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The Journal of Agricultural
Science
cambridge.org/ags
Animal Review
Cite this article: Santos JDCdos, Saraiva EP,
Pimenta Filho EC, Neta GCX, Morais LKC, Teti
HS, Fidelis SS (2023). Neonatal mortality of
lambs in production systems in a semi-arid
environment: main risk factors. The Journal of
Agricultural Science 1–12. https://doi.org/
10.1017/S0021859623000291
Received: 31 October 2022
Revised: 11 April 2023
Accepted: 17 April 2023
Keywords:
Homoeothermy; heat stress; newborn lambs;
radiant thermal environment; tropical areas
Corresponding author:
E. P. Saraiva; Email: edilson@cca.ufpb.br
© The Author(s), 2023. Published by
Cambridge University Press
Neonatal mortality of lambs in production
systems in a semi-arid environment: main
risk factors
J. D. C. dos Santos1,2, E. P. Saraiva2,3 , E. C. Pimenta Filho3, G. C. X. Neta4,
L. K. C. Morais2, H. S. Teti2and S. S. Fidelis5
1
Animal Science Integrated PhD Program, Federal University of Paraiba (UFPB), Areia, PB, Brazil;
2
Research Group
in Bioclimatology, Ethology and Animal Welfare (BioEt), Department of Animal Science, Federal University of
Paraiba (UFPB), Areia, PB, Brazil;
3
Department of Animal Science, Federal University of Paraiba (UFPB), Areia, PB,
Brazil;
4
Integrative Thermal Physiology Laboratory, Department of Animal Morphology and Physiology, São Paulo
State University (UNESP), Jaboticabal, SP, Brazil and
5
Laboratory of Animal Biometeorology, São Paulo State
University (UNESP), Jaboticabal, SP, Brazil
Abstract
The sheep farming has economic and sociocultural importance in semi-arid regions world-
wide. Of the total of 1.2 billion sheep in the world, 0.4 are found in semi-arid regions. In
this review, we have discussed the main risk factors for neonatal lamb mortality and its asso-
ciation with the prevailing environmental conditions of tropical semi-arid regions. Over the
last decades, the average mortality rate of newborn lambs remained relatively constant
(∼15%) around the world. This rate is reported to be higher (Up to 30%) in small-scale
sheep farming systems from developing countries. Overall, the main risk factors of neonatal
mortality include low birth weight, dystocia, adverse weather conditions, inadequate milk,
or colostrum supply, competition between siblings in multiple births, and poor expression
of maternal behaviour. In tropical semi-arid regions, recent findings revealed that newborn
lambs from hair coat sheep breeds are less vigorous to perform the first suckling when
even exposed to moderate cold conditions (18–20°C). On the other hand, the high radiant
heat load in these areas can also challenge the thermoregulation of ewes and newborn
lambs, especially if they are kept in areas without protection against direct short-wave solar
radiation. Under such circumstances, newborn lambs were reported to be hyperthermic.
The heat stress as risk factor for neonatal lamb mortality is a topic that deserves more inves-
tigation, particularly in tropical semi-arid areas, where is expected to be drier and hotter as
consequence of rapid advances in climate change.
Introduction
Sheep farmers produce meat, milk, wool/hair, and skins for national and international markets
(Morris, 2017). In the world, more than 1.2 billion sheep are being produced globally each year
and 0.40 of the effective sheep population is found in semi-arid regions, with greater concen-
tration in India, the Middle East, the highlands of East Africa and South America
(Zygoyiannis, 2006; IWTO, 2022).
In these regions, the systems used for raising sheep vary between and within different coun-
tries. There are three major sheep production management systems in the world, namely
extensive, semi-extensive and intensive production. Generally, the different breeding systems
can provide good results (Morris, 2017). However, to be efficient in the sheep production sys-
tem, a high survival rate of the lambs is necessary, which is influenced by the temperament of
the ewe, the vigour of the newborn, and other factors related to the environment, e.g. nutrition,
human-animal interaction, facilities and thermal environment (Costa et al., 2008; Silva et al.,
2013; Araújo da Silva et al., 2020). The exit of the newborn from the uterine to the external
environment requires profound behavioural, physiological, and neurological adjustments in
the ewe and newborn (Lanfranchi et al., 2016). When compared to adult animals, newborn
lambs are more susceptible to the fluctuations in the thermal environment, as they born wet-
ted, and has a greater body surface area to volume ratio (Jones, 1997; Laburn, 2001;
Lezama-García et al., 2022).
