5
Are mountain passes higher in the tropics?
Janzen’s hypothesis revisited
Cameron K. Ghalambor,1,* Raymond B. Huey,y Paul R. Martin,y
Joshua J. Tewksbury,y and George Wangy
*Department of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins,
Colorado 80523; yDepartment of Biology, University of Washington, Seattle, Washington 98195
Synopsis In 1967 Daniel Janzen published an influential paper titled “Why Mountain Passes Are Higher in the Tropics.”
Janzen derived a simple climatic-physiological model predicting that tropical mountain passes would be more effective
barriers to organismal dispersal than would temperate-zone passes of equivalent altitude. This prediction derived from a
recognition that the annual variation in ambient temperature at any site is relatively low in the tropics. Such low variation
within sites not only reduces the seasonal overlap in thermal regimes between low- and high-altitude sites, but should also
select for organisms with narrow physiological tolerances to temperature. As a result, Janzen predicted that tropical lowland
organisms are more likely to encounter a mountain pass as a physiological barrier to dispersal (hence “higher”), which should
in turn favor smaller distributions and an increase in species turnover along altitudinal gradients. This synthetic hypothesis has
long been at the center of discussions of latitudinal patterns of physiological adaptation and of species diversity. Here we
review some of the key assumptions and predictions of Janzen’s hypothesis. We find general support for many assumptions
and predictions, but call attention to several issues that somewhat ameliorate the generality of Janzen’s classic hypothesis.
Introduction
How climate shapes variation in the physiology,
ecology, and evolution of organisms is a fundamental issue for organismal biologists (Dobzhansky,
1950; Andrewartha and Birch, 1954; Pianka, 1966;
MacArthur, 1972; Brown et al., 1996; Spicer and
Gaston, 1999; Chown et al., 2004a). Biologists have
long appreciated that abiotic (e.g., temperature, solar
radiation, humidity) as well as biotic factors (e.g.,
competition, predation, parasitism) influence the conditions in which organisms can survive, grow, reproduce, and disperse (e.g., Wallace, 1878; Hutchinson,
1957; Dobzhansky, 1950; Pianka, 1966; Porter and
Gates, 1969; MacArthur, 1972; Holt, 2003). Nevertheless, the relative importance of these abiotic and
biotic processes, and of their interactions, remains
unsettled (Dobzhansky, 1950; MacArthur, 1972;
Schemske, 2002; Chase and Leibold, 2003).
Studies of tropical versus temperate-zone organisms
have been central to these debates, largely because latitudinal gradients in climate are striking and co-vary
with conspicuous gradients in species diversity of many
taxa (Wallace, 1878; Dobzhansky, 1950; Pianka, 1966;
Brown and Lomolino, 1998; Willig et al., 2003).
A seminal contribution here is Janzen’s (1967) paper
“Why mountain passes are higher in the tropics.”
This paper developed a conceptual framework for
examining how latitudinal variation in climate should
shape the evolution of physiological tolerances and,
in turn, should determine topographic resistance to
dispersal and, through this, influence geographic
range size. For many biologists, Janzen’s paper provides
a logical—indeed a necessary—starting place for discussions of latitudinal gradients in species diversity,
physiological adaptation, and related phenomena.
Janzen (1967) began by explicitly assuming that
the effectiveness of a topographic barrier to dispersal
depends mainly on the magnitude of the temperature
gradient across that barrier and less on the actual
change in altitude. Thus mountain passes are physiological, not topographic, barriers to dispersal.
Consequently, a mountain pass will be a greater
physiological barrier if there is relatively little overlap
in climate between a low-altitude valley and an adjacent
high-altitude pass.
Janzen next argued that the greater seasonal uniformity of temperature at tropical localities would 1)
necessarily result in low overlap in climate between
valleys and mountain passes and 2) select for organisms
that had narrow tolerances to temperature. He then
linked these assumptions and predicted that tropical
organisms would have greater difficulty crossing
From the symposium “Adaptations to Life at High Elevation” presented at the annual meeting of the Society for Integrative and
Comparative Biology, January 4–8, 2005, at San Diego, California.
1 E-mail: cameron1@lamar.colostate.edu
Integrative and Comparative Biology, volume 46, number 1, pp. 5–17
doi:10.1093/icb/icj003
Advance Access publication January 6, 2006
Ó The Society for Integrative and Comparative Biology 2006. All rights reserved. For permissions, please email: journals.permissions@
oxfordjournals.org.
6
mountain passes (than would temperate-zone organisms) because they would be more likely to encounter
a climate to which they were not adapted. Or, to use
Janzen’s own evocative words, “mountain passes are
‘higher’ in the tropics.” Reduced dispersal across
tropical passes should in turn lead to greater genetic
divergence between populations, enhance allopatric
speciation, and potentially result in greater species
packing along altitudinal gradients.
