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Research article
Reaction norms for age and size at maturity in response to
temperature: a test of the compound interest hypothesis
KLAUS FISCHER1,* and KONRAD FIEDLER2
1Institute of Evolutionary and Ecological Sciences, Section Evolutionary Biology, Leiden University,
P.O. Box 9516, NL-2300 RA Leiden, The Netherlands; 2Department of Animal Ecology I, Bayreuth
University, D-95440 Bayreuth, Germany
(*author for correspondence, tel: +31-71-5274877; fax: +31-71-5274900; e-mail: fischer@
rulsfb.leidenuniv.nl)
Received 10 September 2001; accepted 18 March 2002
Co-ordinating editor: O. Leimar
Abstract. In ectotherms, temperature induces similar developmental and evolutionary responses in
body size, with larger individuals occurring or evolving in low temperature environments. Based on
the occasional occurrence of opposite size clines, showing a decline in body size with increasing
latitude, an interaction between generation time and growing season length was suggested to
account for the patterns found. Accordingly, multivoltine species with short generation times
should gain high compound interest benefits from reproducing early at high temperatures, indi-
cating potential for extra generations, even at the expense of being smaller. This should not apply
for obligatorily monovoltine populations. We explicitly test the prediction that monovoltine
populations (no compound interest) should be selected for large body size to maximise adult fitness,
and therefore size at maturity should respond only weakly to temperature. In two monovoltine
populations (an Alpine and a Western German one) of the butterfly Lycaena hippothoe, increasing
temperatures had no significant effect on pupal weight and caused a slight decrease in adult weight
only. In contrast, two closely related, yet potentially multivoltine Lycaena populations showed a
greater weight loss at increasing temperature (in protandrous males, but not in females) and smaller
adult sizes throughout. Thus, the results do support our predictions indicating that the compound
interest hypothesis may yield causal explanations for the relationship between temperature and
insect size at maturity. At all temperatures, the alpine population had higher growth rates and
concomitantly shorter development times (not accompanied by a reduction in size) than the other,
presumably indicating local adaptations to different climates.
Key words: age and size at maturity, body size, butterfly, compound interest, development, growth,
reaction norm, temperature-size-rule
Introduction
For animals, the considerable and wide-ranging ecological importance of body
size is clearly demonstrated by its effect on e.g. fecundity, metabolic rates,
dispersal, competitive, predatory and defensive abilities, capability to with-
stand starvation and desiccation (Peters, 1983; Calder, 1984; Schmidt-Nielsen,
Evolutionary Ecology 16: 333–349, 2002.� 2002 Kluwer Academic Publishers. Printed in the Netherlands.
1984; Partridge and Fowler, 1993; Blanckenhorn, 2000). Hence, understanding
how natural selection acts in the evolution of this key life-history trait is of
special concern in evolutionary ecology (Partridge and French, 1996). A par-
ticularly important and widespread correlate of size-differences in ectotherms is
temperature. The thermal environment experienced by an individual during
development has a direct effect on final size, with body size decreasing as
temperature increases and vice versa (Ray, 1960; Atkinson, 1994, 1996; Sibly
and Atkinson, 1994; Partridge and French, 1996). At least under controlled
laboratory conditions, this relationship holds almost universally (Atkinson,
1994).
Apart from this environmentally induced phenotypic plasticity, temperature
can have evolutionary effects on body size. Latitudinal and altitudinal clines in
body size with an established genetic basis have been reported for several
animals (e.g. Bryant, 1977; Berven, 1982; Imasheva et al., 1994; James et al.,
1995, 1997; Partridge and French, 1996; Chown and Gaston, 1999; Billerbeck
et al., 2000), with smaller individuals being found in populations at lower
latitudes or altitudes. The frequent recurrence of this pattern suggests that size
clines result from natural selection (McCabe and Partridge, 1997). Although
many ecological variables might be involved as selective agents, direct evidence
for the particular importance of temperature was gained from laboratory ex-
periments. Replicated Drosophila populations kept in long-term culture at
different temperatures evolved to be larger at lower temperatures (Partridge
et al., 1994, 1995), and recent studies suggest that body size itself can be a
target of thermal selection (McCabe and Partridge, 1997; Reeve et al., 2000).
