Meeting Abstract
Hummingbirds fuel their high energy needs with the fructose and glucose in their nectar diets. These sugars are used to fuel both immediate energy needs and to build fat stores to fuel future fasting periods. Most studied vertebrates are relatively adepts at utilizing ingested glucose for energy in most tissues, while fructose is largely metabolized by splanchnic tissues. If and how hummingbirds partition dietary fructose and glucose towards immediate oxidation to fuel foraging behaviour or fat storage is unknown. Using a chronic stable isotope tracer methodology we examined if glucose or fructose are preferentially used for de novo lipogenesis in ruby-throated hummingbirds (Archilochus colubris). Ruby-throated hummingbirds were fed diets with the glucose or fructose enriched with 13C for 5 days and we measured isotope incorporation into fat via the breath 13CO2 signature while fasting (oxiding fat) to trace incorporation into adipose stores. We found higher incorporation of stable isotopes into the fat stores when glucose was enriched compared to fructose suggesting preference for glucose as a substrate for fatty acid synthesis. However, we noted that the apparent turnover of the fat pool in hummingbirds was much slower (t50= 87 h) than that reported in other small vertebrates. To test if this was related to time spent in captivity, we repeated the study with newly caught hummingbirds and found a similar preference for glucose. We also observed a ~70% faster fat turnover in freshly caught birds. The faster rate was significantly correlated to lower body mass (r=0.87, p<0.001) and higher nectar intake (p=0.006). This suggests that there are profound changes to daily energetics in long-term captive individuals relative to their wild counterparts.
