Expression of maternal isotopes in offspring: implications for interpreting ontogenetic shifts in isotopic composition of consumer tissues

When evaluating ontogenetic shifts in isotopic composition of consumer tissues within the context of a dietary analysis, the isotopic starting point of consumers in the population should not be ignored. Neonate isotopic composition may be different from that of juveniles and adults; in general, neonate tissues are built from maternal resources rather than food resources. Thus, the range of isotope values observed within a population of consumers may be significantly impacted by consumer isotope ratios at birth. Long-term goals of my research involve the use of stable isotopes to assess the role diet plays in driving population level differences in life history and demography observed among three pigmy rattlesnake (Sistrurus miliarius; family Viperidae) populations. For meaningful interpretation of field-collected data, it was important to determine starting (i.e., at birth) isotopic compositions of rattlesnakes from the study populations. We quantified isotopic composition of pregnant pigmy rattlesnake scale tissue, isotopic composition of neonate scale tissue and the degree that neonate scale tissue isotopic composition reflected the isotopic composition of maternal scale tissue. Collectively, neonate isotopic composition was highly variable among-litters; average litter delta(13)C values spanned 7 per thousand and average litter delta(15)N values spanned 2.8 per thousand. Over 95 % of the variation in offspring isotopic composition was expressed among litters. Thus, high levels of among-litter isotopic variation were largely due to the retention of a maternal signal. Results of the enclosure study suggest that highly variable isotopic signatures in young animals within field populations could easily reflect the retention of a maternal signal rather than differences in resource utilisation among younger snakes.