May represents the transition from the sometimes-chilly days of spring to the long, hot days of summer here in the northern hemisphere, or the lead into winter in the southern hemisphere (the deer rut in Australia being in full swing now). The third of seven months to have a length of 31 days, the month's name has a couple of possible derivations. Etymologists suggest that it is either derived from the Latin Maius - after the Greek goddess Maia, one of the Pleiades and the mother of Hermes and associated with growth and fertility - or named for the maiores, Latin for “elders”.
A warm and wet March was followed by a settled, warm and largely dry start to April. Despite some sharp showers towards the end of the first week, most saw good sunny spells and it definitely felt like spring was ensconced. The Easter bank holiday weekend was similarly largely warm and dry, with temperatures climbing well into double figures across much of England, before a band of unsettled weather crossed through on Easter Monday.
Things turned wet and very windy, with gale force winds sweeping through courtesy of storm “Noa” during the middle of the second week. The Needles on the Isle of Wight recorded gusts of 96 mph (154.5 kmph), and there was some localised damage and flooding. Another low followed at the end of the week, before unusually warm air built in for the penultimate week, bringing temperatures up to the high teens in western Scotland and western Northern Ireland. This warmth was originally forecast to cover the whole of the UK, but was kept at bay by an easterly airflow bringing cool, wet and windy conditions.
The penultimate weekend and first half of the final week of April was broadly cool and wet, particularly in northern England and Scotland. The northerly airflow spread throughout the UK, bringing overnight frosts, before milder air pushed back to see out April, bringing temperatures in the south into the high teens Celsius. We did see some much-needed rain in the south-west during the final week, which is still in drought from last summer and announced plans to extend a hosepipe ban put in place last year.
Outside of the UK, Argentina's heatwave continued into April. Despite being mid-autumn, temperatures remained widely above 30C (85F) in central and northern areas for the first week of the month before a cold front moved north. There were also two “hot spots” in Europe for much of last month: in the southwest (Spain) and northeast. Cordoba Airport reached 39C (102F), and there were several wildfires across the country. Indeed, across much of Spain and Portugal many records fell. Finland was also unusually warm last month, with 18.1C (64.6F) at Rauma. Much of Europe also remained in drought, having seen very little rainfall so far this year. Mid-April saw new temperature records set widely across south-east Asia. Tak agro hit 45.4C (113.7F), the highest temperature on record for Thailand, while Luang Prabang in Laos reached 41.6C (106.9F) and Mengla reported China's monthly record at 38.2C (100.8F). While the Iberian Peninsula was subjected to a historic spring heatwave, central and eastern Europe remained unusually cold, particularly in Serbia and Montenegro.
As usual, several minor updates have been made to various sections of existing articles, including to include accounts of parakeets predating grey squirrel kittens and records of hedgehog remains in white-tailed sea eagle nests. A new section covering the courtship of the Chinese water deer also went live last month.
News and discoveries
Sleight savvy. Magic is one of the few things in life that can still amaze us, and many other species seem equally duped by sleight of hand - not all primates, though. When a magician with 12 years of experience was employed to do a “French drop” trick (where you pretend to drop something but keep it in your other hand) to three species of New World monkeys, the yellow-breasted capuchins and Humboldt's squirrel monkeys fell for it. The third species, common marmosets, weren't duped. The scientists suggest that, having opposable thumbs like us, the capuchins and the squirrel monkeys were familiar with the conjurer's anatomy, therefore predicting his hand movements, and were fooled as a result.
Self-domesticating elephants? Psychologists at the Max Planck Institute for Psycholinguistics in the Netherlands recently identified that African savannah elephants have 79 genes that we associate with domestication in other mammal species. Self-domestication, where animals adapt to live alongside humans without our intervention, has so far been argued only for bonobos, so this is the first suggestion that pro-social behaviour (i.e., selection of cohabitants based on them being positive, helpful and/or friendly) has played a part in elephant evolution. This finding helps expand our understanding of social evolution in mammals and provides insights into convergent evolution.
