Passage through bird guts causes interspecific differences in seed germination characteristics

Abstract

Summary 1. Seed germination characteristics are often modified after seeds are ingested by frugivores. Factors that are intrinsic either to the plant or to the frugivore's digestive tract are responsible for the great variation observed in germination response. 2. Our objectives were to determine whether and how the seed germination patterns of five common western Mediterranean plant species are affected by seed passage through the guts of their major dispersers, and to elucidate the mechanism by which such patterns are changed. 3. We used captive birds ( Turdus merula and Sylvia melanocephala ) to obtain ingested seeds and compared their germination rate (speed) and germinability (final percent germination) with those of controls (uningested, pulp-removed seeds), controlling for seed age, size and source. Germination was monitored for 2 years in an experimental garden. We evaluated the possible changes in seed traits after ingestion by measuring weight and coat thickness, and by observing seed coat sculpture. 4. Rate of seed germination, but not germinability, changed in all species after gut treatment. The greatest effect was in Osyris , in which germination was much enhanced. A great acceleration of germination, which is likely to translate into a seedling size advantage, was also found in Asparagus . In the other three species tested, germination was slower for ingested than for control seeds. 5. For Rubus and Rubia seeds, we found a different germination response depending upon the frugivore species tested. A different degree of seed coat scarification caused by differences in gut retention time, chemical and/or mechanical abrasion probably account for such responses. 6. In three of the species ( Osyris , Rubia and Phillyrea ), seed weight decreased after gut treatment. Such weight loss was not caused by any change in coat thickness, but may have been because of the scarification and consequent alteration of the seed coat structure. 7. The five Mediterranean species studied germinate when rains are most likely to fall (mostly autumn and spring). The different speed of germination promoted by gut treat- ment within frugivores may increase the probability that seeds can recruit successfully at a given time and in a given place. 8. This study suggests that frugivores contribute to the heterogeneity in germination characteristics not only within plant populations but also within plant communities, each frugivore species having a particular effect on the seeds of each plant consumed.

Full text

Functional
Ecology
2001
15
, 669–675
© 2001 British
Ecological Society
669
Blackwell Science, Ltd
Passage through bird guts causes interspecific differences
in seed germination characteristics
A. TRAVESET,*† N. RIERA* and R. E. MAS‡
Institut Mediterrani d’Estudis Avançats (CSIC-UIB),
*
C/Miquel Marqués 21, 07190 Esporles, Mallorca, Balearic
Islands, Spain and
‡
C/Bisbe Sastre 26, 07011 Palma de Mallorca, Balearic Islands, Spain
Summary
1.
Seed germination characteristics are often modified after seeds are ingested by
frugivores. Factors that are intrinsic either to the plant or to the frugivore’s digestive
tract are responsible for the great variation observed in germination response.
2.
Our objectives were to determine whether and how the seed germination patterns
of five common western Mediterranean plant species are affected by seed passage
through the guts of their major dispersers, and to elucidate the mechanism by which
such patterns are changed.
3.
We used captive birds (
Turdus merula
and
Sylvia melanocephala
) to obtain ingested
seeds and compared their germination rate (speed) and germinability (final percent
germination) with those of controls (uningested, pulp-removed seeds), controlling for
seed age, size and source. Germination was monitored for 2 years in an experimental
garden. We evaluated the possible changes in seed traits after ingestion by measuring
weight and coat thickness, and by observing seed coat sculpture.
4.
Rate of seed germination, but not germinability, changed in all species after gut
treatment. The greatest effect was in
Osyris
, in which germination was much enhanced.
A great acceleration of germination, which is likely to translate into a seedling size
advantage, was also found in
Asparagus
. In the other three species tested, germination
was slower for ingested than for control seeds.
5.
For
Rubus
and
Rubia
seeds, we found a different germination response depending
upon the frugivore species tested. A different degree of seed coat scarification caused
by differences in gut retention time, chemical and/or mechanical abrasion probably
account for such responses.
6.
In three of the species (
Osyris
,
Rubia
and
Phillyrea
), seed weight decreased after gut
treatment. Such weight loss was not caused by any change in coat thickness, but may have
been because of the scarification and consequent alteration of the seed coat structure.
7.
The five Mediterranean species studied germinate when rains are most likely to fall
(mostly autumn and spring). The different speed of germination promoted by gut treat-
ment within frugivores may increase the probability that seeds can recruit successfully
at a given time and in a given place.
8.
This study suggests that frugivores contribute to the heterogeneity in germination
characteristics not only within plant populations but also within plant communities,
each frugivore species having a particular effect on the seeds of each plant consumed.
Key-words
: Frugivory, Mediterranean shrubs, seed ingestion by birds, seed traits
Functional Ecology
(2001)
15
, 669–675
Introduction
The effect that the ingestion of fruits by vertebrate
frugivores has on seed germination has received con-
siderable attention (reviewed in Traveset 1998 and
Traveset & Verdú 2001). Many studies show that ger-
mination is more successful after seeds pass through
the digestive tract of frugivores (mostly birds). How-
ever, such enhancement is not universal, and several
(usually uncontrolled) factors (e.g. retention time in
guts, seed size, seed age, seed source) cause the great
variation found in germination response. The condi-
tions under which germination tests are performed
also influence germination success, and contrasting
results are often found when comparing treated and
†Author to whom correspondence should be addressed.
E-mail: ieaatv@clust.uib.es
FEC561.fm Page 669 Tuesday, September 4, 2001 11:43 AM

