Papagaios, Life & Times on a Speculative Archipelago - Kingrays (Part Two)

Extracts from Toledo S., Augustus P., Galvin D. 2017. ‘Large body sizes in Rhynchothrixinae and Papagotitaninae were driven by predator-prey selective pressures’ Ecology and Evolution.

The Papagotitaninae are a group of lizards that are endemic to the Papagaios Archipelago. Many of them grow to large sizes, with the largest in terms of body length, if not mass, being the macrocaudates. Historically, the largest macrocaudates, including those of the Corneops genus, ranged from 2m to over 6m in length, making them comparable in size to the Australian Megalania whose length estimates range from 5.5-7m (Fry, Wroe & Teeuwisse et al, 2009; Molnar, 2004).

It has long been thought that the papagotitans’ giant size was an example of insular gigantism, wherein individuals isolated on islands increase in size compared to mainland relatives, be this due to a lack of pre-existing predators or greater availability of food. As a result of this phenomenon, there is a temptation to claim any large island species is a result of insular gigantism, something recent studies are starting to reassess.

Hocknull et al (2009) determined that the Komodo dragon, currently found on several Indonesian islands, is in fact a member of a group of large monitor lizards, including the aforementioned Megalania and so had attained its size prior to arrival on the islands it currently reside on. Likewise, the giant tortoises of the Galapagos were discovered to be part of a trend of larger tortoises found in both island and continental biomes during the Pleistocene (Pérez-García et al, 2017) which only survived in the Galapagos due to their isolation and lack of human presence prior to the last five hundred years.

Prevailing beliefs state that Papagaian lizards are descended from small-bodied ancestors that rafted over from Europe in the late Eocene (Novais, 2004). Taking our lead from the above trend, we carried out new analysis of known macrocaudate fossils, along with those of other papagotitans to better understand when gigantism appeared and if it is ancestral to the group or has evolved multiple times. Cladistic analysis shows gigantism arose several times in Papagotitaninae and the macrocaudates were the last group to develop the trait. Further, we suggest that the large size of macrocaudates is a result of interactions between them and their primary prey species: birds of the Rhynchothrixinae group.

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Rhynchothrixinae are a group of kingrays, Papagaios’ only family of passerine birds. Soon after arriving, they split into three distinct lineages (Shaw & Penn, 1999): one leading to the Eukinkraminae waterfowl; one to the wading Gracilirostruminae and fast flying Trychocapinae; and one that contained the finch-like Pseudofringillilinae and the Rhynchothrixinae. The latter pair are united through their adaptation to herbivory, though the common ancestor of these latter groups diverged long ago.

While Pseudofringillilinae took to the buds, seeds and nuts of the trees, rhynchothrix favoured leaves, with fruit making up a smaller percentage of their diet. The earliest rhynchothrix was no larger than a crow but had a rounded beak with a sharp edge, not unlike a tortoise, suggesting the move to leafy vegetation happened quickly. Digesting leaves is nutrient poor and taxing on the digestive system. One can evolve a second stomach and specialised enzymes, like the hoatzin, or – and perhaps the easier option – one can grow large. Within a few million years, fossils show the average rhynchothrix was the size of a swan (Shaw & Penn, 2000).

Growing large has advantages, but makes flight more costly. With leaves abundant, rhynchothrix appear to have become flightless by 30mya. Were there giant, predatory lizards roaming the islands this would surely be an unfit lifestyle. As it happens, new fossils discovered on Milagres suggest the largest macrocaudate at this time was no more than 80cm long (Novais, 2011).

[Note 5: ‘Rhynchothrix’ comes from ‘rhynchos’ beak and ‘thríx’ whisker. In this group, the tuft of feathers above the beak of all kingrays has become a long drooping moustache. This is more common in males, who are also more colourful and larger-bodied, but some species have moustached females too.]

Our analysis of both rhynchothrix and macrocaudate fossils suggest that it was the presence of a large, flightless herbivore that encouraged the evolution of larger predators. There is a gap in the fossil record in the mid-Oligocene, but on one side we have metre long macrocaudates and swan-sized rhynchothrix and on the other, 2m predators and birds approaching 1.2m tall – similar to a small cassowary.

[Note 6: if the size of the lizards seems large in comparison to their prey, remember they are around half tail.]

Unlike other large birds such as the dodo or moa which quickly succumbed to new, external predators, the rhynchothrix had time to adapt to this internal pressure and did so primarily by growing larger and faster. By the late Oligocene, the larger species approached 2m tall. Compared to the similarly-sized ostrich, their necks were shorter and thicker, leading to a larger torso containing a massive, leaf-fermenting stomach.

Since their evolution, many species of rhynchothrix evolved with differences unique to their island and biomes. While all species are social, it is common for birds on the grasslands to live in flocks of ten or more, comprised of two or three related couples and the offspring of the past few seasons. Within these flocks, one bird acting as lookout can see far and protect many. The forest birds meanwhile live in pairs, the young leaving as soon as they are able. This may be due to the denser forests provide more obstructions and disincentivise this lifestyle (Toledo, 2009).

Regardless of their habitat, rhynchothrix are ecological engineers. The flocks of the grasslands trim back any saplings that grow too proud and keep the forest boundaries in check. Meanwhile the woodland species do considerable work in keeping areas of the forest floors clear of smaller plants, reducing predator hiding places in the process. The paths they create between these glades were beneficial to early settlers and in some cases formed the basis of roads and paths still in use today (Spot, 2014).

Further, their tenacity also played a role in the development of mutualism between the clinker shrubs and their protective crabs. There is also evidence that by eating fruit and nuts, they directly created many of the forests they later managed (Ntse, 2015) …