Shrimp chops. Anticipatory plasticity is a phenomenon where a species undergoes irreversible development associated with something that it “expects” to encounter in its environment. New science looking at tadpoles of the Mexican spadefoot toad suggests some amphibians undergo this developmental process. The researchers raised 30 clumps of toad spawn in water replete with fairy shrimp and another 30 clumps in shrimp-free water. Two days later, the shrimp-exposed tadpoles hatched out with jaws that were slightly larger and stronger than those in the control group, allowing them a 12% greater bite force.
Smelling in stereo. Modern-day hammerhead and bonnethead sharks have broad heads that seem to allow them to more precisely home in on scent. The recent discovery of fossilised shark remains in the Moroccan Sahara suggests that this body plan evolved 365 million years ago. Using CT scans to create digital 3D reconstructions of the animals' skulls, the researchers found prominent ridges on both sides of the caudal (tail) fin, like the fast-swimming great white and mako sharks around today, as well as large eyes and a broad upper jaw ending in widely spaced nostrils. The lower jaw was very narrow, however, suggesting a narrow mouth with broad nose. The palaeontologists named the shark Maghriboselache mohamezanei.
Seasonal highlight – It's (w)rabbit season
I suspect most of us have, at some point in our lives, come across a rabbit. You may even have one, or several, sharing your home. Indeed, the People Dispensary for Sick Animals (PDSA) recently estimated that there are 1.7 million pet rabbits in the UK and, worldwide, there are some 40 different breeds. Unfortunately, the humble rabbit often fails to get the attention it deserves, and I hope this month's feature goes some way to addressing that balance.
Rabbit or hare?
In Britain we have three members of the Lagomorpha (literally “hare-shaped”) order of mammals: the brown hare, Lepus europaeus; the mountain (or Irish) hare, Lepus timidus; and the rabbit, Oryctolagus cuniculus. All these animals share a similar basic body pattern and have many behavioural traits in common, which can lead to some problems telling them apart. Indeed, the separation of burrow-living rabbits from the non-burrowing hares was only accepted, taxonomically, in the early 1900s and, prior to this, rabbits were classified as Lepus cuniculus. Rabbits are, however, generally significantly smaller than hares, with proportionally much shorter ears and shorter hind legs. They grow to between 34cm and 50cm (1-1.6 ft.) in length, with another 4-8cm of tail, and weigh in at around 1.2-1.5kg (2.6-5.5 lbs). Brown hares (right), by contrast, grow to 70cm (26 in.) long and can weigh up to 7kg (15.5 lbs). The size of the ears is particularly helpful when separating the species. Hares have ears that are at least the length of their head, while rabbit ears are shorter than their head length. Fur colour is variable but typically grey-brown on the back and blue-grey on the stomach, with a red/ginger patch on the back of the neck. The ear tips are brown, unlike those of hares which are black, and the tail has a black upper surface and is white below.
The European rabbit's Latin name, Oryctolagus cuniculus, translates roughly as 'hare-like tunnel digger' and alludes to a way of life that involves digging sometimes complex burrow systems called warrens. This species evolved in what is now southern Spain at some point during the Middle Pleistocene (781-126 thousand years ago) and spread from there into northern Europe. Rabbits are native to the semi-arid regions of the Iberian Peninsula (i.e., Spain and Portugal) in southwestern Europe, and Morocco and Algeria on the north African continent. Thanks to some help from humans, rabbits are now found widely throughout Europe, from Ireland in the west to Poland and Ukraine in the east and north into southern Sweden. There are also introduced populations in Australia, New Zealand, Chile and the Falkland Islands in the south Atlantic. Part of the rabbit's success has been its adaptability, which has allowed it to colonise a wide range of different habitats including heathland, grasslands, meadows, deciduous woodland, sandy soils (e.g., sand dunes), mixed farmland and even cliffs up to about 500m (1,600 ft), although they do appear to avoid conifer forests.
Carrots are divine. You get a dozen for a dime
In many populations, wild rabbits are essentially nocturnal and tend to show a clear daily cycle of activity: they emerge from their burrows at dusk, feed throughout the night, and return to their burrows during the early morning. Rabbits are, however, quick to adapt to human activity and, in areas where they are not subject to heavy persecution, it is not uncommon to find them out feeding during the daytime. Even in largely undisturbed populations, though, they are found resting or preening above ground during daylight hours and still do most of their feeding and socialising under the cover of darkness.
