60
P.P. Molloy et al.
imposed by limited space or conditions is not to breed at all. Ecologically enforced
aggregations can therefore be viewed as individuals making the best of a bad
situation.
When aggregations are the result of active choice by individuals, rather than a
by-product of habitat shortage (Fig. 3.1a ), we can consider the possible adaptive
signifi cance of aggregating by studying costs and benefi ts of breeding near conspecifi cs. The main potential benefi ts of social reproduction fall into two general categories: enhanced reproduction and reduced predation (Table 3.1 ).
3.2.1 Possible Reproductive Benefi ts
A reproductive benefi t of breeding in groups is increased rates of encounter with
potential mates, which can facilitate mate choice (Loiselle and Barlow 1979 ) . This
benefi t has been suggested particularly for lekking birds, such as great snipe,
( Galinago media ) (Höglund and Alatalo 1995 ) . On leks, males display in small
territories that are usually clumped and that provide no resources to females.
Females examine a number of potential mates, mate with one (or more) and leave
the mating arena to have their young (Andersson 1994 ; Reynolds 1996 ) . The costs
of sampling potential mates when in aggregations can also be reduced because
travel time between mates is low. In barking treefrogs, Hyla gratiosa , for example,
females position themselves near choruses of calling males so that they can detect
the calls of multiple males simultaneously (Murphy and Gerhardt 2002 ) . Individuals
at aggregations can also observe and copy the choices of others (Reynolds and Gross
1990 ; Dugatkin and Hoglund 1992 ) . Examples of mate choice copying in fi shes
have been summarized by Reynolds and Jones ( 1999 ) and include gobies (Gobiidae),
damselfi sh (Pomacentridae) and sticklebacks (Gasterosteidae). These advantages
may be particularly great for females, who are conventionally the choosier sex, and
may generate strong preferences by females for aggregated over solitary males
(Alatalo et al. 1992 ) . Easier mate choice may, however, not be a benefi t for females
in all taxa with lek-like mating systems. In marine mammals such as elephant seals,
Mirounga angustirostris , and in ungulates such as Uganda kob, Kobus kob , leks are
arenas for intense male-male competition for access to mates, which reduces considerably the scope for uncoerced mate choice by females (Andersson 1994 ) .
Breeding in dense aggregations may increase mating rates. For example, the
reproductive success of small male marine iguana, Amblyrhynchus cristatus ,
increases with lek size (Wikelski et al. 1996 ) as did that of both male and female
dance fl y, Empis borealis , in mating swarms (Svensson and Petersson 1992 ) .
Similarly, close proximity between individuals may ensure a high fertilisation rate
of the eggs in many aquatic animals that broadcast their gametes into the water
column (e.g. urchins, Levitan 2004 ) . Conversely, reproductive proximity can also
facilitate extra-pair matings in pair-spawning species, which is a benefi t for those
who engage in it, and a cost to males that lose such matings. In colonial birds and
fi shes, for example, sperm competition can be intense, and individuals can lose a
P.P. Molloy et al.
imposed by limited space or conditions is not to breed at all. Ecologically enforced
aggregations can therefore be viewed as individuals making the best of a bad
situation.
When aggregations are the result of active choice by individuals, rather than a
by-product of habitat shortage (Fig. 3.1a ), we can consider the possible adaptive
signifi cance of aggregating by studying costs and benefi ts of breeding near conspecifi cs. The main potential benefi ts of social reproduction fall into two general categories: enhanced reproduction and reduced predation (Table 3.1 ).
3.2.1 Possible Reproductive Benefi ts
A reproductive benefi t of breeding in groups is increased rates of encounter with
potential mates, which can facilitate mate choice (Loiselle and Barlow 1979 ) . This
benefi t has been suggested particularly for lekking birds, such as great snipe,
( Galinago media ) (Höglund and Alatalo 1995 ) . On leks, males display in small
territories that are usually clumped and that provide no resources to females.
Females examine a number of potential mates, mate with one (or more) and leave
the mating arena to have their young (Andersson 1994 ; Reynolds 1996 ) . The costs
of sampling potential mates when in aggregations can also be reduced because
travel time between mates is low. In barking treefrogs, Hyla gratiosa , for example,
females position themselves near choruses of calling males so that they can detect
the calls of multiple males simultaneously (Murphy and Gerhardt 2002 ) . Individuals
at aggregations can also observe and copy the choices of others (Reynolds and Gross
1990 ; Dugatkin and Hoglund 1992 ) . Examples of mate choice copying in fi shes
have been summarized by Reynolds and Jones ( 1999 ) and include gobies (Gobiidae),
damselfi sh (Pomacentridae) and sticklebacks (Gasterosteidae). These advantages
may be particularly great for females, who are conventionally the choosier sex, and
may generate strong preferences by females for aggregated over solitary males
(Alatalo et al. 1992 ) . Easier mate choice may, however, not be a benefi t for females
in all taxa with lek-like mating systems. In marine mammals such as elephant seals,
Mirounga angustirostris , and in ungulates such as Uganda kob, Kobus kob , leks are
arenas for intense male-male competition for access to mates, which reduces considerably the scope for uncoerced mate choice by females (Andersson 1994 ) .
Breeding in dense aggregations may increase mating rates. For example, the
reproductive success of small male marine iguana, Amblyrhynchus cristatus ,
increases with lek size (Wikelski et al. 1996 ) as did that of both male and female
dance fl y, Empis borealis , in mating swarms (Svensson and Petersson 1992 ) .
Similarly, close proximity between individuals may ensure a high fertilisation rate
of the eggs in many aquatic animals that broadcast their gametes into the water
column (e.g. urchins, Levitan 2004 ) . Conversely, reproductive proximity can also
facilitate extra-pair matings in pair-spawning species, which is a benefi t for those
who engage in it, and a cost to males that lose such matings. In colonial birds and
fi shes, for example, sperm competition can be intense, and individuals can lose a