Thermal Adaptation - Basking

Reptiles are ectothermic animals and rely on external heat sources to regulate their internal temperature (Bogert 1949). This suggests that their means of thermoregulation is primarily behavioural, other than obvious physical structures affecting heat transfer such as skin type/thickness, and skin colour. These physical structures likely contribute to thermoregulation of reptiles in conjunction with behavioural adaptations, but will not be discussed in this post. The primary focus of this blog post is to evaluate the environmental conditions that reptiles are subject to, and discuss the adaptations that enable the effective operation of biological systems within reptiles. An understanding of the adaptations allowing the survival of reptiles in their habitat enables inference into past, present and future evolutionary patterns.

(Fig.1 - Thermoregulation time for reptiles of varying weights)

Reptiles regardless of species or size tend to share common behavioural thermoregulation patterns. Most activity is recorded in the morning with the rising sun (finding an appropriate position and substrate to bask upon), with decrease of activity or normal foraging/resting behaviour from approximately mid-day onwards. This was found in the lace monitor (Varanus varius), which would emerge from shelter and bask in morning sun until reaching 35-36°C, then reduce activity to forage and return to shelter (Seebacher 2004). The 2004 study by Seebacher also found variances in basking time between reptiles of varying weights; the five kilogram lace monitor required over 90 minutes to reach body temperature equilibrium, where a small lizard (0.01kg) required less than 15 minutes (Fig. 1). This is significant in determining the costs of achieving thermal preference, in that the risk of predation increases if basking time increases (Blouin-Demers & Weatherhead 2001). The time in which reptiles reach their thermal preferences is varied throughout species and weights, but the techniques used work the same. It would make logical sense that larger reptiles which take longer to behaviourally thermoregulate have evolved with different adaptations to deal with costs of thermoregulation (predation, oxygen availability, energy costs) than those which can achieve optimum internal temperature relatively quickly and easily.

Reptiles rely on behavioural thermoregulation (basking) in the early hours of the day to maintain optimal internal temperature (Bogert 1949). Basking in the morning enables reptiles to spend the remainder of the day engaging in routine activities (foraging, mating etc.) which require energy and warm, active cells. Larger reptiles require more time basking, presenting risks such as predation, energy loss, and dehydration. However, it is likely that larger reptiles have evolved with very different defense mechanisms to smaller ones to compensate for the cost-benefit scenarios present in reptilian ecological communities.

References:

Blouin-Demers, G. Weatherhead, P. 2001, ‘Thermal ecology of Black Rat Snakes (Elaphe Obsoleta) in a thermally challenging environment’, Ecology, Vol. 82, No.11, pp. 30125-3043

Bogert, C. 1949, ‘Thermoregulation in Reptiles, A Factor in Evolution’, Evolution, Vol. 3, No. 3, pp. 195-211

Seebacher, F. Shine, R. 2004, ‘Evaluating Thermoregulation in Reptiles: The Fallacy of the Inappropriately Applied Method’, Physiological and Biochemical Zoology: Ecological and Evolutionary Approaches, Vol. 77, No. 4, pp. 688-695

Figures:

Fig.1 - Seebacher, F. Shine, R. 2004, ‘Evaluating Thermoregulation in Reptiles: The Fallacy of the Inappropriately Applied Method’, Physiological and Biochemical Zoology: Ecological and Evolutionary Approaches, Vol. 77, No. 4, page 690

Thermal Adaptation - Heat Acclimation in Plants

The survival and reproduction of plants is dependent on availability of water, nutrients and sunlight. Increased environmental temperature can put significant pressures on plants, affecting osmoregulation and photosynthetic ability of the cells (Berry & Bjorkman 1980). Plants inhabiting hot climates possess adaptations to the impacts of heat stress (excessive water loss, chlorosis, increased respiration) (McWilliam & Naylor 1967). Physiological stress in plants as a result of temperature reduces reproductive and individual growth. This blog post will discuss some adaptations allowing plant survival in high temperatures. 

Fig. 1 - Diagram showing processes of photosynthesis and respiration in plant cells
(Morholt & Brandwein)

Plant respiration is the oxidization of glucose in the mitochondrial organelles of plant cells, resulting in production of carbon dioxide (CO²) and water (Fig. 1). Global circulation models (GCM) have suggested that increased environmental temperature increases respiration in plants, resulting in water loss of leaves and roots, photosynthetic inhibition of leaves, and weakened roots (especially with increased soil temperature) (Atkin et al 2005). Increased respiration would also increase atmospheric CO², contributing to the existing greenhouse effect. Atkin et al (2005) contradicted GCM and found that increased environmental temperature does not always result in increased plant respiration and adaptations exist to balance regular CO² production with increased environmental temperature. These adaptations are both intra-cellular and physiological, depending on the species concerned and the niche conditions for its habitat.