In recent years, there has been considerable theoretical interest in the
physiological and evolutionary mechanisms underlying the temperature-size-
rule (e.g. Atkinson, 1994, 1996; Berrigan and Charnov, 1994; Sibly and
Atkinson, 1994; Partridge and French, 1996; Van der Have and De Jong, 1996;
Van Voorhies, 1996; Chown and Gaston, 1999). However, a satisfactory
general explanation is still lacking (Partridge and French, 1996; Chown and
Gaston, 1999) and hypotheses remain contested (cf. Van Voorhies, 1996;
Mousseau, 1997; Partridge and Coyne, 1997; Arnett and Gotelli, 1999). It
remains even unclear whether the similarity of developmental (phenotypic
plasticity) and evolutionary responses of body size to temperature arises from
the same selective pressures or mechanistic constraints (Atkinson, 1996).
Assuming a common physiological process reducing size at higher temper-
atures, but at the same time acknowledging the evidence for temperature acting
as a selective agent on body size (see above), the study of exceptions and
deviations from the broad rule should prove to be particularly useful to en-
hance our understanding on how selection acts on body size. For example,
some insects show opposite size clines, having a larger body size closer to the
equator (e.g. Mousseau and Roff, 1989; Nylin and Svard, 1990; Mousseau,
334
1997). These seemingly contradictory cases can be explained as follows. In
seasonal breeders, there should be selection to allocate as much of the fa-
vourable season as possible to reproduction (Partridge and French, 1996). As
the length of growing season increases, selection should favour increased size at
maturity to maximise adult fitness. With season length further increasing,
however, selection for increased size should be relaxed in favour of the com-
pletion of an extra generation, leading to a ‘saw-tooth’ latitudinal size cline
(Roff, 1980, 1992; Mousseau and Roff, 1989). Thus, an interaction between
generation time and growing season length may account for the occurrence of
opposite size clines (Chown and Gaston, 1999).
In insects with relatively short generation times as compared to season
length, the law of ‘compound interest’ should apply (Cole, 1954; Lewontin,
1965). To maximise Darwinian fitness, there should be a premium on early
reproduction to increase the number of generations per year, even at the ex-
pense of being smaller. This is particularly likely to be important in multi-
voltine organisms in seasonal environments (Partridge and French, 1996). On
the other hand, if generation time comprises a major part of the total growing
season, as is generally the case in the species where larger individuals have been
found at lower latitudes (i.e. butterflies, moths, crickets; see Masaki, 1978;
Mousseau and Roff, 1989; Nylin and Svard, 1990; Gomi and Takeda, 1996),
selection for large body size is likely to be more intense. Finally, populations
with fixed generation time lack the demographic pressure to mature especially
early, since no individual is capable of fitting in additional generations (Meats,
1971). Consequently, selection for large size at maturity to maximise adult
fitness should be strong.
In this context, temperature can have a major influence through its effect on
development time (Honek and Kocourek, 1990) and hence on the number of
generations possible within a season. At high temperatures, indicating fa-
vourable growth conditions and potential for extra generations, multivoltine
populations (or species) with relatively short development times should gain
compound interest benefits from reproducing earlier at a smaller size. The
expected pattern of a reduction in final size as temperature increases agrees well
with the available empirical evidence (see above). For obligatorily monovoltine
populations, in contrast, early reproduction offers no compound interest
benefits regardless of temperature, whereas all advantages of large body size
should remain. Hence, one would predict no or only minor reductions in size to
occur with increasing temperatures.
