Posted by: Andrew Guerin | July 10, 2010

Do sea snakes anticipate the onset of tropical storms?

ResearchBlogging.org

Paul, the ‘Psychic’ octopus who can apparently predict* World Cup football match results, has inspired a lot of silliness attracted a lot of media attention recently. But can any other marine organisms see into the future?

Sea snakes of the genus Laticauda (Fig 1.) are not entirely aquatic – not only must they regularly return to the surface to breath, but they also reproduce on land. They hunt in relatively shallow water, searching among rocks and coral for suitable prey such as crabs and small fish which they paralyse using their venom; they are in fact among the most lethally venomous of snakes, but are not normally aggressive towards humans. They can be rather inquisitive though, which can lead to some unnerving moments for SCUBA divers (it can be quite a shock to notice a venomous snake swimming happily around your legs).

Laticauda_colubrina_Lembeh2Fig. 1. Banded sea krait, Laticauda colubrina (Image source: Wikimedia Commons)

Extremely rough weather can result in difficult conditions for organisms that inhabit coastal marine environments, particularly during tropical cyclones. As well as directly damaging habitats such as coral reefs, the strong winds, high waves, and storm surges could threaten mobile organisms with injury or stranding. In environments that are regularly subject to such perturbations, animals might be expected to seek shelter in rough weather, and sheltering behaviour would be most effective if it began prior to the onset of dangerously rough conditions.

While studying sea snakes in Taiwan, Liu et al. noticed that the numbers of sea snakes they observed among tidal pools and shallow waters declined sharply in the day or so prior to Typhoon Morakot in August 2009 (Fig.2); only one individual snake was spotted on the evening prior to the onset of the typhoon, compared to 20 or more per site on other dates. Once the storm had passed, numbers of snakes returned almost immediately to their pre-typhoon levels, making mass mortality of snakes during storm conditions seem an unlikely explanation for the decline.

There are a few possible explanations. For example, if conditions were worsening prior to the storm proper, this might have made it more difficult for the researchers to spot snakes in the field – which would mean that the observed decline was simply a sampling artefact. On the other hand, if the decline was real and not just due to difficulty spotting snakes, it could have been a response to increasing wave height, rather than anticipation of a coming storm. However, the exodus began before the sea state got rougher than it would on normal windy days. Indeed, analysis of existing data found no correlation between snake abundance and windspeed (which drives wave height). Rainfall, likewise, did not seem to be the trigger for the disappearance of the snakes.

 snakechart Fig. 2 (from Liu et al. 2010). Barometric pressure over sampling period, with numbers of snakes observed on sampling days (solid points) and rainfall (vertical bars). Typhoon Morakot occurred on days 38-40.

When snake abundance was plotted alongside data on barometric pressure, however, the decline in snakes seemed to correspond rather neatly to a drop in pressure recorded prior to the arrival of Typhoon Morakot (Fig. 2.). Observed numbers of snakes were significantly correlated with barometric pressure, suggesting that the drop in pressure provided an early warning of the cyclone’s approach; and this may have triggered anticipatory behaviours that allowed the snakes to survive the storm.

However, a quick and dirty analysis of the plotted data (extracted from Fig. 2. using DataThief) shows that there was no correlation between the number of snakes sighted and pressure under non-cyclone conditions (Fig. 3). This suggests that the relationship is more complex than a simple linear relationship between snake activity and atmospheric pressure; snakes may only have responded to dropping pressure once it fell below a certain threshold.

snake pressure Fig. 3. Potential relationship between barometric pressure and snake sightings; plot on the left includes all data, plot on the right excludes data from just before the storm, showing no correlation in ‘normal conditions’

This is a fairly preliminary piece of observational work, and there are a few flaws in the paper. For example, there is very little detail on the field survey methods used, and the paper is missing some other information that many would consider important to include. It’s still an interesting use of what looks like opportunistic data collected during a longer term project – and definitely worth reporting on. Ideally, a manipulative experimental study would allow a better understanding of the responses of these creatures to changes in surface pressure, and at the very least more data are needed on snake abundances at lower pressures to understand the relationship between pressure and snake activity in the field (the left hand chart in Fig. 3. has only two data points at pressures lower than about 1004 hPa).

Another question, of course, is where do the snakes go? It seems unlikely that they move into deeper water to avoid the worst of the storm, because they might be vulnerable to large predators in open water, and breathing at the surface in cyclone conditions might be difficult even some way offshore. It isn’t likely that they simply hunker down on the seabed either, because they need to return to the surface regularly to breath. The study authors speculate that the snakes make use of cavernous spaces in the local volcanic rock, which would allow them to find safe places to breath while sheltered from the worst of the weather, but the danger inherent in undertaking coastal fieldwork during a cyclone means that it will be difficult to test any such hypotheses. In any case, further observational work will be required to confirm that sea snakes really do anticipate stormy conditions and seek shelter, and to see if this is a general response of sea snakes to extreme weather, or simply an adaptation displayed by a local population.

And of course, nothing described above requires snakes (or any other marine creatures) to have psychic powers.

Y.-L. Liu, H. B. Lillywhite, and M.-C. Tu1 (2010). Sea snakes anticipate tropical cyclone Marine Biology

Direct Link (doi: 10.1007/s00227-010-1501-x) 

*I do realise that the octopus isn’t actually psychic, and can’t actually predict the future – and that snakes, sea-based or otherwise, can’t either.

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Responses

  1. A related observation by shark scientist:

    Running before the storm: blacktip sharks respond to falling barometric pressure associated with Tropical Storm Gabrielle

    M. R. Heupel, C. A. Simpfendorfer and R. E. Hueter
    http://www3.interscience.wiley.com/journal/118870236/abstract?CRETRY=1&SRETRY=0

  2. Thanks for the link Jason. I may well check that out at some point – the paper you link to was also cited in the Liu et al. paper.

    Leads me to wonder how general this kind of response is among marine vertebrates.


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