NOTE
Alien mammals and the trophic position of the
lesser grison (Galictis cuja) in Argentinean
Patagonia
Miguel Delibes, Alejandro Travaini, Sonia C. Zapata, and Francisco Palomares
Abstract: The lesser grison (Galictis cuja), a poorly known mustelid of southern South America, has been described as
a catholic feeder living close to water. Consequently, the potential exists for competition with the recently introduced
American mink (Mustela vison). Nonetheless, like most ferrets the lesser grison can be a specialized mammal hunter,
in which case it could benefit from introduced mammalian prey (the European hare, Lepus europaeus, and European
rabbit, Oryctolagus cuniculus). The diet and trophic position of the lesser grison in Argentinean Patagonia are described
in order to discuss the potential effects of introduced mammalian species on grison populations. Mammals (mainly native rodents and introduced lagomorphs) occurred in 100% of the feces and represented 95% of the biomass consumed.
In one site, European hares and rabbits reached a combined 66% of biomass consumed, whereas in the other site rabbits were scarce or absent and lagomorphs represented 35% of the biomass consumed. We postulate that lesser grison
populations in Patagonia have benefited from the introduced lagomorphs. On the other hand, lesser grisons frequently
occupied river valleys, but did not rely on aquatic or semiaquatic prey. Hence, hunting-habitat preference segregates
this mustelid from the American mink (Mustela vison), which consumes a catholic diet in which aquatic species usually predominate.
Résumé : Le petit grison (Galictis cuja), un mustélidé peu connu de la partie australe de l’Amérique du Sud, a été décrit comme un consommateur à régime très diversifié habitant près de l’eau. Conséquemment, les risques de compétition avec le vison d’Amérique (Mustela vison), introduit récemment, sont possibles. Néanmoins, comme la plupart des
furets, le petit grison peut s’avérer un chasseur de mammifères spécialisé et, dans ce cas, il pourrait tirer profit de
l’introduction de mammifères susceptibles de lui servir de proies (le lièvre européen, Lepus europaeus, et le lapin de
garenne, Oryctolagus cuniculus). Nous décrivons ici le régime alimentaire et la position trophique du petit grison dans
la Patagonie argentine afin d’examiner les effets potentiels des espèces de mammifères introduites sur les populations.
Les mammifères (surtout des espèces indigènes de rongeurs et des espèces introduites de lagomorphes) apparaissaient
dans 100 % des fèces et représentaient 95 % de la biomasse consommée. À un site, les lièvres européens et les lapins
composaient ensemble 66 % de la biomasse consommée, alors qu’à d’autres sites, les lapins étaient rares ou absents et
les lagomorphes comptaient pour 35 % de la biomasse consommée. Nous en déduisons que les populations du petit
grison en Patagonie ont tiré profit de l’introduction des lagomorphes. Par ailleurs, les petits grisons occupent souvent
des vallées de rivières, mais ils ne dépendent pas de proies aquatiques ou semi-aquatiques. Donc, leurs préférences
d’habitat pour établir leurs territoires de chasse assurent à ces mustélidés la ségrégation d’avec les visons d’Amérique
(Mustela vison) dont le régime alimentaire très diversifié est habituellement dominé par des espèces aquatiques.
[Traduit par la Rédaction]
Introduction
There are numerous examples of successfully naturalized
species having strong ecological effects on native fauna and
flora (Herbold and Moyle 1986; Ebenhard 1988; Lever
1994), although there are some contrary opinions
(Simberloff 1981). Most frequently, the effects of invasive
species are detrimental, such as extinctions or reductions in
abundance of native plants and animals caused by introduced predators and competitors (Herbold and Moyle 1986;
Received 8 April 2002. Accepted 19 November 2002. Published on the NRC Research Press Web site at http://cjz.nrc.ca on
21 February 2003.
M. Delibes1 and F. Palomares. Department of Applied Biology, Estación Biológica de Doñana, Consejo Superior de
Investigaciones Científicas, Avenida María Luisa s/n, 41013 Sevilla, Spain.
A. Travaini and S.C. Zapata. Centro de Investigaciones de Puerto Deseado, Universidad Nacional de la Patagonia Austral and
Consejo Nacional de Investigaciones Científicas y Técnicas, Avenida Lotuffo s/n, 9050 Puerto Deseado. Santa Cruz, Argentina.
1
Corresponding author (e-mail: mdelibes@ebd.csic.es).
