September marks the start of both the new academic and ecclesiastical years, and is also the start of meteorological autumn here in the northern hemisphere, and spring in the southern hemisphere. Derived from the Latin septem, meaning “seven” and a reflection of this being the seventh month in the calendar of Romulus, September is home to the autumn and summer equinoxes, depending where you are on the planet. Here in the north the days start getting shorter than the nights, while the nights grow shorter in the south. September also marks the start of the pannage season here in the New Forest, where pigs are let out into the woodlands to eat acorn and beech mast - if you're interested in finding out more, check out my article on the common of mast.
This summer was almost the polar opposite of last year's, being widely at or below average across the country for the whole of the month. July was also a very wet month, with most places seeing about 1.5 times the rain expected for the month; parts of Wiltshire saw just over 2.5 times and Lancashire more than 3 times expected rainfall. July finished on an unsettled, thundery note that continued into August. Indeed, the first weekend of the month came with yellow weather warning for wind and rain courtesy of storm “Antoni”.
We saw something more summery towards the end of the second week, and week three was broadly drier and calmer than the start of the month. Temperatures climbed briefly into the mid-20s Celsius during fourth week, before low pressure, the second named storm of August (“Betty” this time), pushed in, again bringing torrential rain, thunder and, for much of western England, Wales and western Scotland, gale force winds. August ended on a similarly unsettled note, with periodic low pressure systems passing across, bringing rain mostly to the north and west, with Ireland seeing the most persistent. The wind direction also changed to northerly for the Bank Holiday weekend, dropping temperature from the mid-20s to high teens Celsius.
Outside of the UK, parts of China saw torrential rain to start the month. Beijing received 5 cm (1.97 inches) of rain in 24 hours causing widespread flooding. Further east, the heatwave in the Middle East and North Africa heatwaves continued, with temperatures widely in the 40s Celsius at the start of August. Many temperature records, particularly overnight highs, also fell in Japan last month. A state of emergency was declared in Maui and thousands were evacuated and at least 80 people killed as the worst wildfires in memory swept the island. It remained very warm in the Canary Islands, too, with daytime highs widely in the high 30s to mid-40s Celsius and overnight lows of almost 42C () at La Aldea de San Nicolas on Gran Canaria. During episodes of “calima”, the Canary Islands can experience among the hottest nights in the world. In southern California, storm “Hilary”, the first tropical storm to hit the region in 84 years, dropped more than half the annual amount of rain on some areas, causing flash flooding and mudslides.
A few updates to existing content have been made this month, including to the deer digestion article. Two new sections of the water deer article have also gone live now, covering fawn hiding behaviour and suckling.
News and discoveries
Optimizing octopuses. Most of us are familiar with changes in DNA driving evolution, but a few years ago scientists found that cephalopods (octopus, squid and cuttlefish) were able to edit their RNA, which is used to transfer instructions for protein construction. New science from the Rosenthal Lab at the University of Chicago suggests that this remarkable ability allows octopuses to adapt to changing water temperature. As the scientists turned down the thermostat on the tanks, they recorded the octopuses making just over 13,000 changes to their RNA, while leaving their DNA untouched. Most changes happened in the nervous system, suggesting they may help the animals' brains cope with cooling.
Polluted primates. A recent study of the faeces from chimps, baboons, colobus and red-tailed monkeys living in Kibale National Park, Uganda, by scientists at Indiana University has found 97 pollutants. In particular, organochlorides, organophosphates and brominated flame retardants were linked with high cortisol levels, a stress hormone that helps to regulate both metabolism and the immune system, suggesting these chemicals are negatively impacting the health of these mammals.
Interrupting insects? The impacts of human-generated pollutants on the health and wellbeing of a host of animals, including humans, is well known. More recently, attention has fallen on possible detrimental effects of particulate matter, a combination of solid particles and liquid droplets suspended in the air. New data from scientists studying insect navigation and mate-finding found that both activities were compromised when their antennae were contaminated with particulate matter. Given the potential for this matter to be distributed many kilometres quickly, the researchers suggest that this air pollution may be a factor in the decline of insect populations worldwide.
