Seasonal Update (August 2024)

HomeBlog
As we approach the end of summer, our Red deer stags will be turning their minds to breeding, finishing their antler growth and shedding their velvet ahead of the autumn rut. - Credit: Marc Baldwin

Another month of topsy turvy weather that seems to have summed up this summer in the UK. We saw some very warm weather, particularly in the south-east, as London basked in 30C (85F) during the third week, but any hot and sunny weather has been short lived, with multiple low-pressure systems passing over that brought rain and blustery winds with them.

Website update

I started putting the behaviour section of the water deer article online last month, starting with two new sections: a preface to discussing water deer behaviour, and an introduction to sociality in the species.

News and discoveries

A bee forages on French white lavender. New research suggests that air pollution disproportionately negatively affects pollinators such as bees, butterflies, and moths, compared with crop pests, such as aphids. - Credit: Marc Baldwin

AI's climate impact. Internet giant Google delivered its annual sustainability report last month in which it noted that its carbon emissions continued to grow last year, putting their target of becoming net zero by 2030 in jeopardy. The report partially blames the expansion of their Artificial Intelligence (AI) platform for the increase, something that competitors such as Microsoft and Amazon are also battling. It is anticipated that energy requirements of server farms will double between 2022 and 2026 as AI becomes integrated into more applications and devices.

Deer still here. Around 2008, wolves began crossing the border from Canada into Washington, recolonizing the state after having been driven to extinction in the 1930s. Efforts by conservationists have seen the wolf population grow steadily, which has raised questions about what impact these apex predators are having on prey populations. The answer, according to a study published Ecological Applications recently, is: not much. Based on data from 280 radio-collared white-tailed deer, habitat quality was the biggest factor influencing deer populations, and when predators were considered, it was mountain lions that had the most significant impact. These findings contrast with studies in national parks such as Yellowstone that suggest a larger impact from wolves on both deer populations and behaviour.

Polluted pollinators. It's well known that our day-to-day activities pollute the very air that we breathe, and that air pollution has serious consequences for human health. Unfortunately, we're now beginning to recognize that many other species are also affected by our emissions. A new meta-analysis of 120 studies by Reading University published in Nature Communications suggests that air pollutants, including ozone, nitrogen oxides, sulphur dioxide and particulate matter, disproportionately affect pollinators such as bees and moths more than crop pests like aphids. Pollinators experienced a 39% decline in foraging efficiency in the presence of air pollution, while pest species were unaffected at the same levels.

Seasonal highlight – Stag beetles (Lucanus cervus)

I confess to feeling a little jealous every summer when I see friends post photos of stag beetles that have turned up in their gardens on their Facebook news feed. These giant (by beetle standards) insects are an increasingly rare sight in Britain these days, although I think most people are familiar with the imposing critters. Nonetheless, based on some of the feedback from an article Springwatch ran on these insects a few years back, it seems few people know much about the animal itself. With that in mind, this month I want to take you on a brief excursion into the fascinating world of Britain's largest beetle.

A male and female stag beetle (_Lucanus cervus_), illustrating the sexual dimorphism in Britain's largest beetle species. - Credit: Bugman95 (WikiMedia Commons)

A beetle with antlers?

Stag beetles form part of the Coleoptera order (from the Greek meaning 'sheathed wing') of insects, which currently holds about 40% (some 400,000 species) of all insects and a quarter of all animal life known to science. Within the Coleoptera sits the Lucanidae family, in which we find 1,200 to 1,300 species of stag beetles from across the globe. This month it's the greater stag beetle (henceforth, just 'stag beetle'), known scientifically as Lucanus cervus, that is our focus, but Britain is home to two other species that are also commonly referred to as stag beetles. There is the smaller cousin of L. cervus, the lesser stag beetle (Dorcus parallelipipedus) that is found across southern England and south Wales, and Sinodendron cylindricum, which is sometimes referred to as the rhinoceros beetle. Neither of these latter two species matches the stag beetle for size or stature.

