# Phylogenetic ancestral state reconstruction of bilabial trills in the languages of Malekula Island (Vanuatu), and the historical evolution of rare sound changes

Benedict King, Tihomir Rangelov, Mary Walworth

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Benedict King, Tihomir Rangelov, Mary Walworth. Phylogenetic ancestral state reconstruction of bilabial trills in the languages of Malekula Island (Vanuatu), and the historical evolution of rare sound changes. Journal of Language Evolution, 2025, 10 (1), pp.1zaf004. 10.1093/jole/1zaf004. hal-05392898

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# Phylogenetic ancestral state reconstruction of bilabial trills in the languages of Malekula Island (Vanuatu), and the historical evolution of rare sound changes

Benedict King*, Tihomir Rangelov, Mary Walworth

Department of Linguistic and Cultural Evolution Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany

*Corresponding author. Department of Linguistic and Cultural Evolution Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany. E-mail: benedict_king@eva.mpg.de

Associate Editor: Dan Dediu

It is unknown how rare sound changes appear and spread through the lexicon. The bilabial trills of Malekula are one such example of a rare sound, and the recent assembly of a lexical database for Vanuatu (the Vanuatu Voices database) affords a unique opportunity for a quantitative historical analysis. We built a linguistic phylogeny of Malekula languages, and performed phylogenetic ancestral state reconstruction of fourteen semantic values which exhibit bilabial trills to track their historical evolution. We found a surprising degree of dynamism, with trills spreading gradually through the lexicon and showing frequent losses and reappearances. Our results are consistent with frequent borrowing of trills between neighbouring languages. We suggest that the rapid dynamism and evidence for borrowing are explained by the low functional load and identity attachment of trills respectively.

**Keywords:** sound change, bilabial trills, phylogenetics, Vanuatu, Malekula.

## 1. Introduction

Cross-linguistically rare speech sounds can emerge in a language in spite of presumed constraints that would otherwise inhibit their development. These constraints then must be overcome by counteracting forces, either by language internal or external motivations. Once present in a given language's sound inventory, rare sounds may then also easily be lost because of phonetic constraints and/or phonological factors, such as low functional load.

Little is known about the dynamics of how rare sound changes appear and spread through languages, or at what rate they can be lost. Research on rare sounds has tended to focus on whether rare sounds emerge due to language internal or external motivations (e.g. Blust 2005; Bostoen and Donzo 2013; Sands and Gunnink 2019). One recent qualitative study, however, explored more multifaceted approaches to how rare sounds emerge and persist, which

included externally motivated pressures (Rangelov et al. 2023).

Rangelov et al. (2023) identified a number of different factors—phonetic, phonological and social—that supported the emergence and persistence of two cross-linguistically very rare sounds—the prenasalized and plain bilabial trills [mB, β]—in twenty-three related languages spoken on Malekula Island in Vanuatu. These sounds also appear to be easily lost over time (Rangelov 2019; Rangelov et al. 2023).

Bilabial trills are speech sounds that involve oscillations of the lips following a total occlusion of the airflow, which may be preceded by nasal airflow (prenasalization). Just as plosives, bilabial trills may also be voiced or voiceless depending on the laryngeal setting during the occlusion and the release phase.

Malekula Island is home to a third of the approximately seventy languages that exhibit bilabial trills, and it is one of three areas in the world where languages with
<figure><img src="image_2.png" /><figcaption>**Map 1.** The languages of Malekula, colour-coded for the presence of bilabial trills: both bilabial trills present (purple), only the prenasalized trill (red), and no bilabial trills documented (gray). The blue lines demarcate the boundaries between the three subgroups of Malekula languages (Northern, Eastern, and Western), as per Lynch (2016).</figcaption></figure>

bilabial trills are clustered; the other two areas are central Africa and northern Papua New Guinea (Keating 2007; Rangelov et al. 2023). Malekula is home to over forty languages, with an incredible amount of village-level linguistic variation reported (Tryon 1976; Lynch and Crowley 2001; François et al. 2015; Shimelman et al. 2020; Hammarström et al. 2021; Eberhard et al. 2023). Bilabial trills have been documented in twenty-three of these languages (see Map 1).

