The Auk 113(2):310-318, 1996
THERMOREGULATORY
ROLE
UNFEATHERED
NECK
HEAD
IN MALE
WILD
OF THE
AND
TURKEYS
RICHARD BUCHHOLZ •
Department
of Zoology,
University
of Florida
Gainesville,Florida 32611, USA
AI•STRACT.--The
brightly colored,unfeatheredheadsand necksof male Wild Turkeys
(Meleagris
gallopavo)
are generallythoughtto functionin sexualselection.However, studies
in other bird specieshave suggestedthat uninsulatedbody regionsmay servean important
role in heat dissipation.I test the heat-dissipation
hypothesisin Wild Turkeysby experimentally reinsulatingthe headsand necksof Wild Turkeysas though they were feathered.
The oxygenconsumption,thermal conductance,
coolingcapacity,surfacetemperatures,and
coretemperatureof controland reinsulatedWild Turkeyswere comparedat 0ø,22øand 35ø(2.
Head insulationresultedin significantlyincreasedratesof oxygenconsumption,higher body
temperatures,and decreasedcoolingcapacitiesat 35øC,but had no significanteffectat the
other temperaturestested.It appearsthat behavioral changesat low temperatures,such as
tucking the head under the backfeathers,effectivelyprevent the heat lossthat would otherwise be causedby the absenceof feathers.However, if the head were feathere& turkeys
at high temperatureswould be unable to dissipatesufficientheat to maintain thermeostasis.
Thus,given this finding for Wild Turkeys,it canno longerbe saidthat in all casesbareheads
in birds have evolved by sexualselectionalone. Lossof head and neck featbering in Wild
Turkeysand other birdsmay haveallowedthesespeciesto takeadvantageof regionsin time
and spacethat previouslywere unexploitabledue to the dangersof hyperthermia.Received
22 June1994,accepted
27 January1995.
ENDOTHERMS
USUALLY
MAINTAINbody temperaturesabove environmental temperatureat
considerable
energeticcost.To saveenergythey
reduceheat lossto the environmentby insulating themselvescompletelywith fur or feathers. Birds that have
areas of unfeathered
skin
on their headsand necksare an unexplained
exceptionto the pattern of completeinsulation
seen in other endotherms.In carrion-feeding
birds, unfeathered heads often are assumed to
portedby correlativestudiesshowingthat unfeatheredhead and neck skin is maximallyexposed at high temperaturesand that in some
taxa the size of unfeatheredareasis greaterat
low latitudeswhere heat dissipationmay be of
greater importance (Crowe 1979, Buchholz
1994). Highly vascularizedfleshy ornamentation presentsa functional puzzle when species
are distributedover a large latitudinal range in
which they are exposedto both temperature
extremes.Although these speciesmay benefit
by usingtheir fleshystructuresto dissipateheat
be a hygienic adaptation (Welty 1975: 100).
However, in specieswhere the unfeatheredareasare alsobrightly colored,sexuallyselected under hot conditions, the uninsulated nature
functionsare usuallysuspected(Zuk 1991).
of thesestructuressubjectsthe birds to extreme
Despite some studies suggestingthat these heat lossunder cold conditionsand heat gain
areas of bare skin maintain
sublethal
brain tem-
in the presenceof solar radiation. In this study,
peraturesby dissipatingheat via cephalo-cer- ! testthe possiblethermoregulatoryfunction of
vical retes (Crowe and Crowe 1979, Crowe and unfeathered head ornamentation in a species
Withers 1979, LaRochelie et al. 1982), thermo- that commonlyfacesextremesof coldand heat,
regulatory hypothesesfor the evolution and the Wild Turkey (Meleagris
gallopavo).
Wild Turkeys occur over a broad range of
maintenance
of unfeatheredskinrarelyareconsidered.The heat-dissipation
hypothesisis sup- temperatureextremesfrom their southernlimit
in southern
Mexico
to their
northern
limits
• Present address:Department of Biology, NortheastLouisianaUniversity, Monroe, Louisiana71209,
along the border of the United Statesand Canada (Dickson 1992). Males are twice as large as
females,and have brightly coloredunfeathered
USA.
heads and necks (Buchholz 1995). In addition,
310
April 1996]
BareHeadsandThermoregulation
this bare skin is coveredwith polyp like elaborationsof the integument called caruncles.A
thin dewlap extendsfrom the mandible down
to the neck. Perhapsmostdistinctive is the bare,
distensiblefrontal processor snoodthat projectsfrom the foreheadat the baseof the upper
bill.