Semi-arid equatorial zones have marked daily variations in air temperature (e.g. 15 to
40°C), high levels of mean radiant temperature, and solar radiation (Fonsêca et al., 2014).
In equatorial semi- arid areas, newborn lambs delivered at night can face with cold stress,
where thermal radiant conditions favour the transference of sensible heat loss to be greater
than the metabolic heat produced by metabolism (Fonsêca et al., 2019). Newborn lambs
from hair coat sheep breeds were more lethargic (i.e. lambs took longer to perform the first
https://doi.org/10.1017/S0021859623000291 Published online by Cambridge University Press
suckling) when delivered at circumstances of air temperature
between 18 and 23°C (Fonsêca et al., 2014). Indeed, the hypother-
mia and starvation are reported to account for 10% of neonatal
lamb mortality in tropical semi-arid regions (Riet-Correa and
Méndez, 2001; Nóbrega Jr. et al., 2005).
Neonatal lamb mortality does not have one specific cause.
Primary risk factors include low birth weight, dystocia, competi-
tion between siblings in multiple births, adverse weather condi-
tions, and poor formation of mother-young bond (Riet-Correa
and Méndez, 2001; Medeiros et al., 2005; Nóbrega Jr et al.,
2005; McCoard et al., 2014). For example, cleaning the lambs
by the ewes after birth is crucial to prevent the rapid evaporation
of fluids and heat transference from the skin surface of lambs to
the surrounding environment. The thermoregulatory behaviour of
ewes also plays an important role, as it directly influences lambs’
motivation for seeking more favourable microclimate (e.g. seeking
shelter or shade) in order to avoid extreme cold or heat (Terrien
et al., 2011; Fonsêca et al., 2014; Ferner et al., 2017; Lezama-
García et al., 2022).
While several studies have evaluated the negative impact of
extreme cold weather on vigour of newborn lambs from wool
breeds in temperate regions (Steyn, 2022), the heat stress as risk
factor for neonatal lamb mortality has not received attention.
However, in equatorial semi-arid regions, where levels of solar
radiation are high and nearly constant throughout the year, new-
born lambs can absorb high amount of radiant heat and face with
challenges to maintain their thermal equilibrium (Fonsêca et al.,
2016). For instance, newborn lambs artificially exposed to radiant
temperature of 40°C kept their body temperature within narrow
range, but at a cost of excessive rise in respiratory rate (Mercer
et al., 1979). Recently, field studies conducted in equatorial semi-
arid regions reported that newborn lambs from hair coat sheep
breeds were moderately hyperthermic when exposed to high levels
(Up to 1000 W m
−2
) of direct solar radiation (Santos, 2020). In
this review, we aimed at investigating the following topics: (I)
Characterization of sheep farming systems in semi-arid regions;
(II) to discuss the main risk factors for neonatal lamb mortality;
(III) and aspects of thermoregulation of newborn lambs.
Sheep production system in semi-arid regions
The sheep was domesticated over 10000 years ago during the
Neolithic in Central Asia. Domesticated sheep underwent changes
from wild shaggy sheep to Mesopotamian fleeced and furred
domestic sheep. In Asia, Asian sheep, which originated from
the mouflon (Ovis Orientalis), lead to the divergence of ovine
genetic resources due to their recruitment from the wild several
times for domestication (Hiendleder et al., 2002; Bruford and
Townsend, 2006; Rezaei et al., 2010). Currently, there are
1.2 billion sheep in the world (IWTO, 2022;Figure 1). Asia
accounts for over 0.42 of all sheep in the world, followed by
Africa, Europe, Oceania, and the Americas with 0.31, 0.11, 0.08
and 0.07. (Pulina et al., 2018). Among countries, China leads
the ranking with 0.14, followed by Australia, India, Nigeria, and
Sudan, with 0.057, 0.050, 0.035 and 0.033, respectively (Toldrá,
2022).
The utility of sheep farming in the world is multidimensional,
providing a variety of products (e.g. meat, milk, and wool) in a
wide spectrum of sociological and socioeconomic conditions.
