In contrast to the visual, auditory or olfactory senses, the sense of touch (a form of mechanoreception) has been relatively neglected in primate studies. Over the last few decades, our understanding of the ecology of touch, or how an animal explores and exploits its environment using tactile cues (or tactile information), has improved in both primates and non-primate mammals. Touch can take many forms in mammals, including vibrissal (or whisker) touch, hand touch (especially in glabrous fingertips) and skin touch. Comparative studies exploring the relative sensitivities of touch (e.g. vibrissae/face, hands, feet) in primates and non-primate mammals are often challenging without extensive behavioural training (Bauer et al., 2012; Dehnhardt and Dücker, 1996; Dehnhardt and Kaminski, 1995) or complex anatomical studies (Marshall et al., 2014; Mattson and Marshall, 2016a, 2016b; Peterson et al., 1998). We have had success in using anatomical and skeletal proxies to investigate the ecology and evolution of vibrissal touch. Indeed, there are established ecological correlates between touch sensory-end organs, like number and movement, and skeletal landmarks linked with touch acuity (Muchlinski, 2010a; Muchlinski et al., 2018). It is also possible to connect vibrissa movement to differences in behaviour (Dehnhardt and Kaminski, 1995; Grant et al., 2018; Kemble and Lewis, 1982; Muchlinski, 2010b). We have even modelled the evolution of vibrissae, discussing our findings in the context of touch (Muchlinski et al., 2013, 2018). In this chapter, we examine the ecology of face touch (i.e. vibrissal touch) among the lorisids using the above-mentioned lines of evidence. We suggest that the sensory ecology of the lorisids may be more specialised and novel than we initially thought.

Toothcombs have evolved independently in various mammalian lineages, including primates, scandentians and dermopterans, but the presence of a six-toothed toothcomb composed of four lower incisors and two canines (I1, I2 and C1, bilaterally) is a distinctive feature of extant strepsirrhine primates (Yamashita, 2017). There is some variation within the group with respect to the nature of the anterior teeth: indriids have a reduced toothcomb as a result of losing the lower canines (Gingerich, 1977), whereas Daubentonia has only one enlarged, ever-growing incisor, and no canines, in each quadrant (Quinn and Wilson, 2004). Nonetheless, it seems clear that a six-toothed toothcomb is primitive for Strepsirrhini (Hill, 1953b; Martin, 1990; Szalay and Delson, 1979). The earliest fossil strepsirrhine that indisputably possesses a toothcomb is Karanisia clarki (Seiffert et al., 2003), therefore the appearance of this trait can be established by as late as 36.9 million years ago (Mya). Molecular dates for the divergence of crown Strepsirrhini are generally much earlier than this (49.8–77.0 Mya; Pozzi et al., 2014b; Yang and Yoder, 2003, and sources cited therein). If the toothcomb was present in the common ancestor of the group, it could therefore be expected to have appeared much earlier than its first record in K. clarki. However, the fossil record for crown strepsirrhines remains very limited, which means that K. clarki provides the first window into the adaptive context in which the toothcomb evolved.

The first primate-like mammals to appear in the fossil record date to the earliest Palaeocene (Clemens, 2004; Fox and Scott, 2011; Silcox and López-Torres, 2017; Van Valen and Sloan, 1965), and the first primates of modern aspect (euprimates) do not appear until the latest Palaeocene/earliest Eocene (Morse et al., 2019; Ni et al., 2013; Rose et al., 2012; Sigé et al., 1990; Silcox et al., 2017; Smith et al., 2006). However, the most recent molecular estimates for the last common ancestor (LCA) of all living primates suggest that the order originated at some point between the late Cretaceous and the early Palaeocene (approximately between 60 and 70 Mya; Andrews et al., 2016; Herrera and Dávalos, 2016; Seiffert et al., 2018). Later, between 42 and 55 Mya (according to the same sources for molecular dates), Strepsirrhini split into the progenitors of the infraorders Lemuriformes and Lorisiformes (throughout this chapter we use the taxonomy established by Grubb et al., 2003). The Lemuriformes went on to radiate into the vast array of morphologically diverse living and extinct lemurs located on the island of Madagascar, and the lorisiforms split into two families: Lorisidae (pottos, angwantibos, slender lorises and slow lorises) and Galagidae, the bushbabies (Covert, 2002; Martin, 1990; Rasmussen and Nekaris, 1998).

This book started as a conversation in New Orleans back in 2016. A morphologist and a primatologist were sitting at the American Association of Physical Anthropologists annual meeting, bemoaning the scant presentations on lorises (slow lorises, slender lorises, angwantibos and pottos) relative to galagos, lemurs, monkeys and apes. Not only where were the talks on these primates, but where were the books on lorisids? After a few minutes of this talk, we decided it was time to see to it ourselves. Using a cocktail napkin and later a more respectable legal pad, we started sketching out what we would each want to see in a collected, edited volume devoted to what we know about lorises, some of the least understood primates living today. What resulted from that afternoon in a New Orleans bar is this edited volume. The scope is intentionally broad and is primarily divided into sections on evolution and morphology, behaviour and conservation. We also purposefully focused on soliciting short contributions, set as boxes within the text, from young authors doing fieldwork in Asian range countries, places where scientific study and conservation efforts on Loris and Nycticebus, the Asian lorises, is producing previously unknown data on population density and specific challenges to conservation efforts.

Primate communication tends to be divided into four different modalities of behavioural communication – olfactory communication, gesture, facial expression and vocalization – which span four different sensory channels (olfaction, vision, touch and audition). We use the term modality to refer to each type of behavioural communication in this way, but others have used modality to refer specifically to the sensory mode of the stimulus (e.g. Partan and Marler, 1999). We use our broader definition of modality for three reasons. First, one of our aims is to review and examine the classic scientific distinctions made between communicative domains. Second, some types of communication can use more than one sense: for example, gesture can be visual, tactile or auditory. Third, different cognitive mechanisms are thought to underlie both the production and perception of facial expressions, vocalizations, manual gestures and olfactory signals, and so treating them as different modes might be most productive for our purpose.

The aim of this chapter is not to provide a review of what is known about each modality, but instead to give an overview of the methods and general approaches employed. What follows is an outline of the scientific questions, methods for data collection and approaches to analysis within each area of primate communication research. For each modality, there is a section that refers to the brain mechanisms underlying its production and perception. However, it is important to emphasize that the focus here is on the main methods used to study the neural correlates of primate communication, while Chapter 3 discusses the corresponding brain areas and neural circuits in more detail. Importantly, it will soon become clear that the methods (and perhaps more importantly, the scientific questions) can differ between modalities.