Thermal sensitivity has been of scientific interest for almost a century. Despite this, several research questions within this field remain unanswered, particularly regarding the specific distribution of thermal sensitivity to cold across the human body. Additionally, while exercise is known to cause a cold stimulus to be perceived as less unpleasant according to the principle of thermal alliesthesia, less has been reported on the effects of exercise on thermal sensitivity to cold. With applications mainly related to clothing insulation and design in mind, the present research project aimed to investigate thermal sensitivity to cold at whole body segments, as well as within body segments, at rest and during exercise. Additionally, a comparison of thermal sensitivity to cold between genders and between ethnic groups was also performed.

Approximately 75% of menopausal women in the United States experience hot flushes but the etiology of this thermoregulatory disorder is unknown. The dominant theory is that estrogen withdrawal sensitizes thermoregulatory areas of the brain, leading to the inappropriate activation of heat loss effectors in response to mild stimuli. This dissertation examines the circuitry of estrogen effects on thermoregulation. First, a rodent model was characterized. In ovariectomized rats, estradiol treatment decreased tail skin vasodilatation, a primary heat loss mechanism, and raised the ambient temperature threshold for tail skin vasodilatation. These results indicate that estradiol does not alter the maximal ability of blood vessels to constrict and dilate, but rather shifts the threshold for thermoregulatory activation in rats. Using this animal model, we examined how estradiol treatment and ambient temperature affect neuronal activity in brain areas involved with thermoregulation and reproduction. Out of 14 examined regions, only 3 areas were significantly affected by both estradiol and temperature and only the median preoptic nucleus (MnPO) exhibited increased activity at warmer ambient temperature. Interestingly, the effects of estradiol and ambient temperature on MnPO activity closely resembled their effects on tail skin vasodilatation. These results identify the MnPO as a plausible site for the integration of estrogen with skin vasodilatation. In the third study, we examined whether thermoregulation can be modified by neurokinin 3 (NK3) receptors, the dominant receptor for neurokinin B (NKB). Core temperature in ovariectomized rats was decreased by microinfusion of a selective NK3 receptor agonist into the MnPO and adjacent septal areas. This transient hypothermia was accompanied by a lack of homeostatic tail skin vasoconstriction but was not caused by tail skin vasodilatation or a global impairment in thermoregulation. These results demonstrate that thermoregulation in rats is modified by NK3 receptors in brain areas that receive projections from NKB neurons. In humans, menopause is associated with hot flushes and the hypertrophy and increased NKB gene expression in arcuate (infundibular) neurons. We propose a novel theory that estrogen withdrawal causes hot flushes by enhancing NKB release from arcuate (infundibular) neurons onto NK3 receptors.

Thermoregulation is considered a top priority in neonatology due to the fact that relative to adults, neonates have a morphological susceptibility to excessive heat exchange with the environment, and exhibit limited physiological/behavioural responses to thermal strain. Consequently, the environmental conditions in which they are nursed must be tightly regulated to maintain body temperature stable. Neonatal intensive care units (NICU) use radiant warmers (RW) to thermally manage many newborns. However, recent evidence suggests that RW induce intermittent bouts of thermal strain that could adversely affect patients. This warrants further investigation of neonatal heat balance and the pertinent factors affecting it. Conducting an exhaustive audit of heat exchanges affecting the body during standard care under a RW could yield important information that would lead to the improvement of clinical practice in NICUs. The present thesis focuses on neonatal thermoregulatory responses, various body heat exchange mechanisms and processes during standard care under RW.