Since its emergence, most production systems were linked to sub-
sistence, with creations in fragile lands. Over the years, this has
become multifactorial, as climatic conditions, stocking rate, avail-
able area, forage availability, and quality will determine the vari-
ation of production systems, which can be characterized by the set
of technologies and management practices, which lead to consid-
ering the type of animal, breed, the purpose of breeding and
region where the activity is carried out (Holanda Júnior, 2006).
Currently, sheep farming has commercial objectives, which are
characterized by the production of food of high biological value
(Moraes Neto et al., 2003; Braga and Rodriguês, 2005). Despite
the expansion and evolution of systems, a large part of domestic
sheep farming is still done extensively and semi-extensively, which
is characterized by low reproductive performance, low level of
organization, poor use of husbandry techniques and zootechnical
bookkeeping, in addition to seasonality in the availability of water
and food resources (Costa et al., 2014; Simioni et al., 2014;
Table 1).
Figure 1. Number of sheep in the world between 1990
and 2022.
Source: IWTO, 2022.
2 J. D. C. dos Santos et al.
https://doi.org/10.1017/S0021859623000291 Published online by Cambridge University Press
It is estimated that 0.43 of the world’s surface is arid, where
0.40 of the world’s sheep population is found (Kassas, 1975;
Harrington, 1981). Arid and semi-arid regions can be defined
as areas where precipitation, relative to the level of evapotranspir-
ation, is inadequate to sustain reliable agricultural production
(Meigs, 1953). Precipitation in these areas is generally low and/
or erratic. The vegetation consists of grasslands, shrublands,
savannahs, or woodlands, but may also be covered by desert
(Hill and Guerschman, 2020). Anatomical, behavioural, and
physiological characteristics reflect some specificities in the energy
balance of native sheep in arid and semi-arid areas (Fonsêca et al.,
2019). These animals deal with frequent challenges such as low
availability of water resources, supply and quality of food, high
levels of solar radiation, and seasonal and daily temperature
fluctuations.
Both in the extensive and semi-extensive systems, the animals
usually remain in pasture areas during the day and are collected
for installation at night, becoming more susceptible to environ-
mental weather conditions during the daylight. In conditions of
equatorial semi-arid environment, high solar radiation and
mean radiant temperature are the principal factors that affect
the thermoregulation of animals, especially when they do not
have access to shade (Maia et al., 2008; Baêta and Souza, 2010;
Turco and Araújo, 2011). Under such circumstances, during
lambing seasons, the radiative properties are decisive in the
thermoregulation processes. In new-born animals, the pigmenta-
tion of the fur, characteristics of the covering layer, and body size
are decisive for radiant thermal changes. Studies with adult ani-
mals have observed that animals with a dark external surface
are more subject to heat stress than those of light colour, present-
ing greater absorbance of different wavelengths. In contrast,
lighter hairs show greater reflectance, absorbing smaller amounts
of thermal energy (Gebremedhin and Hillman, 1997; Silva et al.,
2003; Maia et al., 2005). The small body size and a relatively large
surface about its mass are striking characteristics in new-born ani-
mals, making them efficient heat sinks and more susceptible to
variations in the thermal environment, when compared to adult
animals (Cain et al., 2006; McManus et al., 2009).
Due to the economic losses generated by the sheep production
system, the survival of lambs has become a worldwide concern.
However, attempts to reduce lamb mortality in recent years
(1970–2014) have not changed significantly, remaining at an aver-
age of 15% in many countries (Fig. 2). For example, in New
Zealand and Uruguay, neonatal deaths account for more than
30% (Sassi and McCosker, 1975; Aspin, 1997); whereas in
England and Australia, deaths vary from 2 to 21% (McFarlane,
1965; Stamp, 1967). In addition, in other semi-arid regions of
the world, mortality rates exceed 30% (e.g. Ethiopia, 46 to 51%,
Bekele et al., 1992; Morocco, 18 to 31%, and Ghana 33%;
Chaarani et al., 1991).
Considering neonatal mortality rates with animals reared in
semi-arid zones, the scenario is practically the same. In studies
with Santa Ines ewes with and without previous parturition
experience, Simplício et al. (1982) reported mortality rates ran-
ging from 5 to 19%, respectively. Lima (1983), when working
with Santa Ines, Morada Nova, and Somalis sheep in the
Northeastern semi-arid region, mentions rates of 13, 18, and
9%. Girão et al. (1998) cite rates of 15% for Santa Ines lambs.
