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Food abundance, prey morphology and diet specialization influence individual sea otter tool use

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By Molly Rickles, SRC Intern

In this study, Fuji looked into how ecological factors impact sea otter’s tool use. Sea otters are known to be near shore foragers that carry food on the surface, and have been observed to use tools to eat their prey. In many other species, tool use has been found to be a response to a lack of food, since the tools allow the animal to eat less preferential species easier. This means that individuals that are found at sites with limited food will have a more specialized diet. Researchers wanted to see if this would be the same with sea otters.

In this study, sea otters were tagged from 2000-2014 in five study areas across California. The individuals were tagged using a plastic cattle tag on their hind flippers as well as with an implanted VHF radio transmitter. The animals were observed as foraging for food when they were found to be repeatedly diving underwater and returning to the surface. The results of these foraging dives provided the data, which included the dive outcome, prey identification, time spent handling prey and if tools were used. Once data was collected, groups were classified based on the most prevalent prey item, which included: Abalone, crab, mussel, clam, urchin and snail. In addition, AIC statistics were made for each model, and then compared to select the best-supported model based on the data.

Figure 1

This figure shows the various sites where sea otters were tagged and researched. Even though many of the sites are close together, each site had different prey species present which presented different scenarios for the sea otters. (Source: Fujii, J. A., Ralls, K., & Tinker, M. T. (2017). Food abundance, prey morphology, and diet specialization influence individual sea otter tool use. Behavioral Ecology. doi:10.1093/beheco/arx011)

It was found that crab was the most common prey item. In addition, in sites where food was limited, sea otters were found to have more diversity in prey. Results showed that adult females were the most common tool-users, and that snails were the most common prey eaten with tools. Sea otters who most often ate snails were found to be more likely to use tools to eat other types of prey, since they were accustomed to using tools to eat snails. This also went the other way, since species that ate prey that did not require tools were less likely to use tools with more difficult to eat species.

Figure 2

This graph shows the predicted probabilities of tool use among the different food groups consumed by sea otters. Sea otters that eat snails have the greatest chance of using tools to consume snails as well as other prey. (Source: Fujii, J. A., Ralls, K., & Tinker, M. T. (2017). Food abundance, prey morphology, and diet specialization influence individual sea otter tool use. Behavioral Ecology. doi:10.1093/beheco/arx011)

These results show that dietary differences among the individual sea otters played a role in tool use, especially when food resources declined. Researchers observed that older sea otters used tools more frequently, showing that tool use requires time and energy spent learning to be successful. In addition, pups raised by females who used tools were commonly seen using the same tools and eating the same prey, suggesting that mothers teach it, which could be an area for future research. This study proved both the necessity hypothesis and the opportunity hypothesis, showing that tool use increases when prey is limited and that tool use improves prey capture, making it a beneficial skill.

Work Cited

Fujii, J. A., Ralls, K., & Tinker, M. T. (2017). Food abundance, prey morphology, and diet specialization influence individual sea otter tool use. Behavioral Ecology. doi:10.1093/beheco/arx011

Trade-offs Between Energy Maximization and Parental Care in a Central Place Forager, the Sea Otter

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By Trish Albano, SRC Intern

Energy expenditure is a key component to consumer foraging strategies. In this study, the influence of prey availability and reproductive limitations were examined, in an effort to determine their impact on the foraging behavior of the southern sea otter (Enhydra lutris nereis). Theoretically, when food is plentiful, it is energetically efficient for a central place forager to exploit prey patches that are closest, therefore minimizing energy expenditure and conserving metabolic rate. The study predicts that as prey becomes more scarce, otters will dive deeper to find prey and the associated energy expenditure will be proportionately greater. Due to the physiological differences between males and females, the study predicts that males will show a greater range of dive depths and durations but both sexes will respond to prey scarcity in the same manner. The demands of pup care in females are also taken into account – it is hypothesized that if pup care plays a significant role in females’ diving behavior, mothers with small pups will stray from normal responses to food scarcity the most because deep, long dives could increase the likelihood of pup loss.