The current thesis examined whether sex-differences in local and whole-body heat loss are evident after accounting for confounding differences in physical characteristics and rate of metabolic heat production. Three experimental studies were performed: the first examined whole-body heat loss in males and females matched for body mass and surface area during exercise at a fixed rate of metabolic heat production; the second examined local and whole-body heat loss responses between sexes during exercise at increasing requirements for heat loss; the third examined sex-differences in local sweating and cutaneous vasodilation to given doses of pharmacological agonists, as well as during passive heating. The first study demonstrates that females exhibit a lower whole-body sudomotor thermosensitivity (553 ± 77 vs. 795 ± 85 W•°C-1, p=0.05) during exercise performed at a fixed rate of metabolic heat production. The second study shows that whole-body sudomotor thermosensitivity is similar between sexes at a requirement for heat loss of 250 W•m-2 (496 ± 139 vs. 483 ± 185 W•m-2•°C-1, p=0.91) and 300 W•m-2 (283 ± 70 vs. 211 ± 66 W•m-2•°C-1, p=0.17), only becoming greater in males at a requirement for heat loss of 350 W•m-2 (197 ± 61 vs. 82 ± 27 W•m-2•°C-1, p=0.007). In the third study, a lower sweat rate to the highest concentration of acetylcholine (0.27 ± 0.08 vs. 0.48 ± 0.13 mg•min-1•cm-2, p=0.02) and methylcholine (0.41 ± 0.09 vs. 0.57 ± 0.11 mg•min-1•cm-2, p=0.04) employed was evidenced in females, with no differences in cholinergic sensitivity. Taken together, the results of the current thesis show that sex itself can modulate sudomotor activity, specifically the thermosensitivity of the response, during both exercise and passive heat stress. Furthermore, the results of the third study point towards a peripheral modulation of the sweat gland as a mechanism responsible for the lower sudomotor thermosensitivity in females.

Student Number : 9613963J - PhD thesis - School of Animal, Plant and Environmental Sciences - Faculty of Science / Animals living in cold environments show physiological, morphological and behavioural adaptations to low temperatures. The African ice rat, Otomys sloggetti robertsi, which is endemic to the southern African Drakensberg and Maluti mountains above 2000m, is an interesting exception since, unlike most alpine small mammals, it does not hibernate or display torpor and is physiologically poorly adapted to low temperatures. It is a strict herbivore, feeding on a low quality diet. Ice rats do show some morphological (e.g. short tails) and behavioural (e.g. communal huddling; constructing underground burrows) adaptations, but little else is known about their biology, particularly how they maximise energy gain to meet thermoregulatory requirements, especially during cold periods. Since feeding represents the primary method of energy gain in endotherms, I studied aspects of the foraging biology of ice rats, including gut structure, foraging patterns and habitat choice. The gut structure of O. s. robertsi is well adapted for a high fibre, herbivorous diet and shows broad similarities with those of its mesic- and arid-occurring relatives. However, O. s. robertsi showed increased dimensions of several foregut organs which may be adaptations for increased energy uptake and/or poor diet quality in alpine environments. Furthermore, females had a larger stomach as well as a longer caecum, small and large intestine in summer than in winter but the gut of males was unaffected; such sexual asymmetry may be related to increased energy requirements of females during pregnancy and lactation. Environmental influences on the aboveground behaviour of O. s. robertsi were investigated by recording the duration of behaviours as well as sequential transitions among behaviours. Ice rats spent most of their day foraging and basking, and much time was spent in their underground burrows. Seasonal comparisons revealed that ice rats spent significantly more time acquiring energy through foraging in winter, whereas they remained below ground for longer periods of time during the middle of the day in summer to escape extreme heat and solar radiation. To understand how low temperatures and predation influenced foraging patterns, the behaviour of ice rats was studied in summer and winter in a population where predators were minimal and in another population which experienced higher levels of predation. Ice rats are central place foragers that travel short distances to forage and display significant seasonal variation in their foraging patterns. In the absence of predation risk, ice rats generally returned to a central place with forage, even though returning to a burrow after foraging in winter was energetically costly. However, these costs must be weighed against