The potential explanatory power of the compound interest hypothesis may
have been underestimated in recent reviews on relationships between temper-
ature and adult size (e.g. Berrigan and Charnov, 1994; Atkinson, 1996). Thus,
we set out here to test the particularly critical prediction of a low or absent
response to experimental temperatures in two obligatorily monovoltine
335
populations of the butterfly Lycaena hippothoe. This may be the first explicit
test of predictions derived from the compound interest hypothesis (as recently
elaborated by Partridge and French, 1996; Chown and Gaston, 1999). The
resulting reaction norms will be compared to those previously described for
two potentially multivoltine Lycaena populations (L. hippothoe sumadiensis, L.
tityrus; Fischer and Fiedler, 2000, 2001), all of which are taxonomically closely
related and ecologically very similar with regard to habitat requirements and
larval host-plant. In these populations the more time-limited (earlier emerging)
males suffered substantial weight losses at increasing temperatures. Females
also responded to temperature, but much more weakly. Furthermore, we point
out differences between both monovoltine populations (originating from the
central Alps and western Germany) which may represent local adaptations.
Material and methods
Study organism
Lycaena hippothoe (Linnaeus 1761) is a widespread temperate-zone butterfly,
ranging from northern Spain in the west throughout much of the northern
Palaearctic region eastwards to the easternmost parts of Siberia and China
(Ebert and Rennwald, 1991; Tuzov, 2000). Throughout its vast range adults fly
in one generation, except for the western Hungarian lowland population,
which is bivoltine (Fazekas and Balasz, 1979; Tolman and Lewington, 1998;
Tuzov, 2000). Larvae (of the last brood) enter diapause overwintering half-
grown, usually in the third instar (Fischer and Fiedler, 2002). The principal
larval host-plant is Rumex acetosa L. (Polygonaceae), a common and wide-
spread perennial herb occurring in various types of grassland. The two pop-
ulations considered in the experiments reported here include the nominate form
L. hippothoe hippothoe from western Germany (Westerwald, 580 m a.s.l.) and
the alpine subspecies L. hippothoe eurydame (Hoffmansegg 1806) from the
central Alps (Senales valley, 1800 m a.s.l.).
Experimental arrangement
Eight and ten freshly emerged females were caught in 1998 in the western
German and alpine population, respectively. For oviposition females were
transferred to Bayreuth University and maintained in an environmental cabi-
net at a constant temperature (25 �C) and long-day conditions (24-h L/D cycle,
L18:D6 throughout). They were placed individually in glass jars (1 l, approx-
imately 10 cm in diameter and 13 cm in height) lined with moistened filter
paper and the jars covered with gauze. Each jar contained a bunch of the larval
336
food-plant R. acetosa (in H2O) as oviposition substrate as well as highly
concentrated sucrose solution for adult feeding. Eggs were removed each day,
pooled (within populations), and maintained in lots of about 100 in glass vials
at a temperature of 20 �C (L18:D6).
After hatching, young larvae were randomly divided among four groups and
exposed to constant temperatures of 15, 20, 25, and 30 �C (photoperiod
L18:D6 throughout). Hatchlings were reared singly in transparent plastic boxes
(125 ml) containing moistened filter paper and fresh cuttings of R. acetosa in
ample supply. The boxes were checked daily and supplied with new food when
needed. In order to even out minor temperature differences within cabinets, the
boxes were shifted around daily. Following the onset of diapause, dormant
larvae were transferred to another cabinet (T 4 �C, photoperiod L8:D16) for
hibernation. After a diapause of about 5–6 months, larvae were again assigned
to the four different rearing temperatures, and reared in the way outlined above
until adult eclosion. For each individual we measured weight (to the nearest
0.1 mg) at the end of diapause (subsample only), larval (post-diapause) de-
velopment time, pupal time, pupal and adult weight, growth rate. Pupae were
weighed on the day following pupation, adults on the day of eclosion after
having excreted meconium. Relative growth rate was calculated according to
Gotthard et al. (1994) as mean weight gain per day: Growth rate = {[ln(pupal
weight) � ln(diapause weight)]/larval time} · 100.
Statistical methods
Within each population, standard two-way ANOVAs were applied to analyse
the effects of temperature and sex on life-history traits. For a subsequent
comparison between both populations we used three-way ANOVAs (factors:
population, temperature, sex). Differences between groups of larvae, pupae or
butterflies were localised using Tukey’s post-hoc comparison (Spjøtvoll–Stoline
variant; significance threshold p < 0.05 throughout). All statistical analyses
were performed using StatSoft (1999). Throughout the text all means are given
±1 SD.
