Biology of the Stag Beetle:" 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.
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.
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.
(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
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.
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).
the eggs in dead tree bark crevices. Females individually lay (Huerta
& Rodríguez, 1988) around 20 eggs of large size (3 mm length;
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 males 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,
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.
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.
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.
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.
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