THERYA, Diciembre, 2010
Juan-Pablo Gallo-Reynoso1 and Charles Leo Ortiz2
Feral cats abound at Isla de Guadalupe; they forage on birds, mice, and placental tissue
as well as carcasses of northern elephant seals (Mirounga angustirostris), Guadalupe
fur seal (Arctocephalus townsendi), California sea lions (Zalophus californianus) and
stranded cetaceans such as Cuvier’s beaked whale (Ziphius cavirostris). We have found
that feral cats are also drinking elephant seal’s milk, stealing it directly from the teats of
nursing females. The amount of energy obtained this way might be signicant for feral
cats in the northern elephant seal rookeries on the island.
Key words: Feral cats, northern elephant seal, milk, Isla Guadalupe.
Los gatos ferales abundan en la Isla Guadalupe, se alimentan de aves, ratones y
tejido placental, asi como también de los cadáveres de elefante marino (Mirounga
angustirostris), lobo no de Guadalupe (Arctocephalus townsendi), lobo marino de
California (Zalophus californianus) y de cetáceos varados como el zio de Cuvier (Ziphius
cavirostris). Se observó que los gatos ferales también se alimentan de leche de elefante
marino, robándola de las tetas de las hembras que están amamantando. La cantidad de
energía obtenida de esta manera por los gatos asilvestrados puede ser signicativa en las
colonias de elefantes marinos de la isla.
Palabras clave: Gatos asilvestrados, elefante marino del norte, leche, Isla Guadalupe.
While censusing, satellite tagging adult males and rototag tagging weaned pups of
northern elephant seals (Mirounga angustirostris) at Playa Norte, Isla de Guadalupe
(14 - 19 February, 2003), we opportunistically observed the stealing of elephant seal’s
milk by feral cats (Felis catus) and western gulls (Larus occidentalis). Western gulls have
been previously reported stealing milk from female northern elephant seals in several
elephant seal colonies (J. P. Gallo-Reynoso personal observations; Pierotti and Annett
1Centro de Investigación en Alimentación y Desarrollo, Unidad Guaymas. Carretera a Varadero Nacional Km. 6.6. Colo-
nia Las Playitas, Guaymas, Sonora, 85480 México. E-mail: firstname.lastname@example.org
2University of California Santa Cruz, Earth and Marine Sciences A 308. 1156 High Street Santa Cruz, CA 95064. U. S. A.
Feral cats steal milk from northern
208 THERYA Vol.1(3): 207-212
FERAL CATS STEAL MILK FROM NORTHERN ELEPHANT SEALS
1995), and this behavior has also been observed in other seabirds such as the Pale-faced
sheathbill (Chionis alba), feeding on southern elephant seal (M. leonina) milk (Favero
1996). However, this is the rst report that focuses on feral cats stealing milk from
The elephant seal harems at Playa Norte (North beach), during the reproductive
season of 2003, were composed of 182 females, 179 pups, 21 adult males, 23 sub-adult
males, several yearlings, and 1,084 weaners. The total population of elephant seals at
this site was estimated at 2,634 individuals (adding one female for each weaner and
each dead pup). The number of pups born on that beach in 2003 was 1,324 (dead pups
accounted for 61 or 4.6 % of total pups). Thus an equal number of milk bearing females
were available, considering that the mean milk transfer to pups equals 137.7 kg during
four weeks of nursing (Ortiz et al. 1984), these females roughly will be delivering to
suckling pups an equivalent of 182,300 kg of elephant seal milk.
Three different cats were opportunistically observed to steal elephant seal’s milk.
A striped male that was observed on three different nights; the other two, a lactating
female and a black male, were only seen on one occasion each. Although observed only
once, this does not mean that these cats did not steal milk before or after the recorded
observations. These three, and another two smaller, marginal cats were observed around
our campsite. We have more observations of the striped male as this cat was easier to
Observations were done mainly at sundown, but once early in the morning and once
at night using several lamps that allowed us to observe the black cat. The stripped cat
was usually moving between female elephant seals at dusk and dawn, while the black
cat was at the harem at night. These cats avoided female elephant seals positioned in
the center of the harem because they responded more to external threats than peripheral
females. Therefore, all of the milk stealing activity was observed to take place on
peripheral females. Cats that were caught licking a teat were chased by the mother of
the pup, and on several occasions by other female elephant seals that either observed
the cat’s approach, or stealing of milk. These cats were also observed to compete with
seagulls for portions of fresh placentas and decomposing elephant seal carcasses.
To be able to get to a female elephant seal’s teat and milk, the stripped cat approached
the elephant seal pup and harassed it by tapping repeatedly on the head with a paw (claws
extended). The pup would then move away from its mother, sometimes protesting, and
as milk continued to drip out from the female’s teat, the cat would lick it. However, we
never observed a behavior that resembled suckling and we only observed that the tongue
licked the dripping milk but none of the three cats were observed touching the female
with their paws. The effort (time) that feral cats dedicated to searching for a female in
good position once entering the harem, and the time spent stealing milk were measured
(Table 1). We estimated that cats were successful at stealing milk 40.7 % of the total time
that they spent looking for a suitable female from which to steal milk.
A similar behavior was carried out by western gulls; they approached a pup and
pecked on the pup’s head, causing it to retreat, and then positioned their beak sideways
to receive the milk dripping from the teat of the female elephant seal. This was observed
several times during our stay at Guadalupe, always during daylight hours. The perpetrators
were adult gulls and second year chicks.