670
A. Traveset
et al.
© 2001 British
Ecological Society,
Functional Ecology
,
15
, 669–675
control (uningested) seeds of the same species under
different conditions (Bustamante
et al
. 1992, 1993;
Figueiredo & Perin 1995; Yagihashi
et al
. 1998; Trave-
set
et al.
2001). Most studies are performed in the lab,
testing germination in Petri dishes usually in growth
chambers, yet these favourable conditions may some-
times obscure significant differences between treat-
ments (Herrera 2000; Traveset
et al
. 2001).
We studied the seed germination response of five
plant species, commonly found in western Mediterra-
nean scrubland, after passage through the digestive
tracts of their major avian dispersers. As germination
responses, we considered (i) length of seed dormancy
(
T
0
, defined as the time elapsed from sowing until first
germination); (ii) rate of germination (defined as the
speed at which seeds germinate, i.e. the number or
proportion of seeds germinated in certain periods of
time), and (iii) germinability (or final proportion of
germination, i.e. proportion of seeds that germinate in
a period long enough to obtain total germination).
Our first goal was to test the hypothesis of Izhaki &
Safriel (1990) that frugivores modify the length of seed
dormancy and rate of germination and that by doing
so, they may be spreading the chances of seedling
survival over time, mainly in environments with unpre-
dictable rain patterns (e.g. the Mediterranean). Our
second objective was to identify the mechanism(s) by
which seed germination responses are modified after
passage through a frugivore. The specific questions of
the study were the following:
1.
To what extent does seed passage through avian
guts affect germination responses?
2.
Does the same plant species respond similarly to
different frugivore species?
3.
Do the germination responses of plants to avian
ingestion depend upon seed traits such as size or
seed coat thickness?
4.
In plant species where birds modify germination
responses of the ingested seeds, is there any seed
trait (in particular weight and coat thickness) that
changes significantly when compared with unin-
gested (control) seeds?
Materials and methods
During the summer of 1997, we collected fruits from
five bird-dispersed plant species common in western
Mediterranean scrubland:
Rubus ulmifolius
(Rosaceae),
Rubia peregrina
(Rubiaceae),
Asparagus acutifolius
(Liliaceae),
Osyris alba
(Santalaceae) and
Phillyrea
spp. (these included the species
latifolia
and
angustifolia
,
which usually hybridise, Oleaceae). Hereafter, we will
refer to them by generic names only. The fruits of all
species were collected from a minimum of 10 individuals.
Recently collected fruits were fed to captive sedent-
ary birds caught in Mediterranean scrubland. These
birds were
Turdus merula
(blackbird) and
Sylvia
melanocephala
(Sardinian Warbler), the most import-
