Thermoregulatory Role of the Unfeathered Head and Neck in Male Wild Turkeys

The Auk 113(2):310-318, 1996 THERMOREGULATORY ROLE OF THE UNFEATHERED HEAD AND NECK IN MALE WILD TURKEYS RICHARD BUCHHOLZ • Departmentof Zoology,University of Florida Gainesville,Florida 32611, USA AI•STRACT.--Thebrightly colored,unfeathered heads and necks of male Wild Turkeys (Meleagrisgallopavo) are generallythought to functionin sexualselection. However, studies in other bird specieshave suggestedthat uninsulatedbody regionsmay serve an important role in heat dissipation.I test the heat-dissipationhypothesis in Wild Turkeysby experi- mentally reinsulatingthe headsand necksof Wild Turkeysas though they were feathered. The oxygenconsumption, thermal conductance,cooling capacity, surface temperatures, and core temperatureof control and reinsulatedWild Turkeyswere comparedat 0ø, 22 ø and 35ø(2. Head insulationresulted in significantlyincreased rates of oxygenconsumption, higher body temperatures,and decreasedcooling capacitiesat 35øC,but had no significanteffect at the other temperaturestested. It appearsthat behavioral changesat low temperatures,such as tucking the head under the back feathers,effectively prevent the heat lossthat would oth- erwise be causedby the absenceof feathers.However, if the head were feathere& turkeys at high temperatureswould be unable to dissipatesufficient heat to maintain thermeostasis. Thus,given this finding for Wild Turkeys,it canno longerbe saidthat in all casesbare heads in birds have evolved by sexualselection alone. Lossof head and neck featbering in Wild Turkeysand other birdsmay have allowed thesespecies to take advantageof regionsin time and spacethat previouslywere unexploitabledue to the dangersof hyperthermia.Received 22 June1994, accepted 27 January1995. ENDOTHERMSUSUALLY MAINTAIN body tem- ported by correlativestudies showing that un- peraturesabove environmental temperatureat featheredhead and neck skin is maximally ex- considerableenergetic cost. To saveenergy they posed at high temperaturesand that in some reduceheat lossto the environmentby insu- taxa the size of unfeatheredareas is greater at lating themselvescompletely with fur or feath- low latitudeswhere heat dissipationmay be of ers. Birds that have areas of unfeathered skin greater importance (Crowe 1979, Buchholz on their headsand necksare an unexplained 1994). Highly vascularizedfleshy ornamenta- exceptionto the pattern of completeinsulation tion presentsa functional puzzle when species seen in other endotherms.In carrion-feeding are distributedover a large latitudinal range in birds, unfeathered heads often are assumed to which they are exposed to both temperature be a hygienic adaptation (Welty 1975: 100). extremes.Although these speciesmay benefit However, in specieswhere the unfeatheredar- by usingtheir fleshystructures to dissipateheat easare alsobrightly colored,sexually selected under hot conditions, the uninsulated nature functionsare usuallysuspected (Zuk 1991). of thesestructures subjects the birds to extreme Despite some studies suggestingthat these heat loss under 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 possiblethermoregulatory function of vical retes (Crowe and Crowe 1979, Crowe and unfeathered head ornamentation in a species Withers 1979, LaRochelie et al. 1982), thermo- that commonlyfaces extremes of cold and heat, regulatory hypothesesfor the evolution and the Wild Turkey (Meleagrisgallopavo). maintenanceof unfeatheredskin rarely are con- Wild Turkeys occur over a broad range of sidered.The heat-dissipationhypothesis is sup- temperatureextremes from their southernlimit in southern Mexico to their northern limits along the border of the United Statesand Can- • Present address:Department of Biology, North- ada (Dickson 1992). Males are twice as large as eastLouisiana University, Monroe, Louisiana71209, females,and have brightly coloredunfeathered USA. heads and necks (Buchholz 1995). In addition, 310 April 1996] BareHeads and Thermoregulation 311 this bare skin is coveredwith polyp like elab- was drawn through the metabolicchamber, pumped orationsof the integument called caruncles.A into glasscolumns filled with soda lime (to remove thin dewlap extendsfrom the mandible down CO2)and silicagel (to removeH20), after which flow to the neck. Perhapsmost distinctive is the bare, rates (g = 20.6 L/min) were measuredby a Brooks distensiblefrontal processor snoodthat pro- $ho-Rateflowmeter. Subsequently, the