The individual steps in Janzen’s model seem logical
and obvious in retrospect, but that was not the case in
the 1960s. Janzen was ahead of the crowd and raised
ideas that would later be embraced as areas of productive research. He was thinking about barriers to dispersal and isolation of populations long before many
biologists became sensitized to population fragmentation. He appreciated the ecological and evolutionary
effects of climate on physiological tolerances and capacities long before the field of evolutionary physiology
existed (Garland and Carter, 1994). And, he challenged
the contemporary dogma of his day (e.g., Dobzhansky,
1950), which held that abiotic (climate) effects
dominated ecological and evolutionary patterns in
the temperate zones, whereas biotic effects dominated
in the tropics. Thus, Janzen provided a novel perspective for the crucial role abiotic effects and physiological tolerance could play in understanding patterns
observed in the tropics.
In the decades since its publication, Janzen’s (1967)
hypothesis has remained at the center of debates of
latitudinal gradients in diversity and ecology (e.g.,
Schemske, 2002); and it has inspired numerous studies
in physiology, biogeography, and evolutionary ecology.
Nevertheless, many of the assumptions and predictions
of this hypothesis have never been systematically tested
or critically evaluated.
Here we revisit Janzen’s (1967) hypothesis. Our
goals are to highlight general patterns and to lay a
foundation for future inquiry, not to present an
exhaustive review. We begin by re-describing the
hypothesis itself. Then we evaluate some of its key
assumptions and predictions. We draw largely on studies of vertebrate ectotherms (amphibians and lizards)
because these taxa are physiologically sensitive to variation in temperature, because they have been studied
extensively from the perspective of Janzen’s hypothesis,
and because these studies have used relatively consistent methodologies, thus facilitating comparisons.
Janzen’s hypothesis: A précis
Janzen’s hypothesis is a consequence of a series
of logical steps. It emerges fundamentally from
considerations of climatic variation, then from the
evolutionary impact of that variation on physiology,
C. K. Ghalambor et al.
and finally from the role of physiology in determining
differences in dispersal and biogeographic patterns
between temperate and tropical environments.
That thermal regimes are more constant in the
tropics compared to the temperate zones has long
been common knowledge. However, Janzen focused
on the consequences of that tropical constancy on
the overlap in thermal regimes of sites separated by
altitude (Fig. 1). He noted that lowland forests in the
tropics are always warm, whereas high-altitude tropical
forests are always significantly cooler. Consequently,
altitudinally separated sites in the tropics will have
little overlap in their thermal regimes at any given
time or even over the course of a year (Janzen, 1967,
pp. 236–237). Temperate zones show a strikingly different pattern because both low- and high-altitude sites
experience marked seasonal variation in temperature.
So, even though high-altitude sites in the temperate
zones are of course colder at any given season than
are low-altitude ones, high-altitude sites can nonetheless be warm in summer, whereas low-altitude
sites can be cool in winter (Fig. 1). As a result, both
low- and high-altitude sites in the temperate zones
have considerable overlap in thermal regimes, at least
computed over a full year (Janzen, 1967, pp. 236–237).
Janzen next explored the physiological consequences
of climate variation. He explicitly assumed that
organisms should evolve physiological adaptations
that reflect the range of climatic variation typically
encountered. Thus, temperate zone organisms would
need to evolve broad thermal tolerances as well as
marked acclimation capacities to cope with the large
seasonal changes in climate. In contrast, tropical
organisms would evolve narrow thermal tolerance
and reduced acclimation responses, appropriate to
the less variable climate of the tropics.
Janzen melded these climatic and physiological considerations into a bold prediction: tropical mountain
passes should be more effective barriers to dispersal
than temperate-zone passes of equivalent altitude,
simply because tropical organisms attempting to
move up (or down) a mountain would likely encounter
temperatures to which they are neither adapted nor
acclimated. By contrast, temperate-zone organisms
should be less constrained by temperature when
moving up or down a mountain pass. Thus, mountain
passes should be physiologically “higher in the tropics”
and impose greater fitness costs to dispersal (Fig. 1).
Janzen (1967) emphasized that he did not intend his
model to serve as an explanation for tropical species
diversity. Even so, his model is relevant to this issue
because his arguments lead directly to the prediction
that altitudinally separated populations in the tropics
will experience reduced gene flow leading to greater
7
Janzen’s hypothesis revisited
low altitude
30
20
high altitude
10
narrow
physiological
tolerance
with
no overlap
increased
costs to
dispersal
over a
climate
gradient
low
gene
flow
high
allopatric
speciation
broad
physiological
tolerance
with overlap
reduced
costs to
dispersal
over a
climate
gradient
high
gene
flow
low
allopatric
speciation
0
Temperature oC
-10
-20
tropical
30
low altitude
20
10
0
high altitude
-10
-20
temperate
Month
Fig. 1 A schematic summary of the primary steps making up Janzen’s (1967) hypothesis. Shown are seasonal changes in air
temperature for low- and high-altitude sites in a tropical and temperate location. Seasonality in the temperate zone
results in the broad overlap of temperatures experienced at high and low altitudes, but a lack of seasonality in the
tropics results in no overlap. Janzen (1967) assumed that this climatic difference should favor broadly overlapping
physiological tolerances between populations separated by altitude in the temperate zone, leading to a reduction in
dispersal costs over an altitudinal gradient. In contrast, he predicted that a lack of seasonality in the tropics should favor
narrow tolerances with little overlap between populations separated altitudinally and increased costs to dispersal.
