Wednesday's stranding and sad death of a 60-ft fin whale in the Outer Hebrides comes all too swiftly after what seems to be an unusually high number of unexplained whale mortalities this year – enough to puzzle any whale CSI forensics team. On 8 September a stranded fin whale died off Cleethorpes beach; on 23 September another fin whale died on the Humber, followed on 29 September by a rare sei whale in the same estuary (although my own research shows this was by no means unique. In 1888, a sei whale was harpooned and killed in the Solent, after following the Isle of Wight ferry from Portsmouth to Ryde). This year, two separate pods of pilot whales stranded in the Western Isles.

Faced with yet another slumped and slowly expiring cetacean on the strand, scientists continue to be mystified by the cause for this run of whale casualties. But slowly, some new clues and possible culprits are emerging. Could global warming be to blame? The food sources on which whales subsist prefer cooler waters, being better able to hold oxygen. Noticeably warming waters may be driving whales, and their food, further north.

Other possible causes for whale strandings – suspected in the pilot whale strandings this year - include parasitical infection of the brain with trematodes, and morbillivirus infection, which has the same kind of effect as canine distemper. Illness or other effects may cause the animals to become disorientated in their navigation. Some scientists speculate that cetaceans set their "travel clocks" by detecting minute changes in the geomagnetic field. Others hypothesise that the anomalies which may lead to strandings could even be created by solar activity known to affect the Earth's magnetic field – most visibly in the aurora borealis and australis. Bad weather may also play its part: strandings increase during and immediately after storms.

Man-made problems may be to blame. Noise from boats, ever louder shipping traffic, seismic surveys for oil and military sonar are known to have sometimes fatal effects on animals that rely so heavily on their sense of sound. More insidiously, heavy metals, PCBs, DDTs and other organochlorines are entering the marine environment. Whales, at the top of the ocean food chain, are the final repository for this toxic cocktail.

Sometimes they become the hapless victims of extraordinary combinations of all of these factors. In one recent instance in the Mediterranean, a group of seven sperm whales were panicked, possibly by the use of military sonar in exercises, into entering waters too shallow for them to feed. Weakened by thirst – whales get their water from what they eat, and so may die of thirst as they starve – the animals' internal systems began to break down their adipose fat in which these toxins were stored, relatively safely. By releasing these toxins into their own blood stream, they were in effect poisoning themselves. Finally the whales ran aground off Italy, where, like the fin whale yesterday, they succumbed to the sheer weight of their own bodies which crushed their internal organs.

Is this what is happening around Britain's shores? As ever with whales, it is difficult to tell. Cetaceans spend all their lives in an environment which is alien to us. Ironically, however, whale strandings can be remarkably helpful. These deaths provide us with invaluable clues to the living animals about which we know so little. A fin whale stranded in Denmark last year, for instance, was thought to be about 15-20 years old, a juvenile. The results of its necropsy, released this summer, show that it was blind, arthritic, and 140 years old – thereby doubling, at a stroke, the known longevity of these animals.

Given that it is believed humpback whales may live to 150 years old, and bowhead and North Atlantic whales up to 200-300 years, their very lifespans defeat our scrutiny. It is a salutary notion: whales may be simply too long-lived for us to study within our limited, human lives.