Savidge 1987). In fact, introduced exotic species are considered the second most important cause of extinctions, immediately behind habitat loss (Macdonald and Thom 2001).
Occasionally, however, successful colonization by exotic
prey may be beneficial to some native predators (Jones
1979).
Usually, the effects caused by invasive species are deduced from observed changes in native species following the
colonization. Nevertheless, it should be very useful to predict the effects of exotic colonizers before the expected invasion. One possibility is to make inferences about the future
ecological relationships of the introduced and native species,
based on a detailed knowledge of their natural history. For
instance, two potential competitors for food resources could
occupy similar feeding niches in allopatry but they would
differ in sympatry following a successful introduction
(Clode and Macdonald 1995). Unfortunately, basic information about their natural history is lacking for most species in
Argentinean Patagonia, a region where many plants and animals have been successfully introduced (Brailovsky and
Foguelman 1991).
The lesser grison (Galictis cuja) is a poorly known Neotropical polecat whose distribution ranges from southern Peru
throughout Uruguay, Paraguay, central Chile, and Argentina
south to Santa Cruz province (Redford and Eisenberg 1992;
M. Delibes, A. Travaini, S.C. Zapata, and F. Palomares, unpublished data). Very few precise data are known about the
foods and habitats used by this mustelid, but two alternative
life histories, based on mostly anecdotal evidence, have been
reported.
On the one hand, grison species are said to live in a great
variety of habitats with water and good cover, and to be active predators, feeding on reptiles, small birds, and rodents
(Redford and Eisenberg 1992). This portrait makes the
lesser grison a potential “victim” competitor of the introduced American mink (Mustela vison), which is spreading
in Patagonia (Pagnoni et al. 1986), lives near water (Allen
1983), is rather generalist in its diet (Dunstone 1993),
weighs almost the same as the lesser grison (lesser grisons
weigh about 1.6 kg, on average (Redford and Eisenberg
1992); male minks average 1.2 kg in Britain but can exceed
2 kg in Canada (Banfield 1974; Dunstone 1993)), and is an
aggressive competitor (Maran et al. 1998; Sidorovich et al.
1999).
On the other hand, because of energetic constraints imposed by their long thin body plan (Brown and Lasiewski
1972; King 1989), many weasels and polecats show a tendency towards specialization on terrestrial mammals as prey.
A few identified prey suggest that in Chile the lesser grison
can be a hunter of small mammals and secondarily of reptiles (Ebensperger et al. 1991). This description would make
the lesser grison less dependent on water in most of its
range, and it probably relies on introduced mammalian prey,
such as European hares (Lepus europaeus) and European
rabbits (Oryctolagus cuniculus).
Our objective is to describe the previously unknown diet
and trophic position of the lesser grison and, in this light,
discuss the effects that the recently introduced American
mink, as well as introduced prey (European hare and European rabbit), could have on its populations.
Material and methods
The diet of the lesser grison was studied through fecal
analysis in sites free of mink. Feces were collected mainly
between November 1994 and February 1995 in two localities, 40 km apart, in the Andean Precordillera in northwestern Patagonia (70°30′–71°30′ W, 39°30′–40°20′ S). The area
includes steep elevational (from 600 to 1400 m above sea
level) and rainfall (from 300 to 1000 mm) gradients
(Pearson and Pearson 1982). The climate is cold, with frosts
throughout most of the year and frequent snowfalls in winter. Topographically the area consists of plains at 700–900 m
above sea level bisected by steep rugged valleys and large
rivers. Vegetation is a mixed steppe of grass and shrubs
dominated by Mulinum spinosum, Schinus molle, Festuca
palescens, Poa sp., and Senecio sp. The localities are Pilolil
(n = 58 feces) and Quilquihue (n = 139 feces). A small sample of 8 feces was collected in various otherplaces. Pilolil is
a high rocky site close to temporary water courses, whereas
Quilquihue is a rocky place in a river valley.
Grison feces were collected in small and medium-sized
caves at the base of rock piles and cliffs. This species builds
latrines, where as many as 15–20 feces can be collected. The
aspect and placement of the grison feces make it unlikely
that they will be confused with those of other carnivores
from the area. In spite of this, we installed a cage trap
(baited with a European hare carcase and blue cheese) by
one of the largest latrines, capturing (and immediately releasing) an adult lesser grison on the third night of trapping.
It defecated inside the cage and its feces were similar to
those collected outside.