A catholic diet is part of the reason rabbits have been able to colonise a wide range of habitats. Their entirely vegetarian feeding habits includes a wide variety of vegetation, including crops such as germinating cereals, young trees, lettuce, cabbages, root vegetables and so forth. Grasses, buds, bulbs and bark are also eaten, particularly during the winter months. Cellulose in plant tissues is a tough structural protein that requires significant effort to digest and, to this end, the rabbit gut houses an ingenious separation mechanism, a wall-like structure called a haustra, that allows the larger particles in the digestate (a thick soupy mix of partially digested food) passing through the digestive tract to be filtered out and re-ingested to ensure maximum digestive efficiency.
The haustra filters out large particles (larger than 100 µm) and sends them back into the caecum, ultimately building up a collection of large particles of undigested food. This food is partially fermented and mixed with a range of microorganisms before being coated in a thick and tough mucus (a vitamin B12-mucoprotein complex, if you're interested). These capsule-type soft faeces, known as caecotrophs, are then passed into the colon and out through the anus to be eaten by the rabbit. Caecotrophs are rich in vitamins, minerals and microbial proteins and are so essential to the rabbit's health that they do not even touch the ground - the rabbit eats them directly from its anus. Lab studies have shown that rabbits rapidly lose condition if they cannot access their caecotrophs and can die if this continues for more than a couple of weeks. The pellets are swallowed whole (no chewing) so the membrane remains intact and thus the pellet remains whole in the stomach for several hours while the microorganisms breakdown the plant material inside, releasing the nutriment. The mucus coating appears very effective at protecting the microorganisms from the highly acidic conditions (pH 1.9) of the rabbit's stomach.
This whole process of reingesting the faeces, or refection, is analogous to the cud chewing seen in many herbivores; it's a mechanism allowing a second chance at digesting food. Once the food particles fall below the 100µm threshold, they are passed out as the small, hard, dry black/brown pellets that many of us recognise as rabbit droppings. These hard pellets are used to mark out the rabbit's territory.
Gee, ain't I a stinker?
Despite how it may appear at first glance, rabbits are highly territorial and use scent to mark out the boundaries of their core areas. Each population (colony) of rabbits is subdivided into several distinct groups; these normally consist of four or five unrelated males (bucks) guarding up to about eight, generally related, females (does), although the social system varies from pairs to up to 20 individuals, depending on the population density. The group will maintain a core area, typically around a breeding warren, that will be defended from neighbouring groups and the boundaries of which are marked with scent.
Scent is deposited from special submandibular glands on the chin and tends to be rubbed on branches and other conspicuous objects within the territory. Urine is also deposited around the territory and the hard pellets may be coated in a scented mucus from the anal glands before they're deposited in shallow scrapes, called latrines, or on anthills (or other similar locations) along the territory border. Scent-coated pellets are generally darker in colour than normal. Females will also mark the entrance to breeding stops with urine and males have been observed spraying urine at females. The largest, most dominant, buck in the group has the most well-developed scent glands and is responsible for most of the scent marking done by the group. In rabbit society scent is used to allude to group identity, sex, age, social status and reproductive condition.
If scent doesn't work to keep intruders out of the home range, rabbits will fight. Before any direct physical contact is initiated, however, the aggressors will use a variety of body language to convey their intentions. A rabbit with ears upright and facing forward, for example, is relaxed, but when the ears turn outwards it indicates that the rabbit is annoyed or tense, while ears facing backwards is a sign that the rabbit is about to attack. A flicking of the back feet is also used to signal annoyance, while high-pitched squeals and the thumping of a back foot are used to indicate alarm. Fights involve combatants jumping and twisting while each attempts to bite or scratch the other. Dominant males from adjacent social groups within the same colony will maintain their territory using a series of displays, including parallel running along, and scraping the ground at, the boundary. In a fascinating article to BBC Wildlife back in 2007, University of East Anglia rabbit biologist Diana Bell described how new social groups form. Dr Bell described how a social 'budding' occurs. When the number of females in the current group exceeds about eight, some of them start restricting their activities to the edge of the territory and it becomes progressively more difficult for the dominant buck to defend both groups. At this point, an immigrant buck will often challenge for dominance of the tearaway does and a new social group is born within the colony. Dr Bell also noted that if the dominant buck is lost from the group, the testes of a fortunate subordinate will dramatically increase in size within a few days, and he will take over.
