Thus, the carotid rete in the Artiodactyla lineage probably evolved at about the same time as the rumen, during the mid-Miocene, about 20 Mya. Recent systematic analysis of the Artiodactyla places the Ruminantiamorpha/Suinamorpha/Camelidamorpha divergence before the appearance of the Tragulidae within the Ruminantiamorpha ( Spaulding et al., 2009), making it likely that the absence of the rete in Tragulidae has been a secondary loss. The rete is absent in the closest relatives to the artiodactyls, the perissodactyls, absent in the oldest ruminant group, the Tragulidae ( Fukuta et al., 2007), but is present in each of the artiodactyl clades: Ruminantiamorpha, Suinamorpha and Camelidamorpha. Venous blood cooled by evaporation from the nasal mucosa drains into the sinus and so cools the arterial blood destined for the brain, generating selective brain cooling ( Baker, 1982 Mitchell et al., 1987) – the reduction of brain temperature below arterial blood temperature ( IUPS Thermal Commission, 2003). The carotid rete consists of an intertwining network of arterioles that lies within a venous sinus at the base of the brain.
Recently, however, another potential contributor to their success has been proposed: the evolution of the carotid rete and the use of selective brain cooling ( Fuller et al., 2008 Mitchell and Lust, 2008). Traditionally, the success of artiodactyls has been attributed to the evolution of improved locomotion and the ruminant digestive tract, adaptations which allowed these ungulates to use widely dispersed forage with a high fibre content ( Janis, 1989 Codron et al., 2008). Since they evolved in conditions that were hotter and drier than present, artiodactyls may be pre-adapted to cope with warmer and drier environments than those currently experienced ( Barnosky et al., 2003 Mitchell and Lust, 2008). Enhanced selective brain cooling in Arabian oryx supports the hypothesis that selective brain cooling would bestow survival advantages for artiodactyl species inhabiting hot hyper-arid environments.Īrtiodactyla, an evolutionarily successful and hugely speciose order of large ungulates, with 90 extant genera, evolved during the Eocene and speciated during a warming period in the Mid-Miocene Climatic Optimum ( Barnosky et al., 2003). The dominant male oryx displayed less selective brain cooling than did any of the other oryx, but selective brain cooling was enhanced in this oryx as conditions became hotter and drier. Arabian oryx displayed a lower threshold (37.8☐.1☌ vs 39.8☐.4☌), a higher frequency (87☖% vs 15☑5%) and a higher maximum magnitude (1.2☐.2☌ vs 0.5☐.3☌) of selective brain cooling than did gemsbok. Selective brain cooling was enhanced in our Arabian oryx compared with another species from the same genus (gemsbok Oryx gazella gazella) exposed to similar ambient temperatures but less aridity. On average, brain temperature was 0.24☐.05☌ lower than carotid blood temperature for four oryx in April. We measured brain and carotid blood temperature, using implanted data loggers, in five Arabian oryx ( Oryx leucoryx) in the desert of Saudi Arabia. Selective brain cooling is a thermoregulatory effector proposed to conserve body water and, as such, may help artiodactyls cope with aridity.
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