DOI: 10.5061/dryad.d7wm37q4m
title: When wax wanes: competitors for beeswax stabilize rather than jeopardize the honeyguide-human mutualism

description: Many mutualisms are exploited by third-party species, which benefit
 without providing anything in return. Exploitation can either destabilize
 or promote mutualisms, via mechanisms that are highly dependent on the
 ecological context. Here we study a remarkable bird–human mutualism, in
 which wax-eating greater honeyguides (Indicator indicator) guide humans
 (Homo sapiens) to wild bees’ nests, in an exchange of knowledge about the
 location of nests for access to the wax combs inside. We test whether the
 depletion of wax by mammalian and avian exploiter species either threatens
 or stabilizes the mutualism. Using camera traps, we monitored feeding
 visits to wax comb made available following honey harvests. We found that
 greater honeyguides face competition for wax from conspecifics and nine
 exploiter species, five of which were not previously known to consume wax.
 Our results support the hypothesis that heterospecific exploiters
 stabilize the mutualism, because wax depletion by these competitors likely
 limits feeding opportunities for conspecific exploiters, favouring the
 early-arriving individual that guided humans to the bees’ nest. These
 findings highlight the importance of the ecological context of species
 interactions and provide further evidence for how mutualisms can persist
 because of, and not in spite of, exploitation by third-party species.
description: <strong>Study
 site</strong> We carried out this study in a 28
 km<sup>2</sup> area within the Niassa Special Reserve in
 northern Mozambique. Our study area is in range of Yao honey-hunters’
 foraging trips from Mbamba village (12º12'S, 38º01'E; ca 2,000
 inhabitants including &gt; 20 regular honey-hunters). Yao
 honey-hunters traditionally reward greater honeyguides after a successful
 honey-harvest by leaving a small pile of beeswax near the harvested bees’
 nest (figure 1), and therefore the main source of wax for greater
 honeyguides in this landscape is that left behind or exposed by humans at
 the harvest site of a bees’ nest. Within this area, the costs and benefits
 of the human-honeyguide mutualism appear to approximate those under which
 it presumably evolved: there is little apiculture and a minimal cash
 economy for buying sugar instead of honey. The habitat is deciduous Miombo
 woodland (dominated by the tree genera <em>Brachystegia</em>
 and <em>Julbernardia</em>) punctuated by granite inselbergs
 and narrow strips of riverine forest along the Lugenda River and seasonal
 tributaries (altitude 400–450 m). The climate is sub-humid tropical with
 mean minimum and maximum air temperatures ranging between 16–33°C in the
 dry season (May–October) and 22–32°C in the wet season (November–April).
 Rainfall begins in November and ends in late April or early May; during
 this period, precipitation averages 250–350 mm per month. Bees’ honey
 stores, which build up throughout the rains with the flowering of dominant
 species, peak in May–June, deplete as the dry season progresses and then
 peak again in November–December following the flowering of trees prior to
 the following rainy season.  Data were collected from 24 September to 25
 October 2015, 29 August to 15 October 2017, 4 November 2018 and 24
 September 2021 to 7 October 2021.
