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Received: 30 January 2020 Accepted: 23 June 2020
DOI: 10.1113/EP088502
RESEARCH PAPER
Are humans evolved specialists for running in the heat? Man vs.
horse races provide empirical insights
Lewis G. Halsey1Caleb M. Bryce2
1Department of Life Sciences, University of
Roehampton, London SW15 4JD, UK
2Botswana Predator Conservation Trust,
Private Bag 13, Maun, Botswana
Correspondence
Lewis G. Halsey,Department of Life Sciences,
University of Roehampton, London SW15 4JD,
UK.
Email: l.halsey@roehampton.ac.uk
Edited by: Michael White
L. G. Halsey and C. M. Bryce contributed
equally to this work.
Funding information
This research received no specific grant from
any funding agency in the public, commercial or
not-for-profit sectors.
Subject Area: Special Issue - Extreme
Environmental Physiology
Abstract
Many mammals run faster and for longer than humans and have superior cardio-
vascular physiologies. Yet humans are considered by some scholars to be excellent end-
urance runners at high ambient temperatures, and in our past to have been persistence
hunters capable of running down fleeter quarry over extended periods during the
heat of the day. This suggests that human endurance running is less affected by high
ambient temperatures than is that of other cursorial ungulates. However, there are
no investigations of this hypothesis. We took advantage of longitudinal race results
available for three annual events that pit human athletes directly against a hyper-
adapted ungulate racer, the thoroughbred horse. Regressing running speed against
ambient temperature shows race speed deteriorating with hotter temperatures more
slowly in humans than in horses. This is the first direct evidence that human running is
less inhibited by high ambient temperatures than that of another endurance species,
supporting the argument that we are indeed adapted for high temperature endurance
running. Nonetheless, it is far from clear that this capacity is explained by an end-
urance hunting past because in absolute terms humans are slower than horses and
indeed many other ungulate species. While some human populations have persistence
hunted (and on occasion still do), the success of this unlikely foraging strategy may be
best explained by the application of another adaption – high cognitive capacity. With
dedication, experience and discipline, capitalising on their small endurance advantage
in high temperatures, humans have a chance of running a more athletic prey to
exhaustion.
KEYWORDS
evolution, persistence hunting, temperature
1INTRODUCTION
‘Humans are the sole species of mammal that excels at long distance
trekking and running in extremely hot conditions’ (Lieberman, 2015).
This claim is the culmination of arguments put forward by a number of
authors including Bramble and Lieberman (2004), Carrier (1984)and
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
© 2020 The Authors. Experimental Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society
Liebenberg (2013) purporting that humans are excellent endurance
runners at high ambient temperatures. Often the explanation given
for this claimed capacity is that it was an adaptation in our ancestors
for foraging – the chasing down of prey over many hours during the
heat of the day, known as persistence hunting. Yet humans do not
possess exceptional cardiovasculatures in contrast to, for example,
Experimental Physiology. 2020;1–11. wileyonlinelibrary.com/journal/eph 1
2HALSEY AND BRYCE
dogs (Canis lupus familiaris) and horses (Equus caballus; see Williams,
Bengtson, Steller, Croll, & Davis, 2015, Figure 1). Even non-elite dogs
and horses have far superior oxygen delivery systems including hearts
that are proportionally much larger, elevated haematocrit levels and
greater muscle mass. Compared with humans, these equine and canine
adaptations facilitate considerably higher maximum rates of oxygen
delivery to the body ( ̇
VO2max; summarized in Figure 3 of Poole &
Erickson, 2011).
If not cardiovascular, what attributes, might humans possess to
make them elite endurance runners, and indeed capable of wearing
down quarry on the move at high temperatures? Liebenberg (2013)
argues that for hominins to have evolved a fitness-enhancing capacity
for endurance running, they would have become adapted for speed,
distance, heat loss and energy economy,among other things. We briefly
consider these arguments and the available data that humans are
adapted for each of these traits.
