Biology • Year 12 • Module 8 • Lesson 2
Temperature Regulation — Endotherm and Ectotherm Homeostatic Adaptations
Build HSC Band 5–6 extended-response technique on thermoregulation strategies, trade-off evaluation, and the limits of homeostatic systems.
1. Data-based extended response — climate change and ectotherm thermoregulation in south-east Australia (Band 5–6)
8 marks Band 5–6
Stimulus. The figure below shows modelled body temperature data for a population of eastern water skinks (Eulamprus quoyii) at two sites in south-east New South Wales: one cool montane site (1000 m altitude) and one warm lowland site (250 m altitude). The data compare current conditions with projected conditions under a +3°C warming scenario by 2100. The preferred body temperature (Tset) for this species is 30–34°C.
Figure 1.1. Modelled mean hours per day that Tᵇ falls within Tₛₑₜ (30–34°C) for Eulamprus quoyii at two sites. Modelled after Kearney & Porter (2009) approach; values illustrative.
Q1. Analyse and evaluate the implications of the +3°C warming scenario for the thermoregulatory success of eastern water skinks at both sites. In your response you must:
- Define thermoregulation and identify the primary strategy used by this ectotherm.
- Interpret the data at each site, describing the direction and magnitude of change in hours within Tset.
- Explain, using lesson mechanisms (basking, shuttling, burrowing), why hours within Tset change in opposite directions at the two sites under warming.
- Use a named Australian ectotherm example (can include the skink or another species) to ground your reasoning.
- Evaluate which site’s population faces a greater long-term thermoregulatory challenge under warming, and justify your judgement.
2. Compare and evaluate — endotherm versus ectotherm thermoregulation as competing strategies (Band 5–6)
7 marks Band 5–6
Q2. Compare and evaluate endotherm and ectotherm thermoregulation as strategies for maintaining temperature homeostasis. In your response you must:
- Define thermoregulation and identify the primary mechanism each strategy relies on.
- Compare the two strategies on at least three criteria (e.g. energy cost, activity range, vulnerability to rapid temperature change, adaptation types used).
- Use at least one named Australian example per strategy (e.g. red kangaroo, echidna / thorny devil, eastern blue-tongue lizard).
- Reach an environment-dependent judgement — not a one-winner ranking — about which strategy is more effective and under what ecological conditions.
Q1 — Sample Band 6 response (8 marks), annotated
Thermoregulation is the homeostatic process that maintains core body temperature within a narrow range (the preferred body temperature Tset) despite changes in the external environment. Eastern water skinks, as ectotherms, rely primarily on behavioural thermoregulation — selecting microenvironments and adjusting activity timing to achieve Tset — rather than internal metabolic heat production. [1 — definition + primary strategy identified]
Data interpretation: At the montane site, hours within Tset increase under +3°C warming, from 3.5 to 5.8 hours per day — an increase of 2.3 hours (approximately 66%). At the lowland site, hours within Tset decrease under warming, from 7.2 to 4.1 hours per day — a decrease of 3.1 hours (approximately 43%). [1 — both sites described with correct direction and magnitude]
Mechanism explaining opposite directions: At the cool montane site, current ambient temperatures are below Tset (30–34°C) for much of the day. Warming brings ambient temperatures closer to Tset for longer periods, making it easier for the lizard to achieve its preferred temperature through basking; it spends more time in the optimal range. At the warm lowland site, current ambient temperatures already approach or exceed Tset during much of the active day. Warming pushes ambient temperatures above Tset, forcing the lizard to retreat to shade or burrow underground for more of the day to avoid overheating — reducing the time available within the preferred range. [2 — mechanism for each site, linked to basking/burrowing and Tset position relative to ambient]
This pattern is consistent with research on Australian reptiles including the thorny devil (Moloch horridus), which already spends midday underground during summer to avoid lethal surface temperatures; under warming, this period of enforced inactivity will lengthen. [1 — named Australian ectotherm example with relevant mechanism]
Evaluative judgement: The lowland population faces the greater long-term thermoregulatory challenge. At the montane site, warming is beneficial (increased time within Tset), at least up to a point. At the lowland site, the lizards are already near the upper end of their thermal tolerance range; further warming will increase the hours during which ambient temperature exceeds the upper critical thermal limit, compressing the activity window so severely that foraging, reproduction, and predator avoidance time all decrease. Unlike endotherms, ectotherms have no internal buffering mechanism to compensate — when the thermal environment exceeds the range that behavioural strategies can correct for, hyperthermia becomes unavoidable. [2 — explicit judgement identifying lowland as greater challenge; justification using thermal limit concept and lack of internal buffering]
Marking criteria.