The obvious consequence is high rates of dispersal and gene flow between populations and a reduction in the potential
for population differentiation and speciation in the temperate zone, whereas, in the tropics gene flow is reduced and
the potential for population differentiation and speciation is increased.
genetic divergence, setting up the conditions that favor
accelerated rates of allopatric speciation (Fig. 1).
Key assumptions
At first glance, Janzen (1967) provides a simple and
elegant hypothesis that links climate, physiology,
and dispersal. However, many of the assumptions
underlying the hypothesis have not been critically
examined. Here we revisit four key assumptions and
then examine the main predictions derived from this
hypothesis.
Assumption 1: The effectiveness of a
topographic barrier depends on the magnitude
of the temperature gradient across that barrier
Janzen (1967, p. 234) proposed that a mountain pass is
a barrier to dispersal primarily because of the climatic
challenges it imposes on the physiology of organisms.
For example, a lowland organism, which should be
adapted to warm temperatures, might not be able to
withstand the low temperatures it would encounter at
high elevations when attempting to cross a mountain
pass. Janzen presented no data to bolster this assumption. Mountains do pose significant barriers to
dispersal in diverse taxa (e.g., Slechtova et al., 2004;
Forister et al., 2004; Funk et al., 2005; Huey and
Ward, 2005). For example, the lowland Puerto Rican
lizard (Anolis cristatellus) survives for only a few hours
at the minimum temperatures occurring at 600 m
(Heatwole et al., 1969). However, it is still not
known whether the cause is the magnitude of the
temperature gradient or some other factor that covaries
with altitude, or even whether the same factors are
most important across taxa. Even so, Janzen’s assumption seems reasonable for many ectotherms, though it
would be less so for endotherms, which are relatively
buffered against environmental temperatures (Porter
and Gates, 1969). Identifying the relative importance
of temperature in constraining dispersal patterns
across altitudinal gradients is of growing interest
given the prospects for climate change (e.g., Porter
et al., 2002).
We see two general ways to test the role of temperature change in limiting dispersal across a topographic
barrier. One approach involves developing theoretical
8
models that integrate operative environments (see
ASSUMPTION 2, below), bioenergetics and physiological
structure, with population dynamics (Dunham et al.,
1989; Porter, 1989; Dunham, 1993; Porter et al., 2002;
Buckley and Roughgarden, 2005). Such models are
increasingly powerful and predict how population
energetics and dynamics change with local climate
(Porter et al., 2002). Thus, one test of Janzen’s assumption is to compute the “potential” altitudinal ranges of
tropical and temperate zone species; if Janzen is correct,
tropical species should have narrower potential ranges
than do temperate zone species.
Alternatively, one might do reciprocal transplants,
such as to transplant low-altitude individuals to various higher altitudes, and then determine empirically
the maximum altitude at which they can grow and
reproduce. An elegant example of this approach is
work by Angert and Schemske (2005) with monkey
flowers (Mimulus) in the Sierra Nevada of California.
If parallel studies were done on related species in both
the tropics and in the temperate zones, one could
not only determine whether tropical species sustain
populations over smaller altitudinal ranges than do
temperate zone species, but also elucidate the role climatic factors play in causing variation in fitness (see
Angert and Schemske, 2005). To our knowledge, such
matched reciprocal transplants have never been done
at different latitudes.
Note that neither approach quantifies dispersal ability per se; rather, they estimate the range of altitudes
over which populations are sustainable. Certainly,
many animals can disperse though environments that
are otherwise unsuitable on a long-term basis, yet few
studies to date have considered how habitat suitability
shapes dispersal and colonization patterns. One weakness of this assumption—as Janzen appreciated—is
that many environmental variables (not just temperature) may influence dispersal patterns and altitudinal
ranges (Porter, 1989; Porter et al., 2002; Gaston, 2003;
Navas, 2005). For example, biotic interactions such as
interspecific competition can also modulate ranges
(Davis et al., 1998a, b; Porter et al., 2002; Case et al.,
2005; Buckley and Roughgarden, 2005). Moreover, different taxa may be limited by different variables—
many plants, for example, may be limited by patterns
of water availability, rather than temperature (Hawkins
et al., 2003). Thus even if insects that rely on these
plants are limited by temperature, they are further constrained by the precipitation requirements of their host
(Huey, 1978). Further, slope, insolation (i.e., amount
of incoming solar radiation), and canopy structure
could also have important interacting influences
(Porter et al., 2002). Thus, testing this assumption
by identifying factors responsible for limiting dispersal
C. K. Ghalambor et al.
and geographic ranges is likely to be a challenging
endeavor.
Assumption 2: Latitude, seasonality, and
altitude influence between-altitude
climate overlap
Janzen’s second assumption is actually an insightful
observation; the overlap in temperature regimes over
a year between low- and high-altitudes is greater in the
temperate zones than in the tropics. He illustrated
this pattern with graphs of seasonal changes in air
temperature at low- versus high-altitude sites from
the tropics and the north temperate zone and showed
quantitatively that the between-altitude overlap in temperature was much greater in the temperate zone than
in the tropics (Janzen, 1967, pp. 235–237).