Balmy baskers. Typically, we think of most “fish” as being cold-blooded. In other words, their body temperature hovers around ambient because they lack the biochemical mechanics to regulate it. Some fast-moving species, such as tuna and a handful of sharks, are known to be able to maintain a body temperature significantly above that of the water. A new study led by scientists at Trinity College Dublin has now added basking sharks (Cetorhinus maximus) to that list. Through dissection of stranded sharks and biologging free-swimming animals, the researchers found muscle temperatures consistently 1C to 1.5C above ambient.
Seasonal highlight – British seals
The red deer is generally considered to hold the title of Britain's largest land mammal. Red deer can reach an impressive size; the 'Exmoor emperor' apparently weighed in at about 140 kg (300lbs) and stood almost three metres (10 ft) tall. There is, however, a mammal in Britain that can weigh three times more than the 'emperor', but is generally not included as a contender because it is not exclusively terrestrial: the grey seal. September is an important month for Britain's seals; some will be giving birth to their pups, while pups born earlier in the season will be preparing to strike out on their own. All seals need to come onshore to give birth and, hence, autumn offers perhaps the best opportunity to get a view of these animals.
Keeping up appearances
Grey and common seals can be difficult to distinguish when seen separately in the water and when dealing with immature individuals. Generally speaking, grey seal coats are less heavily spotted than those of common seals, and greys have longer snouts with nostrils that do not meet at the end (i.e., run roughly parallel). Common seals have a snub nose, rather than the longer straight 'Roman' nose of the greys, and their nostrils are V-shaped, almost touching at the bottom.
The main difference between the species is the size, with common seals being significantly smaller than greys. Adult male common seals reach an average length of about 150 cm (5 ft.) and weigh 88 kg (194 lbs), but can reach 160 cm (5.25 ft.) and weigh 125 kg (275 lbs). Adult male grey seals, by contrast, grow to around 200 cm (about 6.5 ft.) and 230 kg (507 lbs), but can reach 230 cm (7.5 ft.) and weigh 440 kg (970 lbs). Females of both species are about 25% smaller than the males. When out of the water, common seals can sometimes be identified by their classic 'head up, tail up' resting posture. In common with fully marine mammals, seals possess a thick layer of highly specialised fat under their skin called blubber and, in grey seals, this can be up to 6.5 cm (2.5 in.) thick.
All seals moult once a year, replacing their old coat, and to do so they must leave the water. Common seals undergo an annual moult that starts after breeding in July and may last until mid-September, although it usually runs for three to five weeks. During the moult, the animal takes on a mottled appearance with the old dull brown coat gradually replaced by the new steely-grey coat. Grey seals moult earlier in the year, during the spring. Female greys start moulting during mid-January and are normally finished by mid-April, while the males start around mid-February and finish mid-May.
Male seals tend to be thicker set than females, particularly around the neck where they may have scars from fights with other males. The sexes can be more easily identified if the underside is seen at reasonably close range. Both sexes have an umbilical scar around their midriff (belly button) and an anus opening towards the tail; between these lies a male's penile opening, surrounded by stiff hairs.
Getting an idea of the seal's age is, however, decidedly trickier. In his booklet, The Common Seal, Paul Thompson points out that, although juveniles can generally be identified based on their pale cream/fawn coat (white, in the case of grey seals) and small size, by one year old it is impossible to age them reliably from external characteristics. Indeed, the only way to reliably age adult animals is by sectioning a tooth to count the rings in the cementum; a new ring is laid down each year as the animal grows. The oldest grey seal on record was a female who lived to be 46 years old, while the oldest British male grey was 26; few females appear to survive beyond 35 years old, and few males beyond the age of 20. The record holder for the oldest wild common seal was also a female, who lived to 36 years old (the oldest known common seal was “babyface”, a male at the Cornish Seal Sanctuary who died at 44 years old); the oldest wild male was 31. In the wild, however, Thompson considers few males to pass their 20th birthday and few females to live beyond 30 years. Violent fights between males during the breeding season are likely to be at least part of the reason why males generally live shorter lives than females.