Feces were dried and weighed before their components
were separated by hand. Mammals were identified to the
species level based on their hair and bone remains, using a
reference collection and local keys (Chehébar and Martín
1989; Pearson 1995). Birds were identified to the class level,
reptiles to the subclass level, and invertebrates to the order
or family level.
To make our results comparable with most descriptions
of carnivore diets, we express them as frequency of occurrence of each prey type. In addition, we calculated the ingested biomass from undigested matter, using conversion
factors to correct biases caused by differential ingestion
and digestion (Lockie 1959). We consider this a more realistic way to describe the diet of any predatory species. A
conversion factor is the consumed mass of a prey item divided by the dried mass of undigested matter of the same
prey in the feces (Lockie 1959). Usually, conversion factors
are estimated by means of feeding tests, but for lesser
grisons we used those developed by Lockie (1959, 1961)
for red foxes (Vulpes vulpes) and pine martens (Martes
martes). These were used later for other carnivore species,
such as the European badger (Meles meles) (Martín et al.
1995), the Andean hog-nosed skunk (Conepatus chinga)
(Travaini et al. 1998), the Patagonian grey fox (Pseudalopex
griseus) (Zapata et al. 1998), and the American mink
(Ferreras and Macdonald 1999).
We compared the diet of the lesser grison with the known
diet of the American mink in southern South America and in
another area where it has been introduced (England). The
foraging habitats of the grison and mink were determined
Table 1. Prey identified from 205 lesser grison (Galictus cuja) feces collected during summer 1994–1995 in various
localities in northwestern Argentinian Patagonia.
Mammals
Marsupialia
Marmosa pusilla
Edentata
Zaedyus pichiy
Chaetophractus villosus
Rodentia
Akodon spp.
Auliscomys micropus
Calomys musculinus
Ctenomys haigi
Eligmodontia typus
Euneomys sp.
Irenomys tarsalis
Notiomys edwardsii
Oryzomys longicaudatus
Phyllotis darwinii
Reithrodon auritus
Unidentified rodent
Lagomorpha
Ungulatesa
Birds
Eggs
Reptiles
Iguanidae
Eggs
Invertebrates
Insects
Scorpions
Pilolil (n = 58)
Quilquihue
(n = 139)
Miscellaneous
locations (n = 8)
Total (n = 205)
EPB
FO
EPB
FO
EPB
FO
EPB
FO
(94.3)
(100)
(95.4)
(100)
(100)
(100)
(95.2)
(100)
0.9
1.7
—
—
—
—
0.2
0.5
2.8
2.0
(55.3)
26.8
6.4
0.2
—
3.7
1.2
0.4
0.2
1.4
5.8
8.8
0.4
34.7
0.7
2.2
0.1
2.9
0.7
(78.4)
32.4
7.9
0.7
—
7.9
1.4
1.4
0.7
3.6
9.4
11.5
1.4
20.9
1.4
2.9
0.7
—
—
(67.8)
30.3
10.5
—
16.1
—
—
—
—
—
10.9
—
—
32.1
—
—
—
—
—
(91.0)
40.6
14.1
—
21.6
—
—
—
—
—
14.6
—
—
22
—
—
—
2.1
1.5
(49.2)
22.5
5.0
0.1
0.6
3.2
0.9
0.3
0.2
1.2
5.3
9.4
0.5
41.6
0.6
2.5
0.1
2.0
0.5
(69.3)
26.3
5.9
0.5
1.5
5.9
1.0
1.0
0.5
2.9
8.8
12.2
2.9
33.2
1.5
3.9
0.5
—
—
(27.0)
7.1
—
—
1.9
1.4
—
—
—
0.6
3.2
12.1
0.7
65.9
0.5
3.5
—
—
—
(46.6)
10.3
—
—
3.4
1.7
—
—
—
1.7
6.9
15.5
6.9
60.3
1.7
6.9
—
1.4
—
3.4
—
2.1
0.0
4.3
0.7
—
—
—
—
2.0
0.0
3.9
0.5
0.8
0.0
13.8
1.7
0.1
—
2.9
—
—
—
—
—
0.3
—
5.9
0.5
Note: Diet is expressed as estimated percent biomass (EPB) and frequency of occurrence in feces (FO) of each type of prey for
each locality and in total (n is the number of feces). Values for large taxonomic groups are shown in parentheses.
a
Includes Sus scrofa and Ovis aries.
from the habitats used by their most frequent prey, according to the literature.
