Heat is produced throughout 3-D tissues (muscle, liver, brown fat, specialized “heater” organs), but it ultimately leaves the body only via surfaces—skin, appendages, respiratory mucosa (airways/lungs/tongue), and to a lesser extent excreta. Many animals, however, have highly vascularized 3-D structures that act as controllable radiators by shunting blood to their surface, blurring the “organ vs. surface” intuition.

How animals produce vs emit heat

Volumetric heat production (3-D tissues)

Brown adipose tissue (BAT) in mammals: non-shivering thermogenesis via UCP1; robust recent reviews and imaging papers detail its biology in humans and other mammals.

Muscle thermogenesis: routine metabolic heat from contraction; some fishes evolved dedicated heater organs derived from extraocular muscle that warm the brain/eyes by >10 °C above ambient (billfishes, swordfish). Opah even achieves near whole-body endothermy via continuous pectoral fin activity plus retia mirabilia. These are clear examples of 3-D organs whose function is heat production, not direct emission.

Heat emission (ultimately surface-limited)

Thermal windows (vascular “radiators”): large, thin, richly perfused appendages whose surfaces can dump heat when vasodilated. Classic cases:

• Toucan bill—acts as an adjustable radiator; under some conditions accounts for a very large fraction of total heat loss.

• Elephant and jackrabbit ears—convective heat loss dominates when vasodilated (“thermal windows”).

• Penguin flippers/feet—counter-current exchangers tune loss/retention; measured partitioning of heat loss across body surfaces.

Respiratory evaporative heat loss: panting in dogs and many birds increases evaporation from upper airways/tongue—again, it’s mucosal surface evaporation carrying heat out with exhaled air.

Counter-current heat exchangers (retia mirabilia): 3-D vascular networks that reduce unwanted emission by recapturing heat before blood reaches external surfaces (e.g., penguin flipper plexus; carotid rete for selective brain cooling in artiodactyls). These are 3-D structures whose role is to retain heat, not emit it.

Bottom line for your comparison

Heat production is volumetric (BAT, muscle, heater organs, viscera).

Heat emission is surface-mediated (skin/appendage surfaces, respiratory mucosa, sometimes wetted by sweat/saliva). Many “organs that dump heat” (e.g., toucan bill, elephant ear) are best viewed as vascularized radiators whose 3-D anatomy enables control, but the actual transfer to the environment still occurs across their surface via radiation, convection, and evaporation.

Selected references (open-access where possible)

Tattersall et al. 2009 — Toucan bill as adjustable thermal radiator.

Weissenböck et al. 2010; Andrade et al. 2015 — “Thermal windows” concept; elephant/jackrabbit examples.

Wathen 1971 — Jackrabbit ear heat loss: convection can exceed metabolic heat at ~30 °C.

McCafferty et al. 2013 — Emperor penguin surface heat fluxes by body region.

Robertshaw 2006; Goldberg et al. 1981 — Mechanisms and airflow paths in panting.

Shin et al. 2024; Ballinger et al. 2018 — BAT thermogenesis reviews.

Morrissette et al. 2003; Wegner et al. 2015; Runcie et al. 2009 — Fish heater organs and opah endothermy.

Strauss et al. 2017; Mitchell et al. 2008 — Carotid rete and selective brain cooling in artiodactyls.

Many thanks to the researcher Ranjith R. at Nesamony Memorial Christian College for his reply preceding in this posted question. Very helpful. The toucan paper in particular is helpful for a paper I am working on, not only for the role of an outsized beak, but also for the principle of which it is an example.