DOI: 10.5061/dryad.44j0zpck3
title: Correlates of homing performance in Oophaga histrionica

description: Homing is the ability to return to previously visited sites, often to the
 home range. Most studies have focused on the mechanisms used to home, but
 few have addressed the cost-benefit analysis of homing behavior, e.g., by
 testing for associations between homing performance and ecological
 factors. We aimed to study homing ability in males of the poison frog,
 Oophaga histrionica, by testing the general hypothesis that homing
 performance depends upon potential indicators of territory quality or the
 risk of losing it. First, we tested whether return time was related to
 displacement distance, body size, number of courtships during the previous
 month, or distance to nearest neighbors. All males homed and males that
 were displaced 10 meters (m) from their territories returned significantly
 faster than males displaced 25 or 40 m. Yet none of the ecological
 variables affected homing ability. In a second experiment, we tested
 whether males’ homing performance was affected by adding or removing
 acoustic cues, simulating changes in the number, identity, and spatial
 distribution of neighbors. Most displaced males homed within six hours,
 and males exposed to additional loudspeakers (i.e., neighbors) within
 their territories homed more accurately than other males. Our results
 indicate that the homing performance of males is affected by the perceived
 risk of being displaced from their territories.
description: The study was conducted during
 July-August 2006 and June 2007 at El Amargal Biological Station, near
 Arusí, Chocó, Colombia (5’34” N, 77’30” W, between 60–120 m.a.s.l.), which
 the NGO Fundación Inguedé privately owns. At the study sites, males of
 <em>O. histrionica</em> are found calling from fallen trunks
 and roots, more often as discrete aggregations of contiguous territories
 of about 30–70 males. Aggregations appear more common at the clearings of
 the relatively undisturbed tropical rainforest. Due to the high (around
 8000 mm/yr) and almost continuous precipitation, the calling activity
 appears extended throughout most of the year. We conducted our experiments
 on territorial males (i.e., males that were calling for several days) from
 two breeding aggregations, separated by about 1.7
 km. To
 position individual frogs, we laid out a grid of about 96 x 144
 m<sup>2</sup>, subdivided into squares of 8 x 8
 m<sup>2</sup>, throughout the study area. To census
 territorial males, 1–2 people searched daily for calling individuals,
 between 0730 and 1600 hours. Because only males call in this species, male
 frogs were identified by observing spontaneous calling activity or by
 capturing and gently handling them to evoke release calls. The distinctive
 dorsal and lateral color patterns were drawn both on plastic cards in the
 male’s territory and on a field notebook and used to identify each male.
 Photographs were also taken of each male.  A first homing experiment consisted of capturing
 an already known male and displacing it from the capture site inside an
 opaque (black) plastic bag, to minimize visual or chemical cues that might
 provide information to the frog about the displacement direction. A total
 of 39 males were displaced either 10 m (N=13), 25 m (N=13), or 40 m (N=13)
 away from the capture site, in a westward or an eastward direction,
 according to a random number. They were then released on the forest floor,
 by opening the plastic bag, so that they could come out and start moving
 at any time. We searched for displaced males during daily censuses and
 homing performance was measured by estimating return time as the number of
 days (at 0.5 d precision) elapsed between the moment of release and the
 moment when it was found back in its territory, i.e., within 2 m of the
 capture site.  To estimate body size, we measured the
 snout-to-urostyle length of each male on scaled digital pictures with the
 software Carnoy for Macintosh OS X (http://bio.kuleuven.be/sys/carnoy/). To estimate courtship success, we looked for courting males during daily censuses, i.e., males found (mostly calling and visually displaying) with a female within less than 20 cm distance. Courtship success of each male was then estimated as the number of females courted, after ruling out covariation with the number of days it was looked for. To estimate the distance to neighboring territorial males, we positioned encounter sites during daily censuses, built up convex polygons by connecting the furthest points where each individual was found, and measured (on a map) the distance from the polygon’s centroid to the corresponding centroids of the three nearest neighbors. Displacement distance, body size, courtship