 The
Spotted Hyaena (Crocuta crocuta, Erxleben 1777)
Although the spotted hyaena suffers from a bad public image and many
prejudices, it is a keystone species in African savannah ecosystems, and
a fascinating animal to study.
THE CLAN
clan members at communal den Serengeti spotted hyaenas live in large, stable social groups called
clans, which contain between 30 and 80 animals. Female hyaenas remain
in the clan in which they were born, while males usually disperse to another
clan when they are between 2-3 years old. Thus a spotted hyaena clan is
composed of serveral different female lineages (matrilines), their offspring
and immigrant males from other clans.
THE COMMUTING SYSTEM
Contrary to popular belief, spotted hyaenas are not just scavengers,
but excellent hunters, and most of their food is killed by themselves.
hunting spotted hyaenas An important discovery by the spotted hyaena project was the commuting
system of Serengeti hyaenas. This ranging system has not been described
for any other spotted hyaena population in Africa, and thus is unique
to the Serengeti population.
The commuting system combines a residential life in a defended clan territory
with frequent long-distance (40-80km) foraging trips to the nearest large
migratory herds of wildebeest and zebra.
Thus Serengeti spotted hyaenas regularly move between their home territory
and the migratory herds to feed, and as these movements resemble those
of commuters who travel regularly to a city to work, the foraging behaviour
of Serengeti hyaenas has also been called commuting.
Commuting hyaenas can feed on migratory herbivores such as wildebeest
and zebras throughout the year, no matter where the migratory herds are
located.
Furthermore, by hunting on the large migratory herds, the size of spotted
hyaena clans in the Serengeti is not limited by the low number of resident
herbivores within clan territories, and thus Serengeti clan sizes are
much larger than expected.
As spotted hyaenas are the most numerous large predator in the Serengeti,
the discovery of the commuting system has contributed an essential component
to our understanding of predator-prey-dynamics in the Serengeti ecosystem.
REARING CUBS
Spotted hyaena society revolves around the cubs, and the communal den
in which the cubs are raised. Litter size in the Serengeti is normally
one or two, very occasionally three. In the Serengeti, hyaena cubs are
dependent on their mothers' milk for up to 18 months, and cubs rarely
eat much meat before the age of 6 months.
The milk of Serengeti hyaenas is approximately four times as nutritious
as the milk of cows.
When hyaena mothers leave the clan home territory to commute to feed on
distant migratory herds, their cubs are left at the communal den and do
not receive any nourishment until the mother returns several days later.
two one-week-old spotted hyaena cubs with mother in den A mother with a dependent cub is likely to make 40-50 commuting trips,
walking between 2900-3700 km per year. This is about three times the distance
covered by a migrating wildebeest in the Serengeti ecosystem.
FEMALE DOMINANCE
Among mammalian species, female dominance is rare, but spotted hyaena
society is totally female dominated, with even the top ranking male being
socially subordinate to the lowest ranking female. A major benefit of
social dominance in hyaena society is immediate access to food, thus dominant
female hyaenas are the best fed members of their social group, and this
allows them to have a higher reproductive success than subordinate females.
female spotted hyaena displaces male from food The traditional theory that seeks to explain female dominance in spotted
hyaenas assumes that during evolutionary history females became larger
and more aggressive than males, and thus could dominate them in feeding
situations. Aggression and large size were believed to be caused by high
levels of androgens in females, and it was suggested that female hyaenas
developed their unusual "pseudo-penis" or enlarged clitoris
because of elevated levels of androgens in the female foetus.
Factual evidence does not tally with the assumptions of this traditional
theory. The ingrained prejudice that female hyaenas are "hyper-aggressive"
is not supported by data. Serengeti female hyaenas are not larger than
males, and social dominance among both females and males is not related
to large size. New born female cubs do have elevated androgen levels,
but this is also true for male cubs.
Given these facts, can a new theory be proposed to explain firstly, the
evolution of female dominance in spotted hyaenas, and secondly, the evolution
of the female "pseudo-penis"?
The project has outlined a new theory in which the initial masculinization
of females results from the unusual phenomenon of facultative siblicide
in twin litters. Spotted hyaena cubs are born with their eyes open and
their teeth erupted, ready to fight with their sib.
fighting spotted hyaena siblings Conflicts between siblings develop when it is hard for mothers to obtain
food and they fail to provide both cubs with sufficient milk.
To be primed for sibling conflict, both male and female cubs have unusually
high levels of androgens at birth. Male cubs have elevated levels of testosterone,
while female cubs have elevated levels of the hormone androstenedione.
It is possible that this hormone caused the development of the enlarged
clitoris or "pseudo-penis" in females.
Female spotted hyaenas pay a biological cost when they give birth through
their "pseudo-penis" because one cub in a females's first litter
is often still-born. As the "pseudo-penis" entails a cost, it
should be selected against in evolutionary terms, unless it also imparts
some benefit.
greeting spotted hyaenas showing erected "pseudo-penis" A probable benefit of the "pseudo-penis" is that it has provided
females with control over mating. Due to the position and structure of
the "pseudo-penis", males have no chance of mating successfully
without the full co-operation of the female, and Serengeti male hyaenas
certainly invest a large amount of time and energy in courting females.
Since a special relationship with a female is a non-transferrable ressource,
male hyaenas gain little through aggression. This probably explains why
fighting between male hyaenas has rarely been observed during 14 years
of research in the Serengeti and why conflict amongst males is typically
of low intensity.
When dispersing males enter a new clan they have a low social status.
The social status of an immigrant male gradually improves as males above
him in the social hierarchy die, thus social rank improves with tenure
in the clan, and males can be considered to queue for social status. Males
are not idle while waiting in line for improved social status, they spend
these years developing special relationships with particular females.
|
