Introduction to the Special Issue Celebrating the Life and Work of Elke Zimmermann

Elke Zimmermann passed away prematurely on July 25, 2019, at the age of only 61 years (see obituary in the International Journal of Primatology by Radespiel et al., 2019). Over the 34 years of her career as a zoologist and primatologist, she became interested in and published on a large variety of research topics and study species ranging from amphibians to primates and worked with equal dedication on free-ranging animal populations and with standardized experimental approaches in captivity. Encouraged by her father, Elke became interested in zoology very early in life. She published her first article when she was 16 years old on an inventory of reptiles, which she had observed during an excursion with her family in Southern Spain (Zimmermann, 1975). Her first article on primates followed 2 years later, in which, together with her father, she assembled recommendations for the housing of lorises (Nycticebus spp.), squirrel monkeys (Saimiri spp.), and talapoin monkeys (Miopithecus spp.; Zimmermann & Zimmermann, 1977). At that time, all these species were housed in her family’s house and garden. She continued to publish on the biology of primates (Zimmermann et al., 1979, 1980), amphibians (Zimmermann & Zimmermann, 1980, 1981), and reptiles (Zimmermann, 1976, 1978) while she studied biology at the University of Hohenheim. Although she wrote her dissertation on the neurobiological basis of acoustic communication in frogs and regularly published on amphibians until 1994 (Zimmermann, 1990a; Zimmermann & Zimmermann, 1994; Zimmermann et al., 1990), primates became her main interest after her PhD and during her career. She was interested in the evolution of primates, but in contrast to most researchers who focused on haplorrhine primates, Elke investigated the roots of the primate order and specialized on strepsirrhine primates. She always

later, molecular methods to study the habitat use, sleeping site ecology, ranging patterns, social organization and mating systems of mouse lemurs Kessler et al., 2016;Lutermann et al., 2006;Radespiel et al., 1998Radespiel et al., , 2001Radespiel et al., , 2003bRadespiel et al., , 2009Rendigs et al., 2003;Schmelting et al., 2007;Weidt et al., 2004), sportive lemurs (Rasoloharijaona et al., 2003;Rasoloharijaona et al., 2008), and western woolly lemurs (Ramanankirahina et al., 2011;Ramanankirahina et al., 2012). Elke was always convinced that long-term studies are important to a deep understanding of species-specific adaptations and the evolution of lemurs in their complex abiotic and biotic environments (Kappeler et al., 2017). The behavioral and socioecological findings from her long-term work formed the foundation for several important studies of vocal communication and group coordination in solitarily foraging species. She complemented these studies with captive studies that allowed detailed behavioral observations on individuals of known life history, on topics like mouse lemur female dominance (Hohenbrink et al., 2016;Radespiel & Zimmermann, 2001b), kin recognition (Kessler et al., 2012), and female mate choice (Craul et al., 2004;Radespiel et al., 2002). During the past 10 years, Elke became interested in the impact of site-, species-, or sex-specific patterns of habitat use, sleeping site ecology, and sociality on primate parasite load and health (Hokan et al., 2017(Hokan et al., , 2018Klaus et al., 2017Klaus et al., , 2018Klein et al., 2018Klein et al., , 2019, a very appropriate topic for a researcher working in a veterinary school that also has an Institute for Parasitology.

