Bruck (2013) – Decades-long social memory in bottlenose dolphins

1. Bruck (2013) – Decades-long social memory in bottlenose dolphins Robert Reeves

2. Long-term social recognition (LTSR) • The capacity to recognize an individual even with a lack of continued association or a long period of absence • LTSR has to do with memory, so it’s a cognitive capacity.

3. Why do humans have LTSR? • Bruck’s hypothesis: Complex, fission-fusion social patterns co-occur with extensive (even lifelong) kin-independent LTSR. • “a fission-fusion society is one in which the size and composition of the social group changes as time passes and animals move throughout the environment… group composition is a dynamic property.” • Kin-independent LTSR: long-term recognition of an individual you are not closely related to; rules out inbreeding avoidance as cause of cognitive capacity for long-term recognition • Familiar claim: The complexities of social life provide the critical conditions under which a cognitive capacity emerges.

4. Not just humans! (Or elephants) Bottlenose dolphins

5. Some relevant facts about dolphins • Intelligent, long-lived species • Well-documented complex fission-fusion social patterns, individual recognition capacities, and years-long procedural memories • Females tend to stay in a pod; males tend to leave at 1-3 years of age and sometimes form bachelor pods of 2-3 individuals who compete for access to females • “Hierarchical promiscuous” mating system • Good candidates for LTSR given other known examples (humans, monkeys, corvids, elephants) • But how to study LTSR in dolphins?

6. Humans change! Michael Beecher: Age 25 Michael Beecher: Age ??

7. Signature whistle • Individual-specific contact calls given by dolphins most often during periods of separation • Past research (Sayigh et al.) has shown that dolphin signature whistles are acoustically stable for at least 12 years, and Bruck’s research suggests that it may be much longer than that (20+ years, lifelong) • Dolphins have a leg up in LTSR; a stable form of recognition that could last decades

8. Methods • Dolphins are frequently moved between facilities • Take 56 dolphins (43 of them subjects) from a six-institution breeding program and use their preexisting relationships to determine how long they can remember their former tank-mates by their signature whistle

9. Big ranges • Dolphins ranged from four months to 47 years old, and were divided into three age ranges: calf (under 1 year), juvenile (1-6 years), and adult (over 6 years)[sex ratio of 1:1.15] • Dolphins were group housed for as little as three months up to as many as 18.5 years prior to relocation, and lived in different facilities prior to whistle playback testing for as little as six months to as many as 20.5 years • Average duration of group housing: 4 years • Average length of separation: 6 years • Average times animals moved: 1.48

10. Experimental design • Modified habituation-dishabituation experiment • Habituation phase  test whistle  second habituation phase  second test whistle

11. Modified habituation-dishabituation • Habituation • Dolphin self-separates from the group and comes within range of an underwater speaker, which triggers a non-test, unfamiliar whistle call. • Additional playbacks (all from different, unfamiliar dolphins) are spaced out every 5 min. and continue until the dolphin stops responding for 30 continuous seconds. (On average 3 habituation whistles needed) • First test • Dolphin presented with the first test whistle, either familiar or unfamiliar • Behavior classified into one of four levels

12. Modified habituation-dishabituation • Second habituation • Same as first: habituation whistles given until the dolphin ceases to respond • Second test • Dolphin given second test whistle, either familiar or unfamiliar (whichever was not presented in the first test) • Behavior classified into one of four levels

13. Inconsistent labeling of response levels* *Tables mine

15. Significant or not? (in sum)

16. Discussion • Bruck: “I predicted that given the highly social nature of bottlenose dolphins, LTSR would not only be present, but also persistent.” I was right: “dolphins were capable of remembering each other’s whistle for 15 or more years with no decay.” • Humans and dolphins (and maybe elephants) have been demonstrated to have decades-long social recognition and complex sociality; further, there is evidence that extensive social networks correlate with and could promote cognitive development (i.e. the social brain hypothesis).

17. Discussion • This study is another “data point” in the larger picture of whether “complex sociality” and “advanced cognitive abilities” are linked (i.e. again, the social brain hypothesis). • Bruck urges further study of LTSR in other species (including those without fusion-fission dynamics) to elucidate further the “potential connections between sociality and cognition.” • Dolphin LTSR “has implications for other relatively long-lived, large-brained, socially complex systems” like chimpanzees and other apes, hyenas, and elephants. More research needed to see is social information is definitely more salient than non-social information, but the study suggests that “at least social memory in non-humans animals is more resilient than previously thought,” since to Bruck’s knowledge “no systematic study…has shown such long retention of information in a species outside of humans.”

18. http://www.nature.com/news/dolphins-remember-each-other-for-decades-1.13519http://www.nature.com/news/dolphins-remember-each-other-for-decades-1.13519