ant dispersers of these plants in the Balearic Islands.
Between three and 10 individuals of each species were
tested for each plant. Birds were kept in separate cages
(60
×
60
×
60 cm) and were maintained on a diet con-
sisting of beetle larvae, commercial food for insectivo-
rous birds and fruits of various fleshy-fruited species.
At the time of experimental seed passage, only fruits of
the particular species studied were given to them to
avoid any seed-mixing effect on the results. A random
sample of the fruits was depulped manually and the
seeds served as controls. Therefore, we tested the direct
effect of seeds passing through the guts rather than the
effect of pulp removal. Different studies have already
demonstrated the important role that frugivores play
by freeing seeds from the germination inhibitors
present in the pulp (e.g. Temple 1977; Izhaki & Safriel
1990; Barnea
et al
. 1991; Bustamante
et al
. 1993;
Fukui 1995, 1996; Yagihashi
et al
. 1998; Engel 2000;
Traveset
et al
. 2001). All fruits were collected on the
same day and from the same site to avoid any seed age
or source effect on germination.
Five hundred seeds of each species were used as con-
trols, with the exception of
Phillyrea
, for which only
150 seeds could be obtained. For
Rubus
and
Rubia
, 500
seeds defecated by each bird species were gathered,
whereas for
Osyris
,
Phillyrea
and
Asparagus
, a suffi-
cient number of seeds to test was obtained only with
blackbirds (warblers did not usually swallow these
larger fruits; see Table 1). For these three species, we
had 500 seeds of only the ‘blackbird’ treatment.
All seeds were kept in paper bags in dark and dry
conditions until the date of sowing (6 October 1997).
Seeds were planted in pots (12 cm in diameter) previ-
ously filled with horticultural mixture. We spread a
total of 50 seeds on each pot and covered them with a
2 mm layer of the potting mixture. Pots, randomly
assorted in four trays, were covered with a lid of 1 cm
wire mesh to prevent seed predation by rodents. The
trays were placed on the floor of an experimental garden
and surrounded by a fence to reduce possible animal
disturbances. Pots were watered periodically (with the
same amount of water), only when rain was scarce
(mainly in summer) to avoid seed death through desic-
cation. An unknown number of seeds was removed
Table 1. Average seed diameter (± SD) of the species tested
and fresh weight of control (uningested) seeds and seeds
ingested by either Turdus (blackbird) or Sylvia (warbler)
(n = 25 seeds)
Species
Seed diameter
(mm)
Seed weight (mg)
Control Turdus Sylvia
Rubus 1·98 ± 0·03 3 ± 13 ± 14 ± 1
Rubia 3·45 ± 0·07 20 ± 5 15 ± 5** 15 ± 5**
Asparagus 4·37 ± 0·05 44 ± 6 45 ± 6–
Osyris 5·72 ± 0·04 92 ± 14 81 ± 9** –
Phillyrea 4·21 ± 0·06 50 ± 9 44 ± 8* –
*P < 0·05; **P < 0·01 (indicate differences between
treatment and control).
FEC561.fm Page 670 Tuesday, September 4, 2001 11:43 AM

