Tuesday, December 16, 2014

Plesiosaur Machinations II: The Social Sauropterygian

I find persistent natural history myths, anecdotes, and adages a topic of special interest. These  narratives, stories, adages and anecdotes - usually unsupported by detailed field work and not part of the technical literature - are usually just that, fictional stories, and easily dismissed. However sometimes such popular stories, especially when sourced from people who spend much time in the company of the animals in question, may offer some element of truth. For instance I was talking to a friend of mine who regularly goes deep sea fishing and he told me he prefers to use squid as bait - because brown pelicans (Pelecanus occidentalis) assiduously avoid eating squid. I investigated further and came across an interesting video on youtube suggestive of such avoidance and then went to talk to some squid fishermen at the local harbor (in recent years the squid fishery has been thriving in California waters). The squidders all assuredly and confidently replied no - pelicans do not attempt to pilfer or eat the squid from their catch. Gulls yes, pelicans no. Why is this? The fishermen thought is was because pelicans do not want to consume the internal "pen" of the squid. But I found this argument unconvincing, brown pelicans eat whole fish with loads of bone after all. Brown pelicans, unlike the white pelicans in my last post, plunge from height into the water and engulf prey and water in their pouch. The brown pelican then has to drain out the water in its pouch before swallowing the prey. When a pelican engulfs a mouthful of fish - no problem - but if a pelican engulfs a mouthful of squid it's a different story. At this point the squid can latch onto the inside of the pouch with their suckers or even obstruct and block the throat of the pelican  and thus represent not just a nuisance but a real and obvious choking hazard. Again, nothing in the technical literature on this topic but maybe some element of truth to these anecdotal observations.

And now for another interesting adage regarding peculiar natural history behavior, this time from Australia, and that is that abundant lizards, specifically larger skinks, can keep snakes out of an area. And you can find this sentiment repeated at several places on the interwebz. At first glance it seems that it should be easily dismissed. Indeed one can offer the argument that the opposite is more likely - abundant lizards should attract snakes as potential prey. Never the less, can this story offer some element of truth? The answer is yes, yes it can.

King's Skink chasing Dugite

The King Skink: Snakes Beware!!

Lesley Jackes. communal King's Skinks

Sociality in lizards: family structure in free living King's Skinks Egernia Kingii from southwestern Australia (C. Masters & R. Shine)

Abstract. King's Skinks Egernia kingii are large viviparous scincid lizards from southwestern Australia. Although some other species within the genus Egernia are known to exhibit complex sociality, with long-term associations between adults and their offspring, there are no published records of such behavior for E. Kingii. Ten years' observations on a single family of lizards (a pair of adults plus six successive litters of thier offspring) in a coastal suburban backyard 250 km south of Perth also revealed a very stable adult  pair-bond in this species. The female produced litters of 9 to 11 offspring in summer or autumn at intervals of one to three years. In their first year of life, neonates lived with the adult pair and all the lizards basked together; in later years the offspring dispersed but the central shelter-site contained representatives of up to three annual cohorts as well as the parents. Adults tolerated juveniles (especially neonates) and their presence may confer direct parental protection: on one occasion an adult skink attacked and drove away a tiger snake Notechis scutatus that ventured close to the family's shelter-site. Although our observations are based only on a single pair of lizards and their offspring, they provide the most detailed evidence yet available on the complex family life of these highly social lizards.

So, some lizard curious citizen-scientists decided to go all Jane Goodal on a family of King's Skinks living in their backyard and they document a monogamous, multi-generational, highly social, cohabiting, and offspring defending lizard!! And observed an adult attack and drive away a highly venomous tiger snake!! From the paper, regarding predation and the tiger snake interaction:

We have not observed predation nor found carcasses of skinks within the backyard. However, the lizards responded rapidly to birds flying overhead, with the smaller skinks fleeing first while the adults often remained in place on their basking sites. The skinks showed no overt reaction to bobtail lizards Tiliqua rugosa that occasionally moved through the backyard but we witnessed one vigorous interaction with another reptile species in April 1999. The larger adult E. kingii was found entwined and rolling over and over with a tiger snake Notechis scutatus only slighly longer than itself. When the two reptiles separated, the snake fled with the skink in hot pursuit. The skink later reappeared with no overt injuries.