Results
Developmental pathways
As was expected for these monovoltine populations, the vast majority of larvae
entered diapause even under favourable laboratory (and long-day) conditions.
At lower temperatures of 15 and 20 �C this applied to all larvae in both
populations (sample sizes for western German population 96 and 90, for alpine
337
90 and 219 larvae, respectively). At higher temperatures, however, some indi-
viduals chose a direct development. The percentages of direct developers
reached 3.1 at 25 �C (n ¼ 355) and 31.5% at 30 �C (n ¼ 111) in the western
German population, and 5.0 (n ¼ 298) and 17.4% (n ¼ 98) in the alpine one.
Thus, a notable fraction of subitaneous development did occur under a high
temperature of 30 �C only. Directly developing individuals were excluded from
all subsequent analyses.
Reaction norms for age and size at maturity in western German L. h. hippothoe
Post-diapause development time strongly depended on temperature (Table 1;
for statistics see Table 2) and increased more than fourfold from 16 days
(30 �C) to about 66 days (15 �C). This was due to a longer duration of both the
larval and pupal stage at lower temperatures corresponding to lower devel-
opmental rates. Increasing temperature did not affect pupal weight, but caused
a significant reduction in adult weight (by 13.0% in males and by 13.6% in
females over the temperature gradient). The latter was due to higher weights at
15 �C in both sexes, whereas Tukey’s post-hoc test failed to reveal significant
differences in the range from 20 to 30 �C (Figure 1).
All traits differed significantly between sexes (Table 2). Males exhibited
shorter larval and concomitantly total development times than females. Thus,
protandry (earlier emergence of males; cf. Wiklund and Fagerstrom, 1977;
Fagerstrom and Wiklund, 1982; Zonneveld and Metz, 1991) persisted at all
temperatures. This was at least partially caused by invariably higher larval
growth rates of males. Throughout, females gained considerably higher pupal
and adult weights than males. Overall, however, both sexes responded in the
same manner to increasing temperatures. Consequently, the male–female ratio
of adult weight, ranging between 83.8 and 87.3%, remained similar throughout,
leading to parallel reaction norms for age and size at maturity in both sexes.
Reaction norms for age and size at maturity in alpine L. h. eurydame
The data on post-diapause development in the alpine population revealed
patterns very similar to those reported for the western German one. Again
growth rate, larval, pupal and concomitantly total development time depended
strongly on temperature (Table 1; for statistics see Table 3), with lower tem-
peratures causing lower growth rates and longer development times. As in the
former population, temperature did not significantly affect pupal weight, but
caused a slight reduction in adult weight (by 10.6% in males and by 9.3% in
females between 15 and 30 �C). However, in the post-hoc comparison no
significant differences among temperatures in either of both sexes occurred
(Figure 2).
338
As in western German animals, protandry persisted at all temperatures and
was achieved by invariably higher growth rates and shorter development times
in males as compared to females. In contrast to the western German popula-
tion, sex-specific differences in pupal weight did not occur in the alpine pop-
ulation. Females reached, nevertheless, higher adult weights than males
(Fig. 2b; obviously due to a higher weight loss at metamorphosis in males), and
sexual size dimorphism was confined to this stage. However, with regard to
body weight sexes did not respond differently to increasing temperatures.
