High altitude rarefied atmospheres can come as a shock if you're used to life at sea-level. No matter how hard you breathe, initially you're left gasping. However, given time, most species adjust to the new conditions. Gregory Russell from the University of California at Riverside explains that animals living at altitude tend to have larger hearts and lungs than their sea-level relatives. Yet despite this advantage, they don't always perform better than mice living at lower altitudes, possibly due to low oxygen levels limiting the animal's aerobic performance. Which made Russell, Enrico Rezende and Kimberly Hammond wonder how much of an effect developmental history has on an animal's ability to cope with different oxygen levels. The team decided to see how the aerobic performance of adult deer mice was affected by the oxygen level and altitude of their birth(p. 35).
With access to a lab-based colony of deer mice, Russell transported twenty mouse pairs to 3800 m altitude at the White Mountain Research Station ready to see how their high-born young faired relative to low-born mice. At 5 weeks of age, Russell put each of the high- and low-altitude-born mice in a treadmill and measured their aerobic performances. Then he artificially reduced the oxygen levels in the treadmill for the low-altitude mice (to simulate the effects of hypoxia at high altitude) and increased the oxygen for the high-altitude mice (simulating the effects of normoxia at low altitude) to see how they faired. The animals born in low oxygen (hypoxia) at high altitude out-performed the mice born in high oxygen (normoxia) at low altitude. At 5 weeks of age it seemed as if being born at high altitude had given the mice an advantage over the low-altitude pups.
So how would the pups born at high and low altitude adjust when transported to the opposite situation? Russell carefully transported half of the 5-week-old high-altitude pups down to the Riverside labs, and half of the Riverside pups up to the mountains, leaving the other two groups at their birth places. Giving the relocated animals 8 weeks to acclimate to their new environment, Russell again tested all of the animals' aerobic performances in response to hypoxia and normoxia.
Amazingly, pups born at low altitude (normoxia) outperformed all of the other mice after a period of acclimation to high altitude, while the performance of pups born at high altitude (hypoxia) changed little or declined after remaining at altitude or acclimating at low altitude. The team realised that when they had tested the 5-week-old animals, the high-altitude pups must have already acclimated to the hypoxic conditions. And when the team analysed the animals' aerobic performances, it became clear to them that there must be a physiological difference between pups gestated and born at high altitude,and pups born at low altitude. `How the mice accommodate the challenges imposed by high altitude depends on where they are born' says Russell. `We don't know how they do it', he adds `but suspect that high-altitude mice acclimate in a different way'.