Fig. 2 - Representation of the temperature response of respiration in warm-grown (Hot), cool-grown (Cold) and warm grown type plants acclimated to cooler and hotter temperatures
(Atkin et al 2005)

Desert plants reduce respiration and water loss by reducing surface area of leaves. Plants inhabiting Death Valley, California, retained photosynthetic ability up to 43°C before experiencing damage to the chloroplast membranes (Seemann et al 1984). Atkin et al (2005) found that some plants can regulate concentration/amount of enzymes involved in plant respiration, and vary mitochondrial protein concentration, thus changing rates of respiration. Simplified, this means cold-adapted plants can increase respiration with increased temperature, and warm-adapted plants can decrease respiration to reduce water loss and CO² output (Fig.2). This would balance the production of CO² by plants in the atmosphere. However, the ratio of cold-adapted plants to warm-adapted plants in global populations would impact total vegetative CO² output.

Plants inhabit areas of highly varied temperature, light and water availability in different global biomes. This post has listed some plant survival adaptations, as well as impacts of increased temperature on plants and the world. The likely increase in global temperature is an effect of anthropogenic increase in CO² emissions. Studies have shown that plants can somewhat modify respiration and osmoregulation as a result of temperature increase, and these processes may impact the effects of global warming.

References:

Atkin, O. Bruhn, D. Hurry, V. Tjoelker, M. 2005, ‘The hot and the cold: unravelling the variable response of plant respiration to temperature’, Functional Plant Biology, Vol. 32, pp 87-105

Berry, J. Bjorkman, O. 1980, ‘Photosynthetic response and adaptation to temperature in higher plants’, Annual Review of Plant Biology, Vol. 31, pp 491-543

McWilliam, J.R. Naylor, A.W. 1967, ‘Temperature and Plant Adaptation; Interaction of Temperature and Light in the Synthesis of Chlorophyll in Corn’, Plant Physiology, Vol. 42, pp 1711-1715

Seemann, J. Berry, J. Downton, W. 1984, ‘Photosynthetic Response and Adaptation to High Temperature in Desert Plants: A Comparison of Gas Exchange and Fluorescence Methods for Studies of Thermal Tolerance’, Plant Physiology, Vol. 75, No. 2, pp 364-368

Figures:

Figure 1 - E. Morholt, P.F. Brandwein, (date unknown), A Sourcebook for the Biological Sciences

Figure 2 - Atkin, O. Bruhn, D. Hurry, V. Tjoelker, M. 2005, ‘The hot and the cold: unravelling the variable response of plant respiration to temperature’, Functional Plant Biology, Vol. 32, page 88 (Fig.1)

Thermal Adaptation - Increased Bill Size

Endothermic organisms regulate their body temperature by balancing metabolic heat production with heat exchange in their environment (Tattersall et al 2009). Toucans (family Ramphastidae) are endothermic, and have been studied in nature for their large bill size in comparison to body size. Early explanations for large bill size included mating, feeding purposes, and territorial defense (Tattersall et al 2009; Hughes 2014). Techniques of convective heat loss have been discussed previously in this blog, such as gular fluttering, sweating and increased surface area of ears in elephants (comments). This blog post will discuss the growth and thermoregulatory ability of the toucan’s bill, and evaluate its primary ecological function.

The toucan family (Ramphastidae) is entirely distributed in Neotropical areas, with most species inhabiting lowland tropical forests, where high daily temperatures and humidity is frequent (Hughes 2014). The toucan’s body is covered primarily by dense feathers, which remain slightly above environmental temperature, suggesting most areas of body mass are unsuitable for thermoregulation (Tattersall et al 2009). A 2014 study conducted on the development of physical structures relative to body mass, found that maxilla length and depth (upper part of bill) increases at a greater than isometric rate more than any other physiological structure (e.g. cranium length and width, sternum, synsacrum, femur and tibiotarsus) (Hughes 2014). A 2009 study found that the proximal region of the bill (closest to head) dissipated heat at lower environmental temperatures (>16°C - 25°C), with blood flow increasing to distal regions of the bill as temperatures rose toward 30°C and higher. This study also determined that heat loss from the bill can account for 25 – 400% of heat production in adult toucans, depending on environmental variables such as altitude, wind speed, ambient temperature etc. The thermoregulatory capabilities of the toucan bill also far surpass that of elephants ears’ estimated at a range of 9-91% of heat production (Tattersall et al 2009).

Toucans live in tropical ecosystems, with high humidity and high daily temperatures. Studies have shown they are dependent on rapid growth of the maxilla to dissipate heat (Hughes 2014). Their ability to control blood flow between proximal and distal regions of the beak enables relative homeostasis in a varied climate (Tattersall et al 2009). Toucans have evolved with large bill size in order to maintain optimal internal temperature, suggesting that thermoregulation is its primary function. However, this research does not disprove that mating, feeding or any other environmental pressures have selected for large bill size during the evolution of the toucan.