Bilabial trills are relatively easy to articulate in isolation—many infants can produce them at very early age (Blust 2013), but they are difficult to integrate into connected speech, which is the likely reason for why they are so rare (Maddieson 1989). Articulatorily, bilabial trills involve a total closure of the airflow (as in a plosive) followed by a bilabial release and one or more oscillations of the lips. The most common type of bilabial

trill is the prenasalized [mB], which, in most languages, is restricted to before high back rounded vowels. Maddieson (1989) argues that a historical *[mbu] environment led to the development of [mB] in most languages, because the initial nasal phase allowed for a marginal rise in intraoral pressure during the closure phase, and the rounding and protrusion of the lips for the following rounded vowel; these two factors together allowed aerodynamic conditions for spontaneous oscillations of the lips. Lynch (2016) confirmed the **mb** > **mB**/_u_ pathway in Malekula languages. Rangelov et al. (2023) found that the much rarer plain bilabial trill [b], developed from a **\*β** before **\*u** in some Malekula languages, which also provides similar aerodynamic conditions (Table 1). In the relevant Malekula languages, all twenty-three have [mB], while twelve also have [b].
<table><caption>**Table 1.** Examples of prenasalized and plain bilabial trills in Malekula languages as reflexes of historical *m̥bu and *βu sequences, respectively based on Rangelov et al. (2023). The proto forms have been reconstructed for Proto North-Central Vanuatu (Clark 2009).</caption><thead><tr><th></th><th>PNCV *mbuaŋga 'swamp taro'</th><th>PNCV *mbukasi 'pig'</th><th>PNCV *kambu 'fire'</th><th>PNCV *βura-i 'spit'</th><th>PNCV *βura 'full'</th></tr></thead><tbody><tr><td>Ahamb</td><td>na-mBiaŋg</td><td>na/mBwas</td><td>n-xaMB</td><td>βure</td><td>βur 'full, a lot of'</td></tr><tr><td>Lamap</td><td>mBuaŋ</td><td>mBuas</td><td>na-xaMB</td><td>βuj</td><td>βu 'plenty'</td></tr><tr><td>Avava</td><td>mBuaŋ</td><td>a/mBuah</td><td>a-aMB</td><td>-βura</td><td></td></tr><tr><td>Avok</td><td></td><td></td><td>n-xaMB</td><td>βyje/βule</td><td></td></tr><tr><td>Na'ahai</td><td>ni-mBujaŋ</td><td>ni/mBuas</td><td>na-γaMB</td><td></td><td></td></tr><tr><td>Navwien</td><td></td><td></td><td>n-xaMB</td><td>βoraj</td><td></td></tr><tr><td>Neverver</td><td>ni-mBuaŋ</td><td>le/mBuas 'castrated pig'</td><td>na-xaMB</td><td></td><td></td></tr><tr><td>Ninde</td><td></td><td></td><td>n/əMB 'firewood'</td><td></td><td></td></tr><tr><td>Nisvai</td><td></td><td></td><td>n-xaMB</td><td>-βuraj</td><td></td></tr><tr><td>Uluveu</td><td>mBəaŋ</td><td>mBuaj</td><td></td><td>βulaj</td><td></td></tr><tr><td>Unua</td><td>mBuaŋg</td><td></td><td>no-xoMB</td><td></td><td></td></tr><tr><td>Uripiv</td><td>mBuak</td><td></td><td>n-aMB</td><td></td><td></td></tr></tbody></table>

During the articulation of a bilabial trill target, it is easy for the trilling phase to fail, especially in rapid speech. This is common for all trill sounds, because of their very precise necessary aperture size and airflow (Ladefoged and Maddieson 1996). Therefore, bilabial trills often surface as non-trilled allophones, including plosive and fricative articulations (e.g. Dimock 2005; Yoder 2010). Rangelov (2019) measured success rates of bilabial trills in Ahamb, a Malekula language, and found that successful trilling was observed in as low as 26 per cent of the tokens where it was expected in connected speech; in citation forms, success rates were 44 per cent for [b] and 59 per cent for [mB]. Besides, some Ahamb speakers never produced bilabial trills where they were expected. There is also evidence that bilabial trills have been recently lost, or are only preserved in the speech of a few elderly speakers, in at least three Malekula languages: Uripiv (McKerras 2001; Moore 2019), Aulua (Paviour-Smith 2005), and Nahavaq (Dimock 2009). When they are lost, they normally merge with the respective bilabial plosives.

From a phonological point of view, the fact that bilabial trills in Malekula languages are easily lost is also supported by their low functional load. Adopting Salmons' (2021) definition of functional load as 'how much 'work' a contrast does for creating distinctions', Rangelov et al. (2023) use Labov's (1994) criteria of lexical opposition and lexical predictability to determine the functional load of bilabial trills in Malekula languages. Namely, bilabial trills in these languages are involved in few minimal pairs, meaning low lexical opposition; they are also mostly found before /u/, having a skewed phonotactic distribution, which translates

as high lexical predictability. According to Labov, low lexical opposition and high lexical predictability mean low functional load.