Now, there are lots of neat stuff in this paper that will be revisited later but jeez, that tiger snake interaction was pretty cool. Going further from the discussion section:

The observation of an adult E. kingii fighting a tiger snake is particularly interesting. The snake was too large for the lizard to ingest (indeed, adult E. kingii are primarily herbivorous: Richards 1990) and the lizard was too large for the snake to ingest. The obvious inference is that this behavior constituted direct parental protection of offspring (which were ingestible-sized for such a snake, and would be acceptable prey for this snake species: Shine 1987). We are unaware of any previous example of such behavior in lizards or snakes, apart from an increase in aggressive responses to an intruder by postpartum rattlesnakes (Shine 1988; Green et al. 2002; see also anecdotal reports for other viperid snakes, summarised by Greene et al. 2002).

And if you want to see a series of aggressive attacks by a large King's Skink on another venomous snake called a dugite (Pseudonaja affinis) check out the two clips below. Filmed on a beach in Australia: my favorite part is the view of the barely interested Aussies looking on like, "Oh a giant lizard battling a venomous snake on the beach? I thought you were looking at something we don't see every day!!"

In both instances the interaction was clearly not about food, competition or territoriality - the inference to make is that the snake represented a threat to the progeny of the King's Skink. So although lizards in general won't keep snakes at bay here are at least two instances that might support the notion that keeping a stable of King's Skinks in your backyard or on your beach might help keep the local snake population slithering along to other places...

Besides the benefits of parental protection, the progeny of these sociable skinks also are privy to choice basking and shelter sites which may be at a premium in their environment. Looking at another sociable skink, the great desert skink  (Egernia kintorei), also viviparious, we see direct benefit given to offspring in the form of residence in extensive burrowing systems that extend up to 13 meters wide, have over 20 entrances, and offer refuge from predators and thermal extremes.

Lizards Cooperatively Tunnel to Construct a Long-Term Home for Family

All right so we can see that there might be some linkage between live birth, harsh conditions, and lack of critical refuge sites and theses factors are good Darwinian reasons for the unconventional social arrangements of skinks of the genus Egernia. And such behavior has even been suggested to be plesiomorphic for the group as a whole. Are there any examples of social lizards that give birth to live young and live in harsh environs that might offer a more independent test? Well yes, yes there is: the desert night lizard (Xantusia vigilis). The discovery of said animals' social behavior is of some interest to me because the researchers are from my alma mata UCSC, and also it is a California species.

Photo credit Mitch Mulks. Desert Night Lizard
Family Ties Bind Desert Lizards in Social Groups. Science Daily. 
Paper Here

Evidently this fairly small, viviparous lizard is extremely sedentary and usually lives most of its life in a fairly small home range. During winter months parents and offspring cohabited under highly localized fallen Joshua Tree logs and other vegetative debris. Offspring delayed dispersal from 1 to 3 years and remained in close proximity to parents although they foraged for themselves. Theses nuclear family aggregations were shown to be stable over several years although the exact benefit is unknown. But the researchers did highlight the link between live birth and social behavior suggesting kin-based sociality is a by product of live birth.

From the Science Daily article: According to (Alison) Davis (lead author), about 20 lizard species are thought to form family groups, and only two of those lay eggs. Viviparity (live birth) is crucial for the evolution of cooperative behaviors, she said.

"Viviparity provides the opportunity for prolonged interaction between the mother and offspring, which predisposes the animal to form a family group," Davis said. "The importance of parent-offspring interaction fits with what is currently understood about evolution of family groups and cooperative behavior in birds and mammals."

Although only recently revealed, viviparity neatly bookends the evolution of sauropterygian evolution in the Mesozoic. A Nothosaur from the middle Triassic and a polycotylid from the late Cretaceous both provide solid and iron-clad evidence of live birth in plesiosaurs and that the adaptation goes back very early in the lineage. Furthermore, the relative large size and single offspring in utero for the polycotylid suggests a very strongly developed k-strategist. This species at least does not appear to have been birthing high numbers of relatively small, live offspring - but investing heavily on a single, large offspring.

