The Callicam
Current Common Marmoset Research
Current Research - Page 8
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III. Biomedical Research (cont.)
D) Reproductive Constraints
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Subordinate or low ranking male and female marmosets do not reproduce,
however, researchers believe it is because of several reasons
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Subordinate males and females appear not to breed because of the risk of
incest and behavioral limitations whereas subordinate females also have hormonal
constraints
Common marmosets live in social groups in which all group members help to
raise the babies of a single dominant female. In marmoset groups only the
dominant male and female breed whereas lower ranking or subordinate marmosets do
not because their reproduction is repressed. This suppression of reproduction is
the case in many primate and non-primate mammals including yellow baboons.
Dominant marmosets, unlike dominants in other species, however, do not appear to
use harassment as a means to keep the subordinates from breeding.
Researchers have shown that the ovaries of the subordinate female marmosets
are about half the size of the dominant females. Blood samples taken from
subordinates to detail hormone levels also revealed that subordinates do not
ovulate. When the subordinate females are taken out of the presence of a
dominant female and placed on their own they will ovulate. Researchers,
therefore, believe that reproductive suppression in subordinate females is due
to the hormones of the subordinate female. One hormone believed to be involved
is released from the brain and stops the release of hormones stored in
reproductive organs. When researchers gave large doses of this hormone to
subordinate female marmosets they began to ovulate and when the levels of the
hormone were reduced the female stopped ovulating. Researchers believe the lack
of this particular hormone has something to do with the reproductive suppression
of subordinate female marmosets. Researchers have not pinpointed which cues,
visual, behavioral, or olfactory (smell), produced by dominant female marmosets
cause subordinates to stop ovulating and, in turn, stop reproducing.
Like the subordinate female common marmosets, subordinate males also do not
reproduce. Researchers, therefore, wanted to know the causes of male
reproductive suppression. Researchers thought that reproductive suppression of
subordinate males was due to cues given by dominant males or as an incest
avoidance behavior to keep family members from mating. For the study subordinate
males and their fathers were tested either alone or together in a cage and were
joined by another female familiar to them (like their mother or mate) or an
unrelated and non-familiar female. The number of sexual behaviors the males
engaged in was recorded and blood samples were taken to measure hormone levels.
Researchers found that the sons engaged in very low rates of sexual behavior
with familiar females whereas their fathers (the dominant male) engaged in
higher levels. This might imply that the subordinate male does not mate with
familiar females because of the chance of being related. When fathers and sons,
however, were tested individually with unrelated females, both engaged in
approximately the same number of sexual behaviors. This would suggest that
subordinate male marmosets have no problem mating if it is with a non-familiar
female. Finally, the hormone levels were examined and the researchers found no
difference between the levels of fathers and sons. Researchers think that
because subordinate male marmosets do not engage in much sexual behavior with
familiar females and because hormone levels are similar between dominant and
subordinate males that subordinate males are reproductively suppressed to avoid
incest.
Researchers now have evidence to believe that subordinate male common
marmosets do not reproduce as to avoid incest. Subordinate females, however, are
thought to be reproductively suppressed because of hormones. Presently,
researchers are still studying the cause of reproductive suppression in male and
female common marmosets.
Further Reading:
Abbott, D. H., Saltzman, W., Schultz-Darken, N. J., & Tannenbaum, P. L.
(1998). Adaptations to Subordinate Status in Female Marmoset Monkeys.
Comparative Biochemistry and Physiology, 119, 261-274.
Abbott, D. H., Saltzman, W., Schultz-Darken, N. J., & Smith, T. E.
(1997). Specific Neuroendocrine Mechanismd Not Involving Generalized Stree
Mediate Social Regulateion of Female Reproduction in Cooperatively Breeding
Marmoset Monkeys. The Integrative Neurobiology of Affiliation, 807, 219-238.
Baker, J. V., Abbott, D. H., & Saltzman, W. (1999). Social Determinants
of Reproductive Failure in Male Common Marmosets Housed with their Natal Family.
Animal Behaviour, 58,501-513.
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Text by Rebecca Dallwig.