References:

Hughes, A. 2014, ‘Evolution of bill size in relation to body size in toucans and hornbills’, Zoologia, Vol. 31, pp. 256-263

Tattersall, G. Andrade, D. Abe, A. 2009, ‘Heat Exchange from the Toucan Bill Reveals a Controllable Vascular Thermal Radiator’, Science, Vol. 325, No. 5939, pp. 468-470

Thermal Adaptation - Torpor

Large areas of Australian woodlands are subject to bushfire, particularly eucalypt forests. Woodland forests comprise 67% of forest vegetation in Australia, with scrublands, shrub and mixed forest types dominating the northern and eastern coast (Fig. 1)(ERIN 2012 & ABARES 2013).  An estimated 39 million hectares of Australian forest was burnt from 2006 to 2011 (ABARES 2013). Animals inhabiting Australian woodlands must be well-adapted to frequent changes in environmental conditions in order to survive and successfully reproduce.

(Fig. 1 - Terrestrial Ecoregions of Australia (April 2012))

Bushfire is an important ecological process for the germination of native plants, soil nutrient cycling and removal of obstructive ground vegetation, including invasive weeds (IFA 2005). However, bushfire results in increased environmental temperature, and the potential death of native fauna. In the 1950’s, laboratory studies on thermoregulation of the short-beaked echidna (Tachyglossus aculeatus) found that body temperatures of 38°C resulted in fatalities (Brice et al. 2002). However, recent field studies have shown they are able to withstand ambient temperatures of above 40°C, as well as survive large bushfire events (Brice et al 2002 & Nowack et al 2016).

Echidnas survive bushfire and increased environmental temperature by seeking shelter, usually a log, cave or burrow, before entering a state of torpor.  Torpor significantly lowers metabolic rate, and reduces breathing and heart rate. This decrease in cardiovascular and metabolic activity results in effective loss of body temperature (Heldmaier et al 2004). Echidnas subjected to bushfire in Dryandra Woodland, south-east of Perth, reduced their body temperatures on average by 4°C as a result of behavioural sheltering and torpor (Nowack et al 2016). One individual reduced body temperature from over 30°C, to below 20°C for four days after the onset of bushfire. After a period of re-adjustment, body temperature returned to normal as measured within the week prior (Fig.2). Possibly the most important finding from this study was that none of the echidnas in the sample left their home range as a result of the bushfire, although one did die as a result of poor log choice (Nowack et al 2016).

(Fig. 2 -  Body temperature traces of the same echidna (a) 7 days before and (b) 7 days after the fire on 21 April)

The short beaked echidna enters a state of torpor as an adaptation to bushfire, and subsequent increase in environmental temperature. This adaptation is believed to have enabled their survival through global wildfire events, such as those caused by meteoroid impacts during the Cretaceous-Paleogene era, 65.5 million years ago (Nowack et al. 2016). Temperature variation and natural disasters will continue to occur, affecting all species on earth. Thermal adaptations like torpor have and will continue to benefit animals such as the short-beaked echidna as evolution continues over time.

References:

Brice, P. Grigg, G. Beard, L. Donovan, J. 2002, ‘Heat tolerance of short-beaked echidnas (Tachyglossus aculeatus) in the field’, Journal of Thermal Biology, Vol. 27 pg 449 - 457

Department of Agriculture (ABARES), 2013, ‘Australia’s State of the Forests Report’, Five Yearly Report, Australian Government

Environmental Resources Information Network (ERIN) 2012, Department of Sustainability, Environment, Water, Population and Communities, Commonwealth of Australia, Australian Government

Heldmaier, G. Ortmann, S. Elvert, R. 2004, ‘Natural hypometabolism during hibernation and daily torpor in mammals’, Respiratory Physiology & Neurobiology, Vol. 141 pg 317–329

Institute of Foresters of Australia (IFA), 2005, ‘The role of fire in Australian forests and woodlands’, Forest Policy Statement 3.1, pg 1-3

Nowack, J. Cooper, C.E. Geiser, F. 2016, ‘Cool echidnas survive the fire’, Proceedings of the Royal Society B, Vol. 283, pg 1-8

Figures:

Figure 1: 

Environmental Resources Information Network (ERIN) 2012, Department of Sustainability, Environment, Water, Population and Communities, Commonwealth of Australia, Australian Government

Figure 2:
Nowack, J. Cooper, C.E. Geiser, F. 2016, ‘Cool echidnas survive the fire’, Proceedings of the Royal Society B, Vol. 283, pg 1-8