The stag beetle's Latin name, Lucanus cervus, gives a hint to both its appearance and its association with humans. According to Pliny the Elder, Roman scholar Nigidius called the stag beetle lucanus after the south-western region of Italy where they were worn as amulets. This was later used as the insect's genus by “Father of Taxonomy” Carl Linnaeus when he formally described the beetle in 1758. Linnaeus also assigned the species epithet cervus to the beetle because the long mandibles (jaws) spread apart in a wide V-shape, with three inward-pointing hooks near the tip, have a similar appearance of the antlers of Red deer stags (Cervus elaphus). As we shall see, these mandibles are also used in a similar fashion to deer antlers.

A male stag beetle, illustrating the impressive mandibles that are used to wrestle with other males over access to females. - Credit: Marc Baldwin

Based on body size, the late Colchester-based entomologist J.T. Clark suggested that there may actually two subspecies of the stag beetle in Britain—Lucanus cervus cervus, and the smaller Lucanus cervus capreolus (capreolus translates to 'small goat', the genus assigned to the Roe deer)—although this never achieved wide acceptance in the field. Part of the problem is that this species has quite a large size range. Adult males commonly measure 4 to 7 cm (1.6-2.8 in.) long, the largest on record apparently reaching just shy of 9 cm (3.5 in.), which is bigger than females typically get (2.5 to 6 cm/1-2.5 in.). There is also considerable variation in size across Europe. In a 2001 paper to the journal Insect Conservation, for example, a team from the University of London reported that beetles from Spain, Germany and the Netherlands were significantly larger than those from Belgium or the UK. While the reason for this size discrepancy is unknown, the observation that keeping beetles in captivity where food is unlimited doesn't appear to increase their adult body size suggests it's under genetic control. As well as being somewhat smaller, females lack the enlarged mandibles sported by the males and, at first glance, may be mistaken for lesser stag beetles; their smooth/shiny reddish-brown wing case and smaller head can be used to separate them on closer inspection. Males, like females, are black except for their wing cases, which are reddish-brown. Females have small black mandibles, while the much larger ones of males are chestnut brown in colour.

Stag beetles are found throughout western Europe, including a rather patchy distribution in England and Wales. In England, they're locally common in the Thames valley and surrounding areas, southern East Anglia, and the central south coast. Females lay eggs underground and thus prefer soil that is easy to dig, which makes them an uncommon species in chalky areas such as the North and South Downs in southern England. In the UK this species in generally found at elevations of 50 metres (164 ft.) above sea level, but elsewhere in Europe (e.g., Bulgaria) it may be found up to 1,700 m (one mile). Generally, it's thought of as a species of oak woodlands, but it may also be found in hedgerows, parks and gardens. Indeed, a distribution study published in 2011 reported that, in the UK, stag beetles were known to be mostly urban and had a broad range of host plant associations. Oak was still the most prevalent plant association, but still only accounted for fewer than 20% of the records.

In the heat of the night

Perhaps surprisingly for such a large insect, stag beetles can fly using wings that remain tucked under their shiny chestnut carapace while moving around on the ground. This helps males find females across a wider area than they could if only walking was an option, and allows females to search more widely for suitable egg-laying sites. - Credit: Herwig Winter (WikiMedia Commons)

From May until mid or late July, male stag beetles take to the air in search of females. Their quest is an urgent one because this species is what we call semelparous. In other words, they have a single reproductive 'event' before they die. The males emerge before the females and are normally the ones that you see in flight; females can fly but seldom seem to do so. Consequently, it's the males that tend to disperse farthest, linking what many scientists believe would otherwise be dangerously isolated populations. Many sources suggest that male stag beetles migrate about 500 m (one-third of a mile) from their point of emergence, while females move up to about 20 m (66 ft.) in Britain, although studies in Europe suggest that they may move much further. One German study recorded males dispersing an average of 802 m (almost half a mile), while females moved an average of 263 m (863 ft.) from their place of emergence. How far a given beetle moves likely depends on the local population density (if there are more beetles around you won't need to move as far to find a mate) and habitat type, but prevailing weather may also play a role as, generally, beetle activity appears to vary according to ambient temperature.

