BBC Wildlife Magazine - - Grey Seals -


Seals have large eyes with pow­er­ful iris mus­cles that al­low for a huge in­crease in the size of the pupils in re­sponse to low light con­di­tions. In day­light, seal pupils ap­pear like ver­ti­cal sl­its; as light re­duces, pupil size ex­pands dra­mat­i­cally. Grey seals also have a re­flec­tive layer be­hind their retina, the tape­tum cel­lu­lo­sum, which al­lows light to re­flect back into the eye. A flat re­gion of the cornea en­ables the seal to fo­cus clearly be­low and above water. There is de­bate over the abil­ity of grey seals to see in colour, but a 2016 study con­cludes that com­mon, or har­bour, seals can see coloured tri­an­gles and it’s likely the same is true of greys.


Seals use their sense of smell to lo­cate food from some dis­tance. They are also able to de­tect cer­tain chem­i­cals at very low con­cen­tra­tions. Of par­tic­u­lar note, seals seem acutely sen­si­tive to dimethyl sul­phide (DMS), a chem­i­cal pro­duced by phy­to­plank­ton in re­sponse to zoo­plank­ton graz­ing; the distri­bu­tion of DMS at sea is linked to the abun­dance of zoo­plank­ton and thus to that of shoal­ing fish, on which seals feed.


Above water, grey seals have a sim­i­lar hear­ing range to hu­mans, but un­der­wa­ter it ap­pears that their hear­ing range is greater, ex­tend­ing from less than 75Hz to over 75kHz. Their calls have been grouped into gut­tural ‘rups’ and ‘ru­pes’ (100–3000Hz) and lowfre­quency growls (100–500Hz). Other less com­mon vo­cal­i­sa­tions recorded to date in­clude low-fre­quency clicks (3000Hz), as well as loud knocks. The seals are more vo­cal as the breed­ing sea­son pro­gresses. Stud­ies have also shown more seals haulout when re­spond­ing to boat sonar.


Seals have a highly tuned abil­ity to dif­fer­en­ti­ate be­tween sea­wa­ter sam­ples of vary­ing salin­ity. Salin­ity gra­di­ents can be used as a valu­able clue when seals are ori­ent­ing them­selves or for­ag­ing; for ex­am­ple, stud­ies have shown that the sea­sonal de­vel­op­ment of phy­to­plank­ton is linked to sur­face salin­ity con­di­tions.


Seal vib­ris­sae, or whiskers, are in­cred­i­bly sen­si­tive. They have an un­du­lat­ing shape that re­duces vor­tices gen­er­ated be­hind them as the an­i­mals move through the water. The base of each whisker is packed with nerves. Re­search in Ger­many con­cluded that seals could de­tect hy­dro­dy­namic trails left by fish over 100m away.

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