1.1 Running speed and distance
Humans are markedly slow runners compared to many similarly
sized non-primate animals (Garland, 1983; Hirt, Jetz, Rall, & Brose,
2017). This is demonstrably true over short distances, but also over
long distances including, pertinently, when compared to obligate
persistence-hunters and their ungulate prey. African wild dogs
have a mean preferred travelling speed while ranging and hunting
of 8.3 km h−1(Hubel et al., 2016), which is similar to grey wolves
(8.6–10.1 km h−1;Mech,1994), and hound dogs pursuing pumas
(9.7 km h−1; Bryce, Wilmers, & Williams, 2017), while humans that
were observed over eight persistence hunts averaged 6.1 km h−1
(Table 1 in Liebenberg, 2013). Pronghorn antelope can apparently
maintain 65 km h−1for over 10 min (covering some 11 km; Costello,
1969). Race horses ridden in a 161 km race (e.g. the Vermont 100
Endurance Ride) typically average speeds in excess of 11 km h−1while
bearing a rider, the event therefore lasting around 15 h.
1.2 Heat loss and tolerance
Due to the heat generated by active muscles, thermoregulation is
a key challenge during endurance running (Longman et al., 2019).
Evaporative water loss as a means of heat dissipation is by far
the predominant method of temperature reduction in mammals,
usually through perspiring and/or panting (Belval & Armstrong, 2018).
Humans, the only ‘hairless’ animals to possess eccrine sweat glands
(Marino, 2008; McGowan, 1979), are probably the most perspirative
of all species, and thus able to lose heat more rapidly than do other
animals. This difference is even more pronounced in environments
so hot that the body gains heat convectively (Lieberman, 2015;
Lindinger, 1999; Schmidt-Nielsen, 1964). Moreover, reduced body hair
promotes convective heat loss while the upright gait limits radiative
heat stress (Lieberman & Bramble, 2007; Wheeler, 1992). However,
certain cursorial species appear adapted to the heat by facultative
hyperthermia (i.e. adaptive heterothermy) – they tolerate a high heat
New Findings
∙What is the central question of this study?
Do available comparative data provide empirical
evidence that humans are adapted to endurance
running at high ambient temperatures?
∙What is the main finding and its importance?
Comparing the results of races that pit man against
horse, we find that ambient temperature on race
day has less deleterious effects on running speed
in humans than it does on their quadrupedal
adversary. This is evidence that humans are
adapted for endurance running at high ambient
temperatures. We debate whether this supports
the hypothesis that early man was evolutionarily
adapted for persistence hunting.
load. English pointer dogs show no obvious loss in hunting performance
during days that are particularly hot and humid (Davenport, Kelley,
Altom, & Lepine, 2001). Horses, swine, goats and sled dogs can tolerate
their core body temperatures reaching 42◦C (Armstrong, Delp, Goljan,
& Laughlin, 1987; Caputa, Feistkorn, & Jessen, 1986; Hodgson et al.,
1993; Marlin et al., 1996; McConaghy, Hales, Rose, & Hodgson, 1995;
Phillips, Coppinger, & Schimel, 1981; Poole & Erickson 2011), and
both cheetah (Hetem et al., 2013, 2019) and African wild dogs (Taylor,
Schmidt-Nielsen, Dmi’el, & Fedak, 1971) have been recorded in the
wild with core temperatures over 41◦C, although this is uncommon.