success, and distance to neighbors were then used as potential predictors of return time in graphical and statistical analyses. Return time was transformed using the inverse function (1/SQR(y + 1)) to achieve normality, before conducting further statistical tests. To test whether changes in the number and the spatial distribution of neighboring territorial males affect homing performance by <em>O. histrionica</em> males, we manipulated the acoustic environment within or at the territory of focal males. Every day, between 0800 and 0930 hours, 2–4 people searched for calling males, captured 2–4 males/day, and displaced them 10 m from the capture site in a randomly chosen direction. We chose 10 m as the displacement distance because the first experiment (see above) had shown that males are able to home from 10 m away within a few hours, and because restraining the experiment to one day reduced variation in return time due to eventual nocturnal traveling. Six hours after releasing males, the same group of people carefully searched for displaced males within an area of at least 314 m2 around the capture site; because all males were captured while they were calling, the capture site was assumed to be part of the territory. After locating a male, its position was recorded (angle and distance) in relation to the original capture site. Consequently, the output variables for this experiment are travel distance and angle error, the latter expressed as the difference between the expected (arbitrarily set at 0º) and the actual travel direction. The acoustic environment was modified by simulating new territorial males with acoustic playbacks, and by modifying their number and their position in relation to the focal male. By combining the addition of simulated males with the replacement of usual neighbors, we attempted to discriminate between two possible roles of the acoustic environment on homing performance: to indicate the risk of losing the territory to other males and to provide acoustic cues for spatial orientation during homing. Accordingly, experimental males were assigned to each of the following treatments: (a) Replacement: an acoustic stimulus was played back from the capture site of the displaced male; it simulates a situation in which the focal male’s territory was taken over by another male (N=9 males). (b) Neighbor’s identity: two acoustic stimuli were played back from the calling positions of the two nearest neighbors, which were temporally removed from the area; it keeps providing information on neighbor’s position but alters information on neighbor’s identity (N=9). (c) Neighbor’s density: an increase in the number of neighbors was simulated by playing back two acoustic stimuli from randomly chosen positions within 7 m of the capture site of the tested male, and without removing any neighbors; it simulates an increase in the risk of losing the territory to other males (N=11). And (d) Control: the male was displaced without adding or removing any acoustic cue (N=12).  Acoustic stimuli were digitally synthesized by using temporal and spectral characteristics of <em>O. histrionica</em> advertisement calls, previously recorded in the field. On average, calls lasted 164.51 ± 25.3 ms (mean ± SD) and consisted of 48 ± 13 pulses, uttered with a dominant frequency of 2.888 ± 0.145 kHz. Calls were emitted at a rate of 3.026 ± 0.234 calls/sec, with a mean intensity (SPL) of 76,6 ± 1,34. To avoid pseudoreplication of acoustic treatments (Kroodsma 1989; Kroodsma 1990) we never used the same call in two experiments. Call replicates were prepared by randomly modifying call parameters within ± 1 standard deviation of the average (Amézquita et al 2005; Amézquita et al 2006). Calls were stored as .mp3 files in memory cards and played back from Verbatim Smartdisk 95456 MP3 players through Sony SRS_M30 loudspeakers. The acoustic stimuli were broadcast from the time of release to the time of recapture at rates within the natural range of variation of call rate at the study site.  Data on travel direction and angle error were analyzed using standard statistical tests for circular data (Batschelet 1981). To test whether the distribution of travel directions was random we conducted Rayleigh’s tests for each experimental treatment. To test whether travel directions differed from the expected home direction (arbitrarily set as 0º) we conducted V tests. To test for between-treatment differences in average angle error we used a Watson-Williams test and to test for corresponding differences in homing accuracy (i.e., within-treatment variance in angle error) we used a Mann-Whitney test. Tests on circular data were conducted on the software Oriana 4 (Kovach computing services) whereas non-parametric tests were conducted on R.