Rangelov et al. (2023) argue that a multitude of factors supported the persistence of bilabial trills in Malekula languages. The same aerodynamic forces that supported the emergence of [mB] also support its persistence, as the sound is witnessed to be mostly restricted to before /u/. According to Rangelov et al. (2023), phonological factors can also be said to have played a role, including a historical final vowel loss, which resulted in [mB] contrasting with [mB] word-finally, and the presence of a prenasalization distinction in alveolar trills. Similar phonological factors also supported the persistence of [g]. Very importantly, Rangelov et al. (2023) also argue that, due to their salience and visual availability, bilabial trills serve as identity markers for speakers of Malekula languages. This claim is supported by general and Vanuatu-specific tendencies for speech sounds to encode local identities, and the awareness of speakers of the presence or absence of bilabial trills in the speech of neighbouring villages (Crowley 2006). The geographical distribution of languages with bilabial trills on Malekula (Map 1) also suggests this, as these languages form clusters, while the trills are being lost in fringe areas (Of the languages where bilabial trills are reportedly (being) lost, Uripiv and Nahavaq are spoken on the fringes of bilabial trill territory, while in Aulua's case, a crucial role may have had the fact that it was the base of one of the earliest Christian missions on Malekula and thus has had substantial outside influence).

The data of Rangelov et al. (2023) reveals that different languages in Malekula have varying numbers of
lexical items with bilabial trills, ranging from a dozen to a couple of hundred, and these occur across numerous semantic values. This heterogeneity could be explained by one of two possibilities. First, it is possible that bilabial trills appeared early in the languages of Malekula and spread rapidly across the lexicon, followed by an extended period during which trills were gradually lost. Alternatively, bilabial trills as we observe them today may be the result of more dynamic evolution, appearing and disappearing over an extended period and at different times in different lexical items. Lynch (2016) preferred this latter hypothesis, suggesting that because bilabial trills emerge naturally due to aerodynamic conditions it is possible that they developed multiple times independently. However, both hypotheses would produce the heterogeneous pattern of bilabial trills in present-day Malekula, and therefore are difficult to test from a purely qualitative overview of the data.

The bilabial trills of Malekula offer an ideal case study for a quantitative historical linguistic analysis of rare sound changes. The study of Rangelov et al. (2023) with the associated dataset including all known lexemes with bilabial trills in Malekula languages (Rangelov 2022) and the Vanuatu Voices database (Shimelman et al. 2020) provide a complete overview of the distribution of trills in Malekula, as well as consistent data for all Malekula languages. The Austronesian Basic Vocabulary Database (ABVD) (Greenhill et al. 2008) provides basic vocabulary data required to build a linguistic phylogeny for Malekula. This data was coded for cognacy based on established patterns of sound change and the comparative method. Here, we made use of these three sources to perform a historical phylogenetic analysis of bilabial trills in Malekula to demonstrate how rare sounds can emerge and spread through the lexicon. In response to the difficulty of testing the evolution of rare sounds simply using qualitative data, we explicitly tested the early appearance-differential loss hypothesis against the multiple gain hypothesis.

## 2. Methods

### 2.1. Phylogenetic tree

For a phylogenetic analysis, we selected forty-three documents that represented all areas of the island, both geographically and linguistically. These languages appear on Map 1, which also displays Lynch's (2016) subgrouping, based on traditional historical linguistic methods. The twenty-three languages with bilabial trills belong to the Eastern and Western subgroups (Map 1). There is also a clear geographical clustering of languages with bilabial trills in the southern two-

thirds of the island, where almost all languages have bilabial trills, with the exception of the central part of the west coast of the island. For this study, we assume a single tree of Malekula languages. It has been tentatively suggested based on the distinctive nature of Lapita pottery in north-east Malekula that multiple Lapita settlements may have occurred (Bedford 2019). There is also evidence from ancient DNA that an initially undmixed East Asian Lapita population was replaced with Papuan ancestry by subsequent waves of migration from Near Oceania (Posth et al. 2018). Nevertheless, there is no strong linguistic evidence that the languages of Malekula form multiple highly distinct and separate branches of the Austronesian tree.