In comparison, human distance runners and cyclists rarely attain
core temperatures above 40◦C (hyperpyrexia) even in high ambient
temperatures (Laursen et al., 2006; Figures 3 and 4 of Nybo &
González-Alonso 2015; Williams, Wickes, Gilmour, Barker, & Scott,
2014;Valentino, Stuempfle, Kern, & Hoffman, 2016), and heat stress
resulting in failure to complete races (i.e. voluntary fatigue) typically
occurs at around 40◦C (see references in Brück & Olschewski, 1987;
Lindinger, 1999; Nielsen et al., 1993; Nielsen, Strange, Christensen,
Warberg, & Saltin, 1997), although core temperaturesabove 40◦Chave
been observed on occasion in field studies and without necessarily a
deterioration in performance (Byrne, Lee, Chew, Lim, & Tan, 2006;Ely
et al., 2009; Racinais, Périard, Karlsen, & Nybo, 2015). Compared with
sweating, facultative hyperthermia provides the advantage of retained
body fluids and salts (reviewed in Mitchell et al., 2002). Furthermore,
while heightened temperatures in the brain are associated with the
cessation of activity in animals (e.g. goats, Caputa et al., 1986; guinea
pigs, Caputa, Kądziela, & Narębski, 1983; hamsters, Gordon & Heath,
1980), and selective cooling mechanisms in the brain have been found
in some ungulates including horses (Baptiste, 1998; McConaghy et al.,
1995; Strauss et al., 2017), there is no evidence that they are present
in humans or other primates (Brengelmann, 1993; Nelson & Nunneley,
1998; Nybo, 2012). Potentially,increases in brain temperature limit the
HALSEY AND BRYCE 3
motor performance of active humans (Nybo, 2012) and yet they have
no apparent adaptations to defend against this (Nelson & Nunneley,
1998; Strauss et al., 2017).
Consequently, it is not surprising that, contrary to documented
claims, humans are not the only species that can withstand protracted
locomotion in high-temperature environments. The assertion that
‘no horse or dog could possibly run a marathon in 30 degree heat’
(Lieberman, 2015) is demonstrably untrue. In the annual Tevis Cup
race held in California (USA), where horses are ridden over the
Sierra Nevada mountain range for 161 km at an average pace that
can exceed 10 km h−1, mean ambient temperature is often above
28◦C, even 30◦C. In the 2019 Marathon des Sables in the deserts
of Morocco, a now-celebrity pet dog named Cactus, which lived
locally to the race, voluntarily completed several stages including
the ‘double marathon’ stage 4 (76.3 km, video). Its speed averaged
around 9 km h−1on stage 3 (37.1 km), at high daytime temperatures
probably exceeding 30◦C(https://www.marathondessables.com/en/
news/all-runners- are-grey- night-186). Similarly, a small stray dog,
later named Gobi, followed the heels of ultrarunner Dion Leonard at an
average speed of 8.5 km h−1across the rugged Tian Shan Mountains
of northwest China. The pair even averaged a speed of 12.3 km h−1
on Day 3’s 42 km stage in heat exceeding 49◦C (Leonard, 2017). Thus,
while cultural interventions (e.g. water carriage and scheduling work
for cooler parts of the day; Lupo, 2019) are applied to working dogs
in hot environments to limit the chances of heat stress, dogs are non-
etheless typically able to exert moderate endurance activity such as
trotting for extended periods.
1.3 Energy economy of locomotion
Humans are believed to have a number of anatomical traits that
enhance their locomotion efficiency, including relatively long legs and
Achilles tendons, large gluteus muscles and arched feet with short toes
(Bramble & Lieberman, 2004; Lieberman, 2015; Lieberman, Bramble,
Raichlen, & Shea, 2009). However, there is no evidence that humans
or other hominins are energetically economical movers compared to
other primates (Steudel-Numbers, 2003) or other mammals in general,
either when walking or when running. Rather, for a mammal of their
size they have a fairly typical energy expenditure per unit distance
(Halsey & White, 2012). If anything, they are relatively inefficient
runners but efficient walkers. Humans have a net cost of transport
(NCOT; the whole-animal energy cost to move a unit distance) while
walking that is 25% lower than predicted based on the best fit NCOT–
mass relationship across mammalian species, but an NCOT while
running that is 27% higher (Halsey & White 2012). Horses have an
NCOT about 20% above predicted (Wickler, Hoyt, Cogger, & Myers,
2003). All measured canids – the African wild dog and the grey wolf
(both persistence hunters) and mongrel dogs and Walker foxhounds –
have NCOTs close to that predicted for their size (Bryce & Williams,
2017; Halsey & White 2012; Seeherman, Taylor, Maloiy, & Armstrong,
1981; Taylor, Heglund, & Maloiy, 1982; Taylor, Schmidt-Nielsen, &
Raab, 1970).
