Research Article |
Corresponding author: Wolf U. Blanckenhorn ( wolf.blanckenhorn@ieu.uzh.ch ) Academic editor: Philippe Jeanneret
© 2021 Wolf U. Blanckenhorn.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Blanckenhorn WU (2021) Energetic underpinnings of yellow dung fly mating success in the field. Alpine Entomology 5: 61-67. https://doi.org/10.3897/alpento.5.68153
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Foraging provides the basis for animal reproduction, but requires energy and time to be sustained, entailing a trade-off. Whereas females should maximize their time foraging for resources, males should minimize their foraging time by optimizing time budgets to maximize their access to mating partners.
Mark-resight field studies are difficult and hence uncommon for small insects. Yellow dung flies (Scathophaga stercoraria L.) abound on pastures in cold-temperate regions across the northern hemisphere. Adult flies lick nectar from flowers for energy, but require small insect prey to produce eggs and sperm. Males wait for females around fresh cow dung, but at one point also need to replenish their energy and/or sperm reserves in the surrounding vegetation. Their foraging time budgets should depend on their body size, nutritional energy reserves, availability of sperm, competitor and female density.
Marked male dung flies whose nutritional status was experimentally manipulated – water only (null control); water + sugar (energy replenishment); or water, sugar + Drosophila prey (energy and sperm replenishment) – were repeatedly observed on an experimental pasture for an entire day. Both nutrient types were expected to increase the mating success of especially large males.
The total number of resighted males seen copulating was lowest for water-treated flies. Mating success was positively related to body size. The distance travelled between dung pats was greater for males fed sugar or prey and also increased with body size, while pat residence times decreased with size. No differences were found between the sugar- and prey-fed groups. Crucially however, there was no evidence in the field for a time budget or mating advantage of small males when nutrients were limited.
body size, energy reserves, field observations, food manipulation, foraging, mating success, Scathophaga stercoraria, reproduction
Foraging provides the basis for animal life histories (
To understand the mating system of any particular species, time and energy allocation of both sexes to various activities must be investigated (
I here report the results of replicated field observations of individual yellow dung fly males using the classic mark-resight approach, with male nutritional status additionally being experimentally manipulated. Numerous males of various ages and sizes were randomly collected on a given (random) day in the season (cf.
We conducted at total of six field markings (days), three during the spring and three during the fall fly season (
On each observation day, from early morning to late evening, up to 5 people (students of our annual ecology course) regularly screened the entire pasture for individually identifiable marked male flies, particularly all freshly appearing dung pats, which were marked with coloured, numbered flags. Returning to the same dung pat in haphazard, more or less regular intervals, observers noted fly identity (coloured number) and observation time as well as the pairing status of any marked fly seen. From these raw data, I later extracted the following variables ultimately analysed for all marked individuals resighted at least once on the pasture: 1) pairing status (paired or not) as an estimate of mating success; 2) the estimated distance from the previous to the next dung pat occupied, in case an individual switched pats (in m), as an indicator of flight endurance; 3) the number of times a given individual was observed during a given day (anywhere it was seen); 4) the total number of different pats visited by an individual (i.e. pat switches) as an indicator of searching activity, with distances between the pats estimated and averaged (cf. 2 above); 5) the daytime a fly was first and last seen on the pasture as an indicator of total activity time during the experimental day.
Flies were individually marked with numbered (1–99) opalith discs (6 available colours) that were dorsally glued with shellac onto the flies’ thorax. Disks and shellac glue are commercially available for marking honey bees from Bienen-Meier (Künten, Switzerland;
Prior to our field experiment, two tests of our marking technique were conducted in April and May 1998, during which 276 and 447 (respectively) marked but otherwise untreated males of various sizes were released on two different pastures near Fehraltorf during the course of
Three days after marking, ca. 15% of all originally marked flies were resighted on the pasture; two days later this number dropped to 3%, and after 10 days no marked individuals remained to be seen. Neighbouring pastures were also searched. Thus, not least because of many other cow pastures in the vicinity, marked flies disappeared quite fast from the focal pasture. As usual in such assessments, emigration and death could not be distinguished.
The variables defined above were computed and all data aggregated per individual male as the independent statistical unit. All variables were subjected to separate analyses of variance with food treatment as fixed factor (3 levels), initial body weight as continuous covariate (first also including the interaction, which was removed if not significant at P > 0.15), and sampling date as random effect. The number of flies resighted (of all originally marked) were analysed with binomial errors, the total number of copulations per individual were analysed with Poisson errors, and all other variables were log10-transformed as necessary to meet statistical assumptions to be analysed with normal errors.