We built a phylogeny of Malekula languages using established methods of Bayesian phylogenetic inference (see Hoffmann et al. 2021 for review). We downloaded the data from the ABVD (Greenhill et al. 2008). Details of the documents used, together with their sources, are included in Supplementary Table S1. The data comprised 1997 cognate sets organized into 201 semantic values. We performed two phylogenetic analyses, one without constraints, and one with the North, East and West groupings of Lynch (2016) enforced as clade constraints.

The data were analysed in BEAST2.7.3 (Bouckaert et al. 2019). We applied the covarion model (Tuffley and Steel 1998; Penny et al. 2001), which allows cognates to switch between hidden states with slow and fast rates of change. We set the frequencies of the hidden states to (0.5, 0.5), the default in BEAST, and estimated the stable frequencies of the observed states. We applied an ascertainment bias correction because all-zero cognate sets are not observed (Felsenstein 1992), and applied this correction separately to each semantic value to account for differing patterns of missing data. Cognate sets were partitioned into bins based on the number of cognate sets in each semantic value, with bins of one to five, six to ten, etc. cognate sets. We applied a Yule model tree prior with a uniform prior on the birth rate and an optimized relaxed clock (Drummond et al. 2006; Zhang and Drummond 2020; Douglas et al. 2021) with a lognormal prior (-9.5, 2.0) on the mean branch rate and a gamma distribution (0.5, 0.05) on the standard deviation.

Assigning an age to the root of the tree is challenging for the languages of Malekula. Bedford (2019) reports Lapita pottery from Malekula, the oldest radiocarbon date for which is 3070-2851 calBP from the island of Vao. Since this pottery already displays some regional variation from Lapita pottery from other parts of Vanuatu, this could potentially be assumed to represent a minimum age for the beginning of linguistic differentiation of Malekula languages from languages on other islands. However, this is not equivalent to the date at
which the common ancestor of the modern Malekula languages first began to differentiate, which is the event represented by the tree root. This latter date (differentiation *within* Malekula) is necessarily younger from the former (differentiation of Malekula from other islands). Without precise information to date the tree, we applied a conservative normal distribution prior (mean 2,500 BP, standard deviation 250), for the root of the tree, a distribution that permits any date between 2,000 and 3,000 BP. We note that the age of the tree will not affect our conclusions as differences in the age of the tree only scale the absolute rate parameter, not the parameters of interest regarding the dynamics of sound change.

We ran the analysis for 100 million generations across three independent runs. Convergence was confirmed in Tracer (Rambaut et al. 2018), with Effective Sample Size > 200 for all parameters. The log files from the three runs were combined following the removal of 10 per cent burn-in.

## 2.2. Cognate coded trills data

For compiling the datasets, we used the database of lexemes with bilabial trills in Malekula languages (Rangelov 2022), which contains data from phonological analyses by linguists as well as Vanuatu Voices data (Shimelman et al. 2020). In the latter, the presence of a bilabial trill had been double checked against the included audio recordings (cf. Rangelov et al. 2023 for details). Such verification was not normally possible for the data from phonological analyses, as associated recordings are rarely made available, but we trust that bilabial trills are not overrepresented in these data, as may be the case with the alveolar trill in the world's languages (Anselme et al. 2025). Conversely, likely due to their perceived eccentricity, bilabial trills were rather underrepresented in earlier works on Malekula languages and presumed to be (mispronounced) plosives, as for example in Charpentier (1982)'s large lexical atas of South Malekula. We also used Vanuatu Voices data where no bilabial trills were present for a given semantic value of interest.

We identified 16 semantic values from Rangelov et al. (2023), in which bilabial trills tend to occur, that were also covered in Vanuatu Voices: 13 with prenasalized trills and 3 with plain trills. The lexeme for each of the doculects in the tree were collected and assigned to a cognate set following the comparative method. In our database, lexemes were assigned a code corresponding to cognate set. Lexemes with the cognate set that included bilabial trills were additionally divided into those that had bilabial trills and those without. State 0 indicated trills present, State 1 trills absent but lexeme cognate.

States 2, 3, etc. were for additional cognate sets. Two semantic values ('forest' and 'see') were excluded due to insufficient data. This left eleven semantic values with the prenasalized trills (bad, bone, branch, dull/blunt, fire, kill, lake, smell, squeeze, swell, tail) and three with plain trills (flower, spit, white).

## 2.3. Ancestral state reconstruction

We performed ancestral state reconstruction in the phylogenetic analysis software RevBayes (Höhna et al. 2016). We used an Mk model (Lewis 2001) with nine states for all semantic values: two states (0 and 1) representing a cognate set with or without trills, and seven states for other cognate sets.

