Strategies for Hypothermia Compensation in Altricial and Precocial Newborn Mammals and Their Monitoring by Infrared Thermography

Abstract

Thermoregulation in newborn mammals is an essential species-specific mechanism of the nervous system that contributes to their survival during the first hours and days of their life. When exposed to cold weather, which is a risk factor associated with mortality in neonates, pathways such as the hypothalamic-pituitary-adrenal axis (HPA) are activated to achieve temperature control, increasing the circulating levels of catecholamine and cortisol. Consequently, alterations in blood circulation and mechanisms to produce or to retain heat (e.g., vasoconstriction, piloerection, shivering, brown adipocyte tissue activation, and huddling) begin to prevent hypothermia. This study aimed to discuss the mechanisms of thermoregulation in newborn domestic mammals, highlighting the differences between altricial and precocial species. The processes that employ brown adipocyte tissue, shivering, thermoregulatory behaviors, and dermal vasomotor control will be analyzed to understand the physiology and the importance of implementing techniques to promote thermoregulation and survival in the critical post-birth period of mammals. Also, infrared thermography as a helpful method to perform thermal measurements without animal interactions does not affect these parameters.

Keywords: body temperature; brown adipose tissue; neonate welfare; shivering; thermoregulation; vasoconstriction.

Figure 1

Neonate compensatory mechanisms in response to hypothermia. In newborns (illustrated by a naked mole (Heterocephalus glaber) in this figure), when the peripheral receptors responsible for the thermal sensation of cold are activated (e.g., TRPM8 and TRPA1), a neuronal response is generated that involves spinal structures (DRG) and the brain. In the brain, the thermoregulatory center (POA) receives the signal from the LPB. The POA has connections to the DMH, which, in turn, is connected to the rRPA and the IML neurons. Once in the spinal cord, two responses are produced through sympathetic efferents. Firstly, the innervation of blood vessels generates vasoconstriction and heat retention; conversely, the sympathetic release of NE acts on the BAT adrenergic receptors to produce heat. An additional response is shivering, a heat-generating process that depends on the spinal cord’s somatic motoneurons and their terminals. These mechanisms promote the production or retention of heat to protect the body from the consequences of hypothermia. BAT: brown adipocyte tissue; DMH: dorsomedial hypothalamus; DRG: dorsal root ganglion; IML: intermediolateral nucleus; LPB: lateral parabrachial nucleus; NE: norepinephrine; POA: preoptic area of the hypothalamus; rRPa: rostral raphe pallidum; SS: somatosensory cortex; UCP1: uncoupling protein 1; VM: ventromedial hypothalamus.

Figure 2

Morphoanatomical differences in altricial and precocial newborn rodents and their influence on thermoregulation. In altricial or underdeveloped Monodelphis domestica and Rattus norvegicus) and precocial or physically mature species (Cavia porcellus, Acomys cahirinus), morphological characteristics promote or hinder thermoregulation. The lack of fur, low amounts of BAT, uncoordinated locomotion, and thin skin with capillaries close to the epidermis contribute to heat loss and susceptibility to hypothermia in altricial species, contrarily to precocial species. BAT: brown adipose tissue; WAT: white adipose tissue.

Figure 3

Huddling behavior in newborn Wistar rats (R. norvegicus) and New Zealand rabbit pups (O. cuniculus). (A) Thermoregulatory effect of huddling and the influence of the position within the nest in 1-day-old rat pups. The surface temperature of one of the rats in a central position, measured in the dorsal interscapular region (Bx1), shows a maximum, a minimum, and an average temperature of 37.3, 36.8, and 37.0 °C, respectively. In contrast, the temperatures of one rat pup in the nest periphery (Bx2) registered a maximum value of 36.7 °C, a minimum of 36.1 °C, and an average of 36.4 °C. (B) 1-day-old New Zealand rabbit pups huddling with conspecifics (Bx1) in a central position are observed as a white tone in the thermogram. A reddish-yellow tone is observed in newborn rabbits positioned in the group periphery (Bx2). Between central and peripheral positioned rabbit neonates, a difference of up to 2.3 °C in the minimum temperature (39 vs. 36.7 °C) and an average difference of up to 0.6 °C were recorded. These values measured through infrared thermography provide information on the importance of neonate rodents and leporids’ behaviors to preserve heat and to maintain a stable body temperature. The authors took the thermographic images with a FLIR E80 camera with an 18 mm FOL lens at a resolution of 320 × 240 pixels and the ability to accurately measure temperatures from −20 °C to 550 °C/−4°F to 1022°F (emissivity = 0.95, distance = 30 cm).

Figure 4

Thermographic evaluation in altricial and precocial neonates. (A) Dry newborn water buffalo calf (Bubalus bubalis) thermogram. The maximum temperature (shown as a white color in the thermogram) is observed in the facial and thoracic region, while the coolest temperatures (color green and yellow) are observed in the forelimbs and hindlimbs. (B) Dry newborn puppy (Canis lupus familiaris) thermogram. The maximum temperature can be seen in the cranial region of the thoracic and pelvic limbs (white color), while the back and the facial region (yellow color) and the nose (green color) of the newborn represent the coolest regions. (C) The graph exhibits the maximum, minimum, and average temperatures of the water buffalo newborn calf’s thoracic, abdominal, forelimb, and hindlimb areas. (D) The graph displays the newborn puppy’s maximum, minimum, and average temperature in the thoracic, abdominal, forelimbs, and hindlimb areas. A significant difference between the temperatures of both species can be recognized, where the water buffalo newborn calf has the highest values regardless of the evaluated region. Both thermograms were recorded at minute 60 post-birth to compare the thermoregulatory ability of a precocial animal (water buffalo neborn calf) and an altricial animal (puppy dog). The authors took the thermographic images with a FLIR E80 camera with an 18 mm FOL lens at a resolution of 320 × 240 pixels and the ability to accurately measure temperatures from −20 °C to 550 °C/−4 °F to 1022 °F (emissivity = 0.95, distance = 30 cm).