311
was drawn through the metabolicchamber,pumped
into glasscolumnsfilled with sodalime (to remove
CO2)and silicagel (to removeH20), after which flow
rates (g = 20.6 L/min) were measuredby a Brooks
$ho-Rateflowmeter.Subsequently,
the airstreamwas
sampledwith an Applied ElectrochemistryS-3A oxygen analyzer. The temperatureand humidity of the
room air varied little (23.5 + SE of 0.0øCand 61.2 _+
Fleshyheadornamentationin Wild Turkeys 0.2%,respectively).Humidity in the chamberwasnot
and other galliforms often is thought to be controlled.The bird's evaporativewater losswasmeamaintained by sexual selection (i.e. the struc-
suredgravimetrically(i.e. by weighing the silicagel).
Corebody temperaturewasmeasuredby insertinga
thermocouple,tipped with a thin
competition).Ample empirical evidence sup- copper-constantan
portsthiscontention(Brodsky1988,Boyce1990, layer of silicone,into the bird's intestine to a depth
Hillgarth 1990,Ligon et al. 1990,Zuk et al. 1990a, of 20 cm from the cloacalopening.This measurement
b, Spurrier et al. 1991,Zuk et al. 1992, Buchholz was taken immediatelybefore the subjectwas placed
in the metabolic chamberand immediately after it
1995). A role in sexual selection, however, does
wasremovedfrom the chamber.Six surface-tempernot rule out concurrent
functions
for these
ature measurementswere taken: feather, leg, body
structuresin thermoregulation.Although both skin, head skin, frontal caruncle,and dewlap. Surface
speciesof present-dayturkeys(Meleagridinae) temperatureswere measuredwith a bare-tippedtherhave unfeathered heads and necks, the common
mocouple held against the appropriate spot, while
ancestorsof modern turkeyspresumablyhad the subjectwasstill in its holding boxbefore the trial
featheredheads,asdo mostgalliforms.Under- and, again, while it was in the metabolicchamberat
standingwhy the unfeatheredareasof modern the end of a trial. Skin and feathersurfacetemperaturkeysaremaintained,despitethepossiblecosts tureswere measuredon the chestapproximately3 cm
in termsof heatloss,may explainwhy ancestral ventral to the carpal joint of the wing at rest. Leg
temperaturewas measuredimmediately posteriorto
turkeyslost their head feathering over evoluthe third scaledistalto the tarsaljoint on the left or
tionarytime. In this studyI experimentallyin- right leg, depending on which was accessible.Head
sulatethe headsand necksof Wild Turkeysto skin temperaturewas measuredon the back of the
assessthe thermoregulatory trade-offs that head at a point posteriorto the lower mandible. Surmaintainunfeatheredheadsin this species.
facetemperatureof the frontal carunclesand dewlap
tures function
in mate choice and male-male
were measured
METHODS
at the centers of these structures.
Different rates of physicalactivity acrosssubjects
and trials can make it difficult
Subjects
and equipment.--Eight
two-year-old,male
Wild Turkeysobtainedas chicksfrom a game farm
(L&L Pheasantry,Hegins, Pennsylvania)were used
in the metabolictrials. Rearing conditionsare describedin detail elsewhere (Buchholz 1994). There is
no differencein the metabolicratesof Wild Turkeys
from game-farmor free-living sources(Gray and
Prince 1988). The averagebody massof the study
individualswas7.1 kg (range6.4-8.1kg). During the
study period (29 June-27September1993) the birds
to detect the effect of
experimentaltreatmentson metabolicrate. Therefore,
I minimized the bird's activity by conductingtrials
at night in the dark.Metabolictrialslasting2.5 h were
conductedbetween 1900and 0200 EST. All subjects
were given at leastone day betweentrials. Individual
turkeys were testedat the same time of day (either
1900 or 1100) acrossall treatments to minimize circadian effectson matched comparisonsof metabolic
rate.The first 30 min of eachtrial servedasan equilibration period during which the bird calmeddown
were provided with feed (Purina Gamebird Mainteafter handling. The lowest rate of oxygen consumpnance,12%protein)and wateradlibiturn.