Seals are perfectly adapted to their watery home and, although they may appear clumsy on land, they are sleek and elegant when in the water, travelling at a top speed of about 13 mph (21 kmph). Seals are also able to dive to considerable depths while hunting. Around the UK it's unlikely many seals have cause to dive beyond about 75 m (246 ft.), but dives of 200m (656 ft.) have been recorded in Norwegian waters and there is a record of a common seal being caught in a fish trap set at 500 m (1,640 ft.) off the Californian coast. Typically, common seals will make repeated dives over several hours, each lasting up to 10 minutes, while grey seals appear to employ a 'sit and wait' technique (particularly when hunting sand eels), resting on the seabed and waiting for their prey to come to them, with dives typically lasting 12 minutes and the longest recorded dive being 32 minutes.
When they dive, seals lower their metabolic rate to use oxygen more sparingly; their heart rate drops from about 120 beats per minute (bpm) when at the surface to no more than 40 bpm at depth and they become bradycardic (i.e., their heart rate drops to 10 bpm or less) on forced dives, during which they can respire anaerobically for up to half an hour. Additionally, seals have much more of the oxygen-carrying pigments haemoglobin and myoglobin in their blood and muscles than other mammals. A larger muscle mass and blood volume than similar sized mammals, coupled with a larger venous system to store and move all this extra blood around, means more oxygen can be stored for use during diving.
Since the mid-1950s we've known that seals have high levels of carbon monoxide (CO) in their blood. Traditionally, CO has been considered a dangerous gas, but small doses could be beneficial, improving recovery rates following surgery or cardiac malfunction. More recently, scientists found elephant seals (which can dive to depths of 2,300m/7,500ft and hold their breath for 60 minutes) have about 10% of their haemoglobin bound to CO rather than oxygen - 6-10 times higher than normal. The theory is that, under other circumstances, when the seal surfaces and the oxygen-rich blood rushes back into its tissues, a whole barrage of chemical reactions are triggered. The fact that only 90% of their haemoglobin is carrying oxygen, however, means that there are fewer reactions, and this essentially protects the tissue by slowing down the rate of metabolism and reducing the damage that would otherwise occur (e.g., inflammation and cell death).
The vibrissae (whiskers) of seals are fixed in highly vascularised and innervated pads in the skin and provide their bearer with a sense of touch underwater. Seal whiskers were described in detail in a paper to the Journal of the Royal Society Interface earlier this year by University of Rostock biologist Nicola Erdsack and her colleagues. The vibrissae system consists of three groups of whiskers: “mystacial vibrissae” on the snout; 5-10 “supraorbital vibrissae” above each eye; and a single small “rhinal vibrissae” above each nostril. Erdsack and her colleagues found an inbuilt central heating system in the vibrissae pad and special type of blubber that sits underneath the pad to stop the sensitivity of the whiskers being dulled by the cold; seal whiskers can thus maintain sensitivity even in water that's only 1.2C (34F). The fact that these vibrissae pads are heated to keep them working as the ambient temperature drops makes them prone to being a source of heat loss for the seal, which explains why Erdsack and her team found that they were covered in a much higher density of fur than other parts of the body, helping to insulate them. So, how do the seals use their whiskers to find food? Well, Guido Dehnhardt, also at the University of Rostock, and his colleagues elegantly demonstrated this in a 2001 paper to the journal Science.