A comprehensive study by Markus Rink and Ulrich Sinsch of the University of Koblenz-Landau in Germany involved radio-tracking 56 free-ranging stag beetles and observing many more in Germany's Moselle River valley between 2000 and 2005. Overall, Rink and Sinsch collected data on 171 individuals and found that males were consistently active for fewer days than females, an average of 8.5 weeks versus 12 weeks, and both sexes were more active on cooler nights. Indeed, the biologists found that warm temperatures increased the beetles' metabolism such that weight loss was three or four times greater during the warm summer of 2003 than the more temperate two subsequent summers. (The summer of 2003 was on average 4C/7F warmer than that of 2004 or 2005.) Air temperature needed to reach at least 11C (52F) for the beetles to get airborne, although more generally they didn't take to the air until it had warmed to about 14C (57F). Flights were never observed in air temperatures above 26C (79F), although ground activity continued up to about 33C (91F). The time of first emergence of the beetles didn't seem to be affected by air temperature, although studies elsewhere have shown that stag beetles from the southern Alps emerged later than those in the north where it's warmer. Interestingly, in his 1973 German paper on stag beetles, Herbert Ant at the Museum fur Naturkunde in Munster published a description of male and female stag beetles excavating small holes in the soil and staying there during hot periods. Ant thought that this might be the beetles taking up moisture, but Rink and Sinsch's results suggest it may be a thermoregulatory behaviour aimed at preventing the beetles overheating—i.e., aestivation.

Stag beetles are primarily crepuscular (i.e., most active during dawn and dusk) and the males are attracted to light, which means they often end up around houses on summer evenings. It's believed that males track down females by their scent, and some authors have observed males mating with dead females, so strong is their urge to copulate. Once a female has been located, courtship involves the male circling her, with his mandibles raised and opened wide. Should any other males be attending the same female, a wrestling match may ensue. The males will raise their mandibles at each other in a display of aggression and, if neither back down, they clash. The aim is to throw over the opponent and stag beetles are both skilful wrestlers and, for their size, incredibly strong.

Two male stag beetles square up. - Credit: Alwyn Ladell (CC BY-NC-ND 2.0)

The males lock mandibles like red deer lock antlers and one beetle may eventually pick the other one clear off the ground and throw it to one side, or even out of a tree. I've not come across any statistics for fights in this species, but in a paper to the Journal of Insect Behaviour in 2015, Jana Goyens at the University of Antwerp in Belgium and her colleagues described the outcome of fights between a North American species of stag beetle. The researchers found that the largest opponent won in 85% of cases, and the more evenly matched two opponents were the longer the battle lasted. When the researchers blindfolded the beetles, the outcome of the fights didn't change, suggesting that visual information wasn't a necessary for a successful battle. The tarsal (foot) claws of the males are highly curved and have increased height, which helps maintain balance while fighting and provides grip on most surfaces. Among male stag beetles, it seems that aggressive behaviour is correlated with size: larger males are usually more aggressive and prone to fighting than smaller ones.

Bringing up bugling

Once the beetles have mated, they separate. The male may go and look for other mates, or may die shortly after the mating, while successfully mated females tend to return to the spot where they emerged. When the female arrives at her emergence site, she'll assess whether there's sufficient decaying wood available and, if not, she may look for an alternative location to lay her eggs. Once a suitable site has been found, the female will dig down maybe 50 cm (20in.) and lay her eggs either in the rotting wood, or nearby in the soil. On average, a female will lay about 24 eggs - although she may lay between 15 to 36 depending on her size - that will develop in the soil for about a month before the larvae hatch out and begin feeding on the decaying wood. Stag beetles spend most of their life in larval form, typically around four years, but in some locations along as seven years. The reason for this prolonged larval stage is that wood is difficult to digest and poor in nutriment, so it takes the larva a long time to gain sufficient condition to be able to pupate into an adult. Recently, there has been debate among entomologists (people who study insects) as to whether there are three or five distinct stages (or “instars”) to the stag beetle's larval period. The consensus was that there were only three instars, with the third being the longest.