Subjects
were tion (correctedto standardpressureand temperature)
deniedfoodfor 26 to 29 h immediatelyprior to each measuredduring eachof the four subsequent30-min
metabolictrial to insure that they were postabsorp- periodswas used to calculatean averagemetabolic
tive. Postabsorptive
conditionsare necessaryto mea- rate for the entire trial. All individuals were given
surethe basalor minimum rateof metabolism(McNab
two 2.5-h habituationtrials prior to the experimental
1988a).Water was still availableduring the pre-trial trials.Usually,the subjectsrestedquietly during the
period.
experimentaltrials.The followingthreebehaviorpatOxygen consumption and total water loss were
ternswere recordedas presentor absentthrough inmeasuredin an open system(describedby McNab
stantaneoussampling(Martin and Bateson1986) ev1988b).The temperatureof the 329-Lmetaboliccham- ery 30 min: standing; head tucked under feathers;
ber wasregulatedby pumpingwater from a water
bath through the chamber'shollow wails. Room air
panting. Observationswere made with the aid of a
flashlight through a small window in the chamber.
312
RICHARD
BUCHHOLZ
Experimental
design.--Toevaluatethe potentialthertooregulatory
impactthatheadfeatheringwould have
on Wild Turkeys,I determinedthe thermal balance
when the turkey'shead was "bare" (seebelow), and
when it was insulated as though feathered. To approximatethe insulatorypropertiesof head and neck
feathering,the bareheadand neckof the turkeywere
coveredwith a doublelayer (0.6 cm on head, 0.9 cm
on neck)of acrylicsock(Adler "CasualAcrylicCrew";
75% hi-bulk acrylic, 25% stretchnylon) with large
holes for the eyes and the entire bill. The nostrils
were never covered.Any irritational effectsof the
insulatoryheadcoveringon metabolicratewerecontrolledby placinga hoodmadeof thin, nylonnetting
with little insulatoryvalueon the headsof the "bare"
individuals.Thecontrolheadnet andinsulatoryhead
sockingwere held in place with small alligator clips
[Auk,Vol. 113
of eachturkey in the experimentaltreatmentto the
values obtained
from
the same bird in the control
treatment, which serves to minimize the effects of
interindividual variation on the effect of the experimental treatment.Due to schedulingconflictsin the
laboratory,every turkey was testedat 0øCbefore it
was exposedto the other temperatures.The presentation order of the trials at 22ø and 35øC, and treat-
mentsat 0%22ø,and 35øCwas randomized.RepeatedmeasuresANOVAs (AbacusConcepts,Inc. 1989)were
usedto test the effectsof body size (œ= 6.7 kg for
smalland 7.5 kg for large),chambertemperature,and
head insulation on: oxygen consumption(cm• 02'
g-'. h-•); coolingcapacity;total and dry thermalconductances(mW.[cm•]-•.[øC]-'); and changesin body
and surfacetemperatures(øC).The effect of each 30rain samplingperiodwasalsoincludedwhen oxygen
that attached to the back and chest feathers at the base
consumptionwas the dependentvariable. Treatment
of the neck. The efficacyof using head sockingto groups exhibited similar variances (F• tests; all
approximatethe insulationprovidedby normalfeath- P>0.05; Sokaland Rohlf 1981).ReportedP-valueswere
eringwasdeterminedby studyingthe warmingcurves adjusted using Greenhouse-Geisserepsilon values
of the feathered and unfeathered/reinsulated heads
(AbacusConcepts,Inc. 1989).This techniqueconserfor the useof repeatedmeasures
of domesticroosters(Gallusgallus;after Morrisonand vatively compensates
Tietz 1957).
by adjustingthe degreesof freedom. ThermoreguSix dependentthermoregulatoryvariablesmay be latoryvariablesarereportedasf + SE,asappropriate.
affectedby head insulation.Metabolicrate, as indi-
catedby oxygenconsumption
(cm3 O2.g-'.h-*), is a
measure of the work
the animal
does to maintain
RESULTS
thermeostasis.The rate of evaporativewater loss(g/
h) is a measureof the heat lost via evaporation.Metabolicheat production(Hm)andevaporativeheat loss
(H,) can be converted to common units (roW/g) to
The massspecificrate of oxygenconsumption
was significantly lower for large individuals
comparethe coolingcapacityof the animalin differ- acrossall temperatures(œ= 0.4140 -+ 0.0070 vs.
