There's an interesting topic open for discussion this week about tool-use in Shark Bay bottlenose dolphins. What's interesting about these dolphins, found in the East and West gulfs of Shark Bay in Australia, is that they use marine sponges as a hunting tool to catch fish. The bottlenose dolphins basically wear a sponge on their beak while searching the seafloor for food in deep channels of water. These dolphins, called spongers, are unique in that they pass on the trait through vertical social transmission, from mother-to-daughter or from mother-to-son. Just to clarify, vertical transmission is the passing on of a trait (or disease) from the female of the species to the offspring. Vertical social transmission would mean that the mother is teaching a certain behavior to her offspring and not necessarily passing the trait through genetics or teaching it to other adults. Studies conducted by Kopps and Sherwin (2012) showed that horizontal transmission was not the method of transfer. Horizontal transmission involves the transfer of a trait between members of the same species that are not in a parent-child relationship. Horizontal social transmission was ruled out in this case because members of the same species are not teaching the behavior to each other, but rather mothers are teaching the behaviors to their offspring. Although males learn the trait and use it as well, males do not teach the behavior to their children. As of now, it is not fully understood why male dolphins do not teach the behavior, but it would be interesting to speculate, and this is why I have a blog.
Before speculating on this behavior, let's cover some background knowledge. Tool use is found very infrequently in wildlife (the most famous examples include primates), and this example of sponging is the first example in cetaceans. Cetaceans are the order of mammals including whales, dolphins, and porpoises. As of 2011, 55 dolphins had been documented as spongers in the Eastern Bay, with fewer being found in the Western Bay.
The implications of two articles will be discussed this week, one of which investigates the question of whether the sponger dolphins have a tendency to more closely associate with similar others or not (Mann, Stanton, Patterson, Bienenstock & Singh, 2012). This was mostly observational research being conducted and multiple maps and graphs of interactions between dolphins were formed. The researchers studied a 300 square kilometer area in the eastern gulf of Shark Bay, Australia. Researchers conducted "surveys" which are sightings of dolphins during five minute intervals. A dolphin was identified by marking its body and fins. Association between two dolphins was determined if dolphins were within 10 meters of each other. Researchers also recorded ecological, reproductive and demographic data. The research lasted from 1989 to 2010 with over 14,000 of these surveys taking place. The survey sample included dolphins that were sighted more than 11 times across three or more years.
In most taxa, individuals develop behaviors as part of a group, but it is interesting to see that these spongers actually form a subgroup because of the unique behavior that they undergo. By studying graphs of the sponger vs non-sponger centroids (center of masses), it was found that spongers associated with each other more-so than with non-spongers. On the other hand, since non-spongers outnumber spongers in any given area, there is inevitably going to be overlap and interactions between the two groups, but spongers do exhibit strong homophily. Homophily is the tendency of individuals to associate and bond with similar others. Results showed that spongers were more likely to form a clique and had stronger bonds with spongers than with non-spongers.
A more recent experiment conducted by Kopps and Sherwin (2012) actually implemented a computer model to determine when the cultural trait developed in bottlenose dolphins. The computer model was based on a diploid (having two complete sets of chromosomes, one from mother and father), sexually reproducing dolphin population. This experiment ran studies based on 41 possibly observed spongers. The computer model would run simulations for how many spongers should be expected during given time periods. Knowing that there has been sponging going on for at least 30 years, they tried to figure out how long ago sponging actually emerged. Without going into the complicated details, it was basically concluded that horizontal transmission was not a possibility, based on the number of spongers that were found; vertical transmission was the likely method. Also, they determined that the emergence of sponging should have been sometime around 120-180 years ago, ultimately they decided that ~180 years is a reasonable estimate. This is based on the idea that sponging emerged as a single innovation event (it emerged one time). If the event had emerged twice in separate places, then we should have seen a lot more spongers in the bay. It is always possible that sponging was innovated more than once, but lost in some areas. The model showed that the trait is passed on from a single parent through a vertical transmission. The trait would become stable over time through fitness benefits and learning fidelity. Learning fidelity is supposed to mean that learning is reliable from generation to generation and that virtually all daughters are going to learn the sponging behavior from their mothers. Sponging is not, however, a genetic trait (as defined in the typical Mendelian sense of genes being passed on if they confer a reproductive advantage), because the proportion of spongers would be expected to increase. The proportion of spongers has not ballooned rapidly, as would be expected with a behavior that gives a fitness advantage.
An important finding from the model simulations to keep in mind is that sponging could have been innovated more than once, but lost in some cases. When we think of cultural drift (similar to genetic drift) we realize that traits may be lost over time through random variation and chance. In a real event, this would mean that a mother does not have any female offspring who would learn the behavior and pass it on, and so they reach a dead-end in the lineage with male offspring. With this realistic possibility, it is better explained that the behavior was innovated more than once.
There is still an uncertainty surrounding what causes the males not to teach their children the behavior, while females teach the behavior to both the male and female children. In addition, since the male dolphins usually identify with and interact with their male parent more than the female, males in general have lower rates of tool use than females. If the tool use conferred such a great reproductive advantage, then you would expect the behavior to be taught and used a lot more frequently. Further investigation needs to be conducted in order to deduce the peculiar behavior of male spongers. Is it something genetic that leads them to not teach the behavior? Is there any evidence of males teaching their offspring other behaviors, perhaps foraging without a sponge or other swimming techniques, etc.? It may just be the case that males don't teach their offspring anything, and just leave it up to the females. It is possible that adult female dolphins spend more time with their young, and so they are more likely to teach the behavior before males ever get a chance to. We simply don't know exactly why. The door is left open for investigators to step in and uncover the mystery.
Kopps, Anna M., and William B. Sherwin. "Modeling the emergence and stability of a vertically transmitted
cultural trait in bottlenose dolphins." Animal Behaviour 1.16 (2012): n. pag. Sciencedirect.com.
Web. 22 Nov. 2012. <http://www.sciencedirect.com/science/article/pii/S0000347212003806>.
Mann, Janet, et al. "Social networks reveal cultural behaviour in tool-using dolphins." Nature
Communications 3 (2012): 980.