Table 1. Life-history data (mean ± 1 SD) for post-diapause development of L. h. hippothoe and
L. h. eurydame at different temperatures
T (�C) Trait L. h. hippothoe L. h. eurydame
Males Females Males Females
15 (n = 35) (n = 25) (n = 31) (n = 32)
Larval time (d) 37.9 ± 7.0 42.5 ± 3.8 35.2 ± 4.8 38.3 ± 4.3
Pupal time (d) 26.1 ± 2.3 26.9 ± 3.0 24.1 ± 2.3 23.5 ± 2.8
Larval + pupal time (d) 64.0 ± 8.1 69.4 ± 4.4 59.3 ± 4.7 61.8 ± 5.5
Pupal weight (mg) 168.0 ± 20.8 194.8 ± 21.6 180.8 ± 21.6 183.4 ± 17.9
Adult weight (mg) 74.7 ± 11.8 88.7 ± 11.3 73.7 ± 6.9 82.2 ± 10.7
Growth rate (%/d) 8.8 ± 0.9 (21) 8.1 ± 0.8 (20) 9.3 ± 1.2 (17) 8.8 ± 0.9 (27)
20 (n = 32) (n = 34) (n = 28) (n = 43)
Larval time (d) 19.1 ± 1.3 21.0 ± 1.9 16.4 ± 3.3 20.5 ± 3.5
Pupal time (d) 12.7 ± 0.8 13.1 ± 0.8 10.9 ± 0.9 11.7 ± 1.2
Larval + pupal time (d) 31.8 ± 1.7 34.1 ± 1.9 27.3 ± 3.6 32.1 ± 3.9
Pupal weight (mg) 174.0 ± 13.8 184.0 ± 14.9 179.1 ± 19.1 178.1 ± 20.2
Adult weight (mg) 70.2 ± 7.7 80.4 ± 10.0 68.4 ± 11.3 77.5 ± 9.9
Growth rate (%/d) 18.0 ± 1.6 (31) 16.6 ± 1.2 (33) 19.2 ± 2.0 (19) 16.3 ± 2.5 (36)
25 (n = 30) (n = 48) (n = 26) (n = 40)
Larval time (d) 13.5 ± 1.3 14.9 ± 1.5 12.2 ± 0.8 13.1 ± 1.5
Pupal time (d) 8.0 ± 0.4 8.0 ± 0.6 7.3 ± 0.5 7.6 ± 0.5
Larval + pupal time (d) 21.5 ± 1.4 22.8 ± 1.4 19.5 ± 0.9 20.7 ± 1.6
Pupal weight (mg) 171.9 ± 20.7 183.3 ± 20.0 178.6 ± 12.6 187.1 ± 18.9
Adult weight (mg) 66.6 ± 8.1 79.5 ± 8.8 70.4 ± 9.3 81.2 ± 12.7
Growth rate (%/d) 24.8 ± 2.3 (21) 23.0 ± 2.4 (43) 27.8 ± 2.6 (9) 25.9 ± 2.1 (17)
30 (n = 38) (n = 29) (n = 32) (n = 44)
Larval time (d) 9.5 ± 0.9 10.6 ± 1.1 8.8 ± 1.1 9.6 ± 1.0
Pupal time (d) 6.2 ± 0.4 6.4 ± 0.5 6.0 ± 0.3 6.1 ± 0.4
Larval + pupal time (d) 15.7 ± 0.9 17.0 ± 1.3 14.8 ± 0.9 15.7 ± 1.0
Pupal weight (mg) 169.0 ± 17.1 180.2 ± 17.1 175.0 ± 24.9 177.6 ± 22.1
Adult weight (mg) 65.0 ± 9.8 76.6 ± 9.1 65.9 ± 15.1 74.6 ± 10.3
Growth rate (%/d) 33.8 ± 3.4 (24) 32.1 ± 4.0 (23) 36.5 ± 4.4 (10) 34.2 ± 3.7 (24)
Sample sizes differing from those given in headings are reported in parentheses.
339
Hence, male–female ratio of adult weight remained similar regardless of tem-
perature (ranging between 86.7 and 89.7%), again resulting in parallel reaction
norms for males and females.