671
Effect of gut
treatment on seed
germination
© 2001 British
Ecological Society,
Functional Ecology
,
15
, 669–675
from different pots by ants soon after being planted.
When ants were detected, an insect trapping adhesive
(Tanglefoot Co., Gran Rapids, MI, USA) was placed
around the edge of each pot. At the end of the experi-
ment, all remaining ungerminated seeds were counted
in each pot. Pots from which most seeds had been
removed were eliminated from the analyses, leaving a
total of 109 pots.
Seedlings began emerging 1 month after planting;
from that time, they were counted every week or every
month (depending upon season). The experiment
ended in the spring of 1999 (5 May), when no seedlings
had emerged for about a month. Germinated seeds
were removed as they were counted.
To identify the mechanisms by which seed germina-
tion may be modified after gut passage through a fru-
givore, the possible changes that seeds suffer in both
weight and coat thickness were evaluated. A sample of
25 fresh seeds from each treatment and control group
was weighed individually (to the nearest mg). Seed
coat thickness was measured from a minimum of 10
seeds from each treatment and control by means of a
dissecting microscope connected to a computer using
Optimas 6·1 software (Media Cybernetics, L.P., Silver
Spring, MD, USA). Further observations on seed coat
sculpture were made using a scanning electron micro-
scope (Hitachi S-530).
 