O'Keefe & Chiappe 2011

"Many of the animals alive today that give birth to large, single young are social and have maternal care. We speculate that plesiosaurs may have exhibited similar behaviors, making their social lives more  similar to dolphins than other reptiles." Robin O'Keefe, one of the authors of the paper: Viviparity and K-selected life history strategy in a Mesozoic marine reptile. Science 2011.

It should be noted that this view of social plesiosaurs has been met with skepticism. Sharks, which are  also viviparous but don't practice extended maternal care, may be better analogs, some have offered. However no sharks give birth to a single offspring. Plesiosaurs were diverse animals, can we really assert sociality as a whole for the family? Here I partially agree, there was likely a range of smaller offspring-larger brood-less maternal care to larger offspring-smaller brood-more maternal care/extended family bonds among plesiosaurs. But what we are trying to get at is a baseline from which behaviors may deviate towards other extremes. And I think the baseline for aquatic sauropterygian is strongly skewed towards maternal care and sociality. But what about the abundance of immature plesiosaur/elasmosaur recovered from coastal/lagoonal/freshwater deposits/ Could the mothers have been depositing them there and letting them mature on their own in such habitats? Possibly, but various cetaceans such as grey whales/humpback whales are known for utilizing warmer/nearshore/lagoonal habitats to birth their offspring. With occasional stillbirths and/or predator attacks at this vulnerable stage of life we should expect more immature individuals to be represented in such habits.

Of course science, like politics, is a funny thing is that it takes a compelling amount of evidence and effort to overturn previous - even dogmatic - sentiments. And even when this evidence arrives it does not take root immediately but may take several years or even decades to gain traction (case in point feathered dinosaurs). Sometimes it requires the most vocal opponents to well, how should I put it, to die... Who gets to decide what is the most conservative, parsimonious behavior for a group of extinct animals? Plesiosaurs are reptiles and the overwhelming majority of reptiles practice minimal to non-existent parental care. However when we invoke live birth, especially of a single large offspring, the balance of power shifts a bit with a number of viviparous reptiles offering evidence of parental care. When we invoke social behavior and we see that the overwhelming majority of kin based/social reptiles are also viviparous, k-strategists this further speaks to high levels of sociability probable in plesiosaurs. For me the implication is clear - like the skinks and desert night lizards discussed above plesiosaurs were social animals and invested in their offspring (and I am not the first to suggest this). Trying to retrieve a "typically reptilian" social, reproductive, and ecological adaptation for plesiosaurs has become the more untenable stance to take in my view.

Although the obvious comparison to make is between highly social mammalian toothed whales and plesiosaurs I do not think we need to invoke large brained mammalian analogs over what we can glean from contemporary social reptiles such as Egernia and Xantusia. And again, I am not the first to suggest this - O'Keefe: "a more helpful comparison (than mammals) to the plesiosaur may be the monkey skink or the shingleback skink in the Egernia group. These green, scaly lizards give birth to live offspring, one or two at a time, and are among the few known reptiles to function within a social group and care for their young." In short we do not need to try and resolve how a relatively small brained, limited intellect reptile such as a plesiosaur engaged in elaborate social behavior - several extant reptiles offer the context for how such an animal may have behaved socially, and they do not require delphinid levels of intelligence to do so.