Nóbrega Jr. et al. (2005) observed that 4% of deaths would
occur before birth, 30% in the first 24 h, 20% from the first to
the third day of life, and 35% after 3 days of life. Fragkou et al.
(2010) state that the optimal neonatal mortality rate should not
exceed 3%, with 5% being acceptable. However, such values are
not obtained in most sheep production systems in the world.
Thus, for the activity to be based on good management practices
and maintenance of acceptable levels of animal welfare, it is
extremely important to look at the aspects that interfere with
neonatal mortality, especially in production systems with a low
technological level.
Risk factors that interfere with lamb survival in the first
hours of life
Neonatal mortality is a complex problem in sheep farming, not
have a single specific cause (Oliveira and Barros, 1982). The
main causes of neonatal mortality are birth weight, dystocia, sib-
ling competition in multiple births, adverse weather conditions,
inadequate supply of milk or colostrum (e.g. starvation/hypother-
mia complex), infections, and poor ewe behaviour and/or lamb. In
diagnostic necropsies performed in new-born lambs around the
Table 1. Percentage of total occupancy of the production system (% occupation) adopted in the different regions of the world as a function of the total number of
sheep herds
Production system % occupation Country References
Extensive 69 Brazil Costa et al. (2008)
90 Brazil Martinez et al. (2010)
70
85
60
70
Brazil
Chile
Ethiopia
Mexico
Silva et al. (2013)
Toro-Mujica et al. (2015)
Gizaw et al. (2008)
Hernández et al. (2019)
Semi- extensive 29 Brazil Costa et al. (2008)
10 Brazil Martinez et al. (2010)
29
38
33
Brazil
Ethiopia
Mexico
Silva et al. (2013)
Gizaw et al. (2008)
Hernández et al. (2019)
Intensive 2 Brazil Costa et al. (2008)
0 Brazil Martinez et al. (2010)
1
2
6
Brazil
Ethiopia
Mexico
Silva et al. (2013)
Gizaw et al. (2008)
Hernández et al. (2019)
The Journal of Agricultural Science 3
https://doi.org/10.1017/S0021859623000291 Published online by Cambridge University Press
world, from 1970 to 2022, several authors described the main
causes of perinatal mortality (Table 2).
Birth weight is a major contributor to neonatal mortality (Yapi
et al., 1990; Fogarty et al., 2000), e.g. lambs with low birth weight
and from twin births are more susceptible to hypothermia and
starvation (Scales et al., 1986; Ribeiro et al., 2002). In contrast,
heavy lambs from simple births may also be more susceptible to
mortality, especially due to dystocia (Dalton et al., 1980).
Dalton et al. (1980) described a correlation between birth weight
and mortality of new-born lambs, where the mortality rate of
lambs weighing between 2.0 to 2.5 kg was 48%, while lambs
weighing between 4.0 to 2.5 kg 5.5 kg was 14%, and 22% for the
Figure 2. The average percentage of neonatal mortality in
the world between 1970 and 2014. The dotted line refers
to the average rate (Adapted from Dwyer et al., 2016).
Table 2. Main causes of perinatal mortality of necropsied lambs in the world
Country Diagnosis/causes References
Ireland Climatic conditions (21%); predators (21%); accidents (16%); birth weight (18%); ewe
behaviour (18%); others (6%)
Shiels et al. (2021)
New Zealand Dystocia (9,8); starvation-mismothering-exposure (37%); stillborn (45,5%); unknown (5%);
others (2.7%)
Thompson et al. (2022)
Australia Starvation–mismothering (25%); stillbirth (21%); birth injury (18%); dystocia (9%); death in
utero–prematurity (10%); predation (7%); cold exposure (5%); undiagnosed (4%); infection
(1%)
Refshauge et al. (2015)
India Pneumonia (31.4%); digestive disorders (14.6%); starvation (9.6%); endoparasitism (5.0%);
septicaemia and toxemia (10.1%); accidental (2.1%); others (27.2%)
Mandal et al. (2007)
Brazil Abortion (4%); starvation/hypothermia complex (9%); Neonatal infection (37%); malformation
(21%); predation (2%); others (27%)
Nóbrega et al. (2005)
Brazil Hypoterthermia starvation (46,8%); hypothermia exposure (17%); dystocia (12%); stillbirth
(12,5%); others (10,7%)
Hancock et al. (2012)
United States Starvation/hypothermia (27%); stillborn/dystocia (20%); abortion (16%); pneumonia (17%);
other (20%)
Rook et al. (1990)
United States Starvation (35%); pneumonia (39%); trauma (3%); gastrointestinal problem (10%); respiratory
failure (3%); septicaemia (3%); navel infections (1%); other (6%)
Yapi et al. (1990)
United States Starvation (58.3%); infections (28.3%); other (13.4%) Huffman et al. (1985)
Brazil Climatic conditions (78.5%); dystocia (10.5%); predators (4.5%); stillbirth (2.5%) traumatism
(2.5%); congenital defects (0.5%); other (1,0%)
Oliveira and Barros (1982)
Australia Starvation (46.4%); complex exposure (2.1%); dystocia (18.5); congenital defects (9.1%);
predators (2.7%); nutritional deficiency (0,5%); infections (13.9%); pathological conditions
(4.9%); others (1.9%).