The (female) larva of a stag beetle. This species lays eggs underground in areas where there's a good supply of rotting wood for the grubs to feed on. (I really would advise you follow the link to Maria's WikiMedia page, as it contains some amazing photos showing the pupation and emergence of this species.) - Credit: Maria Fremlin (WikiMedia Commons)

A fully-grown stag beetle larva can reach 11 cm (4.3in.) long, with a fairly smooth, creamy-white skin, orange head and legs, and brown mandibles. The larvae are quite chunky, and often exhibit what appears to be a stripe running along their back but is actually the gut contents that are visible through the translucent skin. Larvae have three pairs of “pro-legs” and lack eyes. Overall, it's virtually impossible to distinguish a small stag beetle larva from a lesser stag beetle larva without a hand lens and field guide. Lesser stag beetle larvae are more likely to be found above ground, however, and often in large numbers with adults nearby. When placed on the ground, a stag beetle larva will curl up into a c-shape. (Longhorn beetle larvae, by contrast, remain straight when placed on the ground, while cockchafer larvae arch their backs.)

Larvae feed on decomposing wood by scraping the surface for splinters and are particularly fond of wood infested with white rot fungus—the more decomposed the wood is, the less energy the larva has to expend digesting it and the faster it can grow. The adult beetles cannot feed on solid food, they can only drink the sap from trees or juices from decaying fruit. Indeed, there are scattered reports of adults (particularly males) feeding on sap runs on tree trunks in Europe, but I've not come across any records of this behaviour in the UK. That said, if offered a solution of sugar water, male beetles can sometimes be encouraged to drink it. Being unable/unwilling to feed means that adults must rely on the fat reserves laid down during their larval stages. Once fully grown, the larvae leave the rotting wood and build a large cocoon in the soil in which to pupate. Pupation takes roughly six weeks and begins in the last autumn of the beetle's lifecycle. The newly pupated adult will remain underground throughout the winter, emerging in the following spring.

Adult beetles tend to live for only a few months. Rink and Ulrich found that females lived longer than males, which started dying off during July; most females hung around until mid-August. Part of the reason females live longer may be that they travel less than males and spend the last few days/weeks of their life underground laying eggs, both of which make them less vulnerable to predation. Being underground may also reduce their metabolic costs, as temperatures tend to be lower and more stable within the soil than the air. The researchers also found that males usually died within hours of showing signs of senility, while females often remained motionless at the surface several days before death. Magpies and other corvids are probably the most significant predators of stag beetles, while domestic cats, foxes, owls, woodpeckers, kestrels, shrews, badgers, wild boar and hedgehogs will also take them. Many beetles are killed on the roads.

Conservation concerns

A female stag beetle rescued from a drain on my way to work. Stag beetle populations appear to be in trouble and anything we can do to help, particularly leaving dead wood in gardens, is appreciated. - Credit: Marc Baldwin

Stag beetles have not always been a source of fascination for humans. In his Fauna Britannica, Stefan Buczacki tells how, here in the New Forest, the stag beetle was once called “the Devil's imp” and thought to damage farmers' crops. Consequently, it was not a welcome sight and often pelted with stones. Today, a loss of habitat—particularly a tendency to keep parks and gardens tidy, which can result in a lack of the rotting wood on which their larvae depend—is considered a major factor in the decline in both number and distribution seen over the past 40 years. It is currently listed on the IUCN Red List of Threatened Species as “near threatened”, in Appendix II of the EC Habitats Directive, Appendix III of the Bern Convention, and on Schedule 5 of the Wildlife and Countryside Act (1981). What all this means in practice, however, is that it is a species of conservation concern here in the UK, which makes it illegal to sell them without a license, but it is not a level of protection sufficient to, for example, prevent a building project going ahead. If you accidentally dig up a stag beetle larva, the best course of action is to re-bury it in a safe spot with as much of the accompanying rotting wood as possible. You can also encourage stag beetles into your garden by ensuring there is plenty of decaying wood around. Relatively simple things such as leaving the stumps of felled trees in the ground, creating a log pile in your garden, putting down woodchip, and so forth, can all help stag beetles colonise the area.

(Just a quick reminder: if you've seen a stag beetle this year, please log the sighting with PTES as part of their Great Stag Hunt. Thank you!)

For a round-up of Britain's seasonal wildlife highlights for late summer, check out my Wildlife Watching - August blog.

Related reading