airstreamwas sampledwith an Applied ElectrochemistryS-3A oxy- jectsfrom the foreheadat the baseof the upper bill. gen analyzer. The temperatureand humidity of the room air varied little (23.5 + SE of 0.0øCand 61.2 _+ Fleshyhead ornamentation in Wild Turkeys 0.2%,respectively). Humidity in the chamberwas not and other galliforms often is thought to be controlled. The bird's evaporativewater losswas mea- maintained by sexual selection (i.e. the struc- suredgravimetrically (i.e. by weighing the silicagel). tures function in mate choice and male-male Core body temperaturewas measuredby insertinga competition).Ample empirical evidence sup- copper-constantanthermocouple, tipped with a thin portsthis contention (Brodsky 1988, Boyce 1990, 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 1995). A role in sexual selection, however, does in the metabolic chamber and immediately after it not rule out concurrent functions for these was removedfrom the chamber.Six surface-temperature 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-tippedther- have unfeathered heads and necks, the common mocouple held against the appropriate spot, while ancestorsof modern turkeys presumablyhad the subjectwas still in its holding box before the trial featheredheads, as do mostgalliforms. Under- and, again, while it was in the metabolicchamber at standingwhy the unfeatheredareas of modern the end of a trial. Skin and feather surfacetempera- turkeysare maintained, despite the possiblecosts tureswere measuredon the chestapproximately 3 cm in termsof heatloss, may explainwhy ancestral ventral to the carpal joint of the wing at rest. Leg temperaturewas measuredimmediately posteriorto turkeys lost their head feathering over evolu- the third scaledistal to 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 temperature was measuredon the back of the assessthe thermoregulatory trade-offs that head at a point posteriorto the lower mandible. Sur- maintainunfeathered heads in this species. facetemperature of the frontal carunclesand dewlap were measured at the centers of these structures. METHODS Different rates of physical activity acrosssubjects and trials can make it difficult to detect the effect of Subjectsand equipment.--Eighttwo-year-old, male experimentaltreatments on metabolicrate. Therefore, Wild Turkeysobtained as chicksfrom a game farm I minimized the bird's activity by conductingtrials (L&L Pheasantry,Hegins, Pennsylvania)were used at night in the dark. Metabolictrials lasting2.5 h were in the metabolictrials. Rearing conditionsare de- conductedbetween 1900 and 0200 EST. All subjects scribed in detail elsewhere (Buchholz 1994). There is were given at leastone day betweentrials. Individual no differencein the metabolicrates of Wild Turkeys turkeys were tested at the same time of day (either from game-farmor free-living sources(Gray and 1900 or 1100) acrossall treatments to minimize cir- Prince 1988). The averagebody massof the study cadian effects on matched comparisonsof metabolic individualswas 7.1 kg (range6.4-8.1 kg). During the rate. The first 30 min of eachtrial servedas an equil- study period (29 June-27 September1993) the birds ibration period during which the bird calmed down were provided with feed (Purina Gamebird Mainte- after handling. The lowest rate of oxygen consump- nance,12% protein) and waterad libiturn. Subjects were tion (correctedto standardpressure and temperature) deniedfood for 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. Postabsorptiveconditions are necessaryto mea- rate for the entire trial. All individuals were given surethe basalor minimum rate of metabolism(McNab two 2.5-h habituationtrials prior to the experimental 1988a).Water was still availableduring the pre-trial trials.Usually, the subjectsrested quietly during the period. experimentaltrials. The following threebehavior pat- Oxygen consumption and total water loss were ternswere recordedas presentor absentthrough in- measuredin an open system(described by McNab stantaneoussampling (Martin and Bateson1986) ev- 1988b).The temperatureof the 329-L metaboliccham- ery 30 min: standing; head tucked under feathers; ber was regulatedby pumpingwater from a water panting. Observationswere made with the aid of a bath through the chamber'shollow wails. Room air flashlight through a small window in the chamber. 312 RICHARDBUCHHOLZ [Auk,Vol. 113 Experimentaldesign.--To evaluate the

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