The Arabian Desert is a brutal place to live. Temperatures regularly exceed 45 degrees Celsius, and food is scarce. Many desert dwellers survive by following the rare rains or burrowing during the day. But according to a new paper, the sand gazelle has adopted a unique strategy: It shrinks its liver and heart. Downsizing these oxygen-consuming organs allows the gazelle to breathe less often, decreasing water loss. "The ability of the camel to survive the desert environment pales when compared to the sand gazelle," says evolutionary physiologist and co-author Joseph Williams of Ohio State University in Columbus.
Crowned by its ribbed, S-shaped horns, the goat-sized sand gazelle (Gazella subgutturosa) blends into the landscape with a pale, sleek coat that reflects heat. The animal is already known to conserve moisture by absorbing some of its urine back into its body. But considering its severe existence, Williams and Saudi Arabian National Wildlife Research Center colleagues Stéphane Ostrowski and Pascal Mesochina, wondered if the gazelle had another trick up its sleeve.
The team rounded up 18 wild sand gazelles and brought them to the research center, near Taif in Saudi Arabia. The animals were then divided into three groups: Over four months, six were given as much food and water as they wanted; six were well fed but were starved for the last 4-and-a-half days; and 6 had their food and water reduced by 15% every three weeks.
At the end of the period, the researchers measured the gazelles' resting metabolic rates and the amount of water they were losing per day from respiration and water loss through their skin. Gazelles on the most restrictive diet were losing about a third less water than the controls, and there was not much difference in water loss between the well-fed group and the group that was starved over the last four-and-a-half days.
Then the team got more invasive, sacrificing the first and third groups to measure organ sizes. Those gazelles on the long-term restricted diet had livers and hearts that were, respectively, 54.4% and 79.1% of the size of those of the well-fed group, the team reports in the July/August issue of Physiological and Biochemical Zoology. Both organs are densely packed with mitochondria, the oxygen-consuming organelles that fuel the cell.
The fact that some organs shrink during food restriction isn't unusual, Ostrowski says, but "our study shows that ... sand gazelles decrease the size of several organs ... to a much larger proportion than what could be anticipated from body mass decrease [alone]." In fact, sand gazelles showed the lowest total daily evaporative water loss ever measured in an arid-zone, hoofed mammal, the team reports.
Although the gazelle's adaptation appears advantageous, physiologist Shane Maloney of University of Western Australia in Crawley says there could be long term consequences of shrinking one's heart and liver. "It probably decreases reproductive performance, for example."