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Little attention has been given to the cues that the female Olive Ridley turtles use to nest. From general observations made on nesting beaches, female sea turtles seem to favour darkness, high tide (in fact highest possible water levels), strong wind and rain conditions for ‘coming ashore’, which all contribute to making their task of creating a nest on the upper beach easier and least detectable.

Olive ridley sea turtles, with their pelagic clear-water habitat, would be expected to possess sensitivity to lunar (monthly) rhythms through variation in night-time illumination. It is difficult to imagine how they could be sensitive to tidal rise and fall (diurnal or semi-diurnal rhythms) however. It is generally considered unlikely that organisms are directly sensitive to (the very weak) forces of gravity; animals that are aware of tidal rhythms sense these changes indirectly (for example, benthic dwellers can sense pressure changes associated with tidal changes in water level).  This poses the question of how turtles sense the precise time to go ashore to nest.  Turtles potentially possess four sensing systems that they could use whilst swimming off a beach to determine optimum conditions for landing, viz:

1)     Sight, although turtles are thought to be limited by short-sightedness and poor vision out of water.

2)     Touch, which could provide information on rain and spray conditions whilst swimming with the head emergent.

3)     Motion, which through sensing of accelerations imposed by wave-induced motion in the water column, could enable sensing of wave and swell conditions.

4)     Sound,  to which turtles are known to be sensitive underwater [[1]]. Wave-breaking on an ocean beach is known to be an important source of sound in the coastal environment, and the nature of the sound will vary with tidal level and wave energy. Sound data could provide the necessary information on all the critical parameters favoured by a nesting turtle, with the exception of illumination level. Thus a turtle’s combined capacity for sight and hearing could provide all the requisite information.

 The ocean is an ideal medium for sound propagation, and is a favoured means of sensing and communication for many pelagic organisms, notably cetaceans. Turtles are not thought to use sound for communication, but it is very possible that they use it for sensing conditions around them. This ability is a new concept for biologists however, and personal communication and literature review has revealed no studies of this mechanism.  

Monitoring of sounds in the ocean has been undertaken for decades for military purposes, and there is much published information on the noises emitted by the oceanic processes that turtles may be listening to [[2]]. These include:  

• Sea surface wave conditions. The formation of ‘white horses’ on waves and the associated entrainment of air have a distinctive acoustic signature. Similarly, it is possible to hear rivulets of water ‘running’ the advancing limb of a wave. Factors such as these make it feasible to deduce sea-surface wave conditions by monitoring sound in the water column below the wave.

• It is similarly possible to hear raindrops impacting on the water surface.

• Surf breaking on a beach has a very distinctive set of acoustic signatures, the tonal values of which coincide exactly with turtle hearing range [[3]].

• Much of the surf sound is generated by sediment in motion. Studies [[4]] have shown that the tone of this sound varies with the size of the sediment, which in turn holds a constant relationship with beach gradient. As the beach gradient naturally steepens progressively towards the High Water Mark on sand beaches, changing tones and rhythms within sounds emerging from the surf zone could indicate the high water time to turtles. They may similarly use this sensory mechanism for determining optimum landing sites in terms of beach gradient along the littoral.

A study of the spatial and temporal variability of sound quality emerging from the surf zone of nesting beaches and the correspondence between these sounds and turtle nesting behaviour, could therefore both improve our understanding of sea turtle behaviour, and provide a methodology for improved ability to predict sites and times of peak turtle nesting activity, as part of conservation programmes.

We at Ambios are keen to research this concept, and have initiated a work programme. This began in 2004  with an MSc dissertation (Emma Graham) at Plymouth University (School of Earth Observation Sciences, SEOS). Emma’s dissertation looked at background issues and sound recording and analysis methods. In 2005 the work continued with Saritha Visvalingam  who undertook her MSc dissertation from Leeds University. Sar focused on sound attenuation through the beach sediments (important if nests are relocated from the beach to hatcheries) and sound sources (wave collapse, swash, sediment movement etc).