the benefits of avoiding exposure to low temperatures by feeding under cover as well as the loss of collected food and possible injury associated with aggressive interactions with conspecifics. Under moderate predation pressure in both seasons, ice rats followed a central place foraging strategy to minimise predation risk, always returning to a burrow entrance with forage collected elsewhere. However, when no perceivable threat was observed, ice rats displayed ‘optimal’ foraging patterns in summer similar to those recorded in the absence of predation pressure and only returned to a burrow with forage as distance from that burrow increased, suggesting that ice rats display facultative foraging decision making in response to multiple environmental cues. The distribution of occupied ice rat burrows was correlated against several environmental factors to determine microhabitat requirements. Ice rat burrows were situated in close proximity to herbaceous and wetland plants, but away from woody vegetation, suggesting that habitat choice is related to the presence of food plants and reduction of shade, facilitating short travel distances during foraging as well as promoting basking. Despite the physiological shortcomings of ice rats, the gut structure, foraging behaviour, and habitat choice of the taxon are adapted for life in cold alpine habitats, most likely by maximising energy intake. Similarities in foraging behaviour and habitat use between O. s. robertsi with its closely-related arid-occurring relative Parotomys spp. suggest phylogenetic influences, but it is possibly more a reflection of similar phenotypic responses to the extreme habitats inhabited by these otomyines.

ABSTRACT Phenotypic flexibility in basal metabolic rate (BMR) in response to short-term thermal acclimation was assessed in the Laughing Dove (Streptopelia senegalensis), a common resident bird species distributed throughout most of southern Africa. I hypothesised that S. senegalensis would display flexibility in BMR over short time scales and that this flexibility would be reversible. Additionally, I hypothesised BMR to be repeatable, and that changes in BMR would be correlated with changes in organ mass. I tested these hypotheses by measuring BMR in three groups of 10 birds before and after a short-term (21 day) thermal acclimation period to one of three air temperatures (10o, 22o & 35oC). After acclimation the three temperature groups were randomly divided and reverseacclimated for another 21 days to one of the two thermal environments not yet experienced. After this reverse-acclimation period BMR was measured again. The dry masses of the stomach, kidney, heart, intestines, liver and pectoral muscles of acclimated birds were used to determine possible mechanistic correlates of BMR adjustments. Additionally, by monitoring BMR every 4-6 days during cold (10oC) and heat (35oC) acclimation I was able to assess the temporal dynamics of adjustments in BMR in response to short-term thermal acclimation. BMR was both flexible and reversible in S. senegalensis as a consistent relationship between BMR and acclimation air temperature was observed after acclimation and reverse-acclimation. BMR increased with decreasing acclimation temperature. Furthermore, a significant proportion (25%) of the observed variation in BMR was repeatable in the 22oC group in spite of the change in BMR induced by thermal acclimation. The mechanistic correlate of BMR adjustment in S. senegalensis appears to be metabolic intensity and not organ size, as the only organ to show a significant increase in size was the intestine of the acclimated 10oC group, which was significantly heavier than the intestine of the 22oC group. BMR also decreases in response to the reduction of flight and/or exercise. Since this reduction was not accompanied by a correlated change in organ mass or body mass, the reduction in BMR as a response to captivity appears to be linked to metabolic intensity of the organs and skeletal muscles. In S. senegalensis adjustments in BMR occur during the first 30 days of captivity and thermal acclimation. The response in BMR to acclimation temperature is clearly evident as BMR of the heat-acclimated group was significantly lower than the coldacclimated group after 21 days. During the response period, which lasts approximately 30 days, BMR adjusts as a mechanism to offset the costs of thermoregulation and habituation to captivity while other metabolic parameters such as body mass, body temperature, and minimum wet thermal conductance adjust to captivity and the thermal environment. After 30 days BMR of the cold and heat-acclimated groups converge on 0.68W, indicating that once the associated metabolic parameters adjust and stabilize in response to the thermal environment, BMR continues to adjust to captivity.