Differences in germination rate between treated and
control seeds were examined for each species with a
repeated-measures analysis of variance (

),
whereas dormancy length, final germination percentage
and an additional measure of germination rate used by
several authors (
T
50
, or time elapsed from sowing until
50% germination, e.g. Barnea
et al
. 1991) were com-
pared by means of

s. Each pot was considered
as a replicate. The angular (arc-sin square root) trans-
formation was used to normalize the proportions.
Weights of ingested and control seeds were compared
using a one-way

for each plant species, whereas
coat thickness (six measurements per seed) was com-
pared with a nested

, using treatment as a fixed
effect and seed as a random effect nested within treat-
ment. All means are accompanied by their standard
errors unless otherwise indicated. Data were analysed
using

V. 10·0 (SPSS, Chicago, IL, USA).
Results
 
Rubus ulmifolius
Rubus ulmifolius began germinating soon after plant-
ing (towards the end of autumn), continuing through-
out the winter until the spring of the following year
(Fig. 1). The rate of germination varied significantly
among treatments in the

(
F
2,14
=
4·3,
P
=
0·035, time
×
treatment not significant), although
it was not until day 94 (8 January 1998) that differences
became apparent.
T
0
did not vary among treatments
(
F
2,14
=
0·87,
P
=
0·44), although
T
50
did (
F
2,14
=
3·68,
P
=
0·05). A Tukey’s test showed that control seeds
germinated significantly faster than those ingested
by warblers (Table 2). In contrast, seeds ingested by
blackbirds did not differ from control seeds, with the
exception of day 115, when germination in the former
was significantly lower (Fig. 1). Regardless of differ-
ences in germination rate, final percentage germina-
tion was not significantly greater for control than for
ingested seeds (
F
2,14
=
1·15,
P
=
0·34).
Rubia peregrina
Most germination occurred during the winter of 1998.
Only a minor fraction of seeds germinated during
winter and spring of the following year (after day 220;
Fig. 1). A highly significant effect of seed ingestion by
birds on germination rate was found in this species
(
F
2,19
=
6·47,
P
=
0·007, time
×
treatment not signific-
ant), although the number of germinated seeds at the
end of the experiment did not differ significantly
among treatments (
F
2,19
=
2·11,
P
=
0·15; Fig. 1).
Seeds ingested by blackbirds began germinating
later (
T
0
:
F
2,19
=
2·86,
P
=
0·08) and
T
50
was also longer
for ingested than for control seeds (
F
2,19
=
6·38,
P
=
0·008) (Table 2). Warblers did not significantly
affect either
T
0
or
T
50
; however, seeds ingested by
these birds germinated faster than those ingested by
blackbirds (Tukey’s test).
Asparagus acutifolius
Most germination took place from January to April
1998 (between days 121 and 191; Fig. 1). Seed ger-
mination of this species appeared to be marginally
affected by ingestion in the

(
F
1,14
=
3·51,
P
=
0·08, time
×
treatment not significant), seeds
passed through blackbirds germinating faster than
uningested seeds.
T
50
was also greater for control
than for ingested seeds, although
T
0
was very similar
between the two (Table 2). In winter and spring of
1999 (days 479–595; Fig. 1), a smaller fraction of seed-
lings emerged, although controls caught up with the
ingested seeds. Final percentage germination did not
differ between treatment and control (
F
1,14
=
1·54,
P
=
0·24).
Osyris alba
The influence of avian ingestion on germination
responses in this plant was the greatest of all species
tested. Germination rate was much greater for seeds
passed through blackbirds (F1,17 = 103·84, P < 0·001,
significant time × treatment interaction; Fig. 1). T0
did not differ between control and ingested seeds
FEC561.fm Page 671 Tuesday, September 4, 2001 11:43 AM

672
A. Traveset et al.
© 2001 British
Ecological Society,
Functional Ecology,
15, 669–675
Table 2. Dormancy length (T0) and time elapsed to 50% of total germination (T50) for control (extracted manually from the
pulp) and treated [ingested by either Turdus merula (blackbirds) or Sylvia melanocephala (warblers) ] seeds of five species from
the Mediterranean scrubland. Data are means ± SD; number of pots in parentheses. Each pot contained 50 seeds
Fig. 1. Cumulative proportion of seeds per pot germinating from each treatment and for each of the study species. Mean and
standard error bars (calculated from the original data) are shown. Note different horizontal scales for each species.
Plant species
T0 (days) T50 (days)
Control Turdus Sylvia Control Turdus Sylvia
Rubus 72 ± 13·6 (6) 63 ± 5·9 (4) 74 ± 16·2 (7) 100 ± 5·3 (6) 106 ± 10·5 (4) 116 ± 13·8* (7)
Rubia 57 ± 2·4 (6) 70 ± 14·3 (9) 66 ± 8·7 (7) 94 ± 0·09 (6) 109 ± 11·9** (9) 99 ± 5·3 (7)
Asparagus 129 ± 13·2 (7) 131 ± 8·4 (9) – 215 ± 113·0 (7) 185 ± 111·8a (9) –
Osyris 153 ± 26·0 (9) 132 ± 26·5 (10) – 387 ± 134·2 (9) 140 ± 8·5*** (10) –
Phillyrea 138 ± 0·0 (3) 138 ± 0·8 (4) – 144 ± 9·82 (4) 186 ± 24·5* (3) –
*P ≤ 0·05; **P < 0·01; ***P < 0·001; aP = 0·08 (indicate differences between treatment and control).
FEC561.fm Page 672 Tuesday, September 4, 2001 11:43 AM

