Vertebral differences in Lagenorhynchus

MARIA CONSTANZA MARCHESI

San Francisco

Workshop; Rethinking Lagenorhynchus; 2015

Society for Marine Mammalogy

The RNP Collection started during the seventies by Dr. Natalie Goodall in Tierra del Fuego, Argentina.In 2001 she opened her Museo Acatushún at Estancia Harberton (80km east from Ushuaia) The museum recieves over 30 students during the austral summer months, that volunteer working at the museum and learning from the scientific activities.Nowadays the collection is the biggest collection of postcranial skeletons in South America, with nearly 3000 specimens. For some of the species this is the biggest collection in the world.For example the Commerson?s dolphins. Of those nearly 3000, 965 are commerson?s dolphins, they are very abundant in the collection given that due to its coastal habits are frequently trapped in artesanial fishermen nets along the eastern coast of Tierra del Fuego.Another species that has importance worldwide is the Peal?es dolphin, also biggest collection worldwide.There are only 8 dusky dolphins, as this species has only been observed in the area during the summer. Only three of them are complete and one is a very young juvenile.The most importan specimens are the hourglass dolphins. This species is pretty rare, there are only 17 complete specimens in the whole world and four of those are found in Natalie?s Museum.Currently the museum is runned by Natalie?s Foundation (The RNPG Foundation) conformed by people close to Natalie and with a Scientific Commitee formed by the Researchers and doctoral fellows that worked with her during the last 20 or ten years. As we are still reorganizing after Natalie?s passing away the collection is currently close for external research, until we get everything back on track.In my case I had the priviledge to work very closely with her for over seven years, first as a volunteer for two summers and later as a coordinator for the activities in the museum. It was during my first year as a volunteer that she asked me to reorganize the Lag?s cabinets.While doing that I noticed some remarkable differences in the shape of the vertebrae of the different species. This was what I saw. Not only the size was different (what was to be expected) but the shape of the centrum, the length and inclination of the processes was very distinc for each species. Also, the number of vertebrae changed greatly from one species to another.As I had to start working on my undergraduate thesis I did some digging among literature?put my ideas together and spoke to Nat about my plan...I wanted to investigate how much did these species differed from each other and try to find an explanation for those differences.The first two species I worked with were the Peale?s dolphin and the hourglass dolphin, two closely related species that differ greatly in their habitat preferences.The Peale?s dolphin is the most coastal dolphin of the three Lagenorhynchus species inhabiting the Southern Hemisphere, a close association with kelp beds (Macrocystis pyrifera) has been registered, where it feeds on demersal and benthic prey.The hourglass dolphin is an oceanic circumpolar species. Its basic biology and trophic ecology is poorly known due to its oceanic behavior and the small number of specimens found dead on beaches. It feeds mainly on fish and squid that undertake diurnal migrations. Sightings of this species have been mainly reported for areas with depths greater than 1500m.Cetaceans from different habitats may show differential adaptations due to each habitat?s particular selective pressures; thus, morphological differences are generally correlated with behavioural differences.There is an association between flexibility and swimming speed, and feeding ecology and habitat. In general, greater flexibility results in increased maneuverability. But there is a trade off between the energetic cost of recoil movemnets (pitching) and the work needed to maneuver.In general, complex habitats are associated with greater flexibility and slow maneuvers and pelagic environments with faster, less accurate movementsIt is believed that fast swimming dolphins have a relatively stable morphological configuration.But despite being stable, the vertebral column of dolphins is considered to be a flexible beam and it is also highly variable. Factors affecting flexibility include muscles and ligaments, size and composition of intervertebral disks and vertebral structure and interferenceIn this sense, vertebral morphology varies regionally, and variation can be observed in centrum shape, processes structure and orientation, the number of vertebrae within a particular region and the development of accesory structuresThese authors made a review of the morphological characteristics that would be observed in flexible and stable areas of the column for cetaceans. This way, flexible areas would have small and rounded centrum facets, vertebrae would have a high relative centrum lenght (I will explain a little bit later how you can calculate RCL, but in general it gives information about the area of contact between vertebrae), neural processes would be short and inclined, transverse processes would be short and the metapophyses would be placed low on the neural process. And basically the opossite could be found in a stabe area.Buchholtz and Schur stated that there is a great intrafamily variation especially regarding total count, vertebral shape and neural spine orientation. They proposed some features such as Low TC, long centra and coastal habits as primitive traits; and high TC, discoidal centra and pelagic habitats as derived traits.Woodward in 2006 in here work with mysticites hypothesized that fast swimming pelagic species would have osteological features that would suggest a stable vertebral column.For my study I had three basic questions?and I am going to answer them breifly showing only what