- 1 mark — Defines thermoregulation correctly and identifies behavioural thermoregulation as the primary strategy for this ectotherm.
- 1 mark — Accurately describes the data at both sites, including direction and approximate magnitude of change.
- 2 marks — Explains the mechanism for each site’s opposite direction of change: montane improves because ambient moves toward Tset; lowland worsens because ambient exceeds Tset and forces increased burrowing/shade-seeking.
- 1 mark — Uses a named Australian ectotherm example with a relevant behavioural mechanism (e.g. thorny devil burrowing, eastern water skink, or blue-tongue lizard shuttling).
- 2 marks — Reaches an explicit evaluative judgement identifying the lowland site as presenting a greater long-term challenge, with justification referencing thermal limits and the absence of internal buffering in ectotherms.
Q2 — Sample Band 6 response (7 marks), annotated
Thermoregulation is the homeostatic maintenance of core body temperature within a narrow range despite external temperature change. Endotherms achieve this primarily through internal metabolic heat production (and physiological responses); ectotherms achieve it primarily through behavioural thermoregulation — selecting microenvironments from the external environment. [1 — definition and primary mechanism for each]
Criterion 1 — Energy cost: Endothermy is energetically expensive — approximately 60–80% of a resting human’s daily energy intake goes to maintaining core temperature through continuous metabolic activity. The red kangaroo, for example, must consume significantly more food energy per kilogram of body mass than a comparably sized lizard. Ectothermy is energetically cheap — the eastern blue-tongue lizard’s thermoregulation costs almost nothing in metabolic terms because it relies on free solar energy rather than food-derived energy. [1 — energy cost criterion with examples]
Criterion 2 — Activity range across temperatures: An endotherm can remain active across a wide range of ambient temperatures because its core temperature is maintained by internal mechanisms. Red kangaroos and echidnas can be active at dawn in cold conditions and through warm afternoons without waiting for external warming. In contrast, an eastern blue-tongue lizard cannot be active in cold conditions until its body temperature has risen to its preferred range through basking — restricting its active period. [1 — activity range criterion with examples]
Criterion 3 — Vulnerability to rapid environmental temperature change: Ectotherms are more vulnerable to rapid temperature shifts because they have no internal buffering. When ambient temperature changes faster than behavioural compensation can correct, body temperature follows ambient temperature directly. The 2019 Queensland flying fox mass-death event illustrates what happens even to endotherms when temperature extremes overwhelm their physiological capacity — but for ectotherms, this threshold is reached at a lower rate of environmental change because they lack the physiological toolkit (sweat glands, shivering) to correct it. [1 — vulnerability criterion with relevant Australian example]
Environment-dependent judgement: Neither strategy is universally superior. In thermally stable, resource-scarce environments (e.g. arid Australian desert), ectothermy is highly effective — the thorny devil thrives because solar energy is abundant and the energy savings of ectothermy allow survival on minimal food. Endothermy would be prohibitively expensive under food scarcity. In thermally variable environments where year-round or night-time activity is essential (e.g. alpine zones, polar regions), endothermy is more effective — echidnas remain active in conditions that would render any ectotherm comatose. The two strategies represent different evolutionary solutions to different ecological pressures, not a hierarchy. [2 — explicit environment-dependent evaluation rejecting one-winner ranking, with two contrasting ecological contexts]
Marking criteria.
- 1 mark — Defines thermoregulation and correctly identifies the primary mechanism for each strategy (internal metabolic heat / behavioural microenvironment selection).
- 1 mark — Compares on energy cost (endotherm high / ectotherm low) with mechanism explanation and at least one Australian example.
- 1 mark — Compares on activity range or temperature independence with at least one Australian example per strategy.
- 1 mark — Compares on a third criterion (vulnerability, adaptation types, water/food requirement, etc.) with supporting Australian or factual evidence.
- 2 marks — Reaches an environment-dependent evaluative judgement (not a one-winner ranking): states explicitly which strategy is more effective under which ecological conditions and why; references at least two ecological contexts.
- 1 mark — Uses lesson vocabulary accurately throughout (thermoregulation, endotherm, ectotherm, physiological/behavioural/structural adaptations, homeostasis, set point or preferred temperature).