My doctoral research, which was advised by Carlos Martínez del Rio (Department of Zoology and Physiology, University of Wyoming), examined the physiological processes that nectar-feeding hummingbirds and bats use to maintain both water and salt balance.
Hartman Bakken B. 2008. Vertebrate solutions to the osmoregulatory quandary posed by nectarivory. PhD thesis. University of Wyoming: Laramie, WY, USA.
Floral nectar is a curiously simple food. Depauperate in electrolytes, lipids, protein, and vitamins, it is essentially water and sugar. For the large and phylogenetically diverse number of vertebrates that subsist on it, nectar’s chemical simplicity presents them with a variety of peculiar physiological challenges (Chapter 1). My research focused on one such challenge – the maintenance of electrolyte and water balance. To avoid overhydration, nectar-feeding vertebrates must shed a staggering volume of dilute water when they are feeding. The capacity to do this, however, hinders their ability to conserve body water. Consequently, during fasting periods, nectar-feeding vertebrates are prone to a rapid and severe dehydration (Chapter 4). To understand how nectar-feeding vertebrates resolve this osmoregulatory quandary, I examined the influence that both water excess and stress had on electrolyte- and water-handling processes in hummingbirds (Trochilidae) and leaf-nosed bats (Phyllostomidae).
To avoid overhydration, both nectar-feeding hummingbirds and bats increased water filtration in the kidney and decreased the reabsorption of that water (Chapters 2, 3, 5 and 6). Additionally, to conserve filtered electrolytes, nectar-feeding bats excreted urine with a concentration ~90% lower than that of plasma (Chapter 6). Although nectarivores could also avoid overhydration by 1) having an unusually fast rate of renal filtration and/or 2) modulating water absorption in the gastrointestinal tract, my findings indicate that neither hummingbirds (Chapters 2 and 3) nor nectar-feeding bats (Chapter 6) possess these traits.
The weak urine-concentrating ability of nectar-feeding bats (Chapter 6) and the brisk rate at which hummingbirds lose body water to evaporation (Chapter 4) confirms that these nectarivores risk dehydration when they are not feeding. To alleviate this risk, both nectarivorous hummingbirds and bats limited urinary water losses by reducing renal filtration. In both groups, these reductions were dramatic: hummingbirds ceased whole-kidney filtration during the night (Chapters 2 and 3) and halved it during a short, midday fast (Chapter 2); nectar-feeding bats reduced renal filtration by ~90% during their natural, daytime fast (Chapter 6).
In general, osmoregulatory processes in both nectar-feeding hummingbirds (Chapter 5) and bats (Chapter 6) are very responsive to hydration status. Interestingly, in spite of having considerably different osmoregulatory systems, these distant relatives converged evolutionarily on the physiological traits they use to maintain electrolyte and water balance (Chapter 7). By studying the traits that allow animals to cope with extreme environmental conditions, such as dietary reliance on nectar, our understanding of physiological function can be enriched. My research suggests that nectar-feeding vertebrates might be excellent systems to investigate syndromes of complex physiological disorders, such as acute renal failure and water intoxication, from a mechanistic perspective (Chapter 8).