Biology of the Stag Beetle: " de lo poco conocido y lo mucho por conocer"

(version 3.1, 2005) (See Spanish version for figures)

Working Group on Iberian Lucanidae (S.E.A.), P. O. Box 385, 33400 Avilés - Asturias (Spain)

The Stag Beetle, Lucanus cervus (L.), is "no doubt, the most beautiful and representative [...] beetle" (Rodríguez, 1989). Sentences like the previous one use to open popular articles about Stag Beetle. And afterwards, those articles usually proceed to review what is told to be the well known biology of such a famous species.

In fact, there are lots of things we do not know about basic aspects of the biology of this conspicuous and emblematic beetle. Until very recently, the few available scientific articles dealing with its life cycle were mostly anecdotical descriptions with little or null rigorous quantification. This is even more surprising considering that the Stag Beetle is a such conspicuous, popular and threatened species. To be honest, panic-monger statements such as “According all the indications, its extinction is assured within fifty years." (Huerta & Rodríguez, 1988; Rodríguez, 1989) are not based in any rigorous study.

Luckily, since the first version of this article was written in 1996 a considerable quantitative and qualitative change has occurred in the amount of biological data bout the Stag Beetle. Aside of the clasic monography by Palm (1959), the best available summary about the biology of Middle Europe Lucanidae is Klausnitzer (1995), which provides some general quantitative data in support of many of the topics mentioned. Recently, a national survey has been carried out in the United Kingdom (Percy et al., 2000), with plentiful biological information. In addition, Sprecher-Uebersax (2001) has finished her PhD thesis about the biology of L. cervus in the sourroundings of Basel (Switzerland). Deborah Harvey (UK) is also carrying out a Ph D on the Stag Beetle.

This paper summarizes the available biological data about Stag Beetle and points out to some gaps in such knowledge. Its main goal is to make obvious the need of getting a deeper knowledge of the biology of this beetle. Natural history provides a good start but is not enough; detailed quantitative research is also needed about the biology and ecology of the Stag Beetle. Only with such a knowledge its conservation can be seriously addressed.

Taxonomy and morfology
Lucanus cervus (L.) belongs to Superfamily Lucanoidea, which includes more than 1500 species all over the world (Taroni, 1998). European representatives comprise only 18 species within two families and seven genera (Baraud, 1993; Muret & Drumont, 1999). Just nine species are present in the Iberian peninsula (Español & Bellés, 1982; López-Colón, 2000).

Sexual dimorphism is remarkable in Stag Beetle and is the likely origin of its Spanish common name, “Ciervo Volante” (Flying Stag). Males, aside of being bigger than females, have very developed mandibles. It is considered as the biggest beetle in Europe. Plates and pictures of huge specimens included in many popular articles give us the typical image of this beetle but hide a considerable variation in the body size of the individuals. Total length ranges 30 - 90 mm in males (included mandibles) and 28 - 54 mm in females (Lacroix, 1968, 1969; Clark, 1977; Harvey & Gange, 2003 a).

Morphological variation is not limited to body size but includes some details of mandible shape and the number of lamellae on each antenna (Colas, 1962; Lacroix, 1968; Mal, 1972). This has prompted the distinction of several forms or varieties (van Roon, 1910; see Baraud, 1993 for a recent review). Of doubtful taxonomic value, several authors consider they should be abandoned (Español, 1973). López-Colón (2000), in his review of this species for the Iberian peninsula, has ignored such excesive subdivision in forms and varieties. Notwithstanding, this morphological variability is also present in other species of this family (Arrow, 1937; Otte & Stayman, 1979; Baraud, 1993) and is very interesting from other points of view, as we will see below, when talking about reproduction of Stag Beetle.