673
Effect of gut
treatment on seed
germination
© 2001 British
Ecological Society,
Functional Ecology,
15, 669–675
(F1,17 = 2·96, P = 0·10) but T50 was significantly greater
for the former (F1,17 = 33·91, P < 0·001; Table 2). Most
germination took place during the winter of 1998; only
about 10% of seedlings emerged in the winter and
spring of the following year (days 460 –552; Fig. 1).
The final germination percentage was also much higher
for ingested than for control seeds (F1,17 = 50·67,
P < 0·001). Due to these differences, non-germinated
seeds of each treatment were dissected at the end of the
experiment to examine whether they were intact or
aborted. A large fraction (> 85%) of seeds were filled
with endosperm and appeared viable, and there were
no differences between control and ingested seeds
(t = −0·56, df = 10, P = 0·59).
Phillyrea
Phillyrea spp. began germinating in February 1998
and continued to do so gradually until the end of
spring, although never reaching 40%. A very small
fraction germinated the following winter (days 511–
527; Fig. 1). The germination curves of control and
ingested seeds were not significantly different in the
 (F1,5 = 2·91, P = 0·15, time × treatment not
significant). Seed ingestion by blackbirds did not influ-
ence T0, although T50 was longer for ingested than for
control seeds (F1,5 = 8·95, P = 0·03; Table 2). At the
end of the experiment, however, no differences were
apparent between ingested and control seeds, as a
greater number of ingested seeds had emerged during
the second winter (Fig. 1).
 ,   

Seed weight was influenced by passing through the
digestive tracts of birds in at least three of the five spe-
cies tested (Table 1). In Rubia, Osyris and Phillyrea,
control seeds always weighed slightly more than defe-
cated seeds. This weight loss might be attributed to the
abrasion that seed coat can suffer when passing
through the bird’s digestive tract. However, this change
in weight does not explain consistently the differences
in germination rates found between treated and con-
trol seeds, as germination patterns in Osyris were the
opposite of those in Rubia or Phillyrea.
When we compared seed coat thickness, we found
no significant differences between control and treat-
ments (Rubus: 0·20 ± 0·05 vs 0·23 ± 0·05 mm (Turdus)
and 0·25 ± 0·05 (Sylvia); Rubia: 0·06 ± 0·01 vs
0·06 ± 0·01 mm (Turdus) and 0·05 ± 0·01 (Sylvia);
Asparagus: 0·04 ± 0·01 vs 0·04 ± 0·004 mm; Osyris:
0·18 ± 0·03 vs 0·16 ± 0·02 mm; Phillyrea: 0·22 ± 0·04 vs
0·20 ± 0·03 mm). This indicated that the loss in seed
weight in Rubia, Osyris and Phillyrea after ingestion
was not because their seed coat became thinner.
Scanning electron microscopy of the seed coat
sculpture added relatively little information to the
effect of birds’ gut treatment on germination. Slight
differences were observed in only Osyris and Asparagus,
the two species in which germination was enhanced
after passing through blackbirds. In both species, the
coats of ingested seeds appeared somewhat modified
(more dilated or with scars and crevices) compared
with those of controls.
Discussion
The passage of seeds through the digestive tracts of
vertebrates – particularly birds – is important in deter-
mining their future germination behaviour. Although
it did not affect dormancy length, seed ingestion by
birds modified, to a large or small extent, the speed at
which seeds germinated in all five species examined.
The greatest effect was in Osyris, a result consistent
with that reported by Izhaki & Safriel (1990), who
tested the same species with different frugivorous
birds. Osyris was also the only species that showed a
significant effect of ingestion on final percentage ger-
mination; however, most ungerminated seeds were still
viable after almost 2 years, suggesting that they might
have germinated eventually had the experiment lasted
longer. Our ongoing experiments with this species
planted in the field will assess this possibility. The
ingestion of Asparagus seeds greatly accelerated their
germination as well, and at the end of the germination
period of the first year, the number of seedlings that
emerged from defecated seeds was double that from
uningested seeds. This is likely to translate into a seed-
ling size advantage and, in turn, into a greater prob-
ability of survival – although we would need data on the
mortality factors (dessication, herbivory etc.) acting
on those seedlings to test this hypothesis. Also, a faster
germination implies less exposure to seed predators,
which may represent an important mortality factor for
some species. In the other three species (Rubus, Rubia
and Phillyrea), seed passage through the digestive
system of a bird had the opposite effect: it reduced the
germination rate. The greatest ‘delaying’ effect was for
Phillyrea: during the first year, only 21% of ingested
seeds germinated compared with 38% of the controls.
A germination delay in ingested seeds is uncommon
(see reviews in Traveset 1998; Traveset & Verdú 2001),
and the mechanism by which it occurs remains unknown.
Consistent with the results reported by Barnea et al.
(1991) from the eastern Mediterranean, we found that
even though most of the plant species grow in the same
habitat, their seeds respond differently to ingestion by
birds. Even the same species of bird can affect the
germination patterns of closely related species quite
differently (reviewed in Traveset 1998; Mas & Traveset
1999). One possible reason is that seed coats are
affected differently (scarified) after exposure to grind-
ing in a bird’s gizzard. The degree of such scarification
probably varies greatly among plant species, perhaps
as a function of seed coat thickness, texture or sculp-
ture. Likewise, the effect on a particular seed species
may vary among frugivores (Traveset 1998). In the two
FEC561.fm Page 673 Tuesday, September 4, 2001 11:43 AM

