DOI: 10.5061/dryad.d51c5b06b
title: Direct and indirect phenotypic effects on sociability indicate potential to evolve

description: The decision to leave or join a group is important as group size
 influences many aspects of organisms’ lives and their fitness. This
 tendency to socialise with others, sociability, should be influenced by
 genes carried by focal individuals (direct genetic effects) and by genes
 in partner individuals (indirect genetic effects), indicating the trait’s
 evolution could be slower or faster than expected. However, estimating
 these genetic parameters is difficult. Here, in a laboratory population of
 the cockroach Blaptica dubia, I estimate phenotypic parameters for
 sociability: repeatability (R) and repeatable influence (RI), which
 indicate whether direct and indirect genetic effects respectively are
 likely. I also estimate the interaction coefficient (Ψ), which quantifies
 how strongly a partner’s trait influences the phenotype of the focal
 individual and is key in models for the evolution of interacting
 phenotypes. Focal individuals were somewhat repeatable for sociability
 across a three-week period (R = 0.080), and partners also had marginally
 consistent effects on focal sociability (RI = 0.053). The interaction
 coefficient was non-zero, although in the opposite sign for the sexes;
 males preferred to associate with larger individuals (Ψmale = -0.129)
 while females preferred to associate with smaller individuals (Ψfemale =
 0.071). Individual sociability was consistent between dyadic trials and in
 social networks of groups. These results provide phenotypic evidence that
 direct and indirect genetic effects have limited influence on sociability,
 with perhaps the most evolutionary potential stemming from heritable
 effects of the body mass of partners. Sex-specific interaction
 coefficients may produce sexual conflict and the evolution of sexual
 dimorphism in social behaviour.
description: <strong>Experimental
 animals</strong>: <em>Blaptica dubia</em> is a quite
 large (up to 45 mm in length) sexually dimorphic blaberid cockroach (Wu,
 2013). They typically live in aggregations at high temperature and
 humidity in central and south America (Alamer & Hoffmann, 2014),
 consume vegetative matter, and are ovoviviparous. They are described as
 “gregarious” (Grandcolas, 1998) or “communal” as individuals of the same
 generation cohabit (without shared parental care; Bewick <em>et
 al.</em>, 2017). I purchased an initial colony of <em>B.
 dubia</em> online in March 2021. I maintained them at the University
 of XXX [removed to preserve anonymity] at 28°C, 50% humidity, with a 50:50
 light:dark cycle. I provided them with cardboard egg trays for shelter,
 carrots for hydration, and Sainsbury’s Complete Nutrition Adult Small Dog
 Dry Dog Food (approx. nutritional composition = 1527 kJ energy, 24g
 protein, 12g fat per 100g) for nutrition. Mortality was very low at all
 life stages (0.31% of the adults in the colony died per half week)
 indicating the colony was healthy. I moved newly born nymphs every few
 days to a container of dimensions 610 x 402 x 315 mm of similarly aged
 individuals (density ranged from a few hundred of the earliest instars to
 10-80 of later instars) and maintained them in mixed groups until
 adulthood (seven instars which take approx. 250 days at this temperature;
 Wu, 2013). Upon reaching adulthood I moved them to either single-sex
 groups (again in containers of 610 x 402 x 315 mm) or in small groups of
 two males and four to eight females in a container of dimensions 340 x 200
 x 125 mm for breeding to maintain the stock population. For this
 experiment, I selected 48 unmated males and 48 unmated females from the
 single-sex adult groups. Individuals were all more than five days old, and
 so presumed to be sexually mature (Hintze-Podufal & Vetter, 1996).
 I transferred each individual to a clear plastic box (79 x 47 x 22 mm)
 labeled with its unique ID to allow individual recognition. I gave
 individuals a small piece of carrot for hydration which was replaced
 weekly. <strong>Data
 collection</strong>: I tested individuals in two blocks of 48,
 treating all individuals in each block once as a focal individual and once
 as a partner for a member of the same sex over two days. This means that
 in the first two days 24 males and 24 females were each assayed for
 sociability once and each acted as a partner individual once. On days
 three and four I repeated this with a second block of 24 males and 24
 females. In this way individuals only ever acted as focal or partner
 individuals with members of the same sex in the same block (either first
 or second) and were each assayed for sociability and acted as a partner
 once per week. I repeated this for three weeks, so each individual was
 assayed up to three times as a focal individual and acted as a partner up
 to three times. Some individuals received fewer than three trials if they
 died (n = 5 males and 0 females), in which case I replaced them with a
 member of the same sex from the stock population (who did
 <em>not</em> inherit the same ID and was therefore another
 unique individual). Individuals might also record fewer than three
 measures for sociability if the mesh was breached by either the partner or
 the focal before the trial began (11 females and eight males recorded one
 measure, 48 females and 42 males recorded two measures, 30 females and 54
 males recorded three measures). I assayed sociability in medium-sized
 plastic boxes (200 x 100 x 70 mm) where I glued a fine polypropylene mesh
 (mesh size 0.6 x 0.6 mm, Micromesh, Haxnicks) across the interior 50 mm
 from one end. This creates an arena with a small compartment (50 x 100 x
 70 mm) and a large compartment (150 x 100 x 70 mm) separated by the mesh
 (Fig. 1A). Separating by mesh was necessary to prevent a partner
 individual from imposing close proximity on the focal individual by
 constantly following or attempting to dominate it (Clark <em>et
 al.</em>, 1995), and therefore my assay captures the focal
 individual’s willingness to socialise, rather than the partner’s (Gartland
 <em>et al.</em>, 2022). For the first block, I randomly placed
 12 females and 12 males alone, each into their own plastic box, in the
 large compartment. These were the focal individuals. I then randomly
 placed an individual of the same sex into the small compartment; these
 were the partner individuals. I used individuals of the same sex to ensure
 I was measuring sociability rather than a willingness to mate. I then
 placed these 24 arenas into four large plastic boxes (six in each) which I
 placed underneath a video camera (ABUS IP video surveillance 8MPx mini
 tube camera), so that each video camera recorded six arenas
 simultaneously. I maintained the room the video recordings occurred in at
 20-22°C using portable heaters, while I used a thermometer to record the
 temperature at the start and end of each trial. I was not able to control
 or monitor humidity during the trials. Once all arenas were in position
 and cameras focused, I started the recording and left the room. The lights
 automatically switched off after 40 minutes, and so the trial began 40
 minutes after I left the room, in darkness, which is when <em>B.
