*Aims of This Review*

This review attempts to synthesise Australian research on heat load in sheep and cattle exported by sea. To reduce author bias, we used a systematic search strategy and only included Australian studies. We attempted to synthesise what is known about this process and identify knowledge gaps where future studies are needed.

This is not a narrative review written to advance a political or ethical position. Narrative literature reviews are publications that describe and discuss the state of the science of a specific topic or theme from a theoretical and contextual point of view and the authors do not disclose how they chose to include studies. The authors aimed to report on the outcomes of published and unpublished animal-based studies without applying our own judgement as to what constitutes 'good enough' or 'unacceptable' outcomes for animals in the context of sea transport. This review does not attempt to assess heat load in contexts outside of sea transport and does not consider conditions outside of ships originating in Australia. This review does not attempt to assess animal welfare impacts of sea transport as a whole.

## **2. Systematic Review Methods**

The authors minimised author bias by using a systematic search strategy according to the PRISMA guidelines (see Supplementary Material) to identify relevant journal articles, books, book sections, unpublished reports, conference proceedings, procedural documents and theses.

## *Summary of Literature Reviewed*

We found a total of 93 literature items matching our criteria. These comprised 51 peer-reviewed studies (55% of literature items), 29 unpublished reports, three book chapters, five theses, two conference papers, and three procedural documents. Most literature items found were contemporary, with 86% published since 2000. Further information is provided in Supplementary Material.

#### **3. Background to Australian Sea Transport of Livestock**

Australia has been the largest exporter of live agricultural animals worldwide in recent decades, with the industry valued at AUD\$1659 million in 2016, comprising 304 cattle voyages and 36 ship voyages exporting over one million sheep and over one million cattle that year [15]. Sheep and cattle have been exported from Australia via sea transport for at least 30 years, predominantly to the Middle East and South East Asia. Most sheep exported live from Australia are sourced from Western Australia, and are sent to the Middle East, with the voyages taking an average of 21 days. Most cattle are sourced from Northern Australia, and are sent to South-East Asia, with the voyages taking an average of 5 days. Voyages occur year-round [15]. During these voyages, hot environmental conditions are often encountered particularly during the Northern Hemisphere summer (May–October) [16] and the adverse impact of heat on transported animals has been reported since 1989 [17,18].

#### *Heat Load and Sea Transport*

During the sea transport of livestock from Australia to the Middle East, wet bulb temperature (WBT; an environmental measure dependent on dry bulb temperature and humidity [19]) commonly reaches 30 ◦C and can reach maximums between 32 ◦C and 34 ◦C with little diurnal variation [17]. Reduced diurnal fluctuation in environmental temperature during the shipment period limits an animal's opportunity to lose heat gained during the previous day, and if heat dissipation is less than the accumulation of heat within the body, the animal accumulates a "heat load" [20,21].

Dangerous heat load may occur for transported livestock in any area experiencing high sustained environmental temperatures. This may occur in many areas of Australia, particularly Northern Australia during the Southern Hemisphere summer, and has been documented for exported animals during the Northern Hemisphere summer, at ports in the Persian Gulf [5,16,22].

#### **4. Physiology of Heat Load**

The basic physiology of thermoregulation has been covered in detail elsewhere [23]. Briefly, mammals such as sheep and cattle have complicated homeostatic systems to keep their body temperatures within a reasonably narrow range. The amount of heat produced by an individual of a given mammal species will be influenced by factors such as nutrition (amount, type, and timing of feeding), body size, breed, physiological status, and acclimatisation [20]. Metabolic heat and heat from the environment can increase body temperature but mammals are able to maintain their homeostatic body temperature over a wide range of ambient temperatures by balancing heat loss or gain, and heat production. There are also behavioural responses to increased temperatures, such as changing posture (e.g., stock stand or spread out to increase surface area for heat loss, reduce activity, and seek shade if outside) [24]. However, the accumulation of heat without dissipation may result in mortality or several debilitating physiological changes. Before such adverse animal welfare events occur, acute heat load episodes exhibit a predictable pattern of physiological stages [25].