674
A. Traveset et al.
© 2001 British
Ecological Society,
Functional Ecology,
15, 669–675
cases in which we compared the effect of blackbirds
with that of Sardinian Warblers, we found signific-
ant differences in germination rates: ingestion by the
former affected Rubia but not Rubus seeds, whereas
the opposite was found with ingestion by the latter.
These differences can sometimes be attributed to dif-
ferent retention times in the guts (Barnea et al. 1991;
Murphy et al. 1993), but not always (Barnea et al. 1990).
Gut passage times reported for blackbirds (15–74 min;
Barnea et al. 1991) encompass those reported by
warblers (Traveset, unpublished). All these studies
assume that longer retention times result in increased
abrasion of the seed coat. However, no data exist to
support this hypothesis. Alternatively, some gizzards
may cause a greater scarification than others, regardless
of seed retention time. Besides mechanical abrasion,
the chemical composition of food ingested along with
seeds (with variable water content, pH, secondary
compounds) may also modify seed coat traits, either
directly or indirectly, by affecting retention time in the
guts (e.g. Levey & Karasov 1994; Murray et al. 1994;
Witmer 1996; Cipollini & Levey 1997).
A striking result of this study was that seeds that had
passed through the birds’ guts lost weight, at least in
three of the five species studied. For Rubia, this has
been confirmed with fruits gathered in 2000 from
another site and tested with other bird individuals
(Traveset et al. unpublished). This contrasts with the
only other study (Paulsen 1998) in which seed weight
of control and ingested seeds was compared. Paulsen
found that seeds of Sorbus aucuparia increased in mass
by ≈ 9% after passing through the digestive system of
thrushes (Turdus spp.), and explained this weight
change by water uptake after the mechanical abrasion
of the seed coat; that water uptake accelerated germina-
tion, which led to a greater seedling growth (Paulsen
1998). From our results, we could explain weight loss
in ingested seeds if we observed that the seed coat had
been abraded, either becoming thinner or more porous.
However, we found no evidence that coat thickness
had changed in any of the species, observing a slightly
altered structure only in Osyris and Asparagus. An
alternative explanation for the smaller seed weight of
ingested seeds compared with controls is that birds
selected fruits containing smaller seeds. To test this, we
compared the seed diameter of these species in the con-
trol and treated groups. We rejected this hypothesis, as
we found significant differences only for Phillyrea, and
in the opposite expected direction [i.e. control seeds
appeared to be slightly smaller (4·64 ± 0·07 mm, n = 53)
than ingested seeds (4·88 ± 0·05 mm, n = 93)]. This
supports the idea that seed coat does not become thinner
after ingestion but becomes lighter.
If the weight loss of ingested seeds in Rubia, Osyris
and Phillyrea reflects greater seed coat abrasion, we
might expect that germination rate increased in these
species; but as mentioned above, we found this result
only in Osyris. In the other two species, germination
rate was reduced. In the case of Rubia, this is consist-
ent with Izhaki & Safriel’s (1990) report for Rubia
tenuifolia, a species closely related to R. peregrina, but
inconsistent with the results of Barnea et al. (1991).
Seed size is another trait that influences the effect of
ingestion by frugivores on future germination. A
recent meta-analysis (Traveset & Verdú 2001) showed
that large seeds are more likely to be affected positively
(with greater germination percentages in ingested
seeds compared with uningested ones) than small
seeds. Our findings are consistent with this pattern, as
Osyris and Asparagus are the species with the largest
seeds and are, in turn, the only ones that increased
their germination rate after being passed through
birds. If retention time in the gut is important in influ-
encing seed coat abrasion, we might expect that small
seeds are more likely to be abraded than large ones,
as the former are often retained for longer periods in
an animal’s digestive tract than the latter (e.g. Garber
1986; Levey & Grajal 1991; Gardener et al. 1993;
Izhaki et al. 1995). However, seed coat traits that may
ultimately determine the likelihood of being abraded
are not necessarily associated with seed size. In our
study species, for instance, there was no relationship
between seed size and coat thickness.
The five species examined in this study germinate
when rains are likely to fall (mostly autumn and
spring), although such rainfall is rather unpredictable
in Mediterranean ecosystems. The different speed of
seed germination that ingestion by frugivores usually
promotes in these environments (Izhaki & Safriel
1990; Barnea et al. 1991; Mas & Traveset 1999; Trave-
set et al. 2001; this study) may increase the chances
that seeds can establish and survive as seedlings within
those seasons. Under some circumstances, seedling
survival will be greater if seeds take several weeks to
germinate, whereas in others, an early germination will
be more beneficial. Ingestion by different frugivorous
species, each having a particular effect on germination
performance, may represent an even greater increase in
the chances of seedling survival (Izhaki & Safriel 1990).
We need more data on the stage from seed to seedling
to assess whether seed passage through frugivores’
guts is adaptive in these environments. The possibility
that the passage of seeds of different plant species
through the guts of different birds leads to heterogeneity
in germination characteristics, not only within plant
populations but also within plant communities, cer-
tainly deserves further exploration.
Acknowledgements
We are indebted to Javier Rodríguez for his assistance
in the field and laboratory, to Ferràn Hierro for his help
with the scanning electron microscope photographs, and
to Beatriz Morales-Nin for letting us use her Optimas
software. We are also grateful to Gene Schupp and an
anonymous reviewer for their valuable comments on
an earlier version of the manuscript. This work was sup-
ported by project DGICYT PB97-1174 financed to A.T.
FEC561.fm Page 674 Tuesday, September 4, 2001 11:43 AM