 <strong>Honey-harvests,
 wax and camera trap placement</strong>
 Wax-eating data were collected
 at 26 small piles of wax comb (ca. 0.1–1.5 kg) placed on the ground or a
 horizontal tree log, in a manner reflecting the honeyguide rewarding
 culture displayed by Yao honey-hunters . These were located at 26 bees’
 nests (six nests in 2015, four in 2017 and 16 in 2021) which were found
 during honey-hunts conducted in a traditional manner. Eleven sites from
 2015 and 2017 where a bees’ nest was harvested and camera traps placed,
 were excluded from the analysis either due to camera trap malfunction or
 because the wax was left without harvesting the bees’ nest. To initially
 locate the bees’ nests, one or two researchers (DJL, CNS, JSC) accompanied
 two Yao honey-hunters (one of whom was a main assistant and is a
 co-author: OY) on a honey-hunt as they elicited guiding behaviour from
 greater honeyguides using stereotypical calls. Twenty-two of the 26 nests
 were located by guiding from a greater honeyguide, 2 were located
 opportunistically prior to being guided while walking in the same habitat,
 and 2 were found in the same habitat after unsuccessful guiding by a
 greater honeyguide. All bees’ nest locations were previously unknown to us
 or the honey-hunters prior to the study, and at all 26 sites honey and wax
 were extracted by two honey-hunters using traditional methods (smoke,
 axes). The honey-hunters retained the combs containing honey and left
 behind a pile of wax combs without honey: these included a mixture of wax
 types, including smaller pieces of newly-produced, empty white wax comb
 (most favoured by greater honeyguides), older wax comb with bee brood in
 it, and dark wax comb containing old larval casings and no brood (least
 favoured by greater honeyguides, pers. obs. and [16]). In all cases, the
 wax piles were consistent with those that Yao honey-hunters naturally
 leave as reward following a successful harvest at a bees’ nest they were
 guided to by a greater honeyguide.
 To record animals eating wax
 combs during the day and night, a camera trap (Acorn 6210, Ltl Acorn,
 Denmark, Wisconsin) was set up for 1–9 days at each site (total = 88.8
 trap days; mean ± SE duration per site = 3.41 ± 0.39 days) at a
 height of 80–90 cm above ground and 1.5–2 m from the wax pile, facing
 slightly downwards (26.6 trap days in 2015, 15.7 trap days in 2017, 46.5
 trap days in 2021). Each camera trap was set to trigger with a 5 s delay
 and take photos at 10 s intervals for as long as the camera was
 motion-triggered. The cameras also recorded a video clip of 10 s or 30 s,
 alternating with sets of 3 photos for as long as the camera was motion
 triggered. Coordinates of the bees’ nest were marked with GPS (Garmin
 eTrex 30, Garmin USA) and all camera traps checked every 1–2
 days. For each animal detected, the duration of wax
 feeding events was estimated from image and video clip time stamps.
 Feeding events were defined as contact between the mouth or bill of the
 animal and any part of the wax comb pile. Whenever possible we recorded
 which food type was eaten (wax only, brood only, or wax and brood
 together), but due to the resolution of the images we were not able to
 reliably score which type of wax was eaten by each animal. Short feeding
 events, such as when the animal disappeared prior to a second image being
 taken, and therefore without a reliable ‘end time’, were recorded as
 having a 2 s duration. At one wax site, feeding greater honeyguides
 disappeared to feed inside the log from which the honey and wax was
 harvested, and were observed emerging with small pieces of white wax. For
 these few observations (5 of 39 visits at this site), the period that the
 bird was out of view was recorded as feeding time.
  <strong>Statistical
 analysis</strong> All statistical analyses were carried out using
 R (4.0.3 version). To document the level of competition that greater
 honeyguides face for beeswax, we summarised the following for each wax
 placement site (n = 26): number of wax-eating visits by each species,
 first arriving species, and species which ate the last remaining wax
 (where known). Feeding times for all wax-eating species were plotted over
 24 h and compared to median sunrise and sunset times (generated using the
 R package ‘suncalc’) for our study duration).
