Is collagen from teeth or bones equivalent for isotopic (¹³C, ¹⁵N) diet investigations?

Abstract

Isotopic biogeochemistry (¹³C, ¹⁵N) of fossil collagen is nowadays currently used to infer extinct animals and prehistoric man diet, and a wide developement of this technic is expected in the near future. In prehistoric specimens, only parts of bones and teeth are usually available and we have to be sure that isotopic values from any part of a single skeleton do not differ significantly before comparing isotopic values from different specimens.

Isotopic investigations on recent lower jaws from several mammal species show that δ¹³C values do not differ significantly between bone and teeth collagen but that δ¹⁵N values present an enrichment (up to 2–3 ±) in teeth collagen compared to bone in species with teeth that stop growing (reindeer Rangifer tarandus, sheep Ovis aries, bear Ursus americanus, wolf Canis lupus). On the contrary, species with continuously growing teeth (horse Equus caballus) present almost identical δ¹³C and δ¹⁵N values in bone and teeth collagen. Such δ¹⁵N values variations are very probably linked to teeth replacement chronology and time of weaning in the different species and for each tooth. Species with continuously growing teeth show similar evolution of isotopic variations in teeth and bone collagen during their lifetime.

Quaternary mammals almost 45 000 years old from France present a similar variation pattern for isotopic values in bone and teeth collagen. Cave bears (Ursus spelaeus) from Aldene and Mialet caves present slightly more negative δ¹³C values (-0.3 to -1.1 ±) and significantly more positive δ¹⁵N values (1.5 to 2.2 ±) in teeth than bone collagen from same fossil individuals. On the site as a whole, collagen δ¹³C values for definitive teeth are on average 0.7 and 1.1 ± more negative than for bone and δ¹⁵N values for definitive teeth are on average 3.1 and 1.9 ± heavier than for bone in Aldene and Mialet caves respectively. In Aldene cave, collagen δ¹³C values for deciduous teeth are on average 1.5 ± more negative than for bone and collagen δ¹⁵N values for deciduous teeth are on average 4.4 ± heavier than for bone. In Marillac, reindeers (Rangifer tarandus) present collagen δ¹³C values for definitive teeth almost identical than for bone and collagen δ¹⁵N values for definitive teeth are on average 1.8 ± heavier than for bone. On the contrary, there is no significant difference between δ¹³C and δ¹⁵N values from horses (Equus caballus) bone and teeth collagen, for a single individual as well as on average of several specimens.

δ¹³C and δ¹⁵N values seem to be equivalent between teeth and bone collagen only for species where teeth grow continously during the whole lifetime. In species where teeth stop growing in adult life, teeth collagen δ¹⁵N values seem to be significatly enriched in comparison with bone collagen δ¹⁵N values (1 to 3 ±), probably because of an ¹⁵N-enriched diet (milk) during the first stages of growing and no subsequent renewal of teeth collagen. The enrichment seems to be even more important for deciduous teeth.

Further investigations on recent mammals teeth and bones are thus urged to obtain a more accurate estimate of intra-individual isotopic variations according to individual age in the most studied species, including man.