Spotlight on Zoo Science
April 21,
2003
Choosy Horses of Assateague
Ever
since
Marguerite Henry published Misty of Chincoteague in
1947, countless children have read the story of wild horses
that inhabit Assateague, a barrier island extending 33 miles
along the Virginia and Maryland coasts. The ancestors of
these feral horses probably arrived in the 1600s, when colonists
grazed horses there in an attempt to avoid taxes on the mainland.
Today, the natural beauty of the island is protected—state park, national seashore, and the Chincoteague National Wildlife Refuge cooperate to manage the wildlife—and to balance the needs of the feral horses with the protection of the native plants and animals. Biologists determined that each side of the island, divided at the state line by a fence that keeps the Maryland and Virginia herds separate, could support a total of 150 horses without damaging the fragile ecosystem.
National Zoo research scientist
David Powell studied the social behavior of the Assateague
horses as part of his doctoral
dissertation work at the University of Maryland. He was interested
in mate choice, one of the driving forces of evolution. Most
often, biologists have looked at mate choice from a female
perspective, because females, particularly in mammals, were
assumed to have more at stake in reproduction. Males were believed
to serve primarily as sperm donors and have less at stake,
so biologists had long believed
males
to be less choosy.
The Assateague horses roam the island in harem bands containing one breeding male, one to ten mares, and their recent offspring. Powell’s interest focused on the role social dominance plays among mares. Within each harem band, a dominance hierarchy exists that seems to be related to a mare’s tenure in the harem and to her age. Bonds among harem females are commonly lifelong and most often outlive the tenure of a stallion.
The stallion is not always the most dominant animal. Several stallions may come and go through the females’ lifetimes. When young males are two to three years old, they leave their natal bands and form bachelor groups until they are ready to capture young females dispersing from their own natal bands or challenge and depose a harem stallion. Because dominance correlates with age and tenure, new stallions are often subordinate to some of the mares in the harem band.
Eva Seligsohn, studying the Assateague horses in the 1980s as a Ph.D. student at the University of Connecticut, found that dominant mares were more likely to give birth and produce a foal that survived to independence. Powell relates, “I wanted to determine if I could find patterns that might explain why dominant mares had a reproductive advantage.”
Powell,
shown at left watching horse behavior,
hypothesized that males could be choosing dominant females
as their mates or that females were competing among themselves
for access to breeding opportunities. He spent two breeding
seasons from March to September observing and collecting
data on social interactions between male and female horses
in an effort to answer his question.
Powell made three predictions about how stallions might behave if they were exercising mate choice for dominant mares. First, if stallions were more interested in dominant mares, they might spend more time in close proximity to them. The second prediction is that if males find high-ranking mares attractive, stallions should direct relatively more sexual behavior towards them. Third, stallions may be more likely to investigate urine and feces left by dominant mares than that of subordinate mares.
Urine and feces contain metabolites (by-products)
of reproductive hormones. Many mammals (but not humans) have
a vomeronasal organ in the roof of their mouths. This organ
is believed to have chemosensory abilities. When encountering
an interesting scent or substance, many mammals perform a
behavior called flehmen, in which they raise their heads,
open their mouths, and curl their lips back. Some animals
even take small amounts of the substance into their mouths
and move their heads in such a way as to get that substance
closer to the roof of the mouth where it can be analyzed
by the vomeronasal organ.
Powell thinks that if stallions were using the mares’ urine or feces to track their reproductive cycles, then the males should show more interest in the urine or feces of dominant mares. Because sniffing is an olfactory behavior and flehmen is a chemosensory behavior, males might use either as a way to track reproductive cycles when investigating urine and feces.
If the reproductive skew were due
to female competition, then Powell predicted that dominant
mares would try to stay
close to the stallion. In addition, dominant mares should
be observed harassing or disrupting mating between the stallion
and subordinate mares.
In the data Powell collected during the 1997 and 1998 breeding seasons, he found no differences in stallions’ proximity to dominant versus subordinate mares. However, stallions were more likely to respond to feces deposited by dominant mares than to that deposited by subordinate mares. Although the difference was not statistically significant, stallions directed more low-intensity sexual behavior, like sniffing females, toward dominant mares, and they directed significantly more high-intensity sexual behavior, such as mounts, towards dominant mares as well.
Powell found no evidence that dominant
mares tried to put themselves close to stallions, and perhaps
passively limit
other females’ access to him. There was, however, evidence
of mating harassment in the form of biting, chasing, or kicking
by females.
Over the two breeding seasons, Powell observed 33 mounts, and in ten cases, another group member harassed the mating pair. In seven of those cases, the female dominant to the mating female was responsible for the harassment, and in all of those cases the dominant mare successfully in broke up the mating and prevented breeding.
While Powell’s research revealed that mares compete
amongst themselves for matings and that dominant females
often succeed in preventing others from mating, the extent
of copulation harassment, roughly 30 percent of observed
matings, does not seem high enough to prevent subordinates
from breeding altogether.
Some aspects of stallion behavior suggest that they are more interested in dominant mares as mates. Their interest in feces of dominant mares may indicate that they track their reproductive cycles more closely than those of subordinate mares.
The finding that stallions showed more high- and low-intensity sexual interest in dominant mares also seems to suggest that they find dominant mares more attractive. However, it is possible that their choice of partner when they perform high-intensity sexual behavior, like mounting, could be influenced by dominant mares who could interfere.
Further studies are needed to tease apart the roles of male mate choice and female-female competition in feral horses. Male preference for a dominant mare has been demonstrated previously in domestic horses, but it is not yet clear why dominant mares are more attractive. The fact that dominant mares are often older could mean that they carry genes that promote survival in the harsh barrier-island environment.
David Powell’s research was supported by a Smithsonian
Pre-Doctoral Fellowship, the Abbott and Nelson Funds of the
Smithsonian, the Biology of Small Populations Training Grant
at the University of Maryland, Earthwatch, Sigma Xi, and
the Department of Conservation Biology at the National Zoological
Park.