Population-specific differences in life-history traits
Apart from the effects of temperature and sex analysed above, alpine animals
exhibited shorter larval (three-way ANOVAs for population effects: F1,531 ¼51.8, p < 0.0001), pupal (F1,531 ¼ 116.2, p < 0.0001) and concomitantly total
post-diapause development times (F1,531 ¼ 116.0, p < 0.0001) throughout
than mid-elevation western German ones (Table 1). The shorter development
times correspond with higher larval growth rates in the alpine population
(except for females at 20 �C; F1,359 ¼ 38.3, p < 0.0001). In spite of the shorter
development times, neither pupal (F1,529 ¼ 1.2, p ¼ 0.28) nor adult weight
(F1,524 ¼ 1.2, p ¼ 0.30) differed significantly between both populations. Thus,
the faster development of alpine animals was not achieved at the cost of a
reduction in final weight. Likewise, weight at the end of diapause did not vary
considerably between populations (and sexes), the values being 6.0 ± 1.1 mg
(n ¼ 98) and 6.1 ± 1.0 mg (n ¼ 119) for western German males and females,
and 6.3 ± 0.9 mg (n ¼ 55) and 6.2 ± 0.9 mg (n ¼ 105) for alpine males and
females (two-way ANOVA for effects of population and sex on diapause
Table 2. Two-way ANOVAs for effects of temperature and sex on life-history traits in
L. h. hippothoe
Trait Source n df F p
Larval time (d) Temperature 271 3 263 1205.8 <0.0001
Sex 271 1 263 36.9 <0.0001
Temperature · sex 271 3 263 4.3 0.0059
Pupal time (d) Temperature 271 3 263 2824.6 <0.0001
Sex 271 1 263 4.5 0.0346
Temperature · sex 271 3 263 1.1 0.37
Larval + pupal time (d) Temperature 271 3 263 2606.2 <0.0001
Sex 271 1 263 37.5 <0.0001
Temperature · sex 271 3 263 4.7 0.0032
Pupal weight (mg) Temperature 270 3 262 1.5 0.23
Sex 270 1 262 42.5 <0.0001
Temperature · sex 270 3 262 2.8 0.0386
Adult weight (mg) Temperature 269 3 261 14.8 <0.0001
Sex 269 1 261 104.3 <0.0001
Temperature · sex 269 3 261 0.5 0.71
Growth rate (%/d) Temperature 216 3 208 905.8 <0.0001
Sex 216 1 208 18.8 <0.0001
Temperature · sex 216 3 208 0.7 0.58
Significant p-values are printed in bold.
340
weight; population-effect F1,373 ¼ 5.6, p ¼ 0.02, all p-values in post-hoc com-
parison >0.25; sex-effect F1,373 < 0.01, p ¼ 0.95).
Discussion
Reaction norms for age and size at maturity
Even under favourable laboratory conditions, the alpine and western German
L. hippothoe populations followed an essentially monovoltine lifecycle. Only at
Figure 1. Pupal (a) and adult weight (b) of L. h. hippothoe in relation to temperature. Boxes within
sexes marked by the same letter do not differ significantly (Tukey’s post-hoc comparison after two-
way ANOVA).
341
unrealistically high temperatures combined with long photoperiods a sub-
stantial fraction of subitaneous developers occurred. Our results revealed that
developmental temperature had in neither of these monovoltine populations a
significant effect on pupal weight. Adult weight, however, decreased with in-
creasing temperature, but the response was rather weak in both populations
(weight loss of about 13% in western German and 10% in alpine animals
between 15 and 30 �C). A post-hoc comparison (Tukey’s) failed to reveal
statistically significant weight differences between rearing temperatures, except
for the western German population, where males and females reared at 15 �Cwere heavier than those reared at all other temperatures. Although using a
fairly conservative test, these results nevertheless indicate that weight differ-
ences between individual temperature treatments were small, even though there
was a significant trend towards smaller adult weights with increasing temper-
ature over the whole range.