In summary of the above, humans may well be more adapted for
running, coping with activity in the heat, and locomotion economy
than are other extant primates (Bramble & Lieberman, 2004). However,
given that many of these human adaptations are substantially inferior
to the equivalents of other mammalian species clearly designed for
endurance running, again the evidence does not strongly support the
concept that humans are a species truly evolved for endurancerunning,
or indeed persistence hunting. An alternate possibility is that our
enhanced endurance capacity compared to our primate cousins instead
evolved as an adaptation to decrease the time taken to descend on
carcasses that had been spotted from distance, thus increasing the
calories available to scavenge upon arrival (Bramble & Lieberman,
2004; Lieberman & Bramble, 2007; but see Pickering & Bunn, 2007).
Nonetheless, today a handful of disparately located tribes (thus
probably culturally independent of each other) at least occasionally
procure meat through the persistence hunting of large ungulates (see
Glaub & Hall, 2017 and references therein). Field documentation of
persistence hunting indicates that the factor enabling human hunters
to catch up with their fleeter quarry appears to be the latter’s hyper-
thermia resulting in heat exhaustion (the proximal factor probably
being dehydration of the intracellular fluids; Lindinger, 1999). Indeed,
triggering hyperthermia in their prey seems to be the aim of the
hunters, who undertake pursuit running at the hottest time of the
day, when ambient temperatures can exceed 40◦C (Liebenberg, 2013).
Perhaps, then, despite the heat tolerance adaptations that are present
in many, perhaps all, ungulate species (McConaghy et al., 1995; Strauss
et al., 2017; Yousef, 1976), the human ability to sweat profusely proves
to be a superior heat adaptation under certain circumstances, in turn
making us veritable endurance athletes and persistence hunters in that
moment.
We investigated this proposition by analysing the effects of ambient
temperature on the running speeds of humans and an ungulate model,
the horse, in directly comparable races – events in which the two
species compete against each other, commonly called ‘Man vs.Horse’
(MvH) races. Humans have long debated and tested their ability to
outrun horses on foot, in race distances ranging from 100 m sprints
to 160 km ultramarathons and beyond (Crockett, 2018). In recent
decades, MvH events have been held annually, typically at marathon
(42 km) or ultramarathon (160 km) distances. We took advantage of
yearly variations in weather on race day to describe the relationships
between mean running speed and mean ambient temperature for each
species. We hypothesized that mean running speed would decrease
with increasing ambient temperature in both horses and humans, but
more so in horses, represented by a greater negative slope for the
relationship in this species.
2METHODS
2.1 Data collection
Historical records from MvH races in excess of 30 km were identified
and compiled from eight international competitions, but the data
4HALSEY AND BRYCE
available for most of them were sparse. Ultimately, sufficient
results for analysis (>30 years) were obtained from three races
(Table 1).
From each race, we collated the annual race distance and finishing
times of the top three horses and top three (non-relay) human
competitors. For most races and years, results were available online,
but in some cases (especially finish times >15 years old), we obtained
historical results from race organizers directly. In addition to finish
times, we compiled ambient race-day temperature (◦C) from archival
weather records at the nearest weather station of similar elevation.
Historical race day temperature and precipitation were extracted
from the National Oceanic and Atmospheric Administration’s (NOAA)
National Centers for Environmental Information (NCEI) online
data repository and NOAA’s Global Surface Summary of the Day
(GSOD) records using the R package GSODR (R Core Team, 2019).
In some cases, we used weather data from multiple stations for a
given race to provide full temporal coverage across all race years
(Table 1).