A melatonina desempenha funções fisiológicas importantes, como o aumento da capacidade de termogênese do tecido adiposo marrom. Além dos depósitos adiposos branco e marrom, há um terceiro tipo, as células adiposas beges. Presentes no tecido adiposo branco, apresentam características semelhantes aos adipócitos marrons via um processo denominado browning que está relacionado com a termogênese e aumento do gasto energético. Diante disso, o objetivo deste estudo foi investigar o papel da melatonina sobre a regulação da temperatura corporal em animais idosos obesos da linhagem Wistar que apresentam produção de melatonina prejudicada. Por 16 semanas os animais foram divididos nos grupos controle e suplementado com melatonina e viu-se que os adipócitos marrons e beges atuam de forma coordenada e complementar para garantir a produção de calor de maneira adequada e que a melatonina tem papel importante nos mecanismos termorregulatórios, assegurando maior capacidade de suportar ao frio e, ainda, participando da regulação do balanço energético. / The hormone melatonin plays several physiological functions, one of them is the increase of thermogenesis capacity of brown adipose tissue. In addition to white and brown adipose deposits, there is also a third type, the beige adipocyte, which presents similar characteristics to brown adipocytes, induced in a process called browning. This process is related to thermogenesis and the increase of energy expenditure. Thus, the aim of this study was to investigate the role of melatonin on the regulation of body temperature in elderly obese animals that have impaired production of melatonin. Male-Wistar were divided into control group and group supplemented with melatonin, and the experimental protocol lasted 16 weeks. The main results showed that brown and beige adipocytes act in a coordinated and complementary way to ensure adequate heat production and that melatonin plays an important role in the thermoregulatory mechanisms, ensuring greater capacity to withstand the cold and, also, participating in the regulation of the energy balance.

Anfíbios tem a habilidade de manifestar febre comportamental em ambientes heterotermais durante infecção a um custo metabólico associado à elevação da temperatura corpórea e à ativação do sistema imune. Apesar do custo metabólico, a temperatura corpórea febril otimiza a resposta imune no combate à infecção e aumenta as chances de sobrevivência do indivíduo. Contudo, devido à limitada capacidade de termorregular, os anfíbios enfrentam variações diárias e sazonais na temperatura corpórea e na resposta metabólica de reação à infecção. O nosso objetivo foi medir a variação da resposta metabólica à infecção dentro da variação de temperaturas ecológicas relevantes do sapo Cururu. Testamos a hipótese de que a infecção aumenta as taxas metabólicas do sapo Cururu, mas o custo energético da resposta imune deve ser menor na temperatura febril dos sapos infectados. Para testarmos as hipóteses, nós medimos a temperatura operacional dos sapos no campo, a preferencia termal dos sapos hígidos e a temperatura preferencial dos sapos infectados. Depois, medimos a taxa metabólica e a resposta metabólica dos sapos antes e depois da infecção por LPS nessas temperaturas. Nossos resultados mostraram que as temperaturas ecológicas relevantes dos sapos variaram entre 17°C e 26°C. A temperatura influenciou a taxa metabólica dos sapos, mas só na temperatura preferencial dos sapos hígidos houve custo metabólico associado à infecção. Contudo, na temperatura corpórea dos sapos infectados a resposta metabólica de reação à infecção foi menor, indicando que o controle regulado no ponto de ajustes \"set-point\" da temperatura corpórea durante a infecção coevoluiu com um custo energético otimizado da resposta imune / Anphibians have the ability of manifested behavioral fever in heterothermal environments during infection with a metabolic cost associated to elevated body temperature set-point and due to activation of immune system. Despite the metabolic cost, fever body temperature optimizes immune response to combat infection and increase the survival of the host. However, because of the limited capacity for