Larval life
Larvae are melolonthiform and feed on strongly decayed wood. Thus they are not strictly xylophagous, but saproxylophagous (Dajoz, 1974). This kind of diet is possible because ot their symbiosis with cellulose decomposing bacteria, hold in a dilatation at the back of the gut (Dajoz, 1974, 1980). It is usual to refer to Stag Beetle's high dependence from oaks, Quercus robur; for example, Palm (1959) found it only in Quercus and Fagus. However, this species is very polyphagous and many deciduous tree species have been quoted as food source (Paulian & Baraud, 1982). Percy et al. (2000) mention 43 feeding species in Britain. There are even observations of larvae on palm (Alberto Gayoso, pers. comm.), pine trees (Diego Benavides, pers. comm.) and eucalypts (María Fremlin, pers. comm). Feeding rate is known: larvae weighing 1 g eat 22.5 cm³ per day (Dajoz, 1974). Unfortunately, almost all additional information about larval diet is merely anecdotic and we do not know of any paper studying larval preference by any tree species, or quality of different trees as food. Such quantitative information is badly needed in order to put into context the anecdotical evidence referred to above, as well as to properly assess habitat quality for this species.

Space partition among xylophagous beetle species within a same wood piece has been reported (Simandl, 1993). It is also known that each Lucanid species utilizes a different portion of a tree (Szujecki, 1987). In Sweden, Palm (1959) reported higher frequency of Stag Beetle in stumps above 20 cm diameter, although larvae were also found in logs and dead standing trees. Within the trunks, larvae were mainly present in the roots and in lesser degree in the aerial part; no presence was detected in the branches (Palm, 1959). According Español (1973) larvae come quickly into the wood and use to stay in the underground portion of the stumps. de Ligondes (1959) found larvae burrowing within a stump. Jirí Simandl (pers. comm.) states that larvae live free within the soil, in the contact zone between the humus and the rotten wood. Our observations of stumps of different tree species, larvae were found underground and although the wood had been burrowed, larvae were also present in the ground around the roots. Sprecher-Uebersax (2001) also documents this pattern. Sprecher (2003) reports that young larvae feed close to the main roots of stumps, while older larvae move further away from the roots. Murria Beltrán et al. (2004) found larvae in the mould which accumulates in the base of holes of Quercus cerrioides. Unfortunately, additional literature that could shed light on this topic is written in Polish or Russian, and out of our reach (Mamaev & Solokov, 1960; Pawlowski, 1961, quoted in Szujecki, 1987).

Studies on the succession of organisms involved in wood decay describe Lucanids as appearing in the middle or late phases of this proccess, about five years after tree death (ranging 1 - 10 years, depending on the author: Dajoz, 1974; Szujecki, 1987). Once more, the few studies quoting Stag Beetle are written in Russian or Polish. In general, Lucanidae are not considered as forest pests (Palm, 1959).

Eggs hatch within two to four weeks (Baraud, 1993; Harvey & Gange, 2003 b). Larval life duration is variable, from one to seven years depending on the author (Palm, 1959; Paulian, 1988; Baraud, 1993; Drake, 1994). This slow development is due, on one hand, to the low nutritive quality of rotten wood (low nitrogen content) and, on the other hand, to the large size which must be achieved at maturity. Although it is assumed that all Scarabaeoidea and Lucanoidea have three larval instars (Stehr, 1991) and Klausnitzer (1995) gives measurements of the head capsules for those three instars, Harvey & Gange (2003, b) have found that L. cervus has 5-6 larval instars. Duration of each instar is unknown, although Sprecher-Uebersax (2001) has documented the growth rate of the first larval instars. Harvey & Gange (2003, b) have also provided data for the duration of the first larval instar (about 5 months). Effects of temperature, humidity and amount of nitrogen in the wood on development are also unknown.

Different aged larvae coexist within a same stump (Paulian, 1959; GTLI, pers. obs.) . However, any other details of larval demography are lacking: death rate of each instar, predation or parasitism levels, within- or interspecific competition, cannibalism (which has been reported for other xylophagous beetles). Data presented by Murria Beltrán et al. (2004) are the first confirmation of coexistence of larvae from L. cervus, P. barbarossa and D. parallelepipedus in the same tree, although details about spatial segregation within the tree are not provided.

After last larval molt, in which 10 cm length can be surpassed (Sánchez, 1983; GTLI, pers. obs.), pupation occurs in the soil, near the stump (de Ligondes, 1959; Sprecher, 2003; Harvey & Gange, 2003 b; GTLI, pers. obs. of specimens in terrarium). Pupation occurs within a chamber built with wood pieces, ground and other materials stuck together with saliva (de Ligondes, 1959; Español, 1973; Harvey & Gange, 2003 b; GTLI, pers. obs. of specimens in terrarium). Metamorphosis occurs during autumn and imagoes overwinter within the pupal chamber or in the ground nearby and show up at the end of next spring (de Ligondes, 1959; Rodríguez, 1989; Harvey & Gange, 2003 b; GTLI, pers. obs. of specimens in terrarium). The statement of Paulian (1959) that larvae overwinter before metamorphosis seems to be refuted by these more recent observations.

