CHAPTER 4 FORAGING BEHAVIOUR AND SUBSTRATE DISCRIMINATION IN
4.2.2 Statistical Analyses Foraging Analysis
In the analysis of substrate preference, I considered the following variables:
• The number of visits was the number of times a fish entered a substrate preference zone.
• The total duration was the total amount of time a fish spent in a substrate preference zone.
• The mean duration was the average duration per visit a fish spent in a substrate preference
zone.
• The duration proportion was the proportion of time a fish spent in a substrate preference
zone relative to the total duration of the observation period (10 minutes).
• The relative proportion was the proportion of time a fish spent in a substrate preference zone
relative to the time a female spent in both preference zones (excludes the time in the non
preference zone).
I used one-sample t-tests to quantify the effect of substrate on the response variables. If there
was no significant difference in the response variables between the substrates, a t-test would show
that the difference between the values for the two substrates (e.g., complex - simple substrate) did not
differ significantly from zero. I tested for a difference between substrates in the number of visits, total
duration of time spent on each substrate and the mean duration of each visit. In my analyses, I used
the difference in values between the substrates because this provides a simple way to analyse and
present the data. This method of analysis takes into account that the use of substrates is mutually
I also used general linear models with sex, feeding treatment and hunger level as fixed factors
to test for their effect on the number of visits, total duration of time spent on each substrate and the
mean duration of each visit. For these models, I again used the difference between substrates for
each variable (e.g., difference in number of visits to the complex substrate minus the number of visits
to the simple substrate). I used the difference in values rather than including substrate as a fixed factor
in the models because this makes it easier to detect the main effect of the treatment rather than
having to test for interactions (e.g., a treatment*substrate interaction because the food density on
substrate types differs between treatments so the preferred substrate should vary between
treatments). I also tested for the effect of sex, feeding treatment and hunger level on the relative
proportion of time spent in one substrate preference zone compared to the total time spent in both
preference zones (i.e. total time excludes time spent in the non-preference zone). I used the
proportion of time on the complex substrate. Assumptions of normality were tested for, and satisfied,
in all the models.
I also repeated the models but used only data from treatments 2 (food ratio of 1:3 for
complex : simple) and treatment 3 (food ratio of 3:1 complex : simple). I repeated the model (with
feeding treatment and hunger level as fixed factors) because these two treatments showed the
greatest extremes in the difference in food ratio between the two substrates. It was therefore more
likely than I would detect a treatment effect than in the case where I included the two treatments with
equal food ratio per substrate (as these two treatments should not differ). I tested for sources of
variation in the number of visits, difference in total time spent on a substrate and the mean duration of
each visit as well as the relative proportion of time spent on each substrate. I did not include sex in the
models with the reduced data set because the analysis on the full data set indicated that there was no
effect of sex on substrate-based foraging decisions.
I also conducted separate one-sample t-tests on the relative proportion of time spent on the
complex substrate for each treatment and hunger level. Finally, I used Mann-Whitney U tests to look
4.3
Results
There was no evidence of a foraging preference for either the complex or the simple substrate
(Table 4.3.1). The t-tests showed there was no significant difference in the use of complex and simple
substrates. There was no difference in the number of visits, total time on each substrate or the mean
duration of each visit. There was also no effect of treatment or hunger level on the number of visits,
total time or the mean duration of visits to each substrate. There was, however, a significant difference
between the sexes for the mean duration of visits to complex versus simple substrates (Table 4.3.2,
Fig 4.3.1: F = 9.849, df = 1, p = 0.002). Females preferred the complex substrate (Fig. 4.3.2; t = -
2.989, df = 47, p = 0.004) while males showed no preference for either substrate (Fig. 4.3.3; t = 1.566,
df = 47, p = .124).
There was no significant difference between the sexes in the difference in the number of visits
or total time spent on each substrate, but looking at the difference in the mean total time spent on
each substrate among the treatments does suggest a treatment effect (Fig. 4.3.4). Small sample sizes
make it difficult to detect a statistically significance result so this could be worthy of future
investigation. There was no difference in the relative proportion of time spent on each substrate
between the sex, among treatments or between hunger levels.
Analysing only treatments 2 and 3, individuals still showed no foraging preference for either
the complex or the simple substrate (Table 4.3.3). There was no effect of treatment or hunger level on
the number of visits, total time or mean duration of visits to each substrate type. There was also no
effect of treatment or hunger level on the relative proportion of time spent on each substrate (Table
4.3.4).