Our model separates the different processes of sound change (in this case gain and loss of trills) and lexical replacement, without introducing excessive model complexity. This was achieved by (1) estimating stable frequencies of trills absent and trills present and (2) including separate rate parameters for sound change and lexical replacement. Our analysis followed the graphic-al model (Höhna et al. 2014) presented in Fig. 1, which has four free parameters. To achieve the statistical power required to estimate these parameters, we assumed that the dynamics of trill evolution were consistent across all fourteen semantic values.

The variable *t* represents the relative stable frequencies of bilabial trills present (*t*₀) and absent (*t*₁). This allows for asymmetry in the evolution of trills. For example, values of 0.05 (present) and 0.95 (absent), represents a model in which trills are rarely gained but easily lost. All cognate sets were given equal stable frequencies in our analysis. The stable frequencies of States 0 and 1 (a single cognate set with or without trills) given as (1/8*t*₀, 1/8*t*₁). The other seven cognate sets were assigned a fixed stable frequency of 1/8.

The variable *r* defines the relative rate of gain and loss of trills against the cognate substitution rate, in other words, the rate of change between States 0 and 1 (a sound change) differs from the rate of change between any other pair of states (cognate substitutions, e.g. 0 and 2, 1 and 3, 2 and 6, etc.). When combined with the stable frequencies, this gives the *Q* matrix shown in Fig. 2.

The parameter *w*, is the relative rate of each lexical replacement in each semantic value. In this way the rate of change for each semantic value is allowed to vary, such that lexical replacement rate of the 'fire' (with a single observed cognate set) is estimated to be lower than for example 'swell' (with eight observed cognate sets). Finally, the parameter *λ* is the absolute rate of change, which all other rate parameters modify.

We carried out the ancestral state reconstructions on the full sample of phylogenetic trees, thereby accounting
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Phylogenetic graphical model of the phylogenetic comparative analysis. *Q* represents the transition rate matrix between each state (cognates, with the first cognate divided into trills present and trills absent), *π* the frequencies of each state, *t* the relative frequencies of trills absent vs trills present, *er* the exchange rates between cognate sets, *r* the relative exchange rates of sound change (trills to no trills and vice versa) and lexical replacement, *Ψ* is the empirical tree distribution (output from beast phylogenetic analysis), *λ* the absolute rate of all transitions, *w* the relative rate of each semantic value, *wr* the absolute rate for each semantic value (*λ* * w), *seq* is the observed cognate coded trills data. *N* is the number of semantic values, *i* ∈ *N* refers to the likelihood being calculated iteratively for each semantic value with the corresponding data (*seq*) and rate (*wr*). Note that this depicts the most complex model, in which *t* is estimated.</figcaption></figure>

for uncertainty in phylogenetic relationships. Separate
analyses were performed for each tree set: unconstrained
and Lynch constrained. The Mpotovoro variety
(Glottocode mpot1241) was pruned from the trees prior
to analysis because this variety is not covered in Vanuatu
Voices (This modification should not have any major ef-
fect on the results for two reasons. Mpotovoro is virtually
undocumented and the existing word list from the 1970s
(Tryon 1976) does not show evidence of bilabial trills in
this variety. This is not surprising, given that bilabial trills
would not be expected in Mpotovoro because no varieties
in the Northern Malekula subgroup, of which
Mpotovoro is part (see below/Map), have bilabial trills.
Secondly, Mpotovoro is sufficiently similar to its closest
relatives Vovo and Vao, so no substantial representation
of diversity has been lost in the Malekula tree by remov-
ing it.). The trees were treated as an empirical tree distri-
bution in RevBayes, and trees were sampled from this
distribution using a Gibbs operator rather than a
Metropolis-Hasting operator: this means that new trees
sampled from the empirical tree distribution were always
accepted, rather than sampled in proportion to their pos-
terior probability (the latter would be driven by how well
the trills data fits the tree).

We ran the model for 20,000 generations, sampling every ten. Stochastic character maps for each semantic value were logged at the same frequency. Convergence was confirmed in Tracer, with effective sample size > 200 for all parameters. We processed the results using the R package RevGadgets (Tribble et al. 2022).