Larvae posses a stridulant organ. Sprecher (2003) has studied its morphology, but its function remains unknown.

Adult life
Adult life lasts fifteen days to one month (Paulian, 1988; GTLI, pers. obs. of specimens in terrarium). Little is known about adult mortality sources, aside the fact that they are eaten by several bird species (Kletecka & Prisada, 1993; Percy et al., 2000; Murria Beltrán et al., 2004; J. I. López-Colón, pers. comm.; GTLI, pers. obs.), hedgehogs, foxes, badgers (Harvey & Gange, 2003 b) and squirrels (Harvey & Gange, 2003 b; Maria Fremlin, pers. comm.). Sprecher-Uebersax (2001) has studied imagoes diet. Imagoes feed on the sugary sap that lick from tree wounds or on juices from ripen fruits (Rodríguez, 1989). Females can pierce tree bark with their mandibles to reach the sap (Rodríguez, 1989). Anecdotal information about feeding on other sweet substances (chocolat cake) is given by Muspratt (1960). Harvey & Gange (2003 b) state, however, that adults do not need to feed, relying mostly on their abdominal fat stores, and that food during the adult stage does not significantly contributes to their longevity.

In Asturias (northwestern Spain), imagoes occur from middle June to the end of August or early September, showing higher abundance during July and some between-year variation (Álvarez Laó & Álvarez Laó, 1995). Phenological variation with altitude and latitude is also conceivable. For example, in the United Kingdom imagoes are mainly observed between May and August, with a maximum in June (Percy et al., 2000) and in Sweden, flight period is June-July (Palm, 1959). Males appear a little earlier than females (proterandry) (Percy et al., 2000; Sprecher, 2003; Harvey & Gange, 2003 b; GTLI, pers. obs.). Abundance also shows between-year variation (Paulian & Baraud, 1982). Four year cycles could be present (Drake, 1994) although there is no quantitative study to support this claim.

Crepuscular or nocturnal habits of imagoes have been traditionally noted (Paulian & Baraud, 1982) but it seems to be also some diurnal activity (Álvarez Laó & Álvarez Laó, 1995) that could be more important in Mediterranean areas (Lacroix, 1968; Arturo Baz, pers. comm.). Flight abilities seem, in principle, well developed. Drake (1994) states that only males flight regularly and Percy et al. (2000) also point out to a higher inclination to flight in males. Hallengren (1997) tried to tradiotrack some individuals but mostly failed. Sprecher-Uebersax (2001; Sprecher, 2003) utilised radiotracking to study adult dispersal. Males flough regularly and dispersed up to 200 m. Females never flough and showed shorter dispersal. There are XIX century tales about mass movements (Darwin, 1871; Lacroix, 1968; Paulian & Baraud, 1982). Anyway, atrophy of flight muscles after some time has been reported (Paulian, 1988), which could limit dispersal likelihood.

Males are said to keep territories (Huerta & Rodríguez, 1988) within which they fly looking for females. This story looks doubtful given the observations of groups of males. More likely is the gathering of males around the females, which are probably found by means of sexual pheromones (in fact, a sexual pheromone has been identified, but in the males, Jason Chapman et al., unpubl. data; Sprecher, 2003, was unable to isolate any female pheromone), or at the feeding places. At those places the so well known male fights occur, in which rivals try to make each other to lose balance, and that use to finish with expulsion of one of the fighters. Studies providing quantitative accounts of such fights are completely lacking and, thus, nothing is really known about frequency, duration or real degree of damage suffered by fighters (usually, various authors present these fights as ritual tournaments and minimize the damage suffered, but in other species severe wounds and deaths have been reported; Siva-Jothy, 1987). Hallengren (1997) documents a fight in which one male lost a foreleg. Sprecher (2003) has found deadly combats, but only in the lab where the scape of the loser was not possible.