2.2 Analysis
We calculated running speed (km h−1) in both species based on
race length and finishing time. To investigate the effect of ambient
temperature on running speed for both horse and human competitors,
we fitted two generalized linear mixed models (GLMMs) using the
R packages ‘lmer’, and used ‘lmerTest’ to assign estimated P-values
for interaction terms. Year and race were input as random factors
(with year nested within race), and species as a fixed factor. Our
full model included the interaction of temperature and species, while
our reduced model did not. We report marginal R2to quantify the
variance explained by the fixed factors in the models (thus excluding
the variance explained that is simply due to differences between
the three races) (Ršm; Nakagawa & Schielzeth, 2013). We compared
the two models using Akaike’s information criterion (AIC; Akaike,
1973). We checked the assumptions of linearity, normality and homo-
scedasticity by visual inspection of plotted residuals and utilized a
combined dataset of results from all three races.
3RESULTS
Data from a total of 260 humans and 358 horses racing in three
independent annual MvH events were analysed. As predicted,
hotter temperatures resulted in slower race speeds for both species
(Figure 1). The slope of the relationship between race speed and
ambient temperature was statistically significantly steeper for horses
than it was for humans (horses: −0.11; humans: −0.07, t=3.17,
P=0.002; model R2m=0.21). Thus, per 1◦C increase in ambient
temperature, the race pace of horses decreased by on average
0.11 km h−1and that of humans by 0.07 km h−1– a 36% smaller
decrease for humans. The AICs for the full model and reduced model
were 1740 and 1741, respectively.
TAB L E 1 Summary of the MvH races retained in our analyses
Race
Location (approx. finish
line coordinates)
Years of collated
data (n)
Distance
(km)
Temperature, mean ±SD
(range) (◦C)
Weather
stations used (n)
Station distance
from finish line,
mean ±SD (km)
Man vs. Horse Marathon Llanwrtyd Wells, Wales,
UK (52.107, −3.637)
1980–2019 (38) 33–38 12.3 ±2.5 (7.3–17.0) 2 6.1 ±3.8
Tevis Cup 100 (TC, horses)
and Western States 100
(WS, humans)
Auburn, CA, USA
(38.897, 121.072)
TC: 1962–2018 (57)
WS: 1974–2019 (45)
160 TC: 21.2 ±5.0
(11.1–31.0)
WS: 23.7 ±3.7
(15.0–30.8)
22.1 ±0.6
Old Dominion 100 Woodstock, VA, USA
(38.874, −78.523)
Horses: 1975–2019 (44)
Humans: 1979–2019 (40)
160 Horses: 23.6 ±3.9
(17.2–31.7)
Humans: 22.3 ±3.0
(17.0–29.0)
4 31.0 ±15.8
HALSEY AND BRYCE 5
FIGURE 1 Running speed (km h−1) against mean ambient
temperature (◦C) in three Man vs. Horse races for the first three
horses (black data points) and first three humans (red data points) of
each year. The two continuous lines are the least squares best fit
regressions, and the associated dashed lines and shaded areas are
95% confidence intervals of those fits. (a) Man vs. Horse Marathon,
Wales, UK; (b) Old Dominion 100, Virginia, USA; (c) Tevis Cup 100
(horses) and Western States 100 (humans), California, USA. The y-axis
is truncated to start at 4 km h−1in all three panels
4DISCUSSION
Although horses are substantially larger animals than humans (approx.
500 vs. 70 kg, respectively), they have comparable stride lengths at
endurance running speeds (Figure 4 of Bramble & Lieberman, 2004;
Heglund & Taylor, 1988). However, because they have superior cardio-
vascular systems (Williams et al., 2015), it is not surprising that
horses typically traverse MvH courses more quickly than do human
competitors (Figure 1). Yet the time gap between the two species
closes on hotter days; in the heat, the degree of deterioration in race
performance of horses is greater than that of humans. This finding was
sometimes subtle but always apparent in each of the three race events
we analysed (Figure 1). Being larger, horses have a lower surface area-
to-volume ratio and greater thermal inertia meaning that all else being
equal, they lose heat to a cooler ambient environment more slowly.But
even accounting for size, data provided in Lindinger (1999) on human
and horse sweat rates (the percentage of sweat used for cooling, and
the percentage of heat loss from various routes) indicates that the
rate of evaporative heat loss of horses is about half that of humans.