Going back to the examples of the King's skink, desert skink, and desert night lizards which I discussed earlier I find it important to highlight another factor that may have helped push these lizards into the social realm and that is that critical microhabitats - shelter sites, basking sites, burrows - are at a premium and by living in extended families there was a net genetic benefit to the lizards that cohabited these areas with their kin. Juveniles did not have to disperse until several years of age to new sites and therefore minimized predation risk. The island night lizard depends on highly localized fallen joshua trees/yucca clumps/piles of dead vegetation to seek shelter and food under; the King's skink dependent on shelter sites/basking sites; and the desert skink on large, complex subterranean burrows. Taking these observations and applying the idea of critical refugia to plesiosaurs presents a problem; there is no where to hide in the open ocean. And so, with the observation of the venomous snake wrangling King's skink in mind, I am going to piggyback on that observation and suggest that plesiosaurs were not fleeing from the first sign of danger - but standing their ground, facing, and fighting - en masse - any potential predator. They moved aggressively in phalanx formation to danger. This is the paradigm of the earlier depictions of plesiosaurs I alluded to in my last post where I suggested that these earlier renderings of plesiosaurs - battle ready, pugnacious, and ready to throw down with any other denizens of the deep - offer >much more truth< than more contemporary works that seem to always portray plesiosaurs as the ill-equipped cannon fodder of the much larger, dominant, and aggressive predators they lived with. Pictures like this one, this one, this one, this one, even birds, and this one. It seems everywhere you look plesiosaurs are getting their asses handed to them, usually with those poor, dangly "oh so vulnerable" necks taking the brunt of the abuse. And yes we do have evidence of plesiosaurs being scavenged by sharks, a Tylosaurus swallowing a polycotylid (not even a long necked plesiosaur), and some suggestive tooth marks on the skull of Tuarangisaurus possibly from a pliosaur (Sven Sachs, 2004 Tuarangisaurus australis... Memoirs of the Queensland Museum available online). And I know I read about possible mosasaur damage to a Mauisasaurus but can't find the ref?... But as far as I can tell no suggestion of neck trauma stemming from predation among all those long necked forms....

And I will suggest that, when in phalanx, plesiosaurs were no easy pickings. Even very large predators will give up the hunt when the element of surprise is lost, their prey detects them, and has the armament to fight back. A Hainosaurus that drives home the attack on a phalanx of elasmosaurs and loses one or both eyes in the process has substantially lowered its own value in the  Darwinian sweepstakes. Lions often back off from herds of cape buffalo that stand and fight. Crows mob and drive off eagles. Sea lions will mob and harass great white sharks. And King's Skinks drive off venomous snakes.

Here I depicted a phalanx ( I am just going to start using that term instead of pod and hope it catches on, thanks Bk Jeong) of indeterminate Microcleidus sp. facing down and nipping at an intrepid Rhomaleosaurus sp. As you can see any headlong rush into such a group is going be a painful experience with no guarantee of capture. Furthermore, like herons when cornered and unable to fly, there is no reason that plesiosaurs could not have targeted the eyes of would be assailants. And when visually dependent predators lose an eye, not a good thing. Plesiosaurs were most vulnerable from the rear and you will notice I gave their stumpy, skink like tails eye spots. Furthermore the very solid and almost carapace like constuction of plesiosaur torsoes offered additional protection from rearward attacks. Microcleidus is a pretty cool but seldom discussed plesisoaur. It was basically an elasmosaur before it was cool to be an elasmosaur. Only about 3-4 meters long it had an exceptionally elongate neck, small head, but that head and jaw was nasty.

Microcleidus from Brown, Vincent, Bardett 2013

And below I depicted some more well known characters - a generic elasmosaur type plesiosaur family faces down a generic mosasaur stalking one of the youngsters.

As I went over in my last post, Introducing the Plesiosaur Phalanx Attack, a strong social adaptation in plesiosaurs offers a compelling foraging strategy. And such a social framework also offers a strong defensive strategy. Furthermore, by invoking this defensive adaptation a lot of the problems in plesiosaur anatomy that suggest high vulnerability to predation - long vulnerable neck, low maneuverability, low swimming speed - are squashed. The neck and head are not the most vulnerable spots in this scenario but instead the torso, rear flippers, and tail are. A large, plump tail would serve as a dispensable organ and may have been covered in eye spots or even shed (highly speculative?!?) to distract predators like many modern lizards. And the torso is heavily reinforced with a bony wall of ribs, gastralia, shoulder and pelvic girdles. Instead of a paradoxical animal, vulnerable on all levels to predation, seemingly outside the bounds of Darwinian evolution - a much more dynamic, capable, and combative animal emerges.

In short, a rare example, where the early artists got much more right in some regards than contemporary artists.

Édouard Riou from the 1867 edition of "Journey to the Center of the Earth"

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BK said...

Thanks for the shout-out

Anonymous said...

Interesting idea.
But if Microcleidus can deploy a phalanx as a means of defense, what prevents Rhomaleosaurus from deploying its own phalanx or some other form of group attack on the Microcleidus?

Duane Nash said...

Could be... numbers on numbers.

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