thermoregulation, amphibians can confront daily and seasonal variation in body temperature and in the metabolic response of reaction to combat infection. So, we measured the variation in metabolic response of reaction to infection at ecology relevant body temperature range in Cururu toads. We hypothesized that infection increases metabolic rates of the Cururu toads due to the activation of the immune system at different temperatures, but the energetic cost of immune response is lower at preferred body temperature of infected toads (behavioral fever). To test these hypotheses we measured the operative body temperature in the field, the preferred body temperature of higid toads, and the preferred body temperature of infected toads. After, we measured metabolic rate and metabolic response of the toads before and after injection of LPS at these temperatures. Our results showed that the ecology relevant temperature range of Cururu toads (R. icterica) varies between 17°C and 26°C, respectively, at operative temperature and at preferred body temperature in infected toads when exposed to heterothermal environment. The temperature had the major impact on metabolic rate of the toads during infection. But, at fever body temperature toads decrease the metabolic response of reaction to infection, indicating that the regulated control of body temperature set-point during infection coevolved with an optimized energetic cost of immune response

Em mamíferos, a temperatura corporal (Tc) varia ao longo do dia, de forma rítmica, bem como a atividade locomotora. Essa ritmicidade diária é gerada endogenamente, por osciladores circadianos. Em geral, os valores mais elevados de Tc concentram-se nos mesmos horários em que há atividade locomotora. No entanto, variações diárias de Tc são observadas mesmo em indivíduos imóveis, evidenciando que o ritmo de Tc não é uma mera consequência dos efeitos agudos da atividade locomotora. Este trabalho teve o objetivo de estudar o ritmo circadiano de Tc no roedor Ctenomys aff. knighti (tuco-tuco) e sua relação temporal com o ritmo de atividade locomotora. A caracterização do ritmo de Tc nesse animal é particularmente interessante, pois ele habita o ambiente subterrâneo, que impõe desafios tanto à expressão rítmica (pois os ciclos ambientais são ausentes ou têm baixa amplitude) quanto à termorregulação (devido à características como alta umidade e atmosfera hipóxica e estagnada). Medimos a Tc e a atividade locomotora dos tuco-tucos em condições controladas de laboratório. Na caracterização inicial, constatamos que o ritmo de Tc dos tuco-tucos persiste mesmo em escuridão e temperatura constantes. Quando houve exposição a um ciclo diário de claro/escuro, o ritmo de Tc foi sincronizado, e os valores mais altos ocorreram na fase de escuro. Os ritmos de Tc e de atividade locomotora mostraram-se fortemente associados no tempo. Investigamos, então, se a corrida na roda de atividade modificaria a amplitude do ritmo de Tc, por envolver atividade muscular intensa. Observamos, no entanto, que a amplitude é pouco alterada com a retirada da roda e que na sua ausência os animais substituem a corrida por outras atividades que também causam efeitos agudos na Tc. Em seguida, estudamos a variação da sensibilidade da Tc aos efeitos agudos da atividade ao longo do dia. Através de um método estatístico, que também foi utilizado para filtrar os dados de Tc, verificamos que há maior correlação entre variações de Tc e de atividade no início da noite. Por fim, investigamos se o controle do ritmo de Tc é exercido pelos núcleos supraquiasmáticos, os quais são os osciladores circadianos do ritmo de atividade locomotora. Nossa abordagem utilizou animais neurologicamente intactos, comparando os padrões do ritmo de Tc e de atividade locomotora durante a ocorrência do fenômeno de \"partição do ritmo\". Constatamos que na maioria dos casos os padrões de partição, tanto da Tc como da atividade locomotora são idênticos, sugerindo um controle temporal comum. No entanto, em um dos animais observamos diferenças entre os dois ritmos, sugerindo que esse controle circadiano pode ser ainda mais complexo. / In mammals, body temperature (Tb), as well as locomotor activity, changes during