Consequently core temperature rises much more slowly in humans
than horses, and this probably explains why humans experience a
relatively slow loss of physical capability compared to horses when
running in the heat. Clearly, humans have an exceptional capacity to
dump excess heat through sweating (Lindinger, 1999; Schmidt-Nielsen,
1964).
Can this subtle advantage be enough to proclaim that humans have
a heat adaptation enabling them to out-run prey in hot environments?
The very high sweat rates of humans, the fact that human persistence
hunters tend to select the hottest time of day to hunt, the heat
exhaustion exhibited by their prey, and the reduced detriment of high
ambient temperatures to running speed (Figure 1) together suggest
that heat tolerance during running is key to the success of human
persistence hunters. Yet horses in MvH races are still running more
quickly than humans, even on hotter days, all, of course, while carrying
a human rider and without it being imperative to survival. And data
for wild endurance species such as African wild dogs and grey wolves
indicate that they too travel faster while hunting than do humans
(Hubel et al., 2016;Mech,1994), although ambient temperature
was not reported. Thus in answer to the question as to whether
humans are comparatively well adapted to endurance running at high
temperatures, the evidence suggests no – in absolute terms, even in the
heat humans are not fast runners.
While horses are not a prey species of human persistence hunters,
the MvH comparison highlights that humans will be off the pace in
trying to chase down ungulate prey even at high ambient temperatures.
During persistence hunts, what are the choices that humans can make
that can bridge this performance gap? Human persistence hunting
involves a group of communicating individuals, with only the very
best completing the final stages (Liebenberg, 2013). After spending
time carefully observing a herd, they predominantly target an animal
weakened from age, injury or emaciation, or otherwise an individual
with large, heavy horns (e.g. Liebenberg, 2013). They know that as
they get close to their quarry, it will flee at pace. Although hunter and
hunted cover approximately the same distance, the latter is doing so
through stop–start bursts of intermittent locomotion, which could be
energetically less economic (Kramer & McLaughlin, 2001; Seethapathi
& Srinivasan 2015), thus generating more heat. Furthermore, if pauses
are too short, intermittent locomotion can result in detrimental high-
energy phosphate depletion and lactate accumulation from anaerobic
metabolism (Edwards & Gleeson, 2001; Kramer & McLaughlin, 2001;
Weinstein & Full, 2000). Persistence hunters assess information on
their own physical state and their perceptions of the physical state of
their quarry, then attempt to optimise the pace at which they track
6HALSEY AND BRYCE
their prey so that the prey overheats, and they do not. Finally, humans
persistence hunters have the advantage of hands – some stave off
dehydration-induced fatigue by carrying water on hunts, for example
in ostrich shells (Liebenberg, 2013).
Our synthesis of the current literature, coupled with our analysis
of MvH race times, leads us to the following interpretation, which we
put forward for debate and critique. We humans clearly have a super-
ior capacity for endurance than do other primates (Pontzer, 2017),
allowing us to expend more energy on foraging – an investmenthumans
pay back with a greater calorie intake (Leonard & Robertson 1997;
Pontzer et al., 2016). Nonetheless, compared to a broader range of
species, human endurance is not exceptional. Carnivores in general
travel further than herbivores in search of food (Carbone, Cowlishaw,
Isaac, & Rowcliffe, 2005; Joly et al., 2019) requiring a capacity to
locomote for extended periods, and humans appear to fit this mould
with anatomical adaptations such as a significant bias toward slow-
twitch skeletal muscle fibres (O’Neill, Umberger, Holowka, Larson, &
Reiser, 2017). Yet our general running endurance is not as impressive
as that of many other mammals. Rather than being the elite heat-
endurance athletes of the animal kingdom, humans are instead using
their elite intellect to leverage everything they can from their moderate
endurance capabilities, optimising their behaviours during a hunt to
bridge the gap between their limited athleticism and that of their more
physically capable prey. Our capacity for profuse sweating provides
a subtle but essential boost to our endurance capabilities in hot
environments. This is a slight but critical advantage that our ingenuity
magnifies to achieve the seemingly impossible: the running down of a
fleeter-footed quarry.