Of roughly 540 males originally marked and released (6 dates × 3 treatments × 30 flies/treatment), 231 were resighted (ca. 43%). Of these, 46 were resighted only once (on one dung pat), and 114 occupied only one dung pat for longer time, such that the total sample of resighted males having switched pats at least once was N = 117 (Table
Mean ± SE and statistical significance of all behavioural variables assessed for three nutritional treatments (N refers to total males seen, and males seen on more than one pat). Data for the six observation dates are absolute counts.
water | water, sugar | water, sugar, prey | χ2 / F* | |||||||
---|---|---|---|---|---|---|---|---|---|---|
(N = 60, 26) | (N = 93, 54) | (N = 78, 37) | ||||||||
mean | SE | mean | SE | mean | SE | treat | size | date | txs | |
No. of flies (of 30) resighted | 10 | ±1.81 | 15.5 | ±2.86 | 13 | ±2.21 | 3.18 | – | – | – |
18 May 1999 | 11 | 24 | 8 | |||||||
5 June 1999 | 14 | 14 | 14 | |||||||
15 June 1999 | 8 | 14 | 13 | |||||||
18 October 2004 | 17 | 24 | 21 | |||||||
27 October 2004 | 6 | 8 | 6 | |||||||
4 November 2004 | 4 | 9 | 11 | |||||||
Pr. copulated | 0.22 | ±0.06 | 0.35 | ±0.06 | 0.40 | ±0.09 | 1.98 | 20.0 | 25.1 | – |
Initial body mass (mg) | 25.43 | ±1.16 | 25.87 | ±0.87 | 26.49 | ±1.06 | 0.45 | – | 18.8 | – |
Total time active (min) | 129.1 | ±18.1 | 182.3 | ±17.6 | 146.9 | ±17.0 | 1.17 | 0.09 | 2.21 | – |
Times observed | 4.28 | ±0.47 | 6.55 | ±0.60 | 5.40 | ±0.61 | 2.88 | 0.27 | 5.62 | – |
No. of pats visited | 2.00 | ±0.22 | 2.23 | ±0.15 | 2.00 | ±0.15 | 1.68 | 2.75 | 3.23 | – |
Distance to next pat (m) | 16.41 | ±2.78 | 28.19 | ±2.43 | 23.90 | ±2.68 | 5.24 | 6.34 | 1.93 | – |
Min. pat residence time (min) | 33.23 | ±5.96 | 38.70 | ±4.36 | 36.17 | ±3.45 | 1.14 | 7.59 | 1.66 | 2.18 |
The body mass of the flies did not vary significantly among our three treatments, nor did the total number of daily sightings or the total observation time, as of course should not be the case given random assignment to feeding treatments (Table
The proportion of males resighted tended to be lower for the water treatment (Table
Reproductive success of an organism is a function of intrinsic (e.g. body size) as well as extrinsic, environmental factors (e.g. mate or food availability, weather conditions, etc.). Although foraging success is often far removed from fitness (
Adult yellow dung flies feed on nectar for energy but additionally prey on other insects, and the nutrients derived from these different foraging modes serve different purposes. Sugars are required for fuelling flight, and proteins from prey are used to produce the eggs or sperm needed for reproduction, although sugars can likely be derived also from prey (
Although not quite statistically significant, the total number of resighted individuals and the proportion of males seen copulating following release on their native pasture after food treatment was lowest for the flies only given water. This indicates that these flies were energetically limited and needed to first feed before becoming reproductively active again (Table
While yellow dung flies cannot convert sugars into proteins (contrary to some other insects;
Some of our behavioural traits showed expected time budget differences between feeding groups (Table
Our six observation days were distributed evenly across typical spring and fall fly seasons (
In conclusion, our study shows that even for small insects, mark-recapture studies can be effective for gaining behavioural insights into reproductive success in the field. Ironically, Switzerland is not the best place for such studies because of the high density of cow pastures, rendering the tracking of small marked flies in confined geographic space difficult.
I thank multiple ecology students whose names are long forgotten for performing the field experiments and observations, but especially Dieter Burkhard for conducting invaluable preliminary markings. This work was funded over the years by the Zoological Museum of the University of Zurich and continuous grants from the Swiss National Science Foundations to Wolf Blanckenhorn (last 3100A0-111775). In memory of Paul I. Ward, who died too early.
Data File in Excel for Supplemental publication
Data type: Excel text file
Explanation note: Sheet1 contains the complete data file for the analyses performed in this paper. Variable names are self-explanatory. Sheet 2 contains some extra but incomplete (hence excluded) data from one day in 2005 because the weather did not allow completion.