Janzen’s global climatic template is inarguable, as
it follows directly from the angle of the earth’s axis
of rotation relative to the sun (MacArthur, 1972),
and from the independence of the adiabatic lapse
rate with latitude (Dillon et al., 2005). Even so, several
other climatic issues complicate this pattern and
have implications for the patterns of physiological
adaptation we should expect in temperate and tropical
organisms. We discuss some of these complications
below.
Janzen’s hypothesis is driven by the greater seasonal
variation in temperature in temperate, but not tropical locations. However, marked seasonal variation
occurs mainly at temperate latitudes in the northern
hemisphere (Addo-Bediako et al., 2000; Chown et al.,
2004b), not in the southern hemisphere (Fig. 2A),
where the proximity of the oceans buffers winter temperatures (Addo-Bediako et al., 2000; Chown et al.,
2004b). Thus, had Janzen (1967) compared temperatures from his sites in Costa Rica with sites in either the
southern Andes or southern Africa, he might have been
somewhat less impressed about the “low” height of
temperate zone mountain passes. Moreover, increased
seasonality at high latitudes in the northern hemisphere is primarily driven by cold winter temperatures
(Fig. 2B), as warm summer temperatures vary less with
latitude (Fig. 2C). Clarifying these climatic patterns
is important, because they provide insight into the
kinds of physiological adaptations to temperature we
should expect at a global scale. For example, if large
seasonal changes in temperature select for a broad
physiological tolerance, then organisms at high latitudes in the northern hemisphere should show a much
greater tolerance than organisms occupying equivalent
latitudes in the southern hemisphere: this is indeed the
case in insects (Addo-Bediako et al., 2000). In addition,
because seasonality is primarily driven by cold winter
temperatures in the north, physiological adaptation
9
Janzen’s hypothesis revisited
Fig. 2 Global patterns of temperature, showing that the
latitudinal gradient of seasonality is caused mainly by a
strong gradient in minimum temperature and less by the
gradient in maximum temperature. A) Seasonal range
of monthly temperatures (mean daily maximum of the
warmest month minus mean daily minimum of the
coldest month). B) Mean daily minimum temperature
of the coldest month. C) Mean daily maximum
temperature of the warmest month. In A) colors are
normalized to the site with the least seasonal variation
(set to deep blue). In B) colors are normalized from
the coldest site (set to deep blue). In C), colors are
normalized to the warmest site (set at deep red);
Temperature data are from New et al. (1999) and are
based on 720 by 360 grid.
should be driven more by a greater tolerance to cold
temperatures at high latitudes in the north, rather
than greater tolerance to warm temperatures near
the equator (see below).
Janzen (1967) emphasized the importance of
seasonal changes in temperature. However, diurnal
changes in temperature may also drive physiological
adaptation, especially at high altitudes in the tropics
(see below). Insolation and diurnal temperature
fluctuations are generally greater in low-latitude
mountains than in high-latitude ones and the magnitude of daily variation in temperature at high altitude
in the tropics can match the magnitude of seasonal
variation in the temperate north because of the combination of thin air and the vertical angle of the sun’s
rays near the equator (Mani, 1968; Sarmiento, 1986).
In other words, at very high altitudes in the tropics,
organisms may experience summer-like conditions
during the day and winter-like conditions at night
every day of the year (Mani, 1968). Therefore, with
increasing altitude, tropical organisms might need to
evolve relatively broad thermal tolerances to cope with
the increasing diurnal changes in temperature, contrary
to Janzen’s expectation.
Although seasonal variation in atmospheric temperature is great at high latitudes, organisms living
there might nonetheless be active over relatively narrower ranges of body temperatures or simply escape
unfavorable climatic conditions. Indeed, ectotherms
typically restrict activity periods in seasonal environments (Stevenson, 1985) and spend prolonged periods
of hibernation in protected sites (Mani, 1968).
Similarly, endothermic mammals and birds at northern
latitudes exhibit hibernation and migration strategies
that have no equivalent in the tropics. As a result,
selection for broad thermal tolerances might be somewhat buffered by behaviors that shield organisms
from the seasonal changes in temperature (Huey
et al., 2003).
Finally, Janzen (1967) focused on air temperature
(Ta). However, the thermal environment of organisms
is now known to be better characterized (Bakken, 1992)
by operative environmental temperature (Te, for ectotherms) and by standard operative temperature (Tes,
for endotherms). Because Te estimates the equilibrium
body temperature of a specified ectotherm in a specified spot, it is the preferred index of the thermal
environment for ectotherm physiology and ecology
(Huey, 1991; Dunham, 1993; Porter, 1989). An interesting exercise would be to repeat Janzen’s computation of between-altitude overlap in climate but to
substitute Te for Ta.