4.1 Future research
Of course, further data on the endurance capabilities and adaptations
of prey and predatory animals in the heat would advance our under-
standing of man’s place in the pantheon of persistence hunters. Of
particular intrigue is whether human endurance activity can be limited
because of increases in brain temperature, as is the case for a diversity
of other species. Data on goats and rodents are widely cited (Caputa
et al., 1986; Caputa, Wasilewska, & Swiecka, 1985; Gordon & Heath,
1980), while data on humans are widely desired (Nybo, 2012).
Physical endurance is not necessarily confined to the context
of running and may be a factor in other intensive activities. Across
mammalian species, there is a clear reduction in daily metabolic rate
in those species that inhabit higher mean ambient temperatures
(Figure 2a; Speakman & Król, 2010), suggesting that ambient
temperature limits activity intensity. Is this also the case for humans?
This question is yet unanswered. As a first and tentative approximation,
we collated data from the literature on daily energy expenditure during
high intensity activities, from running and cycling races to military
training and polar expeditions, along with mean ambient temperatures
where available (which otherwise were estimated based on latitude
and time of year). In contrast to the data for other mammals, our
analyses show no relationship between sustainable daily energy
FIGURE 2 Relationships between long-term metabolic rate (MR)
and mean ambient temperature (◦C). (a) Mass-independent field
metabolic rate (kJ day−1) measured in mammals against ambient
temperature of their habitat. Reproduced from Speakman and Król
(2010) with permission. (b) Sustained metabolic rate (MJ day−1)of
people undertaking arduous tasks and events for at least 1 day against
(estimated) mean ambient temperature. The metabolic rate data are
adjusted for body mass, proportion of participants who are male, and
event duration. The black trend line is the best linear fit accounting for
multiple measures within studies; it is dashed to indicate
non-significance (P=0.15). References to these data are: (Ainslie
et al., 2002; Beals et al., 2019; Bircher et al., 2006; Bourrilhon et al.,
2009; Castellani et al., 2006; Clemente-Suárez, 2015; Coker et al.,
2018; Costa et al., 2014; Diaz et al., 1991; Ebine et al., 2002; Edwards
et al., 1993; Forbes-Ewen et al., 1989; Frykman et al., 2003;Fudge
et al., 2006; Geesmann et al., 2014; Glace et al., 2002; Hill and Davies,
2001; Hoyt et al., 1991; Hoyt et al., 1994; Hulton et al., 2010; Johnson
et al., 2017; Jones et al., 1993; Knechtle et al., 2010; Knechtle et al.,
2011; Koehler et al., 2011; Margolis et al., 2013; Margolis et al., 2016;
Mudambo et al., 1997; Mullie et al., 2018; Plasqui et al., 2019; Pulfrey
and Jones, 1996; Rehrer et al., 2010; Reynolds et al., 1999; Ryder et al.,
2004; Schulz et al., 1992; Sjodin et al., 1994; Stroud, 1998; Stroud
et al., 1993; Stroud et al., 1997; Verma et al., 2018; Westerterp et al.,
1986; Westerterp et al., 1992; Westerterp et al., 1994)
HALSEY AND BRYCE 7
expenditure and mean ambient temperature (Figure 2b). Together,
these analyses might very cautiously be interpreted as suggesting that
profuse sweating, which only humans do (perhaps supported by their
unique ability to guarantee the availability of water), is the adaptation
most capable of nullifying the effects of heat during chronic extreme
activity. However, there is very considerable noise inherent in this
dataset on human activities since few of the included studies were
designed to directly address our question of interest, and ambient
temperatures were often not stated and, in many cases, would have
varied considerably over the duration of the activity. Moreover,
other environmental factors can be as important as temperature
in influencing heat load, such as radiant heat, humidity and wind
speed. There is currently a gap in the empirical literature on how the
maximum daily energy expenditure of humans relates to ambient
conditions.