Dennis (1974)
New Zealand Dystocia (32.3%); starvation (26.5%); infections (11.6%); uterine deaths (10.3%); others (19,3%) Hight and Jury (1970)
4 J. D. C. dos Santos et al.
https://doi.org/10.1017/S0021859623000291 Published online by Cambridge University Press
heaviest (i.e. >6.0 kg). Similarly, in studies with Barki sheep,
Sallam (2019) observed a higher neonatal mortality rate in the
lighter animals (19%), followed by those of intermediate weight
(6%) and heavier (3%). Dwyer et al. (2016) determined a ‘U’rela-
tionship between birth weight and the neonatal survival rate,
which shows the interval of 3 to 5 kg as being of optimum
birth weight.
Worldwide, the overall proportion of lamb mortality attribut-
able to dystocia (pooled proportional mortality rate) is 47% (95%
CI: 38%; 55%; Figure 3; Bruce et al., 2021) (Hall et al.1995; Holst
et al.1997,2002; Sargison et al.1997,1998; Kerslake et al.2005;
Everett-Hincks et al.2014; Behrendt et al.2019; Lockwood et al.
2019a,2019b). Dystocia can be caused by hereditary, nutritional,
managerial, infectious, traumatic factors, or a combination of
these (Roberts, 1971). For example, in a study with Morada
Nova sheep, Fonsêca et al. (2014) and Santos et al. (in prepar-
ation) observed that dystocia lambs were slower to get up and per-
form the first feeding; in addition, they received less intensity of
maternal care.
Another contributor to neonatal mortality is the number of
lambs born per birth. Refshauge et al. (2015) and Bruce et al.
(2021) Proportional mortality attributed to dystocia was higher
for singles (70%) than twins (54%) or triplets (59%). Girão
et al. (1998) mention mortality rates of 14% for single lambs
and 30% for twin lambs. This is due to the smaller uterine
space in ewes with multiple births, causing lower birth weight,
in addition to the limited capacity of ewes to provide colostrum
and maternal care in adequate quantity and quality to the new-
born lamb (Medeiros et al., 2005). Colostrum represents a failure
of a passive immune transfer. Lambs are hypogammaglobuline-
mia animals (i.e. dependent on the successful transfer of colostral
immunoglobulins from ewe to lamb). Thus, success in feeding
depends on the ability of the lamb to lift and moves to the
udder, and the ewe’s role is to stimulate and guide her lamb
(Alexander and Williams, 1962; Dwyer, 2008a,2008b).
According to Alexander and Peterson (1961), 15% of lamb
mortalities are the sole responsibility of the s sheep, 33% of
lambs and 52% are the results of their behavioural association.
The onset of maternal-filial behaviour after delivery is induced
and characterized by olfactory and sensory cues provided by the
new-born as licking, low-intensity bleating, and postures that
facilitate the access of the lambs to the udder (Dwyer and
Lawrence, 2005). Negative behaviours of the sheep moments
after birth can be described as not looking for a safe place for
delivery, not being close to the lamb after birth, low intensity of
maternal care (e.g. grooming), and may also show indifference
or hostility to the new-born lamb. Factors such as maternal mal-
nutrition during pregnancy, the presence of predators and inad-
equate handling (human-animal interaction, space availability,
stocking rate) may be responsible for the appearance of maternal
behavioural failures shortly after delivery (Robinson et al., 1999;
Greenwood and Bell, 2003). For example, reduction in the food
supply in the final third of gestation resulted in lighter new-born
lambs, and poor maternal care, such as less time spent cleaning
the young (i.e. grooming), in addition to the appearance of
aggressive targeted behaviours to the neonate (Everett-Hincks
et al., 2005). The lack of maternal care right after delivery
makes it difficult to ingest colostrum and the new-born’s thermo-
regulatory capacity (i.e. the main cause of the starvation/hypo-
thermia complex).