Note, however, that in some other studies exclusively pupal weight (or even
earlier stages) have been used, still showing in the vast majority of cases a
significant reduction in size (cf. Atkinson, 1994). In contrast to these, our results
give only weak support for the temperature-size-rule, and they clearly differ
from those previously reported for taxonomically and ecologically closely re-
lated Lycaena populations, i.e. a western Hungarian lowland population of L.
hippothoe (L. h. sumadiensis; Fischer and Fiedler, 2001) and alpine L. tityrus
Table 3. Two-way ANOVAs for effects of temperature and sex on life-history traits in
L. h. eurydame
Trait Source n df F p
Larval time (d) Temperature 276 3 268 1183.2 <0.0001
Sex 276 1 268 39.9 <0.0001
Temperature · sex 276 3 268 5.3 0.0014
Pupal time (d) Temperature 276 3 268 2215.9 <0.0001
Sex 276 1 268 0.6 0.44
Temperature · sex 276 3 268 2.8 0.0428
Larval + pupal time (d) Temperature 276 3 268 2600.8 <0.0001
Sex 276 1 268 35.8 <0.0001
Temperature · sex 276 3 268 5.1 0.0020
Pupal weight (mg) Temperature 275 3 267 1.7 0.18
Sex 275 1 267 1.7 0.19
Temperature · sex 275 3 267 0.6 0.59
Adult weight (mg) Temperature 271 3 263 6.6 0.0003
Sex 271 1 263 48.6 <0.0001
Temperature · sex 271 3 263 0.2 0.93
Growth rate (%/d) Temperature 159 3 151 798.7 <0.0001
Sex 159 1 151 21.8 <0.0001
Temperature · sex 159 3 151 2.1 0.11
Significant p-values are printed in bold.
342
(Fischer and Fiedler, 2000; see below). The latter two populations were found to
be potentially multivoltine under favourable laboratory conditions. In nature,
L. h. sumadiensis appears to be bivoltine (Fazekas and Balasz, 1979; Tolman
and Lewington, 1998). The alpine L. tityrus population is monovoltine due to
the short vegetation period in the higher altitudes of the Alps, but L. tityrus
populations next (only a few km) to the one considered here are known to be
bivoltine (Scheuringer, 1972) and gene flow is likely to occur (cf. Descimon,
1980). Both populations are hereafter referred to as oligovoltine ones.
In contrast to the monovoltine populations, an increase in temperature
caused significant reductions already in pupal as well as adult weight in directly
Figure 2. Pupal (a) and adult weight (b) of L. h. eurydame in relation to temperature. Boxes within
sexes marked by the same letter do not differ significantly (Tukey’s post-hoc comparison after two-
way ANOVA).
343
developing individuals of both oligovoltine populations (Fischer and Fiedler,
2000, 2001). In the latter, loss in adult weight between the highest and lowest
value amounted to 38.4 and 16.7% for male and female L. tityrus, and to 23.0
and 11.8% in male and female L. h. sumadiensis, respectively. The, overall,
weaker response of the monovoltine populations to increasing temperature
supports our predictions derived from the compound interest hypothesis (cf.
Meats, 1971; Partridge and French, 1996; Chown and Gaston, 1999). Mono-
voltine populations, not able to gain compound interest benefits from maturing
earlier at higher temperatures (indicating potential for extra generations), fa-
voured large size to maximise adult fitness throughout, and achieved, overall,
considerably higher weights than the oligovoltine ones (Fischer and Fiedler,
2002).
Another intriguing difference between oligo- and mono-voltine populations
is that in the former sexes showed divergent reaction norms for age and size at
maturity in response to temperature, whereas in the latter sexes responded in
the same manner to increasing temperatures. At low temperatures, males and
females of the oligovoltine populations achieved similar weights despite the
males’ shorter development time. At high temperatures, however, protandrous
males were forced into a trade-off in which they favoured early emergence over
large size, leading to a distinct weight loss. Female weight, however, was less
affected. Thus, with a rise of temperature males fell increasingly behind females
(Fischer and Fiedler, 2000, 2001). Such sex-related reaction norms were pre-
dicted from sexual selection theory, assuming a selective premium on rapid
development (protandry) for males, and on large body size (fecundity advan-
tage) in females (e.g. Fagerstrom and Wiklund, 1982; Bulmer, 1983; Roff, 1992;
Honek, 1993; Garcıa-Barros, 2000).
The occurrence of such sex-related differences, however, does complicate the
present comparison. Clearly, males of the oligovoltine populations suffered a
higher relative loss in adult weight than those of the monovoltine populations.