Rabbit colonies tend to create underground tunnel systems called warrens in well-drained sandy soil. These may be large and quite elaborate. At the extreme of their range where population densities are low, in northeast Scotland for example, they may dig a single simple burrow, or behave more akin to hares and create a shallow depression (a “form”) under the cover of some bracken/bramble in which to shelter. There is no such thing as an average territory; it varies widely according to rabbit density and habitat type and can be up to 750 square metres (900 sq. yds).
Rabbits have an incredible reputation for being able to rapidly increase their population and there are several reasons for this. Firstly, rabbits are sexually mature early in life, able to breed at about 14 and 16 weeks old for females and males, respectively. Secondly, they are what we refer to as induced ovulators with a postpartum oestrous; in other words, they don't have specific breeding seasons like many mammals (the act of mating stimulates the does to ovulate) and can conceive straight after having given birth. In essence, this means that they can breed at any time of the year and, with a gestation period of 28 to 33 days and an average litter size of five young (called pups or kittens), a female could potentially produce 50 offspring in any given year. Of those 50, ones born early in the year may themselves breed in the same year. In reality, however, Diana Bell and her colleagues never observed a female on their 'rabbit ranch' at the University of East Anglia's campus producing more than 10 kittens in a single year. Indeed, despite not being tied to a specific breeding season, most rabbit kittens are born between February and August, and April-June is the peak time for pregnancies. Furthermore, when conditions are adverse (e.g., in times of drought) females will resorb foetuses, and some studies have observed a large-scale reproductive suppression when the population rises too high. Of course, 'too high' is habitat-dependent and can mean anywhere between 25 and 100 rabbits per hectare—under such conditions subordinate females may fail to produce litters. How this social suppression operates isn't known and, curiously, if rabbit numbers continue to rise beyond this threshold, there comes a point when the reproductive suppression is lifted, and the colony continues to breed as normal!
Pregnant does will compete for the prime breeding chamber(s) within the warren, and these generally go to the does with the highest social rank. Low ranking does are often relegated to digging their own single-chambered breeding den, known as a 'stop', the entrance to which is meticulously covered with vegetation each time the female leaves. The nest chamber is lined with grass, moss and fur pulled from their belly, and into this the pups are born; naked, blind and weighing 30-35g (about one ounce). Overall, the birth lasts about 10 minutes and the female licks each kitten clean as it emerges. Once the litter is complete, the doe will nurse them for less than a minute and then leave. The mother visits the kittens briefly once each day, generally staying for only a few minutes to suckle them; this is an anti-predator strategy. Immediately after birth the kits are very vocal and actively try to suckle and, when the female leaves, they immediately settle down and burrow into the nest lining. The kits start moving around the nest after about seven days and their eyes open at about ten days, shortly after which they will make brief forays out of the nest chamber. Interestingly, studies of rabbits in captivity suggest that the female doesn't take much interest in her kits—she doesn't brood them, she seldom cleans them and doesn't retrieve any that stray from the nest. In addition, when the doe returns to the nest, she simply positions herself over the litter and doesn't appear to give them any help to suckle. By contrast, in the only example of paternal (male-based) care in leporids, male rabbits will defend young from aggressive females, who may attack and even kill strange kittens. In her BBC Wildlife article, Diana Bell noted that there's no love lost between the rabbit kittens; even tiny babies will bicker and bite each other in the nest. Kittens born to high status females are likely to develop a high social status themselves.
Rabbit milk is rich, and this allows the kits to grow rapidly despite the short suckling bouts. It also appears that the milk may play a more important role than simply supplying nutrients to the growing youngsters. In a fascinating paper to the journal Physiology & Behavior in 1994, Agnes Bilko and Eotvos Lorand University in Hungary and her colleagues found that rabbit pups raised in captivity by mothers fed a specific diet showed a clear preference for that diet when they were weaned. The biologists found that when they fed nursing does juniper berries the pups, they suckled were much more likely to eat juniper berries when they took to solids than those suckled by does fed only a standard food pellet. Bilko and her co-workers suggested that aromatic cues to the mother's diet are passed to the kits from the milk and any faecal pellets deposited in the nest.
