Abstract
Nutritional status is a critical element of many aspects of animal ecology, but has proven difficult to measure non-invasively in studies of free-ranging animals. Urinary C-peptide of insulin (UCP), a small polypeptide cleaved in an equimolar ratio from proinsulin when the body converts it to insulin, offers great promise in this regard, and recent studies of several non-human primate species have utilized it with encouraging results. Despite this, there are a number of unresolved issues related to the collection, processing, storage and transport of samples. These include: contamination of samples on collection (most commonly by dirt or faeces), short-term storage before returning to a field station, differences in processing and long-term storage methods (blotting onto filter paper, freezing, lyophilizing), and for frozen samples, transportation while keeping samples frozen. Such issues have been investigated for urine samples in particular with respect to their effects on steroid hormone metabolites, but there has been little investigation of their effects on UCP measurement. We collected samples from captive macaques, and undertook a series of experiments where we systematically manipulated samples and tested the effects on subsequent UCP measurements. We show that contamination of urine samples by faeces led to a decrease in UCP levels by >90%, but that contamination with dirt did not have substantial effects. Short-term storage (up to 12 hours) of samples on ice did not affect UCP levels significantly, but medium-term storage (up to 78 hours) did. Freezing and lyophilization for long-term storage did not affect UCP levels, but blotting onto filter paper did. A transportation simulation showed that transporting frozen samples packed in ice and insulated should be acceptable, but only if it can be completed within a period of a few days and if freeze-thaw can be avoided. We use our data to make practical recommendations for fieldworkers.
Highlights
Nutritional status is a crucial determinant of animal survival and reproduction
C-peptide is produced in an equimolar ratio to insulin when the body converts proinsulin to insulin [1], and a fixed proportion of this C-peptide is excreted in urine, with excretion levels correlated with both plasma C-peptide levels and insulin production in humans [2,3]
Our study simulated a number of different realistic scenarios for fieldworkers working with urine samples for Urinary C-peptide of insulin (UCP) measurement, and evaluated the impact of different potential issues on the concentrations of UCP measured in samples
Summary
Nutritional status is a crucial determinant of animal survival and reproduction. Historically, the measurement of such status in freeranging populations has been limited by the availability of noninvasive tools. UCP levels have been shown to correlate with serum Cpeptide levels in macaques (Macaca spp.) [4] and chimpanzees (Pan troglodytes; [5]), as well as measures of body mass and fat in macaques [4] and bonobos (Pan paniscus; [6]). Feeding experiments in these latter two studies have shown that UCP levels respond to dietary changes, with lower levels excreted during dieting, and higher levels during re-feeding [4,6]. One published study of free-ranging rhesus macaques (M. mulatta) has utilized UCP levels to monitor the direct metabolic costs of different mating strategies [10]
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