Cognitive adaptations in two sympatric mouse lemur species occupying different ecological niches
von Johanna Henke-von der Malsburg
Datum der mündl. Prüfung:2021-05-11
Betreuer:Dr. Claudia Fichtel
Gutachter:Dr. Claudia Fichtel
Gutachter:Prof. Dr. Julia Ostner
EnglischCognition describes an individual’s abilities to perceive, process, and act on information of the abiotic and biotic environment. The investigation of cognitive variation between individuals and species has been of interest for many decades and several hypotheses have been proposed to explain the evolution of cognition. Sizes of certain brain areas covary with the cognitive abilities they process, and socio-ecological factors have been related to various brain size measures. However, covariations of phenotypic cognitive performances with socio-ecological factors are essential to understand the adaptive value of cognitive abilities. While links with social factors have been the focus of comparative studies, which and how cognitive abilities and ecological factors link with each other, has been less systematically investigated. Precisely, it is assumed, that ecological generalists consistently outperform ecological specialists in cognitive challenges, but a respective link lacks systematic empirical evidence. In my first theoretic approach, I reviewed the literature for comparative studies that experimentally assessed cognitive performances among at least two species of the same phylogenetic order. I found that cognitive variation can be related to species-specific dietary preferences, related foraging behaviors, migratory behavior, or habitat complexity. However, other ecological factors, such as the synecology with predators or parasites, or the degree of ecological specialization, have largely been disregarded to be linked with cognitive performances. Only a subset of the reviewed studies specifically mentioned the degree of ecological specialization of the species in comparison. After complementing the respective information using additional literature, I applied a sign-test to estimate whether the degree of dietary or habitat specialization consistently covaries with cognitive performances. Across a total of 34 comparisons, N = 26 cognitive performances differed between the species in comparison. In 62% of these comparisons, the dietary generalist achieved higher performance scores than the relative dietary specialist. Habitat generalists achieved higher scores than relative habitat specialists in 72% of the comparisons. However, ecological generalism was not significantly associated with higher performances that go beyond innovative and flexible learning abilities. It remains therefore questionable, whether generalist species consistently outperform more specialist species across a broader range of cognitive abilities. In my second empirical approach, I applied a conclusive cognitive test battery to two wild primate sister species. The gray and the Madame Berthe’s mouse lemur possess a comparable social system, occur in sympatry at the chosen study site, but are differentially adapted to ecological factors. Essentially, the gray mouse lemur is a dietary and habitat generalist, while the Madame Berthe’s mouse lemur is more specialized along these ecological niche axes. Occupying different ecological niches, while experiencing similar ecological challenges, assumes complex evolutionary processes for ecological adaptations that may be linked to cortical development, and may covary with cognition. Following the Opportunistic Intelligence Hypothesis, the generalist gray mouse lemur should consistently outperform the specialist Madame Berthe’s mouse lemur across the various cognitive tests. Alternatively, species-specific performance levels could covary with specific ecological adaptations. For instance, gray mouse lemurs feed more on gum, a characteristic that has been associated with better self-control. Madame Berthe’s mouse lemurs have larger home ranges and feed mostly on spatially clumped homopteran secretions, which may covary with spatial abilities. In summary, the two species differed in about two third of the assessed performance scores. Gray mouse lemurs were more innovative, which may provide them the advantage to adaptively respond to variable environmental conditions. However, they did not consistently outperform Madame Berthe’s mouse lemurs, which were more active and learned visual and spatial reward contingencies faster. Hence, the experimental results in mouse lemurs parallel the theoretic findings of the reviewed literature, in that generalist species do not outperform specialist species per se. Despite the apparent superposition of generalists with respect to innovative problem-solving, they do not consistently outperform specialists in other cognitive tasks. Rather, performances are better linked to other ecological factors, which might be more accurately disentangled in future studies using metric estimates instead of the categoric classification of ecological specialization. Nevertheless, as the first large scale comparative approach within this exciting research field, the results of my thesis offer substantial insight into the link between cognitive test performances and ecological factors.
Keywords: mouse lemurs; degree of ecological specialization; cognitive evolution; cognition; experimental test battery