Mother sea otter with rare twin baby pups, presumed to have been born just one or two days earlier on June 23-24, 2013. Photo taken 24 June 2013, Morro Bay, CA. AKA Southern sea otter and California sea otter, Subspecies E. l. nereis, Coast of Central California. Wiki http://en.wikipedia.org/wiki/Sea_otter

A mother and pup Southern Sea Otter (E. I. nereis). It has been shown that mother sea otters put a considerable amount of energy into the parental care of their pups. Image credit: Wikimedia Commons

Methods

The researchers utilized TDRs (time-depth recorders) implanted in wild southern sea otters to collect data in 5 locations (Figure 1) (Monterey, Big Sur, San Luis Obispo County, Santa Barbara Channel, San Nicolas Island) along the California coast. TDRs stayed inside the animal for approximately 1 year. Most animals were observed from shore for behavioral data collection and it was noted that across all 5 sites, otters fed on 80 different species of benthic invertebrates. Both resource abundant (low otter density, high food density) and resource deficient (high otter density, low food density) sites were explored.

Figure 1: This figure shows the 5 sites that data was collected for Southern Sea Otter populations. In these locations, otters were fitted with TDRs and observed for study.

Figure 1: This figure shows the 5 sites that data was collected for Southern Sea Otter populations. In these locations, otters were fitted with TDRs and observed for study.

Results

When prey was scarce, otters expanded their selection to include less-preferred prey. It was found that males and females without pups utilized similar foraging strategies and responses to food limitation, making deeper, longer and more energetically costly dives as prey became more scarce. However, females with pups were less likely to exhibit these energetically inefficient diving profiles. Mothers with young pups (less than 10 weeks old) placed parental care at higher priority than foraging when resource limitation would require them to perform energetically expensive dives that would reallocate their energy usage away from their pups. Females without pups in resource-abundant areas made 54% of dives to 2-5 m and 25% of dives to 6-10m whereas females with young pups made 3% of dives to 2-5 m, 52% of dives to 6-10 m, and 29% of dives to 11-15 m (Table 1). This trend of greater-proportion deeper dives was similar in resource-limited areas. This shows that females with pups utilized their energy in such a way that made it worth their while to leave their pups alone if there was a higher likelihood that they would be able to access food during their dive. Overall, the researchers’ hypotheses were supported by the data.

Table 1: This table shows the summary for TDRs that were used to obtain data in the study. The TDRs are site specific and measure resource availability, age range, median age, years of deployment, and duration of TDR recording.

Table 1: This table shows the summary for TDRs that were used to obtain data in the study. The TDRs are site specific and measure resource availability, age range, median age, years of deployment, and duration of TDR recording.

Outcome

This study shows that prey availability is critical for all southern sea otters, but especially for mothers with pups. If resource abundance is depleted, it can become very risky and energetically expensive for otters to forage. This could cause an increase in pup mortality and become a threat to southern otter populations.

Resources
Thometz, N. M., et al. “Trade-offs between energy maximization and parental care in a central place forager, the sea otter.” Behavioral Ecology (2016): arw089.

Sea Otters: A Dwindling Species that Helps to Diminish Our Carbon Footprint

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By Alex Babcock,
Marine conservation student

Sea otters are recognized most for their cute appearance and adorable behaviors; however, this species also serves very important roles in its’ ecosystem. A new study, published in this month’s issue of Frontiers in Ecology and Environment, posits the importance of sea otters and carbon sequestration (Wilmers et al. 2012). We know the ocean is one of our biggest carbon sinks and that sea otters are a keystone species for kelp forest environments (Estes et al. 1978). This study goes one step further: it demonstrates that sea otters, by keeping kelp-grazing sea urchin populations low, can cause kelp ecosystems to have higher net primary productivity (NPP) and higher biomass than kelp ecosystems without sea otters present. In effect, the presence of sea otters causes more CO2 to be absorbed from the atmosphere  (Wilmers et al. 2012).

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