There was, however, a difference in the number of feeding strikes between hunger levels
across treatments (Table 4.3.5). There were no feeding strikes for treatment 1 (as there was no food
present) but there were also no strikes for fish fed to satiation one hour before the trial (Fig. 4.3.5).
The treatment by hunger interaction effect was due to difference between treatment 1 and the other
treatments, as there were large numbers of feeding strikes in fish fed 72 hours before the trial in
Table 4.3.1 The difference between the two substrates for the listed response variables measured as response variable for complex substrate minus response variable for simple complex (n = 96 for all variables). The response variables are the number of visits, total time spent on each substrate and the mean duration of each visit. Difference is measured as response variable for complex substrate minus response variable for simple complex. A significant bias towards the use of a substrate occurs if the mean difference is greater or less than zero (one-sample t-tests).
Response Variable Mean ± SD t df Sig.
Difference in Number of Visits 0.20 ± 2.25 0.863 95 0.390
Difference in Total Time Duration (sec) -19.19 ±206.25 -0.912 95 0.364
Table 4.3.2 General linear models with sex, feeding treatment and hunger level (HL) as fixed factors (n = 96 for all variables). Number of visits, total time spent on a substrate and mean duration of each visit were measured as the difference between complex minus simple substrates. The relative proportion of time on each substrate was the proportion of time spent in one preference zone compared to the total time spent in both preference zones (excludes time in non-preference zone). Variables marked * are significant.
Response Variable Factor df F Sig.
Difference in Number of Visits Sex 1,90 1.961 0.165 Treatment 3, 90 0.138 0.937
HL 1,90 1.487 0.226
Difference in Total Time Duration Sex 1,90 2.150 0.146 T reatment 3, 90 0.506 0.679
HL 1,90 0.000 0.997
Difference in Mean Duration Sex* 1,90 9.849 0.002* T reatment 3, 90 0.607 0.612
HL 1,90 0.006 0.940
Relative Proportion of Duration Sex
1,90 1.332 0.251 T reatment
3, 90 0.223 0.881 HL
Table 4.3.3 General linear models with feeding treatment and hunger level (HL) as fixed factors only for treatments 2 (food ratio of 1:3 complex : simple) and 3 (food ratio of 3:1 complex : simple). N = 96
for all variables). Number of visits, total time spent on a substrate and mean duration of each visit were measured as the difference between complex minus simple substrates. The relative proportion of time on each substrate was the proportion of time spent in one preference zone compared to the total time spent in both preference zones (excludes time in non-preference zone).
Response Variable Factor df F Sig.
Difference in Number of Visits Treatment 1 0.060 0.808
HL 1 1.207 0.278
Difference in Total Time Duration T reatment 1 0.452 0.505
HL 1 0.004 0.951
Difference in Mean Duration T reatment 1 0.832 0.367
HL 1 0.151 0.699
Relative Proportion of Duration T reatment 1 0.148 0.702
Table 4.3.4 General linear models split by feeding treatment and hunger level (HL), n = 96 for all variables). The relative proportion of time on each substrate was the proportion of time spent in one preference zone compared to the total time spent in both preference zones (excludes time in non preference zone). Feeding treatments were: (1) 0:0, complex : simple (no food on either substrate); (2) 1:3, complex : simple (6 food items in the simple tray vs. 18 food items in the complex tray; (3) 3:1 complex : simple (18 food items in the simple tray vs. 6 food items in the complex tray); (4) 1:1, complex : simple (6 food items in the simple tray vs. 6 food items in the complex tray). Hunger level 1 was feeding fish to satiation 72 hours before the trial and hunger level 2 is feeding fish to satiation one hour before the trial.
Response Variable T reatment HL t df Sig.
Relative Proportion of Duration 0:0 1 -0.219 11 0.830
2 -0.187 11 0.855 1:3 1 0.787 11 0.448 2 -0.829 11 0.425 3:1 1 -0.102 11 0.921 2 0.790 11 0.446 V.1 1 -0.187 11 0.855 2 -8.290 11 0.425
Table 4.3.5 The comparison of the total number of feeding strikes during the 10 minute observation period for hunger levels within each treatment (Mann-Whitney) (n = 96 for all variables). Feeding treatments were: (1) 0:0, complex : simple (no food on either substrate); (2) 1:3, complex : simple (6 food items in the simple tray vs. 18 food items in the complex tray; (3) 3:1 complex : simple (18 food items in the simple tray vs. 6 food items in the complex tray); (4) 1:1, complex : simple (6 food items in the simple tray vs. 6 food items in the complex tray).Treatments marked * are significant. Hunger level
1 was feeding fish to satiation 72 hours before the trial and hunger level 2 is feeding fish to satiation one hour before the trial.