Mating behaviour in several American Lucanid species has been studied (Mathieu, 1969) but equivalent work about L. cervus is old and written in German, which make it unreadable to us. Mating duration is disputed: short according to Baraud (1993), a short mating or several mating episodes in a short period according to Mathieu (1969), or lasting even several days according to Huerta & Rodríguez (1988). Recent observations by Jason Chapman et al. (unpubl. data) and GTLI's (pers. obs.) support the last option, or al least a prolonged contact or escort by the male. Mating duration is, probably, very variable and this prolongation could be related to paternity insurance in a competitive environment. Several studies with other insect species show that last male copulating with a female fecundates most of her eggs (Eberhard et al., 1993). Some observations indicate that males mate more than once (Hallengren, 1997; Sprecher, 2003). Harvey & Gange (2003 b) have found that females also mate more than once.

Females lay a single batch of eggs (Harvey & Gange, 2003 b). Although it was said that the eggs were laid in dead tree bark crevices, recent observations by Sprecher (2003) and Harvey & Gange (2003 b) show that eggs are laid in the soil, close to the dead wood, at about 25 cm depth. Percy et al. (2000) list 34 different species as oviposition substrates. Females individually lay (Huerta & Rodríguez, 1988; Harvey & Gange, 2003 b) three to 20 eggs of large size (3 mm length; Baraud, 1993; Harvey & Gange, 2003 b).

Darwin (1871) offered a functional explanation to the obvious sexual dimorphism in this beetle. Males fight to get the females, making of selective value the development of the mandibles as weapons in such fights. Something similar occurs not only in many Lucanids (Otte & Staiman, 1979) but also in other Scarabaeoidea beetles (Palmer, 1978; Cook, 1987), as well as in other insects and, of course, in mammals. In several species of “horned” beetles an advantage of bigger males in getting a mate has been reported (Palmer, 1978; Eberhard, 1979; Brown & Bartalon, 1986; Siva-Jothy, 1987). Males with less developed weapons use to lose fights and to die without mating. This is the basis for the evolution of this trait. In the Stag Beetle, larger males seem to win fights more often (Lagarde et al., 2005; Jason Chapman et al., unpubl. data).

The huge variation in the development of the male’s mandibles has been studied by many authors (Paulian, 1959; Lacroix, 1968,1969; Clark, 1977). These studies showed that mandible size is related to body size and that there is a gradual and continuous transition from smaller individuals with small mandibles to bigger individuals with well developed mandibles. Differences in larval feeding, related to nitrogen content in the decaying wood from which larvae feed, can explain the variable final body size of imagoes, but genetical factors could also been involved (Paulian, 1988). Interaction between genetic and environmental factors in the production of dimorphism in horn length has been studied in Ontophagus (Scarabaeidae) (Emlen, 1994, 1996, 1997; Moczek, 1998; Emlen & Nijout, 1999; Moczek & Emlen, 1999; Hunt & Simmons, 2000). Its generality in other cases of dimorphism is yet to be shown.

In many Lucanidae species (Arrow, 1939; Paulian, 1959; Otte & Stayman, 1979) two clearly different forms of males have been found (this is the famous difference between major and minor males). In these species, a logarithmic plot of mandible size against body size does not gives a single straight line but two separate ones. That means that both kinds of males obey different growth rules and mandible size cannot be attributed to a mere body size difference. Some other factor must be responsible. Eberhard (1980) postulated a mechanism to explain these differences. First, differences in substrate quality in which larvae develop must exist. This produces body size differences between adult males. Small males lose more fights and achieve a lower reproductive success. This selects for an alternative mating behaviour in small males. Instead of fighting for females, they sneak in the places in which females use to be. There they wait for a chance and, while big males fight each other, small males reach females and copulate with them. Incredible as this could sound, this alternative behaviour is a very common phenomenon in species in which males fight for females, from insects (Siva-Jothy, 1987) to fishes and amphibians (Krebs & Davies, 1993).