675
Effect of gut
treatment on seed
germination
© 2001 British
Ecological Society,
Functional Ecology,
15, 669–675
References
Barnea, A., Yom-Tov, Y. & Friedman, J. (1990) Differential
germination of two closely related species of Solanum in
response to bird ingestion. Oikos 57, 222–228.
Barnea, A., Yom-Tov, Y. & Friedman, J. (1991) Does inges-
tion by birds affect seed germination? Functional Ecology
5, 394–402.
Bustamante, R.O., Grez, A., Simonetti, J.A., Vásquez, R.A.
& Walkowiak, A.M. (1993) Antagonistic effects of frugivores
on seeds of Cryptocarya alba (Mol.) Looser (Lauraceae):
consequences on seedling recruitment. Acta Oecologica 14,
739–745.
Bustamante, R.O., Simonetti, J.A. & Mella, J.E. (1992) Are
foxes legitimate and efficient seed dispersers? A field test.
Acta Oecologica 13, 203–208.
Cipollini, M.L. & Levey, D.J. (1997) Secondary metabolites
of fleshy vertebrate-dispersed fruits: adaptive hypotheses
and implications for seed dispersal. American Naturalist
150, 346–372.
Engel, T.R. (2000) Seed dispersal and forest regeneration in a
tropical lowland biocoenosis (Shimba Hills, Kenya). PhD
thesis. University of Bayreuth, Germany.
Figueiredo, R.A. & Perin, E. (1995) Germination ecology of
Ficus luschnathiana drupelets after bird and bat ingestion.
Acta Oecologica 16, 71–75.
Fukui, A.W. (1995) The role of the brown-eared bulbul
Hypsypetes amaurotis as a seed dispersal agent. Research
on Population Ecology 37, 211–218.
Fukui, A.W. (1996) Retention time of seeds in bird guts: costs
and benefits for fruiting plants and frugivorous birds. Plant
Species Biology 11, 141–147.
Garber, P.A. (1986) The ecology of seed dispersal in two
species of Callitrichid primates (Saguinus mystax and
Saguinus fuscicollis). American Journal of Primatology 10,
155–170.
Gardener, C.J., McIvor, J.G. & Janzen, A. (1993) Passage of
legume and grass seeds through the digestive tract of cattle
and their survival in faeces. Journal of Applied Ecology 30,
63–74.
Herrera, C.M. (2000) Flower-to-seedling consequences of
different pollination regimes in an insect-pollinated shrub.
Ecology 81, 15–29.
Izhaki, I., Korine, C. & Arad, Z. (1995) The effect of bat
(Rousettus aegyptiacus) dispersal on seed germination
in eastern Mediterranean habitats. Oecologia 101, 335–
342.
Izhaki, I. & Safriel, U.N. (1990) The effect of some Mediter-
ranean scrubland frugivores upon germination patterns.
Journal of Ecology 78, 56–65.
Levey, D.J. & Grajal, A. (1991) Evolutionary implications of
fruit-processing limitations in cedar waxwings. American
Naturalist 138, 171–189.
Levey, D.J. & Karasov, W.H. (1994) Gut passage of insects by
European starlings and comparison with other species.
Auk 111, 478 –481.
Mas, R.E. & Traveset, A. (1999) Efectes de la ingestió per
ocells sobre la germinació i la dispersió de dues espècies
pròximes de Solanum. Butlletí de la Societat d’Història
Natural de les Balears 42, 69–77.
Murphy, S.R., Reid, N., Yan, Z.G. & Venables, W.N. (1993)
Differential passage time of mistletoe fruits through the
gut of honeyeaters and flowerpeckers. Effect on seedling
establishment. Oecologia 93, 171–176.
Murray, K.G., Russell, S., Picone, C.M., Winnett-Murray,
K., Sherwood, W. & Kuhlmann, M.L. (1994) Fruit laxa-
tives and seed passage rates in frugivores: consequences for
plant reproductive success. Ecology 75, 989–994.
Paulsen, T.R. (1998) Turdus spp. and Sorbus ocuparia seeds:
effect of ingestion on seed mass, germination and growth.
Proceedings of the 22nd International Ornithology Congress,
Durban, South Africa. Ostrich 69, 301.
Temple, S.A. (1977) Plant–animal mutualism: coevolution with
dodo leads to near extinction of plant. Science 197, 885 – 886.
Traveset, A. (1998) Effect of seed passage through vertebrate
frugivores’ guts on germination: a review. Perspectives in
Plant Ecology, Evolution and Systematics 1, 151–190.
Traveset, A., Riera, N. & Mas, R. (2001) The ecology of seed
dispersal of a fruit–color polymorphic species, Myrtus
communis, in the Balearic Islands. Journal of Ecology 89
(in press).
Traveset, A. & Verdú, M. (2001) A meta-analysis of gut
treatment on seed germination. Frugivores and Seed
Dispersal: Ecological, Evolutionary and Conservation Issues
(eds D. Levey, M. Galetti & W. Silva). CAB International,
Wallingford, UK (in press).
Witmer, M.C. (1996) Do some bird-dispersed fruits contain
natural laxatives? A comment. Ecology 77, 1947–1948.
Yagihashi, T., Hayashida, M. & Miyamoto, T. (1998) Effects
of bird ingestion on seed germination of Sorbus commixta.
Oecologia 114, 209–212.
Received 12 January 2001; revised 15 May 2001; accepted
24 May 2001
FEC561.fm Page 675 Tuesday, September 4, 2001 11:43 AM