Biology of the Stag Beetle: Unknown Author " de lo poco conocido y lo mucho por conocer"
Proyecto Ciervo Volante, P. O. Box 385, 33400 Avilés - Asturias (Spain)
The Stag Beetle, Lucanus cervus (L.), is "without any 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; there is barely a handful of scientific papers studying the details of its life history. This is even more surprising considering that the Stag Beetle is a threatened species. This paper summarizes the little that is known about Stag Beetle and, thus, it poses more questions than it solves. Its main goal is to make obvious the need of getting a deeper knowledge of the biology of this beetle. Only then it will be possible to seriously address its conservation. And maybe one will find out whether panic-monger statements such as “According all the indications, its extinction is assured within fifty years." (Huerta & Rodríguez, 1988; Rodríguez, 1989) are justified.
Taxonomy and morfology Lucanus cervus (L.) belongs to Superfamily Lucanoidea, which includes more than 1000 species all over the world. European representatives comprise only 17 species within two families and seven genera (Baraud, 1993). Just nine species are present in the Iberian Peninsule (Español & Bellés, 1982).
Sexual dimorphism is remarkable in Stag Beetle and is the likely origin of its Spanish common name, “Ciervo Volante” (Flying Stag). Males, aside 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 articles provide the typical image of this beetle but hide a considerable individual variation in body size. Total length ranges 30 - 90 mm in males (included mandibles) and 28 - 54 mm in females (Lacroix, 1968, 1969; Clark, 1977).
Morphological variation is not limited to body size but includes certain details of mandible shape and the number of lthe lamellae on each antenna. This variation 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 abandon (Español, 1973). J. I. López-Colón (pers. comm.), in his review of this species for the series Fauna Ibérica, 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). Although its high dependence from oaks, Quercus robur, is usually mentioned this species is very polyphagous and many deciduous tree species have been quoted as food source (Paulian & Baraud, 1982). There are even observations of larvae on palm (Alberto Gayoso, pers. comm.) and pine trees (Diego Benavides, pers. comm.). Unfortunately, almost all available 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. Feeding rate is, however, known: larvae weighing 1 g eat 22.5 cm³ per day (Dajoz, 1974).
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) but available information for Stag Beetle is confusing. According Español (1973) larvae come quickly into the wood and use to stay in the underground portion of the stumps. On the other hand, Jirí Simandl (pers. comm.) states that larvae live free within the soil, in the contact zone between the humus and the rotten wood. We lack of direct experience with Stag Beetle larvae but tales from other people seem to support both of the previous behaviours. Unfortunately, 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). Thus, Lucanids are not considered as tree pests. Once more, the few studies quoting Stag Beetle are written in Russian or Polish.
Eggs hatch within two to four weeks (Baraud, 1993). Larval life duration is variable, from one to five years depending on the author (Paulian, 1988; Baraud, 1993; Drake, 1994). This slow development is due, on one hand, to the low nutritve quality of rotten wood (low nitrogen content) and, on the other hand, to the large size which must be achieved at maturity. Surprisingly, number and duration of larval instars is unknown. Effects of temperature and humidity on development are also unknown. Paulian (1959) states that different aged larvae coexist within a same stump but any other details of larval demography are lacking: death rate of each instar, predation or parasitism levels, within- or interspecific competition (D’Ami, 1981, states that when two larvae meet each other in a gallery, cannibalism will occur!).
After last larval molt, in which 10 cm length can be surpassed (Sánchez, 1983), pupation occurs, either within the wood or in the soil, near the stump. Pupation occurs within a chamber built with wood pieces, ground and other materials stuck together with saliva (Español, 1973). It seems that metamorphosis occurs during autumn and that imagoes overwinter within the pupal chamber and show up at the end of next spring (Rodríguez, 1989). However, Paulian (1959) states that larvae overwinter before metamorphosis.
Adult life Adult life ranges from fifteen days to one month (Paulian, 1988), as supported by our own observations of imagoes kept in captivity. Little is known about adult mortality sources, aside the fact that they are eaten by several bird species (Kletecka & Prisada, 1993; J. I. López-Colón, pers. comm.; pers. obs.). Imagoes feed on the sugary sap that lick from tree wounds or on juices from ripen fruits (D’Ami, 1981; Rodríguez, 1989). Females can pierce tree bark with their mandibles to reach the sap (Rodríguez, 1989).
In Asturias (northwestern Spain), imagoes appear form 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 are also conceivable. Our observations show that males appear a little earlier than females (proterandry). 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. Fight speed reaches 6 km/h (D’Ami, 1981) but dispersal abilities are unknown. 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 likelyhood. Research is also required about whether sexes show differential ability or tendency to fly. Drake (1994) states that only males regularly fly but this seems not very likely. Given the ephemeral nature of larval food source, females must surely move in order to find adequate substrates for laying eggs.
Reproduction Males are said 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, 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 and duration of fights 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).
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). Our observations of pairs kept in captivity 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).
Females lay the eggs in dead tree bark crevices. Females individually lay (Huerta & Rodríguez, 1988) around 20 eggs of large size (3 mm length; Baraud, 1993).
Darwin (1871) offered a functional explanation to the obvious sexual dimorphism in this beetle. Males fight for 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.
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).
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) 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. Unfortunately, data on the behaviour of different size males are lacking. This species is, therefore, a good experimental organism to study one of the most interesting reproductive behaviours.
Conservation problems Progresive abundance decrease 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 consulted scientists. In fact, inclusion of all invertebrates in the Bern Convention was rather polemic and limited to non politically problematic species (Stuart Ball, pers. comm.). If any, this shows a lack of consideration of insects in particular, or invertebrates in general, within conservation policies.
Aside United Kingdom, in which the database about the species is an example to be imitated (Clark, 1966; Joint Nature Conservation Committee, unpublished data), we do not know of dossiers about the status of Stag Beetle in any other European country. Jirí Simandl (pers. comm.) states that it is common in lowlands in the Czech and Slovak Republics. In Spain, the Asociación española de Entomología (Spanish Entomological Society) is currently coordinating the compilation of all available information about all the arthropods listed in the Habitat Directive. Our group is collaborating in this task and, with the help of lot of people, we hope to get a first distribution map for Spain next October. At this moment, it seems that there are numerous populations in the Atlantic coast of Spain and that at least there is another important nucleus at the Gredos and Guadarrama ranges, in middle Spain. Its presence in the southern half of the Iberian Peninsula is doubtful.
This does not mean that concern reasons are lacking. The main concern 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. ilex. 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. All this points to the fact that Stag Beetle is quite tolerant to both habitat fragmentation and degradation. However, in United Kingdom, in which this species persists also in bocage habitats (Drake, 1994) its abundance decrease is remarkable; thus, habitat fragmentation is a real risk. 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) but there is no hard evidence for that. 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.
Final remarks Our goal is to increase, within our possibilities, the knowledge about this beetle species. Aside the database about its Iberian distribution, we intend to study population dynamics, behaviour and reproduction, emphasizing those gaps in knowledge referred to in this article.
As bad as the knowledge of Stag Beetle could be, even less is known about Pseudolucanus barbarossa, a related species, endemic from the Iberian Peninsula and northern Morocco (Baraud, 1993) and that faces same potential threats than Stag Beetle. Situation can be much worse than the one of Stag Beetle because the populations of P. barbarossa seem to be much smaller. Although this endemic species is not present in Asturias, we intend to obtain also data on its distribution and biology.