0.4730 + 0.0130 cm•O2.g-•-h-'; Table 1). The
ent treatments.Cooling capacityis the bird's ability
to dissipatemetabolicallyproducedheat by evapo- rate of metabolismwas not significantlydifferration. It is expressedas a percentage,calculatedas ent for uninsulatedand insulated turkeys at 0ø
the heat lostby evaporationdivided by the heatpro- and 22øC.However, at 35 øC,insulated turkeys
ducedby metabolism(100 H,/Hm; Calderand King exhibited a significantly higher average meta1974).Total thermal conductance(mW.[cm2]-'.[øC]-') bolic rate than uninsulated turkeys (Table 2,
measuresall the heat lost by the animal, including Fig. 1). A significant, three-way interaction of
evaporativeheatloss,and is the inverseof insulation.
head insulation, temperature, and time period
It is estimatedusing the values for heat production,
suggeststhat the effectsof head insulation beand ambient and body temperatures.Dry thermal
camemore pronouncedthe longer the subject
conductanceis a measureof all nonevaporative means
was
exposedto the chamber conditionsat hot
of heat loss: radiation, convection and conduction. If
totalconductance
isexceededby heatproduction,heat
is storedin the tissuesof the animal and body temperaturerises.Eachof thesecomponents
of thermal
balancemay be varied by the animal to copewith
increased head and neck insulation.
temperatures.
Uninsulated turkeys demonstratea significantly greater ability than insulated turkeys to
dissipateexcessmetabolicheat by evaporation
at 35 øC,but not at lower temperatures(Fig. 2).
Thermoregulatorytrials were conductedtwice for
eachturkeyat eachof threeambienttemperatures
(0ø,
Total thermal
This matcheddesigncomparesthe metabolicvalues
tern of conductances at 0ø and 22øC, but had
conductance
increased
with
tem-
perature.It alsowas greater for insulatedbirds
22ø, 35øC),one time as a control, and the other with
overall (Table 1, Fig. 3), but this differencewas
the turkey'sheadinsulated.Thesetemperatures
were
significant
only at the highesttemperature(Tachosento be below, within, and above,respectively,
ble
2).
Dry
thermal
conductancedecreasedwith
the zoneof thermalneutrality(Grayand Prince1988).
increasingtemperaturein the uninsulatedbirds
The temperaturesalso are within the range that turkeys experiencein the wild. I conducted48 trials. (Fig. 4). Insulated birds showed a similar pat-
April 1996]
313
BareHeadsandThermoregulation
TABLE1. Partial results of repeated-measures
AN-
OVAs showingstatisticallysignificantsourcesof
variation in the dependent measures(centeredin
bold) of thermal balance listed.
+1 +1 +1 +[ +1 +1 +1 +1 +1 +1 +1
Source of variation
df
F-value
Oxygen consumption
Size
1
5.96*
Insul x Temp
Insul x Temp x Time
2
4
4.16'
4.15'
Cooling capacity
Temp
Insul x Temp
2
2
135.00'**
8.87*
Thermal conductance (total)
Insul
1
7.19 *
Temp
Insul x Temp
2
2
367.00'**
8.03*
Thermalconductance
(dry)
Temp
Insul x Temp
2
2
9.45*
7.96*
Core body temperature
Insul
1
6.62*
Temp
Insul x Temp
2
2
22.10'*
10.39'
Leg temperature
2
Temp
150.00'**
Feathertemperature
Temp
2
42.01'**
Head-skin temperature
Insul
1
12.09'
Temp
2
16.28'**
Frontal-caruncletemperature
Temp
2
6.39*
Dewlap temperature
Temp
Insul x Temp
2
2
6.83*
7.79**
*, P < 0.05; **, P < 0.01; ***, P < 0.001.
significantly higher values than uninsulated
birds at 35øC.
Head insulation resulted in significantly
greatercore-bodytemperaturechangesof birds
at 35 øC,but not at the lower temperaturestested
(Fig. 5). Head insulationservedto keep head
skin warmer at 0øC(uninsulated,31.2 + 1.0øC;
insulated, 36.7 + 0.3øC), but did not result in
significantlyhigherskintemperatures
at 22øand
35øC.Insulatedturkeys at 22øCwere significantly more likely to have increaseddewlap
temperatures than uninsulated birds (uninsulated, 32.1 + 0.7øC;insulated, 35.2 + 0.3øC in-
sulated),but thiswasnottrueat 0øor 35øC.Body
skin,feather,frontalcaruncle,and leg temperatures all increasedwith increasingambient
314
RICHARD
BUCHHOLZ
22ø(2
OoC
.4
35•
[Auk,Vol. 113
OoC
22oc
35oc
ß
ß
ß
0.6
ß
[3[3[3
[]
ß
0.5[]
[]
[]
[]
[]
ß
[]
[]
[]
12345678
12345
678
2345
678
2345
678
Individual Turkeys
Individual Turkeys
Fig. 1. Oxygen consumptionof eight Wild Turkeyswith headsuninsulated(emptysquares)
and then
with headsinsulated (filled squares)at three ambient
temperatures. Turkeys presented in order of increasedbody mass.