Results
One hundred per cent of the grison feces (n = 205) included
mammalian remains, and mammals represented 95.2% of the
biomass consumed (Table 1). This pattern was observed in
all localities. Birds and lizards and their eggs appeared occasionally. Invertebrate remains occurred in 6.4% of the samples, but in some cases they could have been prey of the
grisons’ prey (rodents of the genus Akodon, which occurred
very frequently, are important arthropod predators; Redford
and Eisenberg 1992). The importance of invertebrates in the
consumed biomass was negligible (Table 1).
Rodents (49.2% in biomass) and lagomorphs (41.6% in
biomass) were the most common mammalian prey. Marsupials occurred in lower proportions, while the scarce remains
of edentates and ungulates (including sheep) suggest that
grisons occasionally consume carrion.
At least 13 species of rodents (Akodon longipilis,
A. xanthorhinus and A. iniscatus are pooled in Table 1) were
represented in our samples. Most of them (Akodon spp.,
Reithrodon auritus, Auliscomys micropus) may be characterized as burrowing species that make tunnels or live among
broken rocks, thus resembling voles and small rabbits of the
Holarctic region (Redford and Eisenberg 1992). Only Oryzomys
longicaudatus seems to require permanent water (Redford
and Eisenberg 1992).
Both species of introduced lagomorphs, the European hare
and European rabbit, occurred in the samples, but unfortunately we were not able to differentiate the two species in
most of the feces (Chehébar and Martín 1989). Lagomorphs
predominated in Pilolil and native rodents in Quilquihue and
in the scattered sample, probably because, at the time of the
study, rabbits still had not arrived (or were very scarce) in
the latter places (Funes 1996).
When the diets of the grison and the mink were compared,
important differences were noted: the former eats mammals
almost exclusively, while the latter has a catholic diet, including more aquatic prey, such as fish, crustaceans, and
160
Can. J. Zool. Vol. 81, 2003
Table 2. Percent of all occurrences in feces of different types of prey and trophic diversity (food-niche
breadth, B) for lesser grisons and American minks (Mustela vison) in several localities.
Lesser grison
n
Mammals
Birds
Reptiles
Fish
Crustaceans
Other arthropods
B
American mink
Pilolil
(Argentina)a
Quilquihue
(Argentina)a
Thames River
(England)b
Southern
Chilec
Nahuel-Huapi
(Argentina)d
58
81.1
5.0
2.5
0.0
0.0
11.4
1.48
139
90.6
2.5
4.4
0.0
0.0
2.5
1.21
184
52.3
23.7
0.0
17.7
4.0
2.3
2.75
109
40.1
2.6
0.0
8.4
48.1
1.3
2.54
60
15.2
11.0
1.2
13.4
36.0
23.2
4.22
Note: n is the sample size (number of feces). Food-niche breadth was estimated as B = 1/Σ pi2, where pi is the
proportion of each category in the diet (Levins 1968).
a
From this paper.
b
From Ferreras and Macdonald (1999).
c
From Medina (1997).
d
From Previtali et al. (1998).
water birds (Table 2). Consequently, food-niche breadth is
always higher for the mink (Table 2).
Discussion
The lesser grison has a wide distribution and uses a great
variety of habitats, from tropical forests in northern Argentina to the Andes Mountains in Chile and cold steppes in
Patagonia (Redford and Eisenberg 1992). It can be expected
that its diet reflects the availability of prey in this variety of
habitats, but we are confident that our results are valid, at
least for cold Patagonia, where minks have just been introduced.
A potential criticism concerns the number of individual
animals contributing to the samples. We are aware that feces
collected from each locality could belong to a limited number of individuals, but the high coincidence of the results
from localities separated by tens of kilometres suggests that
they are highly representative of the diet of the species in
northwestern Patagonia.
At present, the most important prey of the lesser grison in
Patagonia are native rodents and introduced lagomorphs. Although the hunting strategy of the lesser grison has not been
described, it is likely that it is consistent with the general behaviour of all long-bodied mustelids, which have maximized
their ability to search through the burrows and runway systems of rodents and lagomorphs (King 1989). In fact, in the
study area we have observed lesser grisons getting into the
entrances of the underground systems of rodents. In Chile
the species was trained to drive chinchillas (Chinchilla lanigera)
out of rock piles (Redford and Eisenberg 1992), similar to
the way in which domestic ferrets (Mustela furo) have been
trained since time immemorial to flush European rabbits
from their warrens (Lever 1994).