Dehnhardt and his team studied the ability of seals to track objects with their whiskers with the aid of a captive male common seal (called Henry) in a German aquarium. Henry was trained to follow a mini submarine in his pool. When Henry was unimpeded and the sub still running, he quickly and directly approached the target. Then the biologists blindfolded Henry with a black stocking mask and headphones while the sub was run in the pool for a few seconds and then turned off; Henry was then released to find it. When set free, Henry pushed his whiskers forward and made small side-to-side movements of his head as he searched, and he almost invariably found it after taking a more indirect route. The most interesting finding came when the researchers overlaid Henry's movements with that of the sub, finding that they were identical; the seal turning abruptly wherever the sub had turned. When Henry's face was covered with a mask that also impeded his whiskers, he couldn't follow the trail. This strongly suggests that seals can find food in murky water and in darkness by following turbulence left by the object. We know that even small goldfish generate vortices that hang around in the water for 30 seconds or more after they've departed, and it seems that seal whiskers are highly tuned to follow these trails.
Sealed with a fish
Both species of seal are primarily piscivorous—that is, the bulk of their diet is made up of fish—and consume 5-6% of their body weight each day. A wide variety of fish species are taken, including gadoids (cod, haddock, pollock, etc.), clupeoids (herring and sprat), pleuronectids (flatfish) and salmonids (salmon). Records of whole herring and saithe, measuring up to 30 cm (12 in.) long, found in seal stomachs suggests that prey is often swallowed whole, and this is often the impression given by captive seals. In his 1989 booklet on the common seal, Paul Thompson notes that crustaceans (crabs and shrimp) and squid are sometimes taken and, more recently, evidence has come to light that birds may also be predated from time to time. In 2008, for example, members of the public visiting Seattle Aquarium reported seeing a male harbour (common) seal catch and eat a gull that landed in its pool. In addition, in the same year, photos were published in the Daily Telegraph showing a common seal attack, skin and eat a male eider duck in the Firth of Clyde, Scotland. The photographs were taken by local naturalist Phil Kirkham and, in a short article to the journal British Birds in 2008, he described how the behaviour was first brought to his attention by a neighbour who observed a seal killing a duck at Kerrycroy in November 2006. Kirkham notes:
“During winter 2006/07 I saw what I presume was the same seal attacking and killing eiders on numerous occasions and I also found several carcasses along the shoreline. Interestingly, as far as I can ascertain, all the birds attacked have been drakes.”
Some seals may also predate other marine mammals and, in April of this year, an adult male grey seal was filmed killing and eating a harbour porpoise off the Pembrokeshire coast. Furthermore, seals may eat other seals. Both species co-occur on the German island of Helgoland Düne and here a young male grey seal was observed preying upon young common seals. In the period between July and September 2013 and numerous times throughout 2014, predation on young common seals was observed and several carcasses showing similar lesions were found. Similar reports have also emerged from the North Sea in recent years.
Radio-tracking studies of grey seals suggests that hunting trips may involve between two and five days at sea, the time generally spent within 50 km (31 miles) of a favoured haul out site. Although seals need to haul out to moult, rest and digest their food, they can sleep for short periods in the water; they may hang vertically in the water (a position called 'bottling') or lay horizontally ('logging'). If the seal sinks below the surface while dozing, the nostrils automatically close and, when they start to run low on oxygen, an automatic nerve response causes the tail flippers to twitch and return them to the surface.
Sealing the future
Seals spend most of their time in the water, particularly during the winter, but must come ashore to moult, give birth and suckle their pups. In the case of grey seals, they must also haul out to mate, while common seals mate in the water. Breeding behaviour includes slapping the water with fore flippers, which sounds akin to a gun shot, and waving clumps of weed in the air. Male common seals also produce powerful low-frequency vocalisation during the mating season, associated with stereotypic dive displays. Competition for mates appears to occur mainly in the water, with hauled out males of both species generally showing little aggression towards one another.
Common seals mate during July and August but females don't conceive until about October because the blastocyst doesn't implant straight away, remaining instead in suspension in the upper uterus for a couple of months. This process is known as delayed implantation and means that, despite mating during summer and requiring only about eight months to gestate, common pups can be born during June when conditions are likely to be most favourable. Indeed, births appear highly synchronised, with most pups born during the first half of June. Grey seals also undergo delayed implantation and mating can occur anywhere from mid-September until mid-December, with most females implanting during late January or early February. Following an eight- or nine-month gestation, a single pup is born in late August or early September, although there are reports of births as late as December.