We ran our analysis using six different model var-
iants (Table 2). These models differed in two ways.
First, either the unconstrained posterior sample of trees
or the sample with the Lynch (2016) grouping enforced.
Second, the relative stable frequency of trills present

(versus trills absent) was either (1) freely estimated,
(2) fixed to 0.5 or (3) fixed to 0.05. The latter represents
a model in which trills are rarely gained but easily lost,
and we predicted this would lead to ancestral state re-
constructions with an early gain of trills, followed by
multiple losses. We calculated the marginal likelihood
of the six models using stepping stone sampling (Xie
et al. 2011), with twenty steps and $\alpha=0.2$. A preburnin
of 5,000 generations was applied, after which each step
was run for 2,000 generations, sampling every five,
with a burnin of 25 per cent. Models were compared us-
ing their Ln Bayes Factor (Table 2).

3. Results

3.1. Dynamics of trill evolution

We found no evidence for a bias toward either gain or
loss of the bilabial trills. When the relative stable fre-
quencies of trills present and trills absent were esti-
mated, the median values were (0.44, 0.56) for the
unconstrained sample and (0.42, 0.58) for the Lynch
constraint tree sample, with distributions overlapping
0.5 (no bias). The preferred model (Table 2) was the
one in which these frequencies were fixed to (0.5,
0.5). The unconstrained tree sample fit the trills data
better than the Lynch constrained tree (Ln Bayes
Factor 12.62). A model in which the frequencies were
fixed to (0.05, 0.95), was strongly rejected (Ln Bayes
Factor 19.45).

We found strong support for rapid switching be-
tween the presence or absence of trills. The relative
rate of sound changes (gain and loss of trills) and lexical
replacements was (0.95, 0.05), indicating that switch-
ing between presence and absence of bilabial trills oc-
curs 19 times faster than lexical replacement.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Instantaneous transition (Q) matrix of the trills data. States are ordered as follows: Cognate 1-trills, Cognate 1-no_trills, Cognate 2, Cognate 3... Cognate 8. Each of the eight cognate sets has equal stable frequencies (0.125). The two states for the single cognate set with trills have frequencies further multiplied by the relative frequencies of trills versus no trills ($t_0, t_1$) such that they together sum to 0.125. The exchange rate $r_1$ is for intra-cognate transitions—i.e. between trills and no trills. The exchange rate $r_2$ is for inter-cognate transitions.</figcaption></figure>

<table><caption>**Table 2.** Comparison of six models of bilabial trills applied to the bilabial trills data.</caption><thead><tr><th>Model</th><th>Trills frequency</th><th>Tree sample</th><th>Marginal likelihood</th><th>Bayes factor</th></tr></thead><tbody><tr><td>1</td><td>0.44 (0.30–0.57)</td><td>unconstrained</td><td>−760.0436</td><td>1.05</td></tr><tr><td>2*</td><td>0.5</td><td>unconstrained</td><td>−758.9907</td><td>0.00</td></tr><tr><td>3</td><td>0.05</td><td>unconstrained</td><td>−778.4455</td><td>19.45</td></tr><tr><td>4</td><td>0.42 (0.31–0.53)</td><td>Lynch</td><td>−771.7505</td><td>12.76</td></tr><tr><td>5</td><td>0.5</td><td>Lynch</td><td>−771.6085</td><td>12.62</td></tr><tr><td>6</td><td>0.05</td><td>Lynch</td><td>−805.3439</td><td>46.35</td></tr></tbody></table>

Asterisk indicates best-fitting model (Model 2). For Models 1 and 4 the relative stable frequency of trills is estimated and the median and 95 per cent HPD is shown.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Bilabial trills show different histories for different semantic values. Summary of ancestral state reconstruction of prenasalized and plain trills in Malekula languages, using the best fitting model (unconstrained tree sample, equal stable frequencies for trills absent and present—i.e. no bias towards either gain or loss). Pie charts at nodes indicate the posterior probability of each state. States 0 and 1 are a single cognate set split into forms with and without a trill, states 2–8 are different cognate sets, none of which ever show the presence of trills. Results are summarized on the maximum clade credibility tree.</figcaption></figure>

## 3.2. Prenasalized trills

The ancestral state reconstructions refute the hypothesis that bilabial trills appeared rapidly and simultaneously (Fig. 3). Only two semantic values ('dull/blunt' and 'tail') show a greater than 50 per cent probability

of trills present at the root node for Malekula. Two further semantic values ('fire' and 'kill') show evidence for an early appearance of trills via sound change, but with less than 50 per cent support at the root node. In contrast the semantic values 'bone' and 'smell' show sound
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Evidence for multiple gains of bilabial trills. Histogram summarizing the number of independent gains of bilabial trills in each semantic value across the posterior sample of trees and reconstructed histories, using the best fitting model (unconstrained tree sample, equal stable frequencies for trills absent and present—i.e. no bias towards either gain or loss). The counts are separated into sound changes (transitions from State 1 to State 0) and lexical replacement (transitions from any of States 2–8 to State 0).</figcaption></figure>

changes $^{mB}b > mB$ leading to the presence of bilabial trills occurring at a much later time.