For females, however, adult weight loss at increasing temperatures was roughly
the same in both types of populations. We tentatively assume this to be a
consequence of the ambiguous status of these ‘oligovoltine’ populations, where
presumably fecundity selection for large body size in females is still intense,
whereas in males there is a selective premium on protandry (see above). The
potential benefits of larger female size, however, should be much lower in truly
multivoltine populations, where it should pay off to speed up development in
favour of additional generations. Such reaction norms with no or only minor
differences between sexes were generally found in various species and popu-
lations of Drosophila (e.g. David et al., 1994; McCabe and Partridge, 1997) as
well as in other insect species (for review see Atkinson, 1994).
To sum up, mono- and oligo-voltine populations showed different responses
to temperature. The mechanisms underlying these differential responses,
344
however, remain to be uncovered. Reduction in adult size at higher tempera-
tures may either be a consequence of maturing earlier in multi- and oligo-
voltine populations to augment the chance for realisation of an additional
generation (Partridge and French, 1996; Chown and Gaston, 1999). Alterna-
tively, given a common physiological constraint reducing size at higher tem-
peratures (e.g. Atkinson, 1994, 1996; Van der Have and De Jong, 1996;
Ernsting and Isaaks, 1997), there may be selection against weight reduction in
monovoltine populations to maximise adult fitness (cf. Mousseau and Roff,
1989; Roff, 1992). Both explanations would be compatible with the compound
interest hypothesis.
The striking parallelism of reaction norms across species boundaries
(L. tityrus and L. h. sumadiensis) and the pronounced variation between dif-
ferent populations of L. hippothoe clearly speak in favour of an ecological
rather than a phylogenetic explanation. Since we have found sex-specific re-
action norms in directly developing populations facing different environments
(i.e. Alps, lowland western Hungary; see also Leimar, 1996), these should not
be attributed to specific environmental conditions, but may be more wide-
spread in species living under time constraints (and where one sex is more time-
constrained than the other). The complete absence of genotype–environment
interactions between sex and temperature in monovoltine L. hippothoe popu-
lations, which should be less time-constrained than populations trying to
realise a second generation per season, emphasise the notion outlined by Nylin
and Gotthard (1998) that clear trade-offs can be expected in stressful situations
only. Without such time constraints, weight loss should be largely avoided due
to plastic growth, as was found in a number of other temperate-zone butterflies
(e.g. Nylin et al., 1989, 1993; Wiklund et al., 1991; Nylin, 1992; Gotthard
et al., 1994, 1999).
Population-specific differences in life-history traits
At all temperatures, the alpine population exhibited shorter post-diapause
development times than the western German one. Both larval and pupal stage
contributed to this developmental advantage due to higher developmental
rates. As these results are based on experiments under common environment
conditions, they do reflect inherent (i.e. genetic, possibly combined with ma-
ternal) differences between both populations, which may represent local adap-
tations to regional climates (Ayres and Scriber, 1994; Mopper and Strauss,
1998), with selection favouring high growth rates in the alpine environment
with a short growing season.
The shorter development time in the alpine population did not result in a
reduced final size, as is obvious from equal pupal and adult weights in both
populations. Hence, weight loss was avoided due to plastic growth (higher
345
growth rates), and consequently the basic trade-off between development time
and size frequently assumed in life-history theory (e.g. Roff, 1992; Stearns,
1992) did not occur. These results agree with earlier studies on butterflies which
revealed an unexpectedly high degree of plasticity in growth and development
(e.g. Nylin et al., 1989, 1996; Nylin, 1992, 1994; Gotthard et al., 1999), and
stress the importance to consider the triangular nature of relationships between
size, development time and growth rate (see Nylin, 1994; Abrams et al., 1996;
Nylin and Gotthard, 1998).
Acknowledgements
We greatly acknowledge valuable comments by D. Atkinson, W. Blancken-
horn, O. Leimar, and W. Weisser that helped to improve the quality of this
paper. Thanks are due to K. Kaminsky, C. Ruf, and A. Servant for help with
butterfly rearing. This study was supported by grants from the Friedrich-Ebert-
Foundation to K. Fischer.
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