As the birthing approaches, common seal mothers tend to opt for male-free haul out sites and move away from other females to the edge of the colony, so they're less likely to be disturbed during the birth and while their pups are very young. Common seal pups are born in a more advanced state than greys and generally have their first adult coat at birth; the first furry white coat (called the lanugo) is moulted in the uterus and the fur expelled with the placenta. The result is that common pups can swim and dive within hours of birth, while grey pups are land-locked for at least the first two weeks after birth. Indeed, female common seals are frequently observed playing with their young pups in the water, returning to shore periodically to rest and suckle, nuzzling them repeatedly and giving 'piggybacks' to help the pups stay at the surface. While in the water together the mother and pup frequently vocalise to each other and the pups utter calls that seem to disappear from their vocal repertoire once they've independent; these are the so-called 'mother attraction calls' and they're made both above and below the water's surface. The vocal behaviour of common seals has recently been the subject of attention for Université Laval biologist Caroline Sauve and her team who have studied these pinnipeds off the Canadian coast.
Sauve and her colleagues analysed just under 1,100 calls from 88 pups across three breeding seasons and found that the acoustic properties varied according to the age and size of the pup, such that a pup's call appears to be quite specific (like an individual voice) and may allow the mother to identify her youngster. That said, the changing nature of the call as the pup grows up means the mother must adapt to these changes. Putting this to the test, the biologists observed that mothers could indeed recognise the calls of their pups, as distinct from the calls of unrelated pups, as early as three days into lactation. Mothers also memorized previous versions of their pup's calls, considering the age- and size-related differences. In one of several papers published this year, Sauve and her team suggest that the need for common seal mothers to feed while lactating, unlike most other phocids that fast while lactating, has driven the evolution of calls to help mother and infant keep in contact while in the water. It certainly seems that grey seals don't have the same connection to their pups.
During the 1997 breeding season on the Isle of May a team of Scottish biologists recorded vocalizations from grey seal pups and played them back to the mothers. The researchers found that pup vocalizations were both stereotyped and individually distinctive, features that normally imply a system of individual recognition. Playback experiments, however, revealed that mothers didn't respond more to vocalizations of their own pups more than to those of other females' pups. Furthermore, 17 cases of allosuckling (where a mother suckled an infant that wasn't hers) were observed during 68 hours of observation on the colony. It may be that, because grey seal pups don't enter the water until they're about four weeks old and the mothers generally do not leave them to hunt during this period, there may be less of a need to be able to recognise the voice of your own pup. The type of habitat in which the pups are born may also be a factor in vocal recognition. In some areas seals give birth on rocky shores that are flooded at high tide, forcing pups into the water soon after birth; those birthing on sandy beaches are less affected by the tides. Rocky populations may thus have a greater need for well-developed mother-pup recognition than those on sandy shores and, although no data exist for common seals, studies on grey seals at Sable Island off Nova Scotia (Canada), have found that two different colonies differed in how well the mothers could identify the voice of their pups.
Seal pups will suckle for three or four weeks, during which time they will double their 10-12 kg (22-26 lb.) birth weight and lay down a layer of blubber that will sustain them while they learn to hunt. (Seal milk is about 50% fat and only about 1% sugar, with the fat content increasing as lactation progresses.) Weaning marks the end of a brief but intense mother-infant relationship and the pup is left to fend for itself. Survivorship seems highly variable, with some studies suggesting as little as 12% of pups dying each winter and theoretical models implying that this number may be as high as 60%. If the pup survives to its first birthday, its chances of survival increase significantly. Females reach sexual maturity at three or four years old and males at five to six years old.