There is evidence for multiple $^{mB}b > mB$ sound changes within single cognate sets. Figure 4 shows the posterior

distributions for the number of independent gains of trills across the phylogeny for each semantic value, separated into gains via sound change and those that occur via lexical replacement. These estimates show
<table><caption>**Table 3.** Summary of the number of times bilabial trills have been gained in each semantic value across the posterior sample, separated into sound changes and lexical replacements.</caption><thead><tr><th>Semantic value</th><th>Type of appearance of trills</th><th>HPD lower</th><th>HPD upper</th><th>Probability >1 changes</th></tr></thead><tbody><tr><td colspan="5"><strong>Prenasalized trills</strong></td></tr><tr><td>Bad</td><td>Lexical replacement</td><td>1</td><td>2</td><td>0.51</td></tr><tr><td>Bad</td><td>Sound change</td><td>0</td><td>3</td><td>0.54</td></tr><tr><td>Bone</td><td>Lexical replacement</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Bone</td><td>Sound change</td><td>2</td><td>5</td><td>0.96</td></tr><tr><td>Branch</td><td>Lexical replacement</td><td>1</td><td>2</td><td>0.81</td></tr><tr><td>Branch</td><td>Sound change</td><td>0</td><td>1</td><td>0.07</td></tr><tr><td>Dull_blunt</td><td>Lexical replacement</td><td>0</td><td>1</td><td>0</td></tr><tr><td>Dull_Blunt</td><td>Sound change</td><td>0</td><td>8</td><td>0.7</td></tr><tr><td>Fire</td><td>Lexical replacement</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Fire</td><td>Sound change</td><td>2</td><td>8</td><td>0.98</td></tr><tr><td>Kill</td><td>Lexical replacement</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Kill</td><td>Sound change</td><td>0</td><td>8</td><td>0.8</td></tr><tr><td>Lake</td><td>Lexical replacement</td><td>1</td><td>4</td><td>0.73</td></tr><tr><td>Lake</td><td>Sound change</td><td>0</td><td>3</td><td>0.37</td></tr><tr><td>Smell</td><td>Lexical replacement</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Smell</td><td>Sound change</td><td>1</td><td>4</td><td>0.88</td></tr><tr><td>Squeeze</td><td>Lexical replacement</td><td>0</td><td>3</td><td>0.56</td></tr><tr><td>Squeeze</td><td>Sound change</td><td>0</td><td>4</td><td>0.74</td></tr><tr><td>Swell</td><td>Lexical replacement</td><td>0</td><td>2</td><td>0.21</td></tr><tr><td>Swell</td><td>Sound change</td><td>0</td><td>5</td><td>0.71</td></tr><tr><td>Tail</td><td>Lexical replacement</td><td>0</td><td>3</td><td>0.77</td></tr><tr><td>Tail</td><td>Sound change</td><td>0</td><td>6</td><td>0.52</td></tr><tr><td colspan="5"><strong>Plain trills</strong></td></tr><tr><td>Flower</td><td>Lexical replacement</td><td>0</td><td>2</td><td>0.14</td></tr><tr><td>Flower</td><td>Sound change</td><td>0</td><td>4</td><td>0.39</td></tr><tr><td>Spit</td><td>Lexical replacement</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Spit</td><td>Sound change</td><td>1</td><td>5</td><td>0.94</td></tr><tr><td>White</td><td>Lexical replacement</td><td>0</td><td>0</td><td>0</td></tr><tr><td>White</td><td>Sound change</td><td>0</td><td>3</td><td>0.51</td></tr></tbody></table>

90 per cent highest posterior density intervals (HPD) are given along with the probability of two or more changes.

uncertainty, with variation in the number of sound changes reconstructed in different samples from the posterior. However, in all semantic values except ‘branch’ and ‘lake’ there is a probability of at least 50 per cent that two or more sound changes resulting in bilabial trills occurred (Table 3). In ‘fire’ and ‘bone’ there is a greater than 95 per cent probability of multiple sound changes leading to trills.