Fig. 3. Total thermal conductancesof eight Wild
Turkeyswith headsuninsulated(emptysquares)and
then with heads insulated (filled squares)at three
ambienttemperatures.Turkeyspresentedin order of
increasedbody mass.
temperatures,but were not affectedby the inOoC
35oc
2 2oc
ment,headinsuiation
hadnoeffectonthepro-
[]
150
sulation treatment (Table 1).
Across and within each temperature treat-
[]
portion of instantaneousobservationsduring
which the subjectswere standing, panting, or
had their head tucked in back feathers or under
[]
[]
lOO
ß
ß
[]
the wing (Mann-Whitney U-tests,n = 16, all
P > 0.05). Panting only occurredat 35øC.The
frequencyof panting was difficult to observe
because the birds
ß
ß
held
their
necks for-
window was blockedby the bird'sbody. Therefore, it was not possibleto evaluate associations
between panting frequency and thermal balance.Nevertheless,upon opening the chamber
at the end of the 35øCtrials,I observedpanting
and an elongatedsnood(only visible in unin-
50
ß
often
ward and down so that the view from the small
[]
sulated trials) in all individuals. Snood elon-
gation did not occur at other temperatures.
Although the proportion of observationsin
Individual Turkeys
which the subjectswere standing was not inFig. 2. Coolingcapacities
of eight Wild Turkeys fluenced by the insulation treatment, the fre1234
678
12345
678
2345678
quency of this behavior did have someeffect
with headsuninsulated(emptysquares)
andthen with
headsinsulated(filled squares)at three ambient tem- on the dependent thermal variables. The freperatures.Turkeyspresentedin order of increased quency of standing had a significant positive
body mass.
correlation with head skin temperatures at 0øC
April 1996]
BareHeads
andThermoregulation
315
0.4
22oc
OoC
22oc
0oc
35oc
ß
35o(
ß
ß
[]
[]
ß
ß
ß
m•m[][]
[]
o
ß
ß
[]
m•um []
-0.2
[]
[][]
ß
mm[] []•m
I-
m[]
[]
[]
[]
ß
ß
[]
-0.6
2345
678
234
678
12345678
12345678
Individual Turkeys
Fig. 4. Dry thermal conductances
of eight Wild
Turkeyswith headsuninsulated(emptysquares)and
then with heads insulated (filled squares)at three
ambienttemperatures.
Turkeyspresentedin orderof
increasedbody mass.
(both insulation treatments combined, rs= 0.54,
12345678
12345678
IndividualTurkeys
Fig. 5. Changein corebody temperatureof eight
Wild Turkeyswith headsuninsulated(emptysquares)
and then with headsinsulated(filled squares)at three
ambient temperatures.Individuals whose body temperature changed indentically when insulated and
uninsulatedare representedby a squarecontaining
a vertical line. Turkeys presentedin order of increasedbody mass.
P=0.05) and may be associatedwith lower rates
of metabolicheat production,althoughthe latter correlationonly approachesstatisticalsignificance (r,= -0.50, P=0.07). At 22ø and 35øC,
feather temperature inversely correlated with
standingfrequency(rs=-0.66, P=0.01 and rs=
-0.52, P= 0.05, respectively).
Head tucking occurredin 8 of 16 trials by six
of the eight individuals at 0øC. Higher frequenciesof head tucking were positively correlatedwith changesin dewlaptemperature(rs=
0.50, P=0.05), and negatively correlated with
changesin skin temperature (r, = -0.58, P =
0.04), changesin core body temperature(rs =
-0.54, P = 0.05),metabolicheatproduction(r, =
-0.56, P = 0.05), and both total and dry conductance (rs = -0.56, P= 0.05 and r, = -0.66,
P = 0.02, respectively).
temperature. Although the insulated birds
managedto increasedry and evaporativethermal conductancesdespite the head insulation,
the lower cooling capacitiesof insulated birds
isevidenceof their inabilityto dissipateenough
heat to offsetmetabolicheat production.These
results demonstrate
that the unfeathered
heads
and necksof male Wild Turkeys, and possibly
the fleshy structureson the head, contribute to
heat dissipationat high ambienttemperatures.