 dubia</em> is active (Bouchebti <em>et al.</em>, 2022).
 I returned two hours after leaving to end the trial, meaning the trials
 lasted 80 minutes. In the darkness, the cameras automatically switch to
 infra-red filming using infra-red LEDs. For each trial, every ten minutes I
 recorded the proximity of the focal individual (in the larger compartment)
 to the mesh that separated it from the partner individual, giving a
 maximum of eight measures per trial. The distance of an individual to a
 conspecific in this manner is often used to measure sociability (reviewed
 in: Gartland <em>et al.</em>, 2022) and by using the distance
 to the mesh rather than the partner I have a measure solely under the
 influence of the focal individual; the partner cannot directly influence
 the distance by moving closer or further away. If the focal was sat
 directly on the mesh (perpendicular to the floor) I recorded a distance of
 zero, otherwise, I used the hexagons on the bottom of the box to record
 how far the focal individual’s head was from the mesh. Smaller values mean
 a focal individual closer to the partner individual which indicates higher
 sociability. Individuals were in some cases able to bypass the mesh (this
 occurred 95 times before the lights went out and 33 times after they did
 out of 288 trials, the 33 breaches after lights out are still included in
 the analyses with only the measurements before the breach used, see Data
 analysis). I used the video recordings to determine when this happened and
 stopped recording data from the video as soon as either individual
 bypassed the mesh. If either individual bypassed the mesh in the 40
 minutes the lights were on before the trial started then I recorded no
 data from that trial. To avoid mixing individuals up at the end of the
 trial I dotted either the partner or the focal with white paint (Edding
 Extra-fine paint markers). At the end of the trial, I returned all
 individuals to their unique boxes. I then weighed all partner individuals
 to the nearest 0.01 g (Fisherbrand Analytical Balances, readability 0.0001
 g). In the first two days I also weighed the focal individual, but since
 the correlation between an individual’s mass as a focal and its mass as a
 partner either the following or preceding day was 0.994 (Pearson
 correlation, t<sub>38</sub> = 56.894, p &lt; 0.001) I
 stopped doing this to save time. Instead, I entered the mass of the focal
 individual as its mass as a partner individual recorded the same week
 (always within one day). As described above, each individual in the two
 blocks was assayed once as a focal and acted once as a partner for another
 individual of the same sex in that block per week, and this was repeated
 for three weeks. After the third trial, I
 aggregated individuals into four groups of 21-24; all the individuals of
 the same sex from the same block were together, with groups having fewer
 than 24 individuals if any died (I did not replace individuals that died
 with stock individuals as I was only interested in the social network
 position of those with known sociability from the dyadic trials). I gave
 each individual a unique combination of two colours (red, green, blue,
 white, gold) on their wing cases using paint pens (Edding Extra-fine paint
 markers) which allowed me to track them individually (combinations were
 repeated between groups i.e., red-blue featured in each of the four
 groups). I then placed each group into new plastic boxes (340 x 200 x 125
 mm) along with four shelters made from cardboard egg trays (approx. 100 x
 120 mm), each placed vertically at each corner on a long side. Shelters
 were taped to the walls of the box, creating clear space between both the
 shelter opposite it (on the opposite long side) and next to it (on the
 same long side). I placed 2 g of dog food and 10 g of carrot in the centre
 of each box. Each shelter was large enough to accommodate many but not all
 of the individuals, and the number of shelters was considerably less than
 the number of individuals. Therefore, the formation of aggregations in
 shelters was enforced, but individuals could move between shelters and
 therefore could exert some influence on who they co-habited with.
 Regularly after placing the individuals into these groups (after 3, 10,
 14, 18, and 21 days) I recorded which individuals were using the same
 shelter. Individuals who could not be identified were recorded as such but
 they were not used to build the networks. Collecting data in this way
 gives a group-by-individual matrix analogous to those generated by
 observing flocks of birds or herds of ungulates in the wild, and further
 is similar to methods than have been used to generate social networks in
 forked fungus beetles <em>(Bolitotherus cornutus</em>; Formica
 <em>et al.</em>, 2012, 2016, 2020) and maritime earwigs
 (<em>Anisolabis maritima</em>; Vipperman, 2021). While a
 single incidence of sharing a shelter could be due to chance, by
 aggregating these observations I can infer consistent social associations.
 When recording these data, I also updated any paint markings that were
 starting to wear, maintaining individually-recognisable marks for the
 duration of the experiment, and replaced carrot and dog food as
 necessary.
description: R is required to run the analysis code. The script relies on
 various packages which will need to be installed before using.