In some semantic values bilabial trills appear coincident with the appearance of the relevant cognate set, i.e. through lexical replacement (either via lexical innovation or borrowing) rather than via a pathway of

regular sound change. An example is the semantic value ‘branch’. Two ‘origins’ of trills occur within this semantic value, both coincident with lexical replacement and therefore not representing the true origin of trills in these word forms. Due to the limitations of the raw data, which consists of word lists in a fixed set of semantic values, we are unable to trace the trill cognate sets further back in time, and cannot therefore rule out an earlier origin for the bilabial trills in these cases.

We found evidence of multiple ‘origins’ of trills in semantic values through lexical replacement, symptomatic of parallel semantic shift or borrowing. In the
semantic values ‘bad’, ‘branch’, ‘lake’, ‘squeeze’, and ‘tail’ there is a greater than 50 per cent probability of multiple lexical replacements (Fig. 4). It is perhaps not surprising that multiple lexical replacements are found for the semantic value ‘lake’, which may include forms from a number of similar semantic values due to the scarcity of true lakes in Vanuatu.

### 3.3. Plain trills

Ancestral state reconstructions for the three semantic values with plain trills all indicate a relatively recent origin within the East Malekula grouping (Fig. 3). However, in the semantic value ‘flower’ the first appearance of plain trills is reconstructed as occurring through lexical replacement, so an earlier appearance in this cognate set cannot be ruled out. For ‘spit’ there is a high probability of multiple sound changes leading to trills (0.94), whereas results are more ambiguous for ‘flower’ and ‘white’, respectively (0.39 and 0.51, respectively).

## 4. Discussion

Our results conclusively favour a dynamic scenario in which bilabial trills are constantly and repeatedly gained and lost. The hypothesis that trills originated once and were gradually lost in different semantic values is firmly rejected. We also find evidence for a relatively late appearance of plain trills, in support of the hypothesis that these only appear where prenasalized trills are already present (Rangelov et al. 2023).

What factors explain the dynamism of bilabial trills? The rapid switching to and from trills observed here is likely to result from the low functional load of these sounds. In modern languages, bilabial trills are often allophones of the corresponding plosive phonemes; where they are phonemic, they are marginal phonemes (§1). However, what factors contribute to this allophony persisting over presumably centuries, given that bilabial trills are phonetically challenging and presumably easily lost? Of the factors that account for the emergence and persistence of bilabial trills in Malekula languages proposed by Rangelov et al. (2023), most have stable or one-time effects (phonetic environments, phonological processes, Papuan borrowing), except for the identity attachment factor. When speakers attach identity to bilabial trills, this can motivate their stability or (re-)emergence in both specific varieties and specific semantic values. This observation is in line with Ladefoged and Everett's (1996) observation that phonetic rarities play a role that goes beyond the 'widespread phonological processes' that revolve around maintaining meaning

differentiation, i.e. they are more likely to be involved in language-external processes, such as identity marking.

The strong evidence for multiple appearances of trills across all semantic values suggests that bilabial trills are subject to borrowing and subsequent diffusion through multiple varieties to various extents. Trills may have been borrowed into one language variety from a neighbouring variety due to changes in prestige or alliances between communities. We also see this geographic effect in reverse—where trills are lost because of contact with varieties that do not have trills—whose speakers may have perceived trills as comical (Crowley 2006) such as in Uripiv and Aulua. The geography component is also visible in the fact that trills are not found in the northernmost parts of the island, which may reflect a historical split in intra-community alliances between that geographic area and the rest of Malekula (Lynch and Brotchie 2010).

## 5. Conclusion

Our phylogenetic analysis of bilabial trills shows that rare sound changes can occur repeatedly within groups of languages where that sound already exists. Bilabial trills in the case of Malekula are likely to have been subject to frequent borrowing and our results show they spread through the lexicon gradually. We ascribe the apparent maintenance of trills in the face of long-term allophony to identity attachment.

Importantly, the method used here provides a quantitative framework for modelling the diachronic pathways of sounds throughout the history of a given group of languages. Here we have shown the usefulness of this approach for investigating the behaviour and relative timing of rare sounds, but this method can also be applied to other sound changes, whether rare or common, advancing our understanding of sound change more generally.

## Acknowledgements

We thank Isaac Stead for reviewing the data and code.

## Supplementary data

Supplementary data is available at *Journal of Language Evolution* online.

## Data availability

All data and code including Supplementary Table S1 (list of doculects), the cognate-coded trills data, Beast2 xml files for the tree analysis, RevBayes scripts and results from all model variants are available in
the online supplementary information at https://doi.org/10.6084/m9 figshare.25593867

*Conflict of interest statement.* None declared.

## Funding

Funding was provided by the Max Planck Society.

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