While investigations into human persistence hunting have been
conducted through research in the fields of social and biological
anthropology, and both comparative and macrophysiology, there have
been few attempts to gain insight from a psychological perspective.
Animals are believed to enjoy the behaviours that they are adapted for
(Markowitz, 1982; Markowitz & LaForse, 1987). What proportion of
people enjoy running? The percentages appear very low. For example,
New York City has the highest reported number of runners per capita
of any city in the USA, with some 410,000 people reported going for
a run at least once in the previous 12 months (Seligman & Nolan,
2017). The population of New York is about 8.6 million, and thus less
than 5% of New Yorkers do even a modicum of running. While the
nature of Western societies typically eschews the need to run, and it
cannot be assumed that everyone who would enjoy running partakes
in this activity, we might anticipate that if our species was ‘born to
run’ we would find excuses to do so despite the constraints of modern
society and the environment (cf. Speakman, 2020). On the other hand,
some runners report experiencing a ‘natural high’ from high endorphin
or cannabinoid levels (Dietrich & McDaniel, 2004; Schulkin, 2016).
These heightened neurotransmitter levels presumably increase real-
time enjoyment of the endurance activity and promote habitual aerobic
exercise (Raichlen, Foster, Gerdeman, Seillier, & Giuffrida, 2012). But is
this common in voluntary runners, or an experience of the few, and do
persistence hunters also experience this natural high?
While the sport of endurance running is participated in worldwide,
persistence hunting is a dying art. Hunter–gatherers in the Kalahari
Desert of Botswana, for example, have shifted away from a significant
dependence on hunting in all forms (Marshall Thomas, 2006).
Those that continue to hunt often utilize dogs and horses to
improve efficiency (Liebenberg, 2006) – bow-and-arrow and pure
persistence hunting have become increasingly obsolete there and
elsewhere (Liebenberg, 2013). There is now very limited scope for
direct investigations into the physiology, ecology and psychology of
persistence hunting. Nonetheless, we can at least further progress
our understanding of the human experience of running per se; better
comprehension about feats of human endurance in the present can
help us better comprehend the realities of human endurance in the
past.
ACKNOWLEDGEMENTS
We thank Chris Tyler, Hanna Male and the Met Office UK for their
thoughts and support during the development of this article. We also
thank the organizers of the man against horse events in California
(USA), Virginia (USA) and Wales (UK) for contributing historical race
results. We also thank Carli Coco for useful comments on an earlier
version of this manuscript.
COMPETING INTERESTS
The authors declare no competing or financial interests.
AUTHOR CONTRIBUTIONS
Both authors contributed to conceptualization, methodology,
validation, formal analysis, investigation, resources, data curation,
writing, review and editing, and visualization. Both authors have read
and approved the final version of this manuscript and agree to be
accountable for all aspects of the work in ensuring that questions
related to the accuracy or integrity of any part of the work are
appropriately investigated and resolved. All persons designated as
authors qualify for authorship, and all those who qualify for authorship
are listed.
DATA AVAILABILITY STATEMENT
Man vs. Horse race data utilized in analyses.
ORCID
Caleb M. Bryce https://orcid.org/0000-0002-6789-9538
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SUPPORTING INFORMATION
Additional supporting information may be found online in the
Supporting Information section at the end of the article.
How to cite this article: HalseyLG,BryceCM.Arehumans
evolved specialists for running in the heat? Man vs. horse races
provide empirical insights. Experimental Physiology. 2020;1–11.
https://doi.org/10.1113/EP088502