Juan-Pablo Gallo-Reynoso and Charles Leo Ortiz
What is the reproductive cost for a female elephant seal with such a loss of milk transfer
to the pup? It is difcult to calculate how much milk is actually stolen in this way from
any particular female, or from all the females on the beach. We estimate that if there
were 182 females nursing pups at the end of the reproductive season, the amount of milk
available for pups, and with potential to be stolen, was more than 25,000 kg. Assuming
that the milk transferred from mothers to pups is the same as calculated by Ortiz et al.
(1984) of about 137.7 kg of milk to their pups for the whole nursing period of 28.2 days,
or 4.9 kg / day; and taking into account that the stomach capacity of a cat is around 300
g (Grandage 2003), and assuming that they were successful lling themselves up twice
each day, then the maximum amount of milk available to be stolen each day by the three
cats would be 1.8 kg. The variable that intervenes here in favor of the pups is that cats
do not steal milk from the same females; they stole the milk from a different female each
time. Perhaps the amount of milk stolen from an individual female elephant seal is not
signicant for the development of elephant seal pups or the parental investment and
reproductive success of their mothers. On the other hand, cats will try to steal elephant
seal milk due to its availability and its rich energy content which is a large surplus (with
an energy content of 543.8 Kcal/ 100 g, with 54.4% fat, 9 % protein, 32.8 % water and
0.7 % ash (Le Boeuf and Ortiz 1977)), translating into a signicant gain in their survival.
Cats that performed this behavior were in good condition, highly territorial and actively
excluded other cats from the area.
What would be the cost for pups losing time suckling milk (and kilocalories) to
develop? Pups refrained from nursing for a few minutes will not be deprived from large
quantities of milk. The amount of milk that pups might lose for the time they are deprived
from suckling, ranges from 4.6 kg/ day to 4.9 kg/ day (Ortiz et al. 1984). This amount will
not signicantly affect their development if a given female is only perpetrated once a day.
Even if the cat stole that amount of milk from the same female each day for the nursing
period (about 8.5 kg, or 0.74 x 105 kcal) this amount would still not be signicant.
Pups were not signicantly injured by cats, only a few scratches that were healed
in a few days, compared to the pecking of sea gulls on their heads and eyes that can
pose a threat to the defenseless small pups. Also, cats can not spend much time licking
the female; their scaled tongue alerts the female that something is wrong and she will
respond to it by chasing away the cat or turning her belly down, denying access to her
This situation raises some questions of conservation concern to the northern elephant
seals on the island. The feral cat population on Isla de Guadalupe is quite large, cats
are found in all areas of the island (Keitt et al. 2005). They primarily feed on birds, feral
Cat N Effort to obtain milk (min) Time spent licking (min)
Stripped male 3 2.2 ± 0.4 range: 1.8 - 2.8 1.5 ± 0.1 range:1.3 -1.7
Black male 1 2.7 1.2
Female 1 3.1 0.5
Total 5 2.7 ± 0.4 range: 3.1 – 1.8 1.1 ± 0.4 range: 1.7 – 0.5
Table 1. Observations of
feral cat effort (search for
a suitable female in a good
position), and success to
obtain milk (time licking
the perfused milk) from
female elephant seals.
Data shows the Mean ± SD.
210 THERYA Vol.1(3): 207-212
FERAL CATS STEAL MILK FROM NORTHERN ELEPHANT SEALS
mice (Mus musculus), adults and chicks of Laysan Albatrosses (Phoebastria immutabilis),
other birds; pinniped, cetacean (Cuvier’s beaked whale (Ziphius cavirostris)) and feral
goat carrion (Gallo-Reynoso pers. obs. 1993, 2010; Keitt et al. 2005); they also eat the
placenta of elephant seals, Guadalupe fur seals (Arctocephalus townsendi) and California
sea lions (Zalophus californianus). By stealing milk from this threatened species, feral
introduced cats can pose a direct threat by coming into contact with the female’s body.
The concern is that by licking the teats, entero-bacteria, protozoans and viruses can be
passed in the saliva from cats to female elephant seals with the possibility of an infection.
Diseases that could be caused by feral cats on Isla de Guadalupe pinnipeds should be
sought and evaluated, due to the nding of a northern elephant seal with encephalitis,
infected with toxoplasmosis (Toxoplasma godingii) cysts (Dubey et al. 2004), also
present in other marine mammals (Dubey et al., 2003). Toxoplasmosis is known to be
transmitted by cats (Frenkel et al., 1970); if cats result in a threat to elephant seals, then
it is imperative that they be subject to management by the Reserva de la Biosfera Isla de
Guadalupe (Isla Guadalupe Biosphere Reserve).
We wish to thank R. Passion, D. Larsen, D. Crocker, C. Champagne, J. Hasrick, J. Guillén
and O. Maravilla, who participated on the expedition to Isla de Guadalupe and to E.
Coria for her advice regarding cats and their diseases. These observations were carried
out with permits: SEMARNAP/SGPA/DGVS/7095. 07 Octubre 2002. The manuscript
beneted from the suggestions of J. E. Maldonado and two anonymous reviewers.
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Sometido: 15 abril 2010
Revisado: 12 julio 2010
Aceptado: 9 noviembre 2010
Editor asociado Jesús Maldonado