Response Variable Treatment Mean ± SD U Z Sig.
Total Feeding Strikes 0:0, complex : simple
HL1 - 0.00 ± 0.00 HL2 - 0.00 ± 0.00 72.00 0.000 1.000 1:3, complex : simple:* HL1 - 0.00 ± 0.00 HL2 - 2.75 ± 2.05 96.00 -3.599 0.000* 3:1, complex : simple* HL1 -0 .0 0 ±0.00 HL2 - 4.58 ±2.31 78.00 -4.459 0.000*
1:1, com plex: simple*
HL1 - 0.00 ± 0.00
D if fe re n c e i 250 - a o <rs u 3 Q 4> a 0 “ o -250 - o j Females .I... Males Sex
Figure 4.3.1 The difference in mean duration of visits to the complex versus simple preference zones for each sex (F = 9.849, df = 1, p = 0.002).
F
r
e
q
u
e
n
c
y
14 H 12 - j -250 -200 -150 -100 -50 0 50 100 150 200 250Difference in Mean Duration (sec)
Figure 4.3.2 Difference in mean duration of visits by females to the complex versus simple substrate zones (one-sample t-test; t = -2.989, df = 47, p = 0.004). Overall, females spent less time on the complex substrate.
Fr
equency
Difference in Mean Duration (sec)
Figure 4.3.3 Difference in mean durations of visits by males to the complex versus simple substrate zones (one-sample t-test; t = 1.566, df = 47, p = 0.124). There was a non-significant trend for males to spend more time on the complex substrates.
-20 -
-40 -
-60 -
Feeding Treatment
Figure 4.3.4 The means for the difference in total time spent on the complex versus simple. Feeding treatments are: 0:0, complex = simple (no food on either substrate); 1:1, complex = simple (6 food items in the simple tray vs. 6 food items in the complex tray); (3) 1:3, complex = simple (18 food items in the simple tray vs. 6 food items in the complex tray); (4) 3:1, complex = simple (6 food items in the simple tray vs. 18 food items in the complex tray).
5 * r __ <b J t ♦ P N
o 4 H
as
c3
1 _ £o
H 1 — 1 - 0 -e
©
©
©
0:0
1:3
3:1
Feeding Treatment
1:1
Figure 4.3.5 The mean ± SE for total number of feeding strikes per individual in the four feeding treatment clustered by hunger level. Circles designate huger level 1 where fish were fed one hour before the trial, and triangles designate hunger level 2 where fish that were only fed three days before the trial. Feeding treatments were: (1) 0:0, complex : simple (no food on either substrate); (2) 1:3, complex : simple (6 food items in the simple tray vs. 18 food items in the complex tray; (3) 3:1, complex : simple (18 food items in the simple tray vs. 6 food items in the complex tray); (4) 1:1, complex : simple (6 food items in the simple tray vs. 6 food items in the complex tray).
4.4
Discussion
In the previous chapters, I discussed the costs of reproduction: the costs of mating (Daly
1978; Pomiankowski 1988; Iwasa et al. 1991), of mate choice (Hedrick and Dill 1993; Rowe 1994;
Grafe 1997; Watson et al. 1998) and of parental care (e.g., Stanley 2002). In light of this, the
processes whereby animals obtain energy for these activities becomes of interest, especially when
discussing traits that are condition-dependent (Zahavi 1975; Getty 1998). Both optimal foraging
models (reviewed in Pyke et al. 1977; and Schoener 1986) and ideal free distribution theory (Fretwell
and Lucas 1970) discuss individual foraging behaviour in terms of food density. Though these theories
are somewhat oversimplified, it is still reasonable to follow their predictions that individuals will forage
selectively to maximise their energetic gains.
Different habitat structures present animals with differences in food density and detection rate
of prey (Eklov and Diehl 1994; Merilaita 2003). There will often be a higher abundance of prey around
structurally complex habitats (Charnov 1976; Stein and Magnuson 1976) so predators might be
expected to prefer foraging in patches of greater structural complexity. For example, both smallmouth
bass Micropterus doloieuf and pumpkinseed sunfish Lepomis gibbosus select different prey depending
on the substrate on which they are presented and the differing levels of topographical complexity
(Stein 1977; Macchiusi and Baker 1991). Different microhabitat structure and substrate have been
shown to affect the foraging behaviour and growth rate of juvenile fish (Chan et al. 1997; Peake 1999;
Nguyen and Crocker 2006; Scharf et al. 2006).