Assumption 3: Tropical organisms have
narrow ranges of thermal tolerance
independent of altitude
Janzen (1967, p. 241) stated that organisms are less
likely to “. . . evolve mechanisms to survive at a given
temperature if that temperature falls outside of the
temperature regime of the organism’s habitat than if
10
it falls within it.” Thus, tropical organisms should have
relatively narrow thermal tolerances (e.g., difference
between critical thermal minimum [CTmin] and
maximum [CTmax] temperatures). This assumption
can be tested with various lines of evidence. First, determine whether tropical organisms experience narrower
ranges of body temperature (Tb) than do temperate
zone organisms. Second, determine whether tropical
organisms have narrow tolerance zones. Finally, determine whether overlap in thermal tolerance among altitudinally separated tropical populations is reduced
compared to temperate populations. If Janzen is
right, then high-altitude populations in the tropics
should have a narrow tolerance for cooler temperatures
whereas low-altitude populations should have a narrow
tolerance for warmer temperatures.
Testing whether tropical organisms experience a
more narrow range of Tb can be complicated by a
variety of factors. For example, even though temperate
zones have relatively variable thermal regimes, organisms living there might not have relatively variable
Tb: as noted above, many ectotherms have effective
thermoregulatory behaviors that reduce variation in
Tb (Stevenson, 1985), and others simply migrate or
hibernate during cold periods. Unfortunately, data
on latitudinal patterns in Tb variability (on daily or
seasonal bases) have not been compiled systematically. However, despite these complications, the few
available compilations of Tb data are consistent with
the expectation that Tb variability is reduced in the
tropics and increases with latitude in both salamanders
(Feder and Lynch, 1982) and lizards (van Berkum,
1988).
Do high- and low-altitude tropical species have
narrow ranges of Tb? Janzen assumed this was the
case. However, the marked diurnal shifts in operative
environmental temperature at high altitude in the
tropics (driven by high radiant heat loads during the
day and by cold nights) might well increase variance in
Tb. For example, Tb of the lizard Liolaemus multiformis
(at altitude of 4,300 m in tropical Peru) covers a large
range from 7 to 33 C during the day (Pearson and
Bradford, 1976) which is comparable to temperate
zone lizards. To date, the magnitude of such diel shifts
remains to be quantified systematically with respect to
latitude, but this pattern is supported by data on other
lizards (van Berkum, 1988). Moreover, mean Tb often
decreases with increasing elevation in tropical
Anolis lizards (Heatwole et al., 1969; Ruibal and
Philibosian, 1970; Huey and Webster, 1976; Hertz,
1981; van Berkum, 1986) and in tropical salamanders
(Feder and Lynch, 1982). Interestingly, mean Tb also
drops significantly with increasing altitude in
tropical Sceloporus (a group known to use behavioral
C. K. Ghalambor et al.
thermoregulation) but not in temperate zone species
(Andrews, 1998), suggesting that tropical species at
high altitude are either more cold adapted or unable
to maintain a preferred Tb (Andrews, 1998).
Do temperate zone organisms have relatively broad
thermal tolerances, and (if so), do broad thermal
tolerances reflect mainly increased cold tolerance,
as suggested by climate data (Fig. 2)? Using data
from Brattstrom (1968) for amphibians, Snyder and
Weathers (1975) showed that tolerance ranges do
increase with latitude and consistent with the observation that temperate zone seasonality is driven by
cold winter temperatures, this increase is driven by
changes in CTmin (Fig. 3). Van Berkum (1988) and
Addo-Bediako et al. (2000) found similar latitudinal
patterns for lizards and insects, respectively, and
in both cases, changes in CTmin are greater than
changes in CTmax. Furthermore, high-latitude insects
in the Southern Hemisphere have markedly narrow
tolerance ranges compared to high-latitude species in
the Northern Hemisphere, again consistent with the
greater seasonality in the Northern Hemisphere
(Addo-Bediako et al., 2000; Fig. 2A).
Are narrow tolerance ranges characteristic of
high-altitude as well as low-altitude species in the
tropics, and are high-altitude tropical species specialized for lower temperatures than are low-altitude
tropical species? Janzen’s (1967) hypothesis assumes
that both are true and are the underlying reason for
greater species turnover along tropical altitudinal
gradients. However, although lowland tropical species
generally do have narrow tolerance ranges (see above),
results to date show that high-altitude tropical
species in fact can have broad tolerance ranges. For
example, tolerance ranges (Brattstrom, 1968) of some
high-altitude tropical amphibians converge on those
of high-altitude temperate zone species (Fig. 4),
mainly because CTmin declines relatively quickly
within increasing altitude in tropical species. More
recent studies (e.g., Navas, 1996, 2005; Luddecke and
Sanchez, 2002) also suggest that high-altitude tropical
amphibians perform well over broader ranges of
temperatures than do their low-altitude counterparts.
This pattern of increasing tolerance with increasing
altitude in the tropics is also observed in the less seasonal south temperate zone. For example, lizards from
the mountains of central Chile (Carothers et al., 1997)
and southeastern Australia (Spellerberg, 1972), which
are temperate zone areas but with low seasonality
(Fig. 2A), also show an increasing thermal tolerance
range with increasing altitude, mainly because of a
decline in CTmin, suggesting that physiological adaptations to altitude may be similar between the tropics and
south temperate zone.