Effect of the thermal environment on the thermoregulation
of new-born lambs
The external environment that surrounds the animal is composed
of climatic elements such as air temperature, solar radiation,
humidity and wind, which interact with each other, influencing
its thermal balance (Silva, 2000a). Sheep are animals that manage
to maintain their body temperature in narrow levels of variation;
this variation is a consequence of thermodynamic processes and
the balance of generation, input and output of thermal energy.
Figure 4 shows the representation of the heat balance in a new-
born lamb, being described by Eqn (1):
(a)RC+M+Hest+RL+CS- CR+K - ES- ER=Dq(1)
where
(α)
R
C
is the short-wave irradiance, the amount absorbed on
the surface of an animal depending on the absorption coefficient
Figure 3. Proportional mortality ratio due to dystocia as
a function of the number of animals born in studies with
newborn lambs. Adapted from Bruce et al. (2021).
The Journal of Agricultural Science 5
https://doi.org/10.1017/S0021859623000291 Published online by Cambridge University Press
(i.e. variable depending on the radiative properties of the surface),
e.g. new-born lambs in semiarid equatorial zones can be exposed
to high levels of radiation and have their welfare compromised; M
represents the amount of heat generated by the metabolism; e.g. in
new-born lambs the main reserves in the body for heat produc-
tion are brown adipose tissue (i.e. thermogenesis without tremor)
and muscle glycogen (thermogenesis with tremor); H
est
is the rate
of storage of thermal energy; e.g. due to low body weight, new-
born lambs have limited heat storage capacity;
(α)
R
L
is the amount
of thermal energy exchanged between animal and environment
through long wave radiation; e.g. due to the high levels of average
radiant temperature found in semiarid equatorial regions, animals
almost always gain heat via long wave radiation; C
S
is the rate of
heat exchanged per convection on the animal’s surface; C
R
is an
index of heat eliminated per respiratory convection; E
S
represents
a heat transfer by skin evaporation (can be represented by the rate
of evaporation of amniotic fluid shortly after delivery and sweat);
E
R
it is an elimination of heat through evaporation through
the respiratory tract; Lastly, Δq”it is a variation of the internal
thermal energy over time.
In newborn animals, the inefficiency of the thermoregulatory
system after birth makes them ‘immature’homoeotherms, losing
and gaining heat more easily. This fact occurs because these
animals have less thermal insulation and a higher body
surface/volume ratio than adult animals. New-born lambs are
born damp and have thin skin, scarce subcutaneous tissue and
more superficial peripheral circulation (Nowak and Poindron,
2006). In addition to the thermal gradient existing after delivery
between the uterine environment and the external environment,
e.g. the temperature inside the uterus with approximately 39°C
for an external environment with temperatures that can reach
negative values (Mercer et al., 1979).
In regions with a semiarid equatorial environment, high levels
of air temperature, solar radiation, and average radiant tempera-
ture can be challenging for new-borns in the first hours of life.
These animals can absorb an excessive amount of heat via thermal
radiation, especially when the farrowing occurs in a pasture envir-
onment and/or in management centres without providing shade
(Neves et al., 2009). On the other hand, even though it is an inter-
tropical region, cold nights (daily thermal amplitude of± 15°C) can
favour a rapid transfer of sensitive heat to the environment and
may even be greater than the amount of metabolic heat produced
by the animal (Fonsêca et al., 2019). In addition, wind and rain can
make the heat balance more expensive, by recruiting optional
thermogenesis to replace the excess heat lost, if the animal does
not have the opportunity to find shelter (DeShazer et al., 2009).
To maintain thermal balance after birth, the new-born needs
‘fuel’to increase its metabolic rate, in an attempt to match the
Figure 4. Heat balance of new-born lambs. Source: personal archive.