Contrary to expectations,under cold con-
ditionshead and neck insulationdid not significantly reduce thermal conductance or increasemetabolic heat production. Under cold
conditions,free-living Wild Turkeysoften contract the skin at the back of their necks, effec-
tively drawingthe featheredskinat the baseof
the neck up and over much of the usually bare
areasof the back of the neck (pers. obs.). The
A dramatic cost of insulated heads and necks
captive Wild Turkeys in my study exhibited
occursin male Wild Turkeysat high tempera- similar behavior, possiblyexplaining the abtures.Insulatedbirdshad higher metabolicrates sence of a difference in thermal conductance
and markedly increasedcore body tempera- between uninsulated and insulated birds at 0øC.
tures relative to uninsulated males at the same
Becausewinter starvation can be an important
DISCUSSION
316
RICHARD
BUCm•OLZ
sourceof mortality for turkey populations in
the northern part of their distribution (Healy
1992a),reducing heat lossfrom the head may
enhanceturkeysurvivorship.At night, thermal
conductancemay be further decreasedby tucking the head under the wing or backfeathers.
In my study, four of the eight uninsulated in-
[Auk,Vol. 113
ent temperature(ca. 30øC)and exposureto direct sunlight may reduce male fertility by as
much as 10%in broad-breastedbronze turkeys,
the domesticbreed mostsimilar in appearance
to Wild Turkeys(Kosinand Mitchell 1955).Wilson and Woodard (1955) found that all domestic
Ptarmigan (Lagopus
spp.), which live at high
turkeys were subjectto hyperthermia at ambient temperaturesabove32øC;this was particularly true of large males.In addition, body temperature and water consumptionby domestic
turkeyswere inverselycorrelatedwith the percent of shade cover provided at ambient temperaturesabove35øC(Wilson et al. 1955,Wilson
and Woodard1955).Wild Turkeysexperienced
heat stressat 35øCin the laboratoryduring my
study.All malesrespondedto hot chambertemperaturesby panting,droppingtheir wings,and
extendingtheir necksand snoods.The smallest
male even becamefrantic at the very end of
both high-temperaturetrials and was removed
immediately. Behavioralchangesthat occurin
free-living wild malesunder hot conditionsalso
suggestthat activity is limited by high ambient
latitudes
temperatures.
dividuals
at 0øC were
seen with
their
heads
tucked during at least one of the observation
periods,and threeof thesehad lower metabolic
rates than the remaining individuals. LaRochelie et al. (1982) found a similar effect of
head tucking in BlackVultures (Coragyps
atratus), which also have unfeathered heads. Additional
studies of the effects of cold environ-
mental conditions(e.g. low temperature,high
wind speed,low insolation)and artificialhead
insulationon headtuckingand thermalbalance
in Wild Turkeysare neededto confirm the efficacyof this tacticfor reducingheat loss.
Anatomicaladaptationsusedto modify heat
losshave been describedin other avian species.
and altitudes
where
the difference
be-
tween body temperatureand ambient temperMales call ("gobble")to attractfemalesmost
ature can be large (e.g. > 60øC),often have often before dawn and during early morning
featheredlegsand feet (Johnsgard1983).Gulls (Hoffman 1990). This is especiallytrue in the
(Laridae) have counter-current heat-exchange presenceof clear skies (Healy 1992b), when
mechanisms that reduce heat loss from the feet
males would risk greater heat and insolation
under cold conditions (Baudinette et al. 1976). later in the day. Mature male Wild Turkeys in
The Wood Stork (Mycteriaamericana)and Tur- northern Florida seem to avoid bright sun at
key Vulture (Cathartes
aura)use their unfeath- any time of the year and, in summer,often are
ered legs to dissipateheat at hot temperatures found standingin heavy shadewith their dewand are able to enhance this mechanism of heat
laps and necksbright red and extended,while
loss by defecating on their legs to promote panting heavily (pers.obs.).Also, the malesare
evaporativeheat loss(Kahl 1963,Hatch 1970). more reluctant to flee under these conditions
Ducks may use the large surfacearea of their and canbe approachedmore closelythan when
bills to dissipateheat (Hagan and Heath 1980). it is cooler.Femalesengagedin gular fluttering
The Wild Turkey is the only speciesin which under hot conditions but did not seem disthe value of unfeathered
heads and necks for
tressedby the heat. From these cursory obserheat dissipationhas been demonstratedexper- vationsit appearsthat the sexesdiffer in their
imentally.
susceptibilitiesto heat stress,possiblyresulting
Previous studiesof Wild Turkey metabolism in fitnessconflictsbetweenthe sexesduring the
have ignored the metabolismof Wild Turkeys breeding seasonwhen malesand femalesmust
at temperaturesabove 25øC(Gray and Prince be together.