Biodiversity and Phylogenetics
From the very beginning of her career, Elke was fascinated by the extraordinary biodiversity of tropical biomes (Masters et al., 1994;Nash et al., 2013;Zimmermann, 1998). Therefore, when starting to work on Madagascar in the early 1990s, she was determined to contribute to our understanding of the evolution of the behavioral, ecological, reproductive, and morphological diversity within the endemic radiation of lemurs, on intraspecific and interspecific levels. This interest sparked her work in Ankarafantsika National Park, and also across a broader geographic scope in the rest of northwestern to northern Madagascar and in other parts of the island (Rakotoarison et al., 1997). As a result, and very early on, she and her students described a new mouse lemur species, the golden-brown mouse lemur (Microcebus ravelobensis), found first beside Lake Ravelobe in Ankarafantsika National Park . The new species description was based on distinct phenotypic features such as fur coloration and tail length that distinguish this cryptic taxon from its sympatric congener, the gray mouse lemur (M. murinus), and from the Goodman's mouse lemur (M. lehilahytsara) from Andasibe (formerly called M. rufus), the only reddish mouse lemur species known at that time from eastern Madagascar ). This discovery was later supported by molecular and multiple other traits (Braune et al., 2008;Pastorini et al., 2001;Schmelting et al., 2000;Radespiel et al., 2003a).
In conjunction with the work of other groups in other parts of the island and based on many months of demanding fieldwork in remote and understudied regions of Madagascar (Olivieri et al., 2005;Randrianambinina et al., 2003bRandrianambinina et al., , 2010Rasoloharijaona et al., 2005), Elke's work triggered a large scale and ongoing taxonomic revision of the genus Microcebus, which led to the current recognition of 24 species (Poelstra et al., 2021). It became clear that many rivers and altitude act as effective gene flow barriers for many mouse lemurs (Microcebus spp.) and sportive lemurs (Lepilemur spp., n = 26 species), and Elke contributed to seven further species descriptions in these two genera (Andriaholinirina et al., 2006;Olivieri et al., 2007;Craul et al., 2007).
The extraordinary species diversity in nocturnal lemurs was long overlooked due to their cryptic life style (Zimmermann & Radespiel, 2014) and was repeatedly criticized due to a general scarcity of divergent phenotypic features (Tattersall, 2007), which motivated Elke until her very end to illuminate species divergence in other relevant traits, such as reproduction, behavior, and communication (Evasoa et al., 2019;Hasiniaina et al., 2020;Mendez-Cardenas et al., 2008;Rina Evasoa et al., 2018). Besides her genuine scientific interest in the evolution and adaptations of cryptic nocturnal lemurs, Elke was alarmed by the ongoing destruction, degradation, and fragmentation of lemur habitats all over the island. She was convinced that research and conservation are tightly linked and benefit from each other. While the effective conservation of species requires a solid knowledge of their ecological requirements, and the effective protection of a given forest can directly benefit from the onsite presence of researchers, our ability to continue our research on free-living primates depends on their long-term survival. In this context, Elke regarded it as her responsibility to supervise and support many Malagasy students of primatology and hoped for a better future that can only be built by a well-trained and dedicated next generation.

Acoustic Communication
Vocal communication is the research topic that spanned Elke's whole career, and she became a world expert on the vocal communication in strepsirrhine primates. Even in her first publications, Elke complemented her ethograms with the description of vocalizations either using onomateopoetic descriptions (Zimmermann & Zimmermann, 1977) or adding sonograms, which was technically challenging at the time (frogs: Honegger et al., 1985;, 1988bZimmermann et al., 1990). In 1981, Elke wrote her diploma thesis about the vocal repertoire of Senegal galagos (Galago senegalensis) and Sunda slow lorises (Nycticebus coucang). She noticed that the animals opened their mouth, but she could not hear any sounds. She borrowed an ultrasonic detector from a colleague and her brother wrote a computer program to analyse ultrasound (Zimmermann, 1995). Using this, Elke provided the first evidence that primates are able to produce ultrasonic vocalizations, such as bats, dolphins, whales, rodents, and insectivores (Zimmermann, 1981). Later, she was the first to document ultrasonic sounds in the mouse lemurs of Madagascar (Zimmermann, 1995a).