6 J. D. C. dos Santos et al.
https://doi.org/10.1017/S0021859623000291 Published online by Cambridge University Press
inflows and outflows of thermal energy, especially when born at
lower temperatures. Studies with several species of mammals
have shown that new-born animals increase their metabolism to
a maximum rate (i.e. summit metabolism), such as piglet
(Mount and Rowell, 1960; Noblet and Le Dividich, 1981); rats
(Taylor, 1970; Mortolla and Dotta, 1992); rabbits (Girard,
1990); calves (Okamoto et al., 1986; Steinhoff, 2010); goats (Celi
et al., 2008; Páleníková et al., 2014); and lambs (Alexander,
1962,1970; Eales and Small, 1980; Slee et al., 1987; Plush et al.,
2016). The main energetic substrates for the metabolism of new-
borns are brown adipose tissue, muscle glycogen and colostrum
intake (Mercer et al., 1979). Lambs with lower birth weights reach
their maximum metabolic rate at higher temperatures when com-
pared to lambs born with higher weights. This greater susceptibility
in light animals occurs due to the lower supply of energy fuels.
Eales and Small (1980) observed that new-born lambs
increased their metabolic rate and rectal temperature when sub-
jected to cold conditions, however, as soon as the metabolism
reaches its maximum production, the rectal temperature drops
slowly (Fig. 5). In addition, the metabolic rate peak occurred
between 60 and 100 min; in this interval, if the new-born does
not obtain energy from the ingestion of colostrum, the body
reserves are depleted, which can lead to the death of the new-born
by starvation/hypothermia complex (Hamadeh et al., 2000).
Many studies have been conducted to verify the effects of the
thermal environment on the thermoregulation of new-born lambs
(McCoard et al., 2014; El Hadi et al., 2016; Labeur et al., 2017;
Santos, 2020). Most of these studies were carried out in a temper-
ate environment. McCoard et al. (2014) state that in conditions of
a thermoneutral environment or mild cold stress, an adequate bal-
ance of internal heat generation, through the production of ther-
mal energy from brown adipose tissue and heat loss can prevent
hypothermia after birth. On the other hand, in more extreme cold
conditions, thermogenesis with tremors using muscle glycogen
reserves can be essential for maintaining the thermal balance of
new-born lambs (Plush et al., 2016). Hypothermic lambs have
greater lethargy, tend not to follow their mother, and may show
signs of distress, compromising their survival (Defra, 2004). In
our studies with Dorper, Santa Ines and Morada Nova lambs in
the semi-arid region, we observed that newborn lambs in colder
seasons (T
a
< 25) took longer to carry out the first feeding
(Fig. 6; Fonsêca et al., 2014; Santos et al., in preparation).
Dwyer and Morgan (2006) observed that lambs with a longer
time to perform the first suckle had a lower rectal temperature
at 1 h and 24 h after birth. Santos et al. (in preparation) observed
that coat surface temperature and rectal temperature correlated
positively with the mean radiant temperature pattern (Fig. 7).
In addition, dark-coated lambs tend to have higher coat surface
temperatures, which can generate greater absorption of radiant
thermal load.
Research has also shown the ability of new-born lambs to
maintain body temperature within narrow ranges under
Figure 5. Metabolic rate, rectal temperature, and water tank temperature when measuring basal metabolic rate and maximum metabolic rate in a 4-hour-old lamb
with 4.60 kg of body weight (Adapted de Eales and Small, 1980).
The Journal of Agricultural Science 7
https://doi.org/10.1017/S0021859623000291 Published online by Cambridge University Press
Figure 6. Mean of latency to suckle (min) both in studies in
the semi-arid region with lambs in the semi-arid region as a
function of air temperature classes. (Adapted Fonsêca et al.,
2014 and personal archive).
Figure 7. Least square means of mean radiant temperature
(°C), hair coat surface temperature (°C) and rectal tempera-
ture (°C) both for Dorper and Santa Inês lambs across the
24 h. The data are presented as mean ± SEM. Source: per-
sonal archive.
8 J. D. C. dos Santos et al.
https://doi.org/10.1017/S0021859623000291 Published online by Cambridge University Press
conditions of maximum fluctuation in air temperature. For
example, the body temperature levels of new-borns up to 24 h
old of the Dorper and Santa Ines breeds fluctuated from 36°C
to 41°C under temperature conditions ranging from 20.7 to 36°
C (Santos et al., in preparation). Likewise, Riesenfeld et al.