I think are the most relevant results.To answer them I employed traditional morphometric methods, I took 14 measurements on each vertebra starting form the first thoracic. To make comparisons between species I calculated relative measurements, such as: RCL (according to Buchholzt 2001), Relative Transverse Process Widht, Relaitve Neural Process Height, Metapophyses development.Then I performed bivariate graphic analyses and discriminant analyses.What was important for me was, how I was going to subdivide the vertebral columns because the traditional classification reflects morphological variation for terrestrial vertebrates but not for aquatic vertebrates. So I chose to subdivide the columns into the functional regions proposed by Buchholtz in 2005, based in vertebral shape and processes inclination. For my work I actually used both classifications and confirmed that dividing the skeleton into the traditional regions would not show the real morphological differences that I was observing. The functional subdivition has more regions: including a neck, a Thorax or Chest, a Torso (that is subdivided into an anterior, a mid and a posterior torso) the Tail stock and the Fluke area.Here are the results for the comparison of the proportion of the skeleton occupied by each region. Asterisks show the regions significantly bigger obtained from a Mann-Withney U Test, and the numbers correspond to the mode of the number of vertebrae in a region. Where there are no number is because both species have the same quantity of vertebrae in that region.On the left side it?s the traditional subdivision and the only significant difference was on the Cervical region being bigger for the Peale?s dolphin. On the right side we have the functional subdivision, the most remarkable results are those found in the mid torso, being bigger in the hourglass dolphin.Here we can see a graphic for Relative centrum lenght versus total centrum lenght. Higher RCL values are translated into less contact area between vertebra, so based on this plot we can observe that there are two flexible areas, one in the middle of the Thorax (around the 14th vertebra) and one located in the middle of the tail stock (corresponding to different vertebrae in each species PD 53 and HD 57). There are also two stable areas (low values of RCL) one corresponding with the mid torso and the other one at the base of the fluke.We can also see that Peale?s dolphin have greater RCL values all along the column: meaning this, that contact facets are smaller for the Peale?s dolphin and this could be translated as a more flexible column than the one of the hourglass dolphin.Relative process length (left) and their inclination (Rigth) also seemed to be very different.Transverse processes are relatively longer for the Hourglass, specially in the begginig of the torso. This ncreases the insertion area where the muscles longissimus and multifidus are originated. It is in this region where the longissimus muscle develops forces of greater magnitude that are transmitted to the posterior region of the column.Even though the degree of development of the neural process was similar in both species, the variation pattern of the RNPH values was different for each species. Greater regionalization of the Peale?s dolphin column could be indicating a greater diversity of movements in the mid torso, suggesting a greater maneuverability.Given the morphology and inclination of the processes, the areas where the rotational potential is greater are larger in the Peale?s dolphin. change the angle of attack of mandibles and tail with relative independence, increasing the efficiency to capture prey in complex coastal habitats.Inclination values are employed to separate the column functionally, transtion between anterior and mid torso is signaled by a reversion of the spnie inclination. Processes of the mid torso are more strongly bent forward in the hourglass dolphin, resulting in a greater mechanical advantage for the muscles inserted on them.Based on the vertebral morphology previously observed, For discriminant analyses I also subdivide the thorax intro three.There was a high interspecific differentiation in the morphological characters in the posterior thoracic region and the torso; especially in the mid torso (Mahalanobis distances (DH) maximum values between groups; considering the species as grouping variable) posterior torso had significant discrimination but none of the variables explained the observed interspecific differentiation, with DH between groups as one of the smallest for all the proposed models.The tail stock region also allowed discriminating between species by differences in the relative centrum length TPI explains the multivariate discrimination in the thorax c, and in the anterior and mid torso of the vertebral column. Differences in Neural processes inclination observed graphically were not detected by the discriminant models. In both species there is a regionalization of the vertebral column: we could distinguished three stable areas (cervical, mid torso and base of the fluke) and two flexible areas (mid thorax and tail stock).Despite this similar pattern, vertebral morphology in each species is distinctive and appears to be related with the species trophic niche and with its habitatThe results suggest that the Peale?s dolphin skeleton has vertebral features associated with greater flexibility (e.g. more rounded vertebrae). The greater regionalization of the Peale?s dolphin column could be indicating a greater diversity of movements in the mid torso, suggesting a greater maneuverability. Given the morphology and inclination of the processes, the areas where the rotational potential is greater are larger in the Peale?s dolphin. These areas allow this species to change the angle of attack of mandibles and tail with relative independence, increasing the efficiency to capture prey in complex coastal