Table Of ContentSITE-BASED MATING SYSTEM IN A TROPICAL HARVESTMAN
BY
GISELLE MORA
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1991
.
ACKNOWLEDGEMENTS
I have benefited during the writing of this thesis and
my graduate studies from the encouragement, comments and
criticisms of many people. It has been with their help that
I was able to successfully complete this project. I would
like to thank the members of my committee, H. Jane Brockmann,
Jonathan Reiskind, John Sivinski, John Anderson and William
Eberhard for their valuable comments throughout the various
stages of my research and for their criticisms during the
final revision of this work. Special thanks go to my adviser
H. Jane Brockmann. She has contributed immensely to
organizing the material, to improving the quality of this
thesis and in many other aspects of my education. Linda Fink
has also contributed with ideas, comments and has been a
supportive friend throughout. Julio Arias helped with
statistical analyses.
The Department of Zoology, University of Florida, has
been a stimulating and supportive place for conducting my
graduate studies. I would like to thank Dr. Frank Nordlie,
Mrs. Lynda Everitt and Mrs. Carol Binello for logistic
support
I have also received much support and encouragement
while at the Smithsonian Tropical Research Institute in
Panamd. Drs. Egbert Leigh and Donald Windsor acted as
Smithsonian advisors of this project. Egbert Leigh has
supported this research since 1983 and gave me the confidence
for continuing during the darkest moments of my field work.
I thank him for not letting me forget the value of natural
history. Hector Barrios assisted me in the field during the
1987 season. Argelis Rom^n and Georgina de Alba provided
logistic support and help with many matters during my stay in
Panama. Rayneldo Urrutia coordinated many administrative
aspects of my residence at BCI. I overlapped with many
researchers during my residency at BCI; many of them
contributed with observations, criticisms and friendship. In
special, I appreciate the encouragement I always received
from Phil DeVries.
The support of my family and friends have brightened my
years as a student. I thank my entire family for their
support and love. In special, I thank my parents for always
letting me try to be all what I thought I could be. I also
thank Juan Carlos Vargas, Julio Arias, Clara Sotelo, and
Catherine Langtimm for their friendship and support
especially during the writing of this thesis.
My husband Henry put up with me, fed me, and kept his
and my spirits high when the writing of this thesis got
rough. I thank him for never losing his sense of humor and
for helping me finish.
I have been supported by a Noyes Predoctoral Fellowship
from the Smithsonian Tropical Research Institute, by teaching
and research assistantships from the Department of Zoology,
University of Florida and by a scholarship from the Consejo
Nacional para Investigaciones Cientificas y Tecnol6gicas,
Costa Rica.
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii
ABSTRACT vii
INTRODUCTION 1
BACKGROUND AND METHODS 5
Natural History and Reproductive Biology of Opiliones 5
Reproductive Biology of Zyaopachylus albomarainis 12
Study Site 14
Study Population 16
General Methodology 16
Definitions 18
NESTS AND MALE SUCCESS 22
Nests and Male Success: General Descriptions 23
Patterns of Nest Use 28
Patterns of Nest Success 32
Male Life History and Behavior: General Descriptions 39
Components of Male Success 43
Why do Males Abandon Nests? 50
Nests or Males? A Male Switching Experiment 52
Discussion: Patterns of Nest and Male Success 55
FEMALE ASSOCIATIONS WITH NESTS AND FEMALE SUCCESS 80
Types of Visits and Female Condition 81
Patterns of Female Associations 87
Female Associations and Nest/Male Measurements 97
Components of Female Success 99
Discussion: Females Association with Nests 103
DO FEMALES ASSOCIATE WITH MALES OR NESTS? 120
Methods 121
Results: Female Residency Before and After the
Switchings 123
Discussion: Females Associations With Nests 123
v
THE NEST SITES 127
Rainfall 129
Fungus 138
Predators 143
What Accounts for the Differences in Nest Success? 146
SITE-BASED MATING SYSTEM OF Zyaopachvlus albomarainis 162
The Mating System of Z.- albomarainis 165
The Evolution of the Site-based Mating System of Z.-
albomarainis 176
LITERATURE CITED 181
BIOGRAPHICAL SKETCH 189
vi
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
SITE-BASED MATING SYSTEM IN A TROPICAL HARVESTMAN
By
Giselle Mora
December, 1991
Chairperson: H. Jane Brockmann
Cochair: Jonathan Reiskind
Major Department: Zoology
Zyaopachylus albomarainis Chamberlin (Arachnida,
Opiliones: Gonyleptidae), a tropical harvestman, is the only
arachnid exhibiting paternal care. Males construct mud nests
on trees that females visit to court the resident male and
oviposit. I conducted a two-year field study on Barro
Colorado Island, Panama to identify and evaluate components
of reproductive success and to characterize the mating system
of a natural population of this species. The mating system
of these harvestmen is polygynandrous, with females laying
eggs in several male nests and males collecting eggs from
several females. Females mature a few eggs at a time
throughout a breeding season lasting from June to December.