Prince of a Thousand Enemies
Nobody is entirely sure how many rabbits there are in the UK. Back in 1995, there were an estimated 30 million, the number having risen to 40 million by 2004. More recent guestimates suggest there may be 45-50 million in the country today. Whatever the true count, the National Farmers' Union estimates that rabbit activity (digging, eating crops, etc.) causes £100 million of damage per year. The perceived threat to the agricultural industry is so great that much work has gone into finding ways to control numbers or even eradicate rabbits altogether. As we have seen, though, rabbits have an impressive potential to replace their numbers, which makes most control methods very labour intensive and hence expensive. Rabbits are prey for about 45 different species of predators—including foxes, badgers, weasels, domestic dogs and cats and various birds of prey—and there has long been debate over what part predators can play in reducing rabbit numbers. A variety of different studies on enclosed populations has provided mixed results. The consensus, however, is that predators can do little to reduce a healthy rabbit population, although they may be able to maintain a lower population or reduce the growth rate of a recovering one. Studies in Australia, for example, have found that foxes can slow the increase in rabbit numbers, and in one instance, where rabbit populations were recovering from a severe drought, the numbers increased four-times faster at sites where foxes were removed than at sites with foxes. In some cases, the presence of predators may also be able to expedite a population decline that is occurring for other reasons, but there is no convincing evidence that predators alone can reduce rabbit numbers. This makes sense, logically, because most predators are seasonal breeders and, as such, do not have the capacity to rapidly increase their numbers in response to more rabbits.
It appears that rabbit populations are largely driven by climate (which affects habitat quality and thus food availability), accidents (burrow collapse and drowning take a significant toll), agricultural development and disease. Other factors, such as predation, competition with other herbivores (e.g., hares) and hunting take a back seat until the population is stressed by one of the aforementioned factors. In most populations climate seems to have the largest impact, but under certain circumstances disease can take a devastating toll. This is more apparent where diseases have been deliberately released by humans in a bid to reduce rabbit numbers.
In the late 1800s the prime minister of New South Wales and his “rabbit minister” put up a bounty of £25,000 (equivalent to some £2 million today) to anyone who could come up with a biological solution to the rabbit plague that was out-competing sheep for grazing in Australia and New Zealand. Many people tried and failed to come up with a solution, including Louis Pasteur's nephew, Adrien Loir, who was apparently thwarted in his attempt to bring a chicken cholera bacterium into Australia by border control. It was several years later when, in 1936, a biologist working in Cambridge put forward a plan to test a “new disease” that had killed significant numbers of rabbits in South America on the 10,000 rabbits on the island of Skokholm off the Welsh coast. The pathogen was released into Skokholm's rabbit population but had hardly any impact on the numbers. Despite the setback, the same disease was released in Australia in 1950, with much more significant results. This was the myxomatosis virus and it had a catastrophic impact on the population. We now know that on Skokholm a key ingredient was missing from the jigsaw—there were no rabbit fleas to spread the virus. Granted, Australia lacked the fleas too, but here it was readily spread by mosquitoes and, where mosquitoes were scarce or absent, rabbits were manually infected by methods I will not recount here to prevent distressing my readers. Within three years the myxomatosis pestilence had killed 99.8% of Australia's rabbits.
Hearing of the “promising” results of the myxomatosis virus in Australia, a French scientist procured a vial of the disease from a bacteriological lab and inoculated two rabbits he caught on his land—within six weeks 98% of the rabbits on his estate were dead and within a year so were most of the rabbits in France! It is thought that birds carried the virus across the English Channel, and it arrived in Britain during the autumn of 1952. The UK government tried to contain the outbreak after its initial detection in rabbit populations in Kent (eastern England), but in many cases it was deliberately spread by farmers who wanted to reduce the number of rabbits on their property. It has been estimated that some 99% of Britain's rabbits were killed by this virus during the 50s. The myxomatosis virus is spread by invertebrates (mainly fleas and mosquitoes in Europe) and manifests as a swelling of the eyelids and nose, slight at first but worsening over time; lesions subsequently appear below the ears. The unfortunate rabbit is left blind and deaf, and its ultimate survival depends on the weather, predation level, age and any genetic resistance it may have (more on this in a moment).