habitats. The osteological features observed in the Peale?s dolphin specimens would be indicating that this species sacrifice speed for maneuverability, a necessary requirement for its coastal habitat. A larger relative area of the fluke would benefit this coastal species since high maneuverability requirements need flukes with greater areas in relation to the body volume (Woodward 2006). Altogether, the greater flexibility leads to an increased maneuverability that could be beneficial for foraging in highly heterogeneous environments or for social behavior such as those of group feeding (Woodward 2006). The kelp beds of the algae Macrocystis pyrifera, where this species is frequently seen, and the coastal topography place numerous barriers to straight movements. Moreover, capturing benthic prey requires turns in limited spaces that would be mostly restricted if the column was more stable. The hourglass dolphin has markedly disk-shaped vertebrae all along the column. The stability in the mid torso is reinforced both for the ?lumbarization? of the anterior and posterior vertebrae and for the higher vertebral count in the region. Besides this, the morphology of the processes and the metapophyses suggest a greater mechanical advantage for the swimming muscles inserted mainly in this mid area.As the mid torso involves a larger proportion of the skeleton than in the Peale?s dolphin, the stability of the hourglass dolphin column is enhanced, acting as an ?oscillatory beam? and storing potential energy. This elastic energy could replace part of the muscle work required to accelerate or decelerate the fluke, acting as a spring.In this species the flexible tail stock represents a larger proportion of the skeleton than in the Peale?s dolphin. This would increase the degree of vertical displacement of the fluke, increasing propulsion capacity. Aditionally the tail stock of this species has a well-developed ventral kill in both sexes and this could be associated with smaller drag forces as the tail stock cuts through the water. A corporal design mostly adapted for stability during swimming would contribute in minimizing energy expenditure and increasing thrust efficiency for steady swimming during feeding behaviors and migration. The greater stability of the hourglass dolphin could also be involved in reducing the effect of external forces while this species swims in the turbulent waters that it frequents.From this work we can evidence a high degree of plasticitty in vertebral morphology when compared with variations levels observed in other delphinid generaThis remarks the importance of the environmental complexity and discordance in the particular development of adaptation that allows maximizng the available resources in coastal and pelagic speciesFor my PhD thesis I am including four species and I am employing geometric morphometrics. Beside the Peale?s and the hourglass dolphin I am studying the Commerson?s dolphin and the dusky dolphin. The first one is a coastal species endemic os southern Argentina and the second one is a platform species.First of all I should be able to subdivide the column of the dusky and the commerson?s dolphin as I did with the Peale?s and the hourglass, for that I did an exploratory analyses with ten adult specimens of each species. Here are the graphical representation of the proportion of skeleton of each region for the four species, asterisks mark significant differences from a Kruskall-Wallis Test, white ones are differences with the Peale?s dolphin, black ones differences with the hourglass dolphin, and the red ones differences between dusky and Commerson?s dolphin.From this we can extract that coastal spp (Peale?s and Commerson?s) tend to have relativelly bigger thorax and anterior torsos, and smaller mid torso and Tail stock. While the platform species has intermediate relative sizes for this regions. We should remember that the Tail stock of the hourglass dolphin was significantly bigger than that of the Peale?s but there is not a significant difference in the size of the tail stock of the dusky dolphin when compared with the other two, indicating that it is intermediate in size.We should also notice that the number of vertebrae in the mid torso is bigger in pelagic species, thus adding stability to this stable region.When we analyze relative centrum length values versus total centrum lenght we find that the values in the mid torso of the dusky dolphin is very similar to that of the hourglass dolphin, but with the flexible areas being intermediate between the hourglass and the dusky dolphin.When analyzing the variation pattern for the Commerson?s dolphin we can notice important differences. First both thorax and anterior torso are notably more flexible than in the other three species, with the stable area limited to a small part the mid and posterior torso. Second, the tail stock is considerably less flexible than in the other species?.. I am still trying to understand the biomechanical importance of this differences, and might be related with counteracting a high flexibility of the anterior body or with this species capacity to move in platform waters where they have also been observed feeding (especially adults).For the rest of my PhD I will employ a three dimensional approach by using a microscribe to obtain landmarks on nine vertebrae along the skelenton, I chose those nine vertebrae based on the functional division of the skeletons.I have five different landmark configurations depending on the region I am analyzing but landmarks for the centra are always the same to allow comparison throughout the column within each speciesWith those data I am performing PCA and CVA with the residues from the PCA, I am using the residues due to the small sample size that I have. I am investigating the posibility of doing some simulation with the 3D data I am obtaining but that is in a very early stage of dveelopment.