From 2003 to 2010, Elke was the speaker of the interdisciplinary research group "Acoustic Communication of Emotions in Nonhuman Mammals and Man: Production, Perception and Neuronal Processing" granted by the German Research Foundation, which brought together psychologists, biologists, and musicians to study the principles of emotional communication (Altenmüller et al., 2013). She and her students contributed studies of the encoding of arousal in the vocalizations of tree shrews (Tupaia belangeri; Schehka et al., 2007;Schehka & Zimmermann, 2009, mouse lemurs (Microcebus spp.; Scheumann et al., 2007b;Zimmermann, 2010Zimmermann, , 2018, and cats (Felis silvestris catus; Konerding et al., 2016;Scheumann et al., 2012). She was particularly interested in the evolution of human laughter, demonstrating a phylogenetic relationship of laughter across ape species (Davila Ross et al., 2009, 2010 and its emotional contagion in orang utans (Pongo pygmaeus;Davila Ross et al., 2008). Combining research in humans and animals, she also studied how humans perceive the emotional valence of animal sounds Scheumann et al., 2017a).

Lateralization
Because Elke was interested in the evolution of language, she also was interested in the extent to which manual lateralisation contributed to the evolution of language. She investigated manual lateralisation in mouse lemurs (Microcebus spp.; Leliveld et al., 2008;Scheumann et al., 2011;, tree shrews (Tupaia belangeri; Joly et al., 2012;Maille et al., 2013), and cats (Felis silvestris catus; Konerding et al., 2012). To study the lateralized processing of conspecific vocalizations, she and her students adapted the head turn orientation paradigm as a noninvasive method to investigate the lateralization of brain processing (Konerding et al., 2017;Leliveld et al., 2010;.

Cognition and Neuroethology
Elke investigated the cognitive skills of mouse lemurs (Microcebus spp.), including multimodal predator and prey recognition (Kappel et al., 2011;Piep et al., 2008;Sündermann et al., 2008), spatial memory (Joly et al., 2008;Joly & Zimmermann, 2007), learning (Fritz et al., 2020aJoly et al., 2014;Schmidtke et al., 2018a), and personality (Fritz et al., 2020a;Schmidtke et al., 2020). Due to a personal case in her family, in her later years Elke became interested in Alzheimer research. She was part of the EU-funded research project DEVELAGE, which investigated the extent to which mouse lemurs qualify as an animal model for Alzheimer disease. With her postdocs, she adapted the CANTAB (Cambridge Neuropsychological Test Automated Batteries) chamber for mouse lemurs, which is a neuropsychological test chamber based on a touch screen paradigm to investigate memory function and decline in humans and animals without linguistic cues. Using this test battery, they established protocols and trained mouse lemurs to perform visual discrimination tasks to test the effect of aging on their learning performance and memory (Joly et al., 2014;Schmidtke et al., 2018a). She compared the results to neuropathologies identified using in vivo MRI (Fritz et al., 2020a, b;Schmidtke et al., 2020). She also combined her behavioural studies with veterinary studies of aging (Dubicanac et al., 2018;Dubicanac et al., 2017;Schmidtke et al., 2018b).

This Special Issue
Elke Zimmermann's broad scientific horizon forms the umbrella for this special issue of the International Journal of Primatology. This issue brings together 10 studies conducted by her former students, friends, and colleagues whose work overlaps with her research interests and whose diverse scientific scope represents a last tribute to Elke's own work. The contributions include research on the reproduction, feeding ecology, ranging, population ecology, conservation, and communication of lemurs and other nonhuman primate species.
The first contribution, by Radespiel et al. (2021), contains a detailed analysis of the long-term reproductive dynamics of the two mouse lemur species (Microcebus spp.) that co-occur in Ankarafantsika National Park. While basic species differences in reproductive schedules of these two species emerged very early on in Elke's longterm project (Schmelting et al., 2000), a more comprehensive evaluation of various possible drivers (e.g., climate, photoperiod, body mass, age) of the observed interannual, interspecific, and interindividual variations in female reproductive activation and conception likelihoods became possible when integrating 4,321 reproductive records collected over 24 years. The authors interpret the species differences in reproductive schedules as the result of divergent evolutionary histories of the two mouse lemur species in different parts of Madagascar.