(1988) in a study with Merino lambs observed that rectal tem-
perature fluctuated from 38 to 40°C under temperature conditions
ranging from 25 to 37°C. However, these authors mentioned that
they allow normothermia to be achieved at the cost of excessively
increasing the rate of respiratory and cutaneous evaporation, refut-
ing a theory of ‘immature thermoregulators’for new-born lambs.
Faurie et al. (2004) found that the neonatal ability to maintain a
low body temperature, even during the hottest hours of the day, is
available with the ability to sweat and pant a few hours after birth.
Under conditions of high radiant thermal load, a large part of
the studies was carried out on adult animals, and there is still a
lack of data to understand the thermoregulation of lambs and
the factors that affect the heat balance of animals raised in
these conditions. Silva et al. (2010) tried to quantify the thermal
radiation absorbed by the surface in dairy cows raised on pasture
in semiarid. Maia et al. (2015) investigated the effect of coat col-
our on goats exposed or not to solar radiation; and noted that
black or white goats need protection from solar radiation to main-
tain thermal balance. In addition, when managed without this
protection from the sun’s rays, their physiological responses are
altered, especially in black goats, when the solar irradiance is
greater than 800 W/m
2
. Likewise, Fonsêca et al. (2019) evaluated
the effect of the radiant thermal environment on the bio-thermal
responses of Morada Nova ewes and observed that the minimum
body temperature for 24 h was correlated with the minimum tem-
perature of the environment, indicating that heat conservation
strategies were probably important for Morada Nova ewes when
the rates of sensitive heat loss exceed the heat generated by metab-
olism. Santos et al. (in preparation) in studies with lambs in a
semi-arid region, it was observed that the thermal radiation envir-
onment in an equatorial semi-arid region is a challenge for the
thermoregulation of new-born lambs. The authors observed that
the vigour and surface temperature of the coat were directly influ-
enced by the radiant thermal load, which could even generate
hyperthermia when exposed to solar radiation.
Conclusion and prospects
Information in the literature on the main survival risks of new-
born lambs is abundant; studies show that the world average of
neonatal-perinatal mortality reaches 15%. Mortality rates are
often attributed to various causes such as birth weight, dystocia,
adverse weather conditions, inadequate milk or colostrum supply,
sibling competition in multiple births, hunger/hypothermia com-
plex, and poor ewe or lamb behaviour. Most of the studies that
investigate the factors that influence the thermal balance of new-
born lambs were developed in laboratory conditions and temper-
ate climate regions, which almost always emphasized the effect of
cold as the greatest challenge to homoeothermy of new-borns in
the first hours after birth. Research on thermoregulation and fac-
tors that affect the heat balance of new-born lambs in equatorial
semi-arid regions is scarce. However, some studies already seek to
understand how these animals deal with conditions of high mean
radiant temperatures.
Acknowledgements. We are grateful to Empresa Paraibana de Pesquisa,
Extensão Rural e Regularização Fundiária (EMPAER) for the infrastructure
of the facilities for experimenting. The authors are grateful for the support
of the Study Group on Bioclimatology, Behaviour and Animal Welfare
(BIOET) at the Federal University of Paraíba, Brazil. In addition, we are
grateful for the financial support (grants) granted by the National Council
for Scientific and Technological Development (CNPq) and the
Coordination for the Improvement of Higher Education Personnel
(CAPES), Brazil.
Author contributions. José Danrley Cavalcante dos Santos: Contributed
substantially to the conception, and design of the article and to the interpret-
ation of the pertinent literature. She wrote the article.
Edilson Paes Saraiva: Contributed substantially to the conception and
design of the article. Oversaw the writing process. Critically reviewed the art-
icle for important intellectual content.
Edgard Cavalcanti Pimenta Filho: Critically reviewed the article for
important intellectual content.
Geni Caetano Xavier Neta: Contributed substantially to data collection and
writing.
Larissa Kellen da Cunha Morais: Contributed substantiallyto data collection.
Humberto da Silva Teti: Contributed substantially to data collection.
Sérgio da Silva Fidelis: Contributed substantially to data collection.
Financial support. The authors are grateful to the National Council for
Scientific and Technological Development (CNPq).
Conflict of interest. The authors declare there are no conflicts of interest.
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