Thoracic limb bone development inSotalia guianensis(Van Beneden 1864) along the coastline of Espírito Santo, Brazil

Abstract

The skeleton is often the only remaining structure of the Guiana dolphin, Sotalia guianensis, after decomposition of carcasses. This study investigates the bone development of Guiana dolphins beached on the coastline of the state of Espírito Santo, Brazil. External measurements of 43 thoracic limbs were obtained. Internal structures (radius, ulna, and humerus) were also measured. Dual-energy X-ray absorptiometry (DXA) was used to evaluate bone mass. The variables concerning the thoracic limb were tested using the Akaike information criterion to scale the best growth model when correlated with age and by the allometric model when they were correlated with total body length. The efficacy of DXA was also tested. The Brody growth model (best fit) showed that the thoracic limb stopped growing around the age of 2, while total body length ceased to grow at the age of 5.5. The thoracic limb presented early growth (negative allometry) compared with total body length. The methodology used to measure bone mass was efficient when considering ash weight. No difference in bone density was observed between the right and the left forelimb (P > 0.05), male and female (P > 0.05), or between dolphins found in the 3 sites we monitored. The deposition of bone mass was high in the early stages of life, and stabilization occurred at around the age of 13. O esqueleto muitas vezes é a estrutura que resta da espécie Sotalia guianensis, boto-cinza, devido ao processo de decomposição em que as carcaças são encontradas. O objetivo do presente estudo é entender o desenvolvimento ósseo dos animais encontrados encalhados no litoral do Espírito Santo. Para isso, 43 pares de nadadeiras peitorais foram submetidos às mensurações externas e de estruturas ósseas internas (rádio, ulna e úmero) e à técnica de absorciometria de raio-x de dupla energia (DEXA) para massa óssea. As variáveis das nadadeiras foram testadas pelo critério de Akaike para escalonamento do melhor modelo de crescimento quando correlacionadas com a idade, e para o modelo alométrico quando correlacionadas com o comprimento total (CT). Foi testada a eficácia da DEXA. através do modelo de crescimento de Brody (melhor ajuste) e pode-se observar que as nadadeiras param de crescer por volta dos 2 anos de idade e o comprimento total do corpo aos 5.5 anos, apresentando essas estruturas crescimento precoce (alometria negativa) em relação ao CT. A metodologia aplicada para mensuração da massa óssea mostrou-se eficaz quando correlacionada com peso das cinzas. Não houve diferença de densidade óssea entre nadadeira direita e esquerda (P > 0.05), macho e fêmea (P > 0.05) e nem entre os animais dos três diferentes locais de ocorrência, pois todos estavam dentro do mesmo intervalo de confiança. A deposição da massa óssea é acentuada no início da vida com posterior estabilização por volta dos13 anos de idade.

Variation of age and total length in Sotalia guianensis (Van Bénéden, 1864) (Cetacea, Delphinidae), on the coast of Espírito Santo state, Brazil

“…Age estimates of cetaceans stand is a good tool to characterize populations and to understand the biology of individuals (Pinedo and Hohn, 2000;Butti et al, 2007;Azevedo et al, 2015;Carvalho et al, 2015). In this sense, the ability to estimate the age of small cetaceans is extremely important in natural history studies (Hohn et al, 1989).…”

Bone mineral density of the common bottlenose dolphin radius: A primary skeletal site for clinical bone densitometry and preliminary descriptive data set using archival specimens

“…The intraindividual bilateral symmetry observed in BMD provides support for the use of either pectoral flipper to assess BMD for an individual dolphin and lends support to the use of all radii available, regardless of whether they are left or right pectoral flippers. Similarly, no difference in bone density was observed between left and right thoracic limbs in a study of Guiana dolphins, Sotalia guianensis (Azevedo et al, 2015). Bilateral symmetry is beneficial in a practical sense because, due to spatial constraints, routine necropsies and tissue archival protocols at many institutions include retention of partial skeletons, and under field conditions it is not always possible to access the same flipper for every animal.…”

A review of age estimation methods in marine mammals with special reference to monodontids

“…More relevant is that the resulting age estimates from BMD for an individual are imprecise (see Butti et al 2007), rendering this method suitable for relative but not absolute age. This imprecision may result from both individual variation in BMD and the poor predictive power of length as an indicator of age for animals older than juveniles as noted by Azevedo et al (2015). Furthermore, BMD may be influenced by other factors, e.g., overall health of an individual and/or ingested pollutant such as cadmium.…”