 To test prediction (i) of
 hypothesis one (<em>competitors deplete the wax before greater
 honeyguides feed</em>) we calculated the proportion of sites at
 which greater honeyguides fed, the frequency of feeding visits per hour,
 and the proportion of visits which resulted in the wax pile being depleted
 for each species. Then, using data for the eight most frequent wax-eating
 species (defined as those with &gt; 5 visits: greater honeyguide;
 scaly-throated honeyguide, <em>Indicator
 variegatus</em>;
 lesser honeyguide, <em>Indicator minor</em>; striped bush
 squirrel, <em>Paraxerus flavovittis</em>; African civet,
 <em>Civettictis civetta</em>; honey badger,
 <em>Mellivora capensis</em>; yellow baboon,<em> Papio
 cynocephalus</em>; Meller’s mongoose, <em>Rhynchogale
 melleri</em>) we fitted a univariate Cox’s proportional hazards
 survival model for arrival at wax (the event) for greater honeyguides and
 the other seven competitor species (pooled) using the R-package
 ‘survival’. The response term was time since wax placement, and a binary
 variable denoting whether the final visit time was unknown (right-censored
 events). The proportional hazards assumptions of this Cox regression model
 were met (checked by visual inspection of proportional hazard plots and
 tested using the cox.zph function in the ‘survival’ package; all
 <em>p</em> &gt; 0.05). The results are presented as hazard
 ratios (HR) with corresponding 95% confidence intervals
 (CI). To test prediction (ii) of hypothesis one
 (<em>most important competitors are diurnal</em>), we tallied
 the number of visits of species which consumed wax, and the time of day at
 which depletion (time when final piece of wax is consumed) occurred. To
 test prediction (iii) of hypothesis one (<em>diurnal competitors
 consistently displace greater honeyguides from a wax resource</em>),
 we first calculated the proportion of visits by diurnal competitors at wax
 (scaly-throated honeyguides, lesser honeyguides, striped bush squirrels,
 yellow baboon and slender mongoose) which were simultaneous with greater
 honeyguides, then calculated the proportion of greater honeyguide feeding
 visits which were cut short by either of these five competitor species,
 and the number of greater honeyguide visits which were immediately before
 or after competitor species (within 10 s).
 To test prediction (i) of
 hypothesis two (<em>greater honeyguides are the first-arriving
 species after the wax has been exposed</em>), we calculated the
 proportion of sites where greater honeyguides arrived first, along with
 the previous Cox’s proportional hazards survival model of arrival at wax
 by greater honeyguides and competitor species. To test prediction (ii) of
 hypothesis two (<em>the majority of greater honeyguide feeding
 events occur before major wax competitors</em>), we first observed
 that the larger-bodied mammals (&gt;1 kg) appearing in the images were
 honey badger, yellow baboon, Meller’s mongoose and African civet.
 Preliminary observations suggested that although civets frequently visited
 wax sites, they were messy and often left substantial wax available, only
 depleting all available wax at 2 of 26 sites. For this analysis we
 therefore defined the major wax competitors as yellow baboon, honey badger
 and Meller’s mongoose, and counted the number of greater honeyguide visits
 to wax that occurred before or after the visits of these species, and also
 the number of greater honeyguide visits spent looking for pieces of wax
 after the wax was depleted. These counts were fitted as the response term
 in a generalised linear mixed effects model (GzLM) with a Poisson
 distribution. The number of camera trap days for each interval was
 included as an offset to account for variation in sampling effort, and wax
 placement site was included as a random term in addition to an observation
 level random to account for overdispersion. Similarly, we compared the
 feeding durations of greater honeyguides before and after the arrival of
 major wax competitors using a GzLM with a Gamma distribution (selected due
 to the data having positively-skewed errors) with time spent feeding as
 the response term and visit interval (before major wax competitors, after
 major wax competitors, after wax depletion) as the predictor, and wax
 placement site as the random term. For both models we report chi-squared
 statistics of an analysis of variance (ANOVA) between the model of
 interest and the null model. Assumptions of normality for both GzLMs were
 assessed by visual inspection of the distribution of residuals. Effect
 sizes (estimated marginal means) were calculated using the R package
 ‘emmeans’. Feeding rates for greater honeyguides were calculated by
 dividing the total number of feeding visits by the total number of
 daylight hours the wax was available for (daylength was calculated using
 ‘suncal’ package), over all sites. This was repeated for visits before and
 after major wax competitors. 
description: R Statistical Program is require to open .R code and .csv
 files.