliar frog enters the neighbourhood and begins calling, the local settled males react strongly, changing their calls from a more subdued version they use among their settled peers, to a more strident and aggressive version when an interloper is about. The reason for this is probably that an interloper may throw the whole neighbourhood into turmoil if he were to try to squeeze in a territorial plot of his own or usurp one of an already settled male. Should this happen it may be necessary for the whole process of staking out territorial boundaries to happen all over again, taking away energies from a more important activity - making new frogs.

That's not to say that Golden Rocket Frogs are entirely antisocial outside of the breeding season, rather they appear to engage in nonaggressive visits between territories. This kind of non-breeding sociality is somewhat unusual among frogs and the purpose of it is not at all understood. Clearly, Golden Rocket Frogs, like so many other tropical organisms, have a vast and unusual natural history just waiting to be discovered.

Bromeliad savannah habitat at Kaieteur Falls National Park

Photo: Mark Conboy