11
Janzen’s hypothesis revisited
Temperature (˚C)
40
30
10
CTmin
CTmax
0
-10
0
10
20
30
40
50
60
Latitude
Fig. 3 Changes in CTmin and CTmax as function of latitude in frogs (data from Brattstrom 1968). CTmin (filled circles) is
highly negatively correlated with latitude (p < 0.05). CTmax (open circles) represent individuals acclimated between
26 C and 30 C (if more than one value existed for a species the mean was taken). CTmax is not correlated with latitude
(p > 0.70).
Temperature (C)
40
30
CTmin-max tropical
10
CTmin-max temperate
0
-10
0
500
1000
1500
2000
2500
3000
Altitude (m)
Fig. 4 Change in CTmin and CTmax in temperate and tropical frogs as a function of altitude (data from Brattstrom 1968).
CTmin decreases with increasing altitude in tropical (p < 0.01), but not temperate (p > 0.05) frogs, although the
interaction between the two is not significant (p ¼ 0.12). CTmax was not significantly correlated with altitude (p > 0.05)
in both tropical and temperate species.
Compilations of temperature and tolerance data
are obviously limited, but available patterns are
generally consistent with Janzen’s (1967) assumptions.
Specifically, north temperate zone species have more
variable Tb than do tropical species; and temperate
zone species have relatively broad thermal tolerances,
primarily because they are much more cold tolerant.
Nevertheless, tropical species living at high altitude
can have variable Tb and can also be relatively cold
and warm tolerant, probably reflecting the consistent
cooler temperatures and marked diurnal, rather than
seasonal, shifts in temperature (see also Gaston and
Chown 1999). Thus, not all tropical species have narrow
tolerance ranges. What are needed now are systematic
studies that explore how altitude affects Tb variation,
tolerance zones and “performance breadths,” for
12
example, VanDamme et al. (1989) of tropical and
temperate-zone representatives of diverse taxa.
Assumption 4: Tropical organisms evolve
limited acclimation responses
Janzen (1967) predicted that tropical organisms would
not only have relatively small tolerance zones, but also
have limited acclimation responses. Janzen presumably
assumed that acclimation is favored only in seasonal
environments, where the benefits of physiological compensation would outweigh the costs (e.g., Hoffmann,
1995) of maintaining the capacity to acclimate.
Only a year after Janzen (1967), Brattstrom (1968)
reported that temperate and tropical amphibians had
similar ranges of acclimation ability. (Note: Most of
Brattstrom’s tropical species died (85%) when
acclimated to low temperature (5 C), whereas most
of his temperate zone species survived (25%). Thus,
acclimation responses were measured over a relatively
broader range of temperatures for the temperate zone
species, confounding this tropical versus temperatezone comparison.). However, subsequent studies
generally support Janzen’s expectation (1967) that
acclimation responses increase with latitude. Feder
(1978, 1982) found that all temperate zone amphibians
(N ¼ 22) showed significant acclimation of metabolism to temperature, but that only one of seven tropical species did so. Similarly, Tsuji (1988) showed that
two populations of a temperate zone lizard showed
greater metabolic acclimation to temperature than
did a related tropical species.
Whether acclimation ability varies with altitude
for a broad range of organisms (and does so differently
in the tropics and the temperate zones) is currently
unclear, simply because too few studies are available.
Interestingly, however, those tropical amphibians
that do show acclimation responses are from high
altitude (see Brattstrom, 1968). Moreover, Patterson
(1984) found that high-altitude (but not lowaltitude) populations of the lizard Mabuya striata
from tropical Africa exhibit significant thermal acclimation in resting metabolic rate. In contrast, Rogowitz
(1996) found no difference in acclimation between
high- and low-altitude Anolis from Puerto Rico; but
the maximum altitude on Puerto Rico is less than
1400 meters.
Although empirical data are limited, tropical
organisms—at least low-altitude ones—seem to show
relatively limited acclimation responses as Janzen
(1967) expected. Even so, some tropical species at high
altitude may experience selection for enhanced acclimation in response to diurnal rather than seasonal
fluctuations in temperature.
C. K. Ghalambor et al.
Main predictions: Tropical organisms have
reduced dispersal across elevational gradients
and have reduced between-altitude overlap
of their distributions
Janzen’s (1967) main prediction is that mountain
passes in the tropics are more effective “physiological”
barriers to dispersal than are passes in the temperate
zones. If so, two patterns should be evident. First,
tropical species should have relatively reduced rates
of dispersal up and down mountains. Second, tropical
species should have relatively restricted altitudinal
ranges, such that between-altitudinal faunal and floral
overlaps would be reduced.
Is dispersal reduced along tropical mountains?