What about Stag Beetle? Although presence of minor males has been accepted for a long time, studies quoted above did not find any support for two different growth patterns. However, Eberhard & Gutiérrez (1991) and Lagarde et al. (2005) found it, by using special statistical analyses and a large sample size. Our own data do not support such conclusion (Álvarez Laó et al., 1995). In our oppinion, this species is in a transition stage, not having developed a clear difference in alternative strategies. Our idea is based on Kawano (1989), who established the existence, within Lucanidae, of all transitional grades from species lacking sexual dimorphism to those with sexual dimorphism plus male polymorphism. Solution to this conundrum must wait to detailed studies of fight and mating behaviour in this species.

Conservation problems
Progresive decline of Stag Beetle in middle Europe prompted its inclusion in the Bern Convention as protected species and in the appendix IIa from the Habitat Directive (Viejo Montesinos & Sánchez Cumplido, 1994). These decisions did not relay in any detailed study but in the personal oppinion of the asked scientists. In fact, inclusion of all invertebrates in the Bern Convention was rather polemic and was limited to non politically problematic species (Stuart Ball, pers. comm.). The background information of the European Union report (van Helsingden et al., 1996) was very superficial. If any, this shows a lack of consideration of insects in particular, or invertebrates in general, within conservation policies.

The status of the species in Europe differ from country to country. It has gone extinct in Denmark and Estonia. In the United Kingdom a decline is suspected for the last decades (Clark, 1966; Percy et al., 2000; see, however, Pratt, 2003, for a very critical view of this purported decline). Status in Latvia is not optimistic (Dmitry Telnov, pers. comm.) and is critic in Luxembourg (Marc Meyer, pers. comm.). In the Netherlands the species does not seem to have declined from the 1980s (Cuppen, 1992). Jirí Simandl (pers. comm.) states that it is common in lowlands in the Czech Republic and Slovakia. In Spain, the Asociación española de Entomología (Spanish Entomological Society) coordinated the compilation of all available information about all the arthropods listed in the Habitat Directive. GTLI collaborated in that task and, with the help of lot of people, a first distribution map for Spain was obtained. That map is the only currently available tool to assess the conservation status of Stag Beelte in Spain. Based on that map, the threat level of Stag Beetle is Spain was considered as LC (Least Concern) using the IUCN criteria.

This does not mean that concern reasons are lacking. The main candidate is habitat loss. Although usually this species has been considered to be dependent on old oak woodlands (Quercus robur), in the Iberian peninsula it is also present on other Quercus species, such as Q. pyrenaica and Q. rotundifolia. In any case, its dependence on mature woodlands is not clear either. In Asturias (northwestern Spain) it is present in bocage areas, in which meadows are interspersed with small woodlands and hedges. It occurs also in urban parks and Eucalyptus plantations, suppossedly because of the presence of deciduous trees as Chesnut, Castanea sativa, scattered within such plantations. In United Kingdom, this species persists also in bocage and suburban habitats (Drake, 1994; Percy et al., 2000). All this points to the fact that Stag Beetle is quite tolerant to both habitat fragmentation and degradation, although no study has tested this point. Probably, availability of dead wood is more important than the kind or conservation status of the forest, but this should be assessed quantitatively. An altitudinal limit around 600 m is often mentioned (Jirí Simandl, pers. comm.) but this is plainly wrong, at least south from the Cantabrian range in Spain.

Another additional threath usually mentioned is collection (Sánchez, 1983; Huerta & Rodríguez, 1988; Rodríguez, 1989). Despite not being included in the CITES, trade of this species is forbidden in the United Kingdom (Percy et al., 2000). It si difficult to assess the situation in Spain. On one hand, SEPRONA (a branch of the Spanish police in charge of nature protection) does not have any knowledge of illegal trade involving Stag Beetle (José Delgado, pers. comm.). On the other hand, some people has told us about Stag Beetle being sold in some petshops and stamp collection shops. Frequency of these activities and the real impact on natural populations are unknown. In insect conservation literature, harvest is often considered little important (Pyle et al., 1981) as a source of species extintion, even at a local level. In any case, we could face a legal gap in this respect because Stag Beetle is not included in CITES.

Finally, negative effects of pesticides or road casualties on Stag Beetle populations have not been studied in any detail.

All our work would have been impossible without the kind help of a growing number of people (particulars, conservationist groups, amateur entomologists, academics and several official organisms). It is impossible to mention them all here but we do not forget their valuable collaboration. To all of them, our most warm acknowledgement.

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