To conserve, or not to conserve
For centuries seals were hunted in the UK for their skin and meat and, in the 17th Century, the church declared that seals were fish so their meat could be eaten during Lent. Seal skins had various uses, including in the production of shoes and horse saddles, while their oil was used to fuel lamps and feed cattle. During the late 1960s, however, the low number of common seals recorded at haul out sites in Shetland raised concerns that populations were being over-exploited and led to the passing of the Conservation of Seals Act in May 1970, which imposed a closed season (1st June until 31st August) during which neither species could be hunted in the UK without a license. Since the creation of the Act, the closed season has been extended in some locations such that the seals are now protected year-round (e.g., in Scotland's Moray Firth) and, although the Act doesn't apply in Northern Ireland, seals are protected throughout the year in the Republic of Ireland under the Wildlife Act of 1976. More recently, on 5th May 2009, the European parliament voted to ban products derived from seals (excluding those hunted by indigenous communities) being imported, exported, or even transported through any of the 27 member states.
Britain's seals face many threats, and it is pollution of the marine environment that is probably the most ubiquitous. Seals are sometimes found entangled in fishing nets, pups have been seen struggling to make it ashore while being battered by plastic bottles and other debris, and plastic bags have been found in the stomachs of adult seals. Pollutants such as DDT, PCBs and organochlorides have also been found in seals, and these chemicals can suppress their immune system and lower their fertility.
There is also a widely held perception that seals can be a danger to fish stocks. Generally speaking, perceived competition with fishermen is two-fold—operational (damage to fishing gear) and biological (competing for the same fish)—and seals can, under license, be shot by fishermen if found in the vicinity of fishing gear or fish farms. Some conservationists have suggested that many seals are shot illegally by fishermen trying to protect their catch or farms, but the extent to which this happens remains unknown. Certainly, salmon farms in Scotland have reported problems with seals attacking their fish, and at the end of last month the Scottish government released figures showing that 63 seals were shot in 2019 and 79 in 2020 by salmon farm owners and the industry estimated that more than half a million caged salmon were taken by seals in 2020 alone. A spokesman for one of the farms recently told the BBC that damage to cages or loss of fish because of seal activity had cost the company £3 million (4.1m EUR or 4.6m USD) in only two years.
More generally, it is difficult to know what the true impact of seals on fish stocks is, not least because it seems to vary considerably in different areas. In the North Sea, for example, it is estimated that grey seals take about 3% and 4% of the total sand eel and cod stocks each year, respectively, but in Western Scotland their impact on cod, haddock and whiting appears to be much greater. In a report to the European Parliament's Committee on Fisheries, Seals and fish stocks in Scottish waters, published in 2010, Ian Boyd and Philip Hammond of the Sea Mammal Research Unit at St Andrews University concluded that the available information strongly suggests that the effects of predation by both species of seals on overall marine fish abundance is likely to be insignificant. Boyd and Hammond explain that:
“... we do not know if the increase in grey seal abundance is directly related to the declines in fish stocks. However, given that the overall relative capture rate by seals is much lower than that of fisheries it is more likely that fisheries are the main cause of declines in stock sizes.”
That said, a paper to the Journal of Applied Ecology earlier this year by a team of Scottish fishery biologists suggests that, once over-exploited by fishing trawlers, grey seal predation may significantly hamper the recovery of Atlantic cod stocks even if human fishing effort is reduced to zero.
I hope this month's feature has inspired you to get out to try and catch a glimpse of your local seals, or perhaps just to carry out a beach clean to remove some of the litter than can cause so many problems for these enigmatic marine mammals. If you find a seal that you're concerned about (e.g., a pup that appears abandoned, or an animal that is injured or in distress), please contact the National Seal Sanctuary on 01326 221361 or the British Divers' Marine Life Rescue on 01825 765546. Please report any dead seals in the West Country to the Cornish Marine Strandings Network on 0345 201 2626.
For a round-up of Britain's seasonal wildlife highlights for early autumn, check out my Wildlife Watching - September blog.