Unfortunately, such dispersal rates of tropical and
temperate zone species have never been systematically compared, at least to our knowledge. However,
molecular markers are increasingly being used to
track patterns of gene flow; and this may represent a
future opportunity to quantify the magnitude of dispersal patterns. The few studies along these lines so far
hint that gene flow may be reduced in the tropics as a
whole and also between tropical populations separated
by altitude; Martin and McKay (2004) found that tropical species exhibit greater isolation by distance than
do temperate species, consistent with an expectation of
reduced dispersal in the tropics. In addition, tropical
populations show reduced gene flow and greater isolation by distance in various insect (e.g., Eber and
Brandl, 1994; West and Black, 1998; Aulard et al.,
2002) and plant species (Arias and Rieseberg, 1994;
Murillo and Rocha, 1999; Thomas et al., 2002).
Nevertheless, much more data are required before
we know whether latitude influences altitudinal resistance to dispersal, much less the mechanisms behind
those patterns.
Are altitudinal ranges relatively restricted in the
tropics? Here available data are strongly supportive:
this pattern seems general and is documented in comparisons of herpetofaunas (e.g., Heyer, 1967; Wake and
Lynch, 1976; Huey, 1978; Navas, 2002), birds (e.g.,
Terborgh, 1977; Rahbek, 1997; Rahbek and Graves,
2001; Herzog et al., 2005), and plants (e.g., Smith,
1988; Lieberman et al., 1996). Similarly, betweenaltitudinal faunal similarity of amphibians and reptiles
is reduced in the tropics (Wake and Lynch, 1976;
Huey, 1978; Fig. 5).
These biogeographic patterns are consistent with
Janzen’s (1967) predictions, but again the underlying
mechanisms for these patterns remain largely untested.
To be sure, the mechanisms limiting ranges are more
complex than outlined in Janzen (1967), who focused
more on current dispersal patterns and not necessarily
Janzen’s hypothesis revisited
13
Fig. 5 Patterns of between-altitude faunal overlap for lizards, snakes, and frogs versus latitude. Faunas separated
by altitude are much more similar in the temperate zone than in the tropics (based on data in Huey, 1978).
on geographic ranges and distributions. Indeed, altitudinal ranges are often limited by biotic factors,
not just by physiological ones (Davis et al., 1998a,b;
Gaston, 2003; Navas, 2005). For example, limits to
ranges are thought to be constrained—or at least
influenced—by biotic factors such as interspecific competition (Case and Taper, 2000; Case et al., 2005),
predation (Dekker, 1989), and parasitism (Briers,
2003). Thus many factors are likely to influence
altitudinal range limits, and the primary factors setting
these limits may vary even among close relatives
(Carothers et al., 1997, 2001).
Discussion
Was Janzen (1967) right? Are mountain passes higher
in the tropics? A definitive answer to this question
remains elusive because of the difficulty in linking
patterns to underlying processes. Nevertheless, considerable evidence supports many of the major
assumptions and predictions.
Not surprisingly, Janzen’s (1967) global climatic
template is valid for temperature. Temperate zone
sites do show much greater seasonal variation in ambient temperature than do tropical sites. Moreover,
altitudinally separated sites in the temperate zones
have greater overlap in ambient temperature than do
similarly separated sites in the tropics. However, the
seasonality of the temperate zones is now realized to
be primarily a Northern Hemisphere phenomenon
(Fig. 2), because the proximity of southern landmasses
to oceans buffers climatic extremes there (see also
Addo-Bediako et al., 2000; Chown et al. 2004b).
Thus tropical mountain passes may be higher than
north temperate zone passes, but they are probably
less so compared with south temperate zone ones.
Moreover, high-altitude tropical sites can also experience greater daily fluctuations in temperature compared to similar altitudes in temperate locations.
In any case, Janzen’s global climatic template needs
to be recomputed using operative environmental temperatures rather than ambient temperature (Bakken,
1992) and to allow for expression of behavioral and
other adaptations that buffer variation in ambient
temperatures (Stevenson, 1985; Cossins and Bowler,
1987; Huey et al., 2003).
Janzen (1967) expected that the observed variation
in climatic patterns would influence the evolution of
physiological capacities. Specifically, he expected that
body temperature variation, thermal tolerance ranges,
and acclimation capacities would all increase with
latitude. Available data generally support this expectation. However, some tropical species living at higher
altitudes also appear to experience variable body
temperatures, have broad tolerance ranges, and can
acclimate to temperature; a result not anticipated by
Janzen but still consistent with his general assumption
that organisms adapt or acclimate to the temperatures
they normally encounter.
What about Janzen’s (1967) prediction that dispersal
up and down a tropical mountain should be restricted
relative to that in the temperate zones? To our knowledge, this prediction has never been directly tested.
However, many tropical species have greater isolation
by distance (Martin and McKay, 2004), do occupy
relatively narrow altitudinal distributions (Wake and
Lynch, 1976; Huey, 1978), and do show reduced overlap in altitudinal ranges (Huey, 1978: Lieberman et al.,
1996; Rahbek and Graves, 2001). This is consistent
with the prediction that altitudinal dispersal is more
restricted in the tropics than in the temperate zones
(Wake and Lynch, 1976; Huey, 1978).