In the second contribution, Randrianarison et al. (2022) studied the feeding ecology of six habituated groups of indris (Indri indri) in Maromizaha Protected Area in eastern Madagascar, which Elke had visited and supported. The authors quantified the diet of indris over 9 years, during which they were mainly folivorous and had a very diverse diet without large seasonal fluctuations. Furthermore, they provide details of the vegetation structure, phenology of feeding trees, and phytochemical content of leaves and fruits in the diet of indris. Interestingly, the authors also observed regurgitation and reingestion of vomitus in four of six groups of indris and hypothesized that this rather unusual behavior may help to detoxify their diet, which contained secondary compounds, such as alkaloids and condensed as well as hydrolyzable tannins.
The third contribution, by Longondraza et al. (2022), investigated patterns of scent marking in diademed sifaka (Propithecus diadema) in the same study site, Maromizaha Protected Area. The authors observed five sifaka groups over 14 months to test whether marking and overmarking serve territorial defense and is influenced by sex, rank or season. They found that sifakas scent marked at a higher rate in peripheral and overlapping areas, supporting the territorial defense hypothesis, and scent marking was performed (a) more often by males than females, (b) more often by dominant than subordinate individuals, and (c) regularly during the migration period when males typically disperse. The authors therefore suggest that scent marking, and in particular overmarking by males, may also serve to claim mating resources, consistent with a mate-guarding strategy, whereas females may rather scent mark for self-advertisement.
In the fourth contribution, Steffens et al. (2022) investigated patterns of co-occurrence of the gray mouse lemur (Microcebus murinus) and the golden-brown mouse lemur (M. ravelobensis) in Ankarafantsika National Park by applying trapping and visual survey methods in 58 different forest sites situated in forest fragments (n = 42) or in continuous forest sites (n = 16). This large-scale dataset, to which Elke contributed in earlier years, was collected over 18 years, and was assembled by a collaboration between Canadian, German, Portuguese and Malagasy scientists. While the golden-brown mouse lemur generally preferred continuous forest habitat over forest fragments and yearly encounter rates were negatively associated with rainfall, the gray mouse lemur had higher encounter rates in the drier habitats available in forest fragments than in the continuous forest, but yearly encounter rates were positively associated with yearly rainfall. These divergent ecological preferences and responses to fragmentation and rainfall suggest different ecological adaptations and niches for the two species that may be related to the nonexclusive effects of different dietary needs, microhabitat requirements, tolerance toward varying degrees of aridity, and vagility over dry open spaces.
The fifth contribution, by Lehman and Mercado Malabet (2022), investigated whether lemur responses to forest edge effects undergo seasonal variations in four lemur species (Peyriéras woolly lemur (Avahi peyrierasi), red bellied lemur (Eulemur rubriventer), Ranomafana Bamboo lemur (Hapalemur griseus ranomafanensis), and mouse lemurs (Microcebus sp.) in Vohibola III Classified Forest in southeastern Madagascar. This topic is highly relevant to our understanding of lemur responses to habitat fragmentation. All three larger lemur species were rather edge adverse in general, and were found on average more than 520 m away from edges. Although all species were found closer to edges during the cold dry season than during the warm wet season, this difference was significant only for one species, the Peyriéras' woolly lemur, that has a rather selective, folivorous diet. The authors related their species-specific findings to differences in feeding habits, ecological plasticity and space use, as well as human interventions in and close to these habitats.
The sixth contribution, by Davila-Ross et al. (2022), investigated the effects of approaching motorboats on stress-related behaviors in the proboscis monkey (Nasalis larvatus) in a riparian forest area in Sabah, Malaysia. The authors systematically compared the behavior of male and female members of one-male multi-female groups under varying travel speed and distance between the boat and the groups. They showed that proboscis monkeys show stress-related behaviors when boats come close (<60 m) to the groups, which has important conservation implications, as boats are also often used for ecotourism in the region.