Ebenhard (1988) underlined the fact that very little is
known about the role of introduced birds and mammals as a
new food source for native predators; our data indicate that
the lesser grison in Patagonia could have benefited from the
introduced lagomorphs. The European hare was introduced
in Argentina at the end of the 19th century (Grigera and
Rapoport 1983) and spread all over the country, occupying all
habitats in Patagonia. European rabbits invaded western Argentina from Chile around 1950 (Howard and Amaya 1975) and
were arriving in the study area, following the river valleys
(Funes 1996), at the time we were studying the grison’s diet.
As predicted, European hares and rabbits are important prey
for the lesser grison. For grisons, rabbits can be expected to
be more rewarding prey than hares, as they reach higher
densities locally and make complex warrens. Also, rabbits
were the most frequently consumed prey of grisons in the
small sample (21 feces and prey remains) from central Chile
analysed by Ebensperger et al. (1991). However, lagomorphs
were also important prey of grisons in Quilquihue, where
hares were present, but there were probably no rabbits, at
the time of the study (Funes 1996).
The description of the lesser grison as a generalist predator living close to rivers does not fit the habits of the species
in Patagonia. To be sure, lesser grisons do occupy river valleys, such as the study location in Quilquihue, but even there
they do not rely on aquatic or semiaquatic prey. Thus, habitat preference seems to establish an ecological segregation
between the lesser grison and the American mink, as the latter is a semiaquiatic species (the permanence of water seems
to be the variable that most determines habitat suitability for
the species; Allen 1983). Dependence on water is clear in all
studies of the mink’s diet in South America and other places
(Table 2), making it more likely that the mink competes with
the huillín (Lutra provocax) or the coastal feline otter (Lutra
felina) than with the lesser grison. Besides, the diet of the
introduced American mink is always characterized by high
diversity, as shown in Table 2 and in the numerous Eurasian
examples recently quoted by Jçdrzejewska et al. (2001).
However, 42% of the mink feces analysed by Previtali et
al. (1998) and 40% of those analysed by Medina (1997) in
Chile included mammals (the prey species were not mentioned in the studies). Similarly, introduced minks eat many
rabbits on the coast of Scotland (Birks and Dunstone 1984),
and they use rabbit warrens extensively in Britain (Macdonald et al. 1999). This means that locally (i.e., close to permanent water), the American mink could affect populations of
Delibes et al.
rodents and rabbits, and secondarily those of the grison.
However, permanent-water locations are rather scarce on the
Patagonian steppe, except for large rivers, lakes, and along
the coast (where minks have just become established in Argentina and Chile; Medina 1997). Hence, in most of the
lesser grison habitats in Patagonia, conditions seem to be unsuitable for minks.
We are aware that our results suggest only some of the
several potential effects of introduced species on grison populations (Macdonald and Thom 2001). Ecological relationships are far more complicated in nature, and indirect effects
can be expected. For instance, the introduction of sheep,
hares, and rabbits could harm native rodents, and indirectly
grisons. Moreover, introduced prey could benefit the culpeo
fox (Pseudoalopex culpeo; Crespo 1975), which preys on
grisons (M. Delibes, A. Travaini, S.C. Zapata, and F. Palomares,
unpublished data; evidence of this kind of intraguild predation among carnivores is growing (e.g., Palomares and Caro
1999)), in which case the spread of new prey species could
also, indirectly, harm the lesser grison. Also, the American
mink is a victim of Aleutian disease (Dunstone 1993), and
Mañas et al. (2001) recently suggested that this viral disease
can also affect other species of mustelids, possibly including
the lesser grison. More research is required to explore all of
these possibilities.
Acknowledgments
We are very grateful to M. Funes, O. Monsalvo, J.A.
Donázar, O. Ceballos, and F. Hiraldo for help with fieldwork. Our thanks are extended to Gerardo Aleñá, Gabriel
and Miguel Anz, and Victor Soleño, who permitted us to
work on their lands. Logistic support was provided by the
Centro de Ecología Aplicada del Neuquén (Argentina), especially by A. del Valle. P. Ferreras, R.C. Soriguer, E. Revilla,
V. Nams, and A. Rodríguez kindly revised previous drafts of
the manuscript. S. Conradi helped in many ways. Financial
support was provided by the Instituto de Cooperación
Iberoamericana of the Ministerio de Asuntos Exteriores (Spain)
through the Programa de Cooperación Científica con
Iberoamérica. A. Travaini had a postdoctoral fellowship from
the Ministerio de Educación y Ciencia (Spain).
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