Unusual patterns in ¹⁵N blood values after a diet switch in red knot shorebirds

Abstract

When a diet switch results in a change in dietary isotopic values, isotope ratios of the consumer's tissues will change until a new equilibrium is reached. This change is generally best described by an exponential decay curve. Indeed, after a diet switch in captive red knot shorebirds (Calidris canutus islandica), the depletion of ¹³C in both blood cells and plasma followed an exponential decay curve. Surprisingly, the diet switch with a dietary ¹⁵N/¹⁴N ratio (δ¹⁵N) change from 11.4 to 8.8 ‰ had little effect on δ¹⁵N in the same tissues. The diet-plasma and diet-cellular discrimination factors of ¹⁵N with the initial diet were very low (0.5 and 0.2 ‰, respectively). δ¹⁵N in blood cells and plasma decreased linearly with increasing body mass, explaining about 40 % of the variation in δ¹⁵N. δ¹⁵N in plasma also decreased with increasing body-mass change (r ²=.07). This suggests that the unusual variation in δ¹⁵N with time after the diet switch was due to interferences with simultaneous changes in body-protein turnover.

Keywords:

• blood cells
• diet switch
• discrimination factor
• nitrogen-15
• plasma
• regression model
• shorebirds
• turnover rate

Acknowledgements

We thank Maarten Brugge for his care of the red knots and help with blood sampling. Blood sampling would not have been possible without the aid of Pieter Honkoop, Anita Koolhaas, Luisa Mendes, and Jeroen Reneerkens. The experiment complied with the Dutch Law on Experimental Animals and was approved by the Experimental Animals Ethics Committee (approval number DEC-2000.04).