In addition to predicting that tropical mountains
should be “higher,” Janzen also predicted that tropical
valleys should be “lower” for high-altitude species
(Janzen, 1967, p. 243). However, the evidence to
date suggests that high-altitude tropical species
have broader thermal tolerances than do low-altitude
14
species, primarily because they have relatively greater
tolerance to cold (Figs. 3 and 4; Navas, 2005).
Therefore, resistance to dispersal up versus down tropical mountains may be asymmetric. Lowland tropical
species may be restricted to low altitude because of
their limited tolerance to cold (e.g., Heatwole et al.,
1969); but upland tropical species, which do have
high heat tolerance as well as cold tolerance, should
be able to move to relatively low altitudes.
How can we reconcile these patterns of thermal tolerance with the observed narrow altitudinal bands
occupied by many upland tropical taxa? One possible
explanation could involve evolutionary trade-offs
between broad thermal tolerances and the competitive
environment. For example, if broad thermal tolerances
evolve at a cost to performance and to competitive
ability (Huey and Slatkin, 1976; Gilchrist, 1995),
then high-altitude species might be unable to disperse
to lower elevations because they are competitively
inferior to lowland species, not because they are
physiologically incapable of surviving there. Indeed,
range limits along elevational and other environmental
gradients often reflect interactions between physiological tolerance and competitive interactions and are
common in a wide range of taxa (Connell, 1961;
Bovbjerg, 1970; Jaeger, 1971a, b; Morse, 1974; Chappell,
1978; Bertness, 1981a, b; Connell, 1983; Robinson and
Terborgh, 1995; Griffis and Jaeger, 1998; Martin and
Martin, 2001). Reciprocal removal studies might be an
ideal way of determining whether biotic interactions
prevent high-altitude species from moving down a
tropical mountain and physiological constraints limit
low-altitude species from moving up.
Short-comings and caveats
The empirical data reviewed here represent an attempt
to bring together a disparate literature on climate, thermal tolerance, acclimation ability, geographic ranges,
and patterns of diversity. Unfortunately, no single
study has examined all of the assumptions and predictions of Janzen’s hypothesis; so our data are necessarily
cobbled together from diverse studies, many of which
were motivated by concerns other than Janzen’s hypothesis (1967). This is hardly a strong foundation for
comparative studies. Moreover, the comparative data
we review here needs to be re-analyzed using phylogenetically based comparative methods (Felsenstein,
1985; Garland et al., 1999); and future studies also
need to control for parental and environmental effects
that can confound the genetic basis of trait values
(Garland and Adolph, 1991).
A more difficult challenge in testing many of the
predictions of Janzen’s hypothesis is that similar
predictions emerge from other biogeographical,
C. K. Ghalambor et al.
climatic, and historic hypotheses for latitudinal variation in population differentiation and speciation. For
example, historic patterns of glaciation, lower energy
at higher latitudes, and/or colder temperatures during
the winter may cause higher rates of population extinctions, leading to higher recolonization rates at high
latitudes (Martin and McKay, 2004). This process of
extinction and recolonization can degrade both local
adaptation to climate and population differentiation,
resulting in similar patterns as those predicted by
Janzen (Martin and McKay, 2004). Nevertheless, systematic tests of the assumptions and predictions of
Janzen’s hypothesis provide an opportunity to merge
studies of climate, physiology, evolutionary ecology,
and biogeography under a common conceptual
framework.
Final thoughts
We have focused our review primarily on studies of
vertebrate ectotherms, a group that should be sensitive
to the climatic (Porter and Gates, 1969) and physiological concerns raised by Janzen (1967). Whether
other taxa show congruent patterns needs to be determined. Plants might show even more pronounced patterns: plants have limited ability to use behavior to
avoid environmental influences and thus may experience stronger selection for physiological tolerance as
well as greater population isolation (Bradshaw, 1965;
Huey et al., 2002). Endotherms, on the other hand,
might show less pronounced patterns, because these
organisms are relatively well buffered from climatic
concerns (Porter and Gates, 1969). Birds, with their
high mobility, might be even less impressed by the
height of tropical mountain passes. These questions
are important not only for testing the generality of
whether mountain passes are higher in the tropics
but also for generating testable hypotheses for linking
climatic variation to the physiology, ecology, and
evolution of species. In the face of a rapidly changing
climate, the ability to make informed decisions about
how certain groups (plants vs. animals) or certain
communities (tropical low elevation vs. tropical high
elevation) might respond is a pressing problem for
organismal biologists. These are all appealing issues,
and stand as a legacy of opportunities opened by
Janzen (1967).
Acknowledgments
We thank Dan Janzen for his many contributions
to tropical biology and especially for Janzen (1967).
We thank Doug Altshuler and Robert Dudley for
inviting us to participate in this symposium and
SICB for partial support. This manuscript was
Janzen’s hypothesis revisited
improved by comments and discussions with Robert
Ricklefs, Dionna Ghalambor, Helen Sofaer, Che del
Agua, and an anonymous reviewer. This research
was funded in part by NSF grant IBN-0111023 to
CKG and NSF grant IOB-0416843 to RBH.
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