The seventh contribution, by Scheumann et al. (2022), investigated the role of vocalizations as a third-party vocal intervention signal in proboscis monkeys (Nasalis larvatus). The data for this contribution stem from Elke's project on vocal communication in proboscis monkeys, which she started in 2010. Using vocal recordings from free-living proboscis monkeys, the authors showed that bray vocalizations by the male in one-male/multi-female groups occur significantly more often after agonistic shriek calls and terminate sequences of agonistic vocal displays in 65% of cases. Additional video and audio recordings in Labuk Bay Monkey center showed that brays occur more often after female-female than after offspring-offspring conflicts. The authors suggest that brays work as a vocal third-party intervention in a species living in the high canopy with limited visibility.
In the eighth contribution, Coye et al. (2022) compared the vocal repertoires of two primate species, Diana monkeys (Cercopithecus diana) and Campell's monkey (C. campbelli), to investigate evolutionary drivers of primate vocal communication. These two species are closely related but differ in socioecological factors, such as predation and group size. The authors found similarities in the overall vocal repertoire of the two species but also unique vocal units. These nonshared vocal units were uttered in the predation context, suggesting that predation might be an evolutionary driver for the diversification of alarm calls.
In the ninth contribution, Valente et al. (2022) perform a cross-species comparison of the vocal repertoire of two sympatric strepsirrhine Malagasy primate species, diademed sifakas (Propithecus diadema), and indris (Indri indri) to explore acoustic diversity at the evolutionary origin of primates. To compare the vocal repertoires, they used modern mathematical approaches such as t-SNE reduction technique and hard clustering. A cluster analysis based on data for both species found distinct clusters differentiating the call types from the two species. Call types of both species clustered together in only 1 of 16 clusters. This indicates high vocal diversity between the two species. The authors argue that instead of a single factor, a combination of several factors such as phylogeny, environmental constrains and social complexity shaped vocal diversity in the Indriidae.
The tenth and final contribution, by Fichtel et al. (2021), investigated whether vocalizations are an honest signal of physical condition in grey mouse lemurs (Microcebus murinus). First, the authors showed that a manipulation of the dietary regime (restricted vs. nonrestricted) affects the body mass of the animal. Second, they investigated the effect of body mass on the acoustic parameters of two agonistic call types, the grunt and the tsak. For the grunt, but not for the tsak, they found a correlation between body mass and spectral parameters. Heavier animals produce grunts with more energy in the low frequency part than lighter individuals. This suggests that the acoustic structure of grunts varies according to physical condition and that grunt vocalizations therefore can be considered an honest signal of fitness in mouse lemurs.

Conclusions
The contributions assembled in this issue provide new insights into various speciesspecific adaptions to ecological factors or social constraints, very much alike the research approach that Elke Zimmermann often took in her work. While some of them relate seasonal variations in the habitat or human disturbance to feeding, reproduction, space use, or sympatry, others examine social and environmental influences on olfactoric or vocal communication. Although most studies focus on a better understanding of the proximate mechanisms that regulate behavioral variation, all of them consider the evolutionary processes that may have generated the observed patterns. Binz, H., Zurhorst, C., Zimmermann, E., & Rahmann, H. (1990). Neuronal substrates involved in processing of communicative acoustic signals in tree shrews -a 2-deoxyglucose study. Neuroscience Letters,112 (1)  Sex-specific patterns of age-related cerebral atrophy in a nonhuman primate Microcebus murinus. Neurobiology of Aging, 91, 148-159. https:// doi. org/ 10. 1016/j. neuro biola ging. 2020. 02. 027 Hafen, T., Neveu, H., Rumpler, Y., Wilden, I., & Zimmermann, E. (1998). Acoustically dimorphic advertisement calls separate morphologically and genetically homogenous populations of the grey