Females exhibited a random pattern of nest visitation but the
vii
distribution of ovipositions was not random. This mating
system resembles those of many substrate-nesting, egg-
guarding fish. It is site-based since both male and female
reproductive success depends upon their association with
successful nesting sites. Breeding success of males was
measured by (a) the number of eggs acquired during his
residency (b) the number of juveniles hatched, and (c) the
number of resident females associated with the male and his
nest. The ability of males to acquire and hold a nest at a
good nesting site accounted for much of the variability in
male breeding success. Male reproductive success was not
correlated with body size. Switching experiments revealed
that resident females continued to associate with the site
regardless of the identity of the male at that site.
Ecological variables, including rain, occurrence of fungal
attack and incidence of predation may account for the success
of some nesting sites over others. I hypothesize that this
mating system evolved from no parental care. A limited
availability of safe oviposition sites may account for the
evolution of male territoriality, male egg-guarding and
female attraction and fidelity to nesting sites.
viii
CHAPTER 1
INTRODUCTION
The harvestman Zyaopachvlus albomarainis (Opiliones:
Gonyleptidae) is the only arachnid exhibiting paternal care
(Mora 1990). Their reproductive behavior is remarkable for
an arachnid. Males guard eggs in nests made out of mud and
tree bark. Besides building a nest, males take over or use
abandoned nests. Females visit nests, court males, oviposit,
and leave the eggs for the male to guard. These behaviors
are unique among opilionids.
I present here the results of a field study that focuses
on the variation in reproductive success of males and females
and the factors that contribute to such variation. The
objectives of the study were to identify and evaluate
components of reproductive success and to characterize the
mating system of a population from Barro Colorado Island,
Panama. My ultimate goal was to gain understanding of
selective pressures that have shaped this mating system. The
importance of this study lies in documenting the reproductive
behavior of the only arachnid with paternal care. It also
allows a test of current ideas on the influence of increased
paternal investment on male and female mate choice and
relative variation in reproductive success.
1
2
The first model explaining the factors contributing to
the variation in reproductive success was proposed by Darwin
(1871). He recognized two types of selective forces acting
on the context of reproduction. Intrasexual selection is the
competition within one sex for individuals of the opposite
sex and intersexual selection is the preferential choice for
mating partners by one sex. Darwin considered intrasexual
selection to be more common and more pronounced in males and
intersexual selection to be an attribute of females.
Although this distinction has proven useful, most biologists
now recognize the difficulty of separating the two (Maynard
Smith 1987).
Trivers (1972) elaborated Darwin's idea by arguing that
the sex with the greatest parental investment will become a
limited resource for the opposite sex. He recognized as
parental investment those activities of an animal that
increase the probabilities of present offspring survival and
reproduction at the expense of the parent's ability to have
additional offspring. Consequently, the sex investing the
least will compete among themselves in order to mate with
individuals of the sex investing the most. Since females
generally produce fewer and energetically more expensive
gametes than males, their parental investment is often
greater. This produces the general pattern of male-male
competition and female mate choice. When a male invests, he
may become a limited resource for females and the pattern may
be reversed. Also, phylogenetic and ecological factors will