Males gobble on warm days in winter, sug1988,Oberlaget al. 1990).The adaptivebenefit
of unfeathered heads demonstratedhere sug- gestingthat they are more responsiveto warm
geststhat peak effective temperaturesduring temperaturesand less dependent on photopethe reproductiveseason,especiallyin habitats riod to stimulate breeding than are females
without shade, may limit Wild Turkey distri- (Healy 1992b).Also, in most areasof their disbution or population density. Theseresultsare tribution, male turkeys exhibit a peak in gobreinforcedby early studieson the temperature blingbehaviorseveralweeksbeforefemalesnest
requirementsof domesticturkeys. High ambi- (reviewed by Hoffman 1990).The degreeof sex
April1996]
Bare
Heads
andTherrnoregulation
differences in reproductive timing may vary
with latitude. Davis (1994: 117) found that Wild
317
Threadgill DissertationFellowship from the College
of Liberal Arts and Sciencesat the University of Florida.
Turkeysin centralTexasgobblesynchronously
with female receptivity, while southern Texas
LITERATURE CITED
malespeak in gobbling well beforefemalesare
usuallywilling to mate.The remarkableability
ABACUS
CONCEPTS,
Inc. 1989. SuperANOVA. Berkeof hen turkeysto storespermfor severalmonths
ley, California.
(Verma and Cherms 1964) may allow females BAUDINETTE,R. V., J.P. LOVERIDGE,K. J. WILSON, C.
to mate well before they would normally ovuD. MILLS, AND K. SCHMIDT-NEILSEN.1976. Heat
late and thereby circumvent the reduction in
fertility experiencedby malesat warmer temperatures(Kosin and Mitchell 1955). Of course
loss from the feet of Herring Gulls at rest and
during flight. Am. J. Physiol. 230:920-924.
BOYCE,
M.S. 1990. The red queen visits SageGrouse
sexualdifferencesin reproductivetiming might
be explainedby other ecologicaldifferencesbe-
BRODSKY,
L.M.
tween the sexes. Nevertheless
there are several
lines of evidenceto suggestthat the daily and
seasonaltemporal patterning of male display
behavior may be shapedto avoid overheating.
Males are faced with a thermoregulatory
quandaryunder hot, sunny conditions.Resting
quietly in the shade maintains sublethal body
temperatures,but doesnot allow feeding, fighting for access
to mates,or displayingto females.
Theselatter activitiesare alsofunctionally and
adaptivelynecessary,
butresultin metabolicheat
productionand exposureto solarradiation.Field
studiesof the behavior of Wild Turkeys relative
to environmental conditions, including radiative heat load and wind speed, are needed to
understand
how males trade-off
thermal
needs
with feeding and mating success.
The resultsof
my studysuggestthat the bare headsand necks
of male Wild Turkeys enable them to manage
these conflictinggoalsmore successfully.
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ACKNOWLEDGMENTS
I gratefully acknowledge the guidance of my doctoral committee: H. J. Brockmann, E. Greiner, R. Kil-
tie, D. Levey and B. McNab. J. Anderson,F. Bonnacurso,P. Fuller, B. McNab, and R. Samudio provided
invaluableadvice,assistance,
and equipment.C. Restrepo provided statisticaladvice and moral support.
Thanks go to F. Nordlie, F. Tomson, the Department
of Zoology, the Florida Museum of Natural History,
and C. Wilcox and her crew for providing housing
facilities or animal care. The National Wild Turkey
FederationprovidedWild Turkey transportboxes.My
researchwas funded by the Department of Zoology
at the University of Florida, a Frank M. Chapman
Memorial Fund Grant in Ornithology from the American Museum of Natural History, a Grant-in-Aid of
Researchfrom Sigma Xi, the Animal BehaviorFund
of the University of Florida Foundation, and a
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