Some 40 years after the first 'myxi' outbreak, Asian scientists found an even more deadly virus doing the rounds at rabbit farms in China after it was apparently imported from Germany in commercially bred angora rabbits. The virus attacks the liver, causing necrotizing hepatitis, resulting in severe internal bleeding; death occurs shortly afterwards as internal organs shut down. The disease was, rather aptly if somewhat macabrely, named Rabbit Haemorrhagic Disease, or RHD for short. Symptoms include a fever, blue colouration of the skin, anorexia and frenetic behaviour. This is a highly contagious disease that can take a heavy toll on rabbit populations. The first case was in China during 1984 and it spread to Europe two years later, detected in Italy during 1986 from where it appears to have spread to Russia and much of Europe by 1991. RHD arrived in the UK in 1992 and there was an outbreak in the Channel Islands in 1993 before it finally arrived in Ireland during 1995. Australian scientists imported the virus from Europe in 1991 and began field trials on an island population of rabbits in 1995. Perhaps unsurprisingly, the virus jumped to the mainland during the following year, at which point the Australian government decided that trying to stop the virus was impossible and they declared it a legal biological control agent and re-named it to the less intimidating Rabbit Calicivirus Disease (RCD). RHD is also spread by mosquitoes and fleas and, again, received human help as farmers collected the organs of dead rabbits, ground them up and spread them on their fields. As with myxi, the virus was catastrophic for the population, with a mortality rate of 90-95%.
You might be thinking that, despite the significant declines in rabbit populations caused by myxi and RHD, rabbits are still a prominent component of the countryside both here and in Australia and New Zealand, and you'd be right. One of the benefits of having a short generation time (i.e., being able to build your numbers up quickly) is that even if only a few rabbits survive exposure to a virus and subsequently develop immunity, the population can recover in a relatively short period of time. This is exactly what has happened. Studies in Norway, for example, have found that some rabbits do indeed develop immunity to myxomatosis virus and, in Australia, the mortality rate dropped from 99% to about 40% before RHD was released. Even after RHD had so dramatically reduced the population in New Zealand, by 2011 about 40% of rabbits were surviving exposure to the virus. Over the years scientists have experimented with increasingly virulent strains of these viruses, but rabbits have been quick to adapt. Indeed, even after the release of viruses, the use of poisons, gassing, warren destruction, shooting, trapping, and so on, rabbits are still a common Antipodean species. In the Iberian Peninsula, by contrast, a combination of myxi, RHD, over-hunting and habitat fragmentation is proving too much for the rabbit and populations have crashed, fuelling fears that they may even become extinct in the region.
Should a rabbit manage to out-manoeuvre its predators, avoid exposure to viruses and be fortunate with the habitat in which it finds itself, it may expect to live for three or four years, although about three-quarters will die in the first three months after birth and, of the remainder, half will die before their first birthday. Overall, only about 13% live past the age of one. That said, many pet rabbits will live nine or ten years and the oldest wild rabbit on record, tagged in Poland, made to the grand old age of twelve. In her 2003 book, Rabbits and Hares, Annie McBride makes the startling revelation that, on average, any given rabbit population is made up of completely new individuals every two years!
Finally, the question of when the rabbit first arrived on British shores is a contentious one. There's a lot of history behind it, but overall, I think Naomi Sykes and Julie Curl sum the situation up nicely in their 2010 study of the rabbit's history in Britain:
“When the historical, iconographic, zooarchaeological and landscape evidence are considered together, the obvious conclusion to draw is that modern populations descend not from animals introduced by either the Romans or the Normans but from individuals brought to Britain as part of a fully-fledged and pan European 'coney culture' that included warrens and ferrets, all of which appear in Britain at approximately the same time: the late twelfth century.”
So, evidence of successful Roman and Norman rabbit introductions in Britain is dubious to say the least, although there is compelling evidence to suggest the Romans may have tried.
For a round-up of Britain's seasonal wildlife highlights for late spring, check out my Wildlife Watching - May blog.