the day, exhibiting a rhythmic pattern. This daily rhythmicity is generated endogenously by circadian oscillators. Usually, the highest Tb values occur simultaneously to locomotor activity. Nevertheless, daily Tb changes are observed even in immobile individuals. This fact is evidence that the Tb rhythm is not a mere consequence of the acute effects of locomotor activity. The present work,which os performed in the research center CRILAR, in Argentina, had the objective of studying the circadian rhythm of Tb. in the rodent Ctenomys aff. knighti (tuco-tuco) and its temporal relationship with the locomotor activity rhythm. The characterization of the Tb rhythm in this animal is especially interesting because it inhabits the subterranean environment, which poses challenges to both rhythmic expression (because environmental cycles are either absent or have low amplitude) and thermoregulation (due to high relative humidity, hypoxic atmosphere and limited ventilation). We measured Tb and locomotor activity of tuco-tucos under controlled laboratory conditions. An initial characterization showed that the tuco-tuco\'s Tb rhythm persisted even in constant darkness and constant temperature. This rhythm synchronized to a daily light/dark cycle, with highest values occurring during the dark phase. Tb and locomotor activity rhythm were robustly associated in time. Then, we investigated whether the wheel running would modify the Tb rhythm amplitude, because this behavior involves intense muscular activity. However, we observed that the amplitude is only slightly altered upon running-wheel removal and that in the absence of the wheel tuco-tucos substitute running with other behaviors which also have acute effects on Tb. Applying a statistical method, which was also used to filter Tb data, we verified that there is a stronger correlation between Tb changes and activity in the beginning of the night. Finally, we investigated whether the Tb rhythm is also under control of the suprachiasmatic nuclei, which are knowingly the circadian oscillators for the locomotor activity rhythm. We used an approach which involved neurally intact animals, by comparing the rhythmic patterns of Tb and locomotor activity rhythms during the occurrence of a phenomenon called \"splitting\". In most cases, splitting patterns of both Tb and locomotor activity rhythms were identical, indicating a common temporal control of these two variables. Nevertheless, we observed, in one animal, differences between the two rhythms, which suggest that the circadian control might be even more complex.

INTRODUÇÃO: O excesso de adiposidade é considerado um fator que prejudica a termorregulação durante exercício no calor; entretanto, isso não tem sido observado em alguns estudos que compararam adolescentes obesos e magros. Tal diferença entre estudos pode ser devida à falta de controle de fatores tanto individuais, como nível de condicionamento aeróbico, quanto metodológicos, como nível de hidratação e produção de calor metabólico. OBJETIVO: Testar a hipótese de que meninos obesos com condicionamento aeróbico e grau de hidratação similares aos de magros, apresentam prejuízos nas respostas termorregulatórias ou perceptivas ao se exercitarem num mesmo %VO2pico ou com mesma produção de calor metabólico. MÉTODOS: Foram recrutados para este estudo 50 meninos púberes, sendo quatro desses excluídos por critérios de sobrepeso. No total, 46 (23 obesos e 23 magros), aclimatizados ao calor e com similar VO2pico por massa muscular, participaram do protocolo experimental. Entre esses, 41 meninos foram categorizados de acordo com seu condicionamento aeróbico: 20 (10 obesos e 10 magros) apresentavam alto condicionamento – “highfit” (HIfit); e 21 (10 obesos e 11 magros), baixo condicionamento – “lowfit” (LOfit). Todos pedalaram por dois períodos de 25 min, separados por 10 min de intervalo, dentro de uma câmara ambiental (35°C, 40-45% de umidade relativa), em duas sessões diferentes apenas quanto ao alvo de intensidade do exercício: 1) intensidade relativa (IR) a 40-45% do VO2pico; e 2) produção de calor metabólico fixada (PCMF) em 5,5 W.kg-1. A temperatura retal (Tretal), a temperatura da pele (Tpele) e a frequência cardíaca (FC) foram avaliadas continuamente, e variáveis perceptivas (de esforço, sensação térmica e conforto térmico) foram registradas a cada 5 min. A massa corporal foi medida antes e após cada 25 min de exercício para calcular o balanço hídrico e o volume de suor. Os meninos mantiveram-se hidratados, ingerindo, no intervalo de 10 min, um volume de água semelhante às perdas de suor decorrentes dos primeiros 25 min de exercício. RESULTADOS: O aumento na Tretal até o final do exercício foi discreto e similar entre obesos e magros, mesmo quando os grupos foram categorizados em HIfit e LOfit. Tal similaridade foi observada também no aumento da Tpele entre os grupos. No geral, a FC foi maior nos obesos em alguns momentos da sessão IR e ao longo de quase todo o exercício da sessão PCMF. Mas, quando categorizados pelo condicionamento aeróbico, foi maior em OB-LOfit vs. magros (HIfit e LOfit) no minuto 40 da sessão IR, e maior em OB-LOfit vs. MA-HIfit no minuto 50 da sessão PCMF. Entretanto, isso não foi acompanhado pela percepção de esforço nas duas sessões de exercício, nem mesmo pela sensação de calor ou desconforto térmico. CONCLUSÃO: Quando o alvo de intensidade para o exercício foi fixado pela PCM ou relativo ao VO2pico, e a hidratação foi controlada, nenhuma desvantagem termorregulatória ou de percepção ao calor foi observada por parte dos obesos, independentemente do seu condicionamento aeróbico. / INTRODUCTION: Excess adiposity is considered a factor which impairs thermoregulation during exercise in the heat. However, in adolescents, previous studies showed either similar or greater increase in rectal temperature (Tre) in obese compared to lean while exercising in the heat. The differences amongst studies may be due to lack of controlling factors such as metabolic heat production (Hprod), aerobic fitness and hydration levels. PURPOSE: To test the hypothesis that obese compared to lean adolescent boys with similar aerobic fitness and hydration levels will have greater Tre increase when exercising in the heat in a given Hprod. METHODS: Fifty pubertal boys were recruited for this study; four of these were excluded because overweight criteria; and a total of 46 (23 obese e 23 lean pubertal), heat-acclimatized, and with similar VO2peak by total muscle mass, participated in the experimental protocol. Among these, 41 boys were categorized according to their aerobic fitness: 20 (10 obese e 10 lean) were high fit (HIfit), and 21 (10 obese e 11 lean) were low fit (LOfit). All performed two 25-min exercise bouts, separated by 10-min rest in a controlled hot environment (35°C, 40-45% RH), in two sessions which only differed as the exercise intensity target: 1) relative intensity (RI) 40-45%VO2peak; and 2) metabolic heat production fixed (HProd) of 5.5 W.kg-1. Tre, skin temperature (Tskin) and heart rate (HR) were evaluated continuously, and perceptual variables (thermal sensation and thermal comfort) were recorded every 5 min. Body mass was measured before and after each exercise bout to calculate water balance and sweat volume. Boys kept hydrated as water volume intake at rest (between bouts) was similar to sweat losses from the 1st bout. RESULTS: The increase in Tre was discreet and similar between obese and lean boys at the end of exercise, even when the groups were categorized as HIfit and LOfit. It was also observed to Tskin increase. Overall, HR was higher in obese at times of the RI session, and for almost the entire Hprod session. But, when categorized by VO2peak, HR was higher in obese LOfit vs. lean (HIfit and LOfit) in the RI session (in 40th min) and higher in obese LOfit vs. lean HIfit in the Hprod session (in 50th min). However, this was not accompanied by the perception of effort in two exercise sessions, not even by the heat sensation or thermal discomfort. CONCLUSION: When the intensity target of exercising was fixed to Hprod or relative to VO2peak and hydration was controlled, as well as fitness level, no difference was found in the increase in Tre or sweat volume between groups. Therefore, our hypothesis was not verified, because no disadvantage in thermoregulatory or heat perception was observed in the obese group, regardless of aerobic fitness.