There was no overall effect of habitat complexity (substrate) on foraging preference in
Mogurnda mogurnda. Females, however, showed a preference for simple substrates while males
showed none. There was no interaction effect of sex on treatment or hunger level, indicating this was
a general trend across the various experimental groups. It is unclear why females should display this
preference but males should not. (Abrahams and Dill 1989) reported a variety of responses in guppies
depending on their sex and the predation risk level, and other studies have shown a wide range of
responses to variation in resource abundance from no effect of food density (Cerri and Fraser 1983;
Fraser and Huntingford 1986; Holbrook and Schmitt 1988) to a positive correlation (Gilliam and Fraser
1987). It possible that females could have been at a stage in their reproductive cycle where they were
producing eggs and the higher energetic expenditure led them to a foraging strategy that maximised
the number of food items. Food items were easier to find in the simple habitat, thus lowering searching
and handling time and increased the number of successful food strikes. As the preference was not
fact that individual variation should be taken into account in such studies; foraging behaviour can be
influenced by many factors such as predation risk, social interactions, reproductive state, and even
phylogenetic factors (reviewed in Perry and Pianka 1997).
There was a clear interaction between the effect of hunger level and food density on the
number of food strikes between substrates. There were no feeding strikes for treatment 1 (as there
was no food present) but there were also no strikes for fish fed to satiation one hour before the trial.
The difference lay between treatment 1 and the other treatments, as there were large numbers of
feeding strikes in fish that had not been fed for three days before the trial in treatments where food
was present. Hunger has been shown to be a driving force of foraging effort (Beukema 1968; Sih
1982). My result supports the fact that M. mogurnda showed no substrate preference rather than they
were simply non-responsive to the experiment and disinterested in feeding.
In a recent study, Webster and Hart (2004) showed that hunger-motivated three spined
sticklebacks Gasterosteus aculeatus prefer to forage in complex substrates. My results are not
equivalent to their findings, as hungry sticklebacks showed a marked preference for complex
microhabitats when food densities were equal between substrates, and when density was greater on
the complex substrate. Consistent with my results, however, satiated fish showed no preference, nor
was there a preference for complex substrates when food density was higher on simple substrates.
Webster and Hart (2004) explained their results as adaptive because increased substrate complexity
provides more refuges for prey, and is associated with higher macroinvertebrate densities (Kelly 1996;
Scharf et al. 2006). Increased prey density around complex microhabitat has, however, also been
explained by a reduction in predator’s ability to capture prey by decreasing manoeuvrability or the
visibility (Babbitt and Jordan 1996; Babbitt and Tanner 1997; Baber and Babbitt 2004). This has been
supported by studies in the laboratory (Nelson and Bonsdorff 1990; Gotceitas and Brown 1993;
Gotceitas et al. 1995; Fraser et al. 1996; Lindholm et al. 1999; Bartholomew et al. 2000) and in the
field (Tupper and Boutilier 1995; 1997; Beukers and Jones 1997; Heck et al. 2003). As such, prey
encounter and capture rates are usually negatively correlated to the increase in structural complexity
of the microhabitat (Savino and Stein 1982; Russo; 1987; Diehl 1988; Tatrai and Herzig; 1995 Mayer
et al. 2001).
A reverse preference or no effect of substrate could therefore also be predicted as the
increased handling and searching times of smaller prey in more complex structures could decrease
the associated value of a food item. Optimal foraging theory predicts that allocation time spent
searching for and handling prey will change the energy return for different sized food items (Werner
times are equal for all sizes (Milinski 1982). Searching and handling times are rarely equal, however,
and are certainly not in structurally different microhabitats. In addition, prey employing cryptic
colouration have been shown to prefer darker substrates e.g., the lotic mayfly larvae (Tikkanen et al.
2000), and tide pool sculpins (Houtman and Dill 1994). If the diet of M. mogurnda includes cryptic
species of prey that choose habitat patches to maximise camouflage and avoid predators, the further
increase in searching and handling time may reduce a preference for complex substrates.
Habitat aside, several species of generalist feeding fish have been shown to selectively
choose food based on their size (e.g., Ivlev 1961; Galbrait 1967; Brooks 1968; Hall et al 1970; Werner
and Hall 1974; Munk 1992; Forrester et al. 1994; Luo et al. 1996; Hyvarinen and Huusko 2006).
Recent foraging models are more comprehensive, and awareness of the complexities of the demands
of an organism has increased. Empirical studies are developing to further explore foraging theory.
Ultimately, the degree to which habitat complexity mediates predation may depend on prey
antipredator responses, the predator posing the threat, and the quality and quantity of the habitat
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