65
3 Why Spawn in Aggregations?
individuals (Colin and Clavijo 1988 ) , also forms large feeding groups during
the non-reproductive season (Morgan and Kramer 2004 ) . This is also the case
for the yellow and blueback fusilier, Caesio teres (Bell and Colin 1986 ) , and
white trevally, Pseudocaranx dentex (Afonso et al. 2008a ) , as well as other
schooling species.
Although higher mate encounter rates could be demonstrated easily by comparing
the number of intersexual interactions during and away from spawning aggregations, this would not tell us much about the evolution of spawning aggregations. By
defi nition, spawning aggregations involve high population densities (and thus,
presumably, high encounter rates with mates), but it would be diffi cult to determine
whether higher encounter rates at aggregations are actually the evolutionary driver
of formation of spawning aggregations. It is possible that, because individuals that
reproduced in aggregations are guaranteed to encounter a mate at that time, they are
‘free’ to exploit sex-specifi c resources during the non-breeding season. This may be
the case for several shark species, such as nurse sharks, Ginglymostoma cirratum ,
scalloped hammerheads Sphyrna lewini and sand tiger sharks Carcharias taurus ,
which exhibit both mating in aggregations and sexual segregation out of the mating
season (reviewed by Sims 2005 ) . The evolution of spawning aggregations could
therefore be a cause, rather than a consequence, of the distribution of non-spawning
individuals.
3.3.2 Mate Choice Facilitation or Cost Reduction
If spawning aggregations have evolved because females prefer situations that facilitate mate choice, then we should see evidence of greater ease of choice. Females
often select mates by assessing the quality of courtship displays, and courtship commonly occurs at spawning aggregations. For example, males of several epinepheline
groupers exhibit intricate displays involving colour, swimming patterns, sound and
physical contact (e.g. Colin 1994 ; Pelaprat 1999 ; Erisman et al. 2007 ) . Female
dusky groupers, Epinephelus marginatus , spawning in Corsica, France, visit or are
visited by several courting males but spawning occurs with only a small number of
these visitors (Pelaprat 1999 ) , implying that mate selection occurs. Similar displays
and apparent mate selection have been observed in many other aggregating
taxa, including parrotfi shes (Scaridae) (Clavijo 1983 ; Gladstone 1986 ) , angelfi shes
(Pomacanthidae) (Moyer et al. 1983 ) and jacks (Carangidae) (Graham and Castellanos
2005 ) (Fig. 3.3 ) (also see Chaps. 12.2 and 12.18 ).
Courtship is also observed in non-aggregating species, so the presence of displays
at aggregation alone is not strong support for mate choice facilitation. The key question
is whether aggregating females are able to fi nd, inspect and compare potential mates
more easily than their non-spawning relatives. A number of observations made in
relation to aggregation size or density would be consistent with this idea. With
increasing aggregation size/density, the time taken by females to select a mate
should decrease, but mate inspection rate should increase, as should the probability
3 Why Spawn in Aggregations?
individuals (Colin and Clavijo 1988 ) , also forms large feeding groups during
the non-reproductive season (Morgan and Kramer 2004 ) . This is also the case
for the yellow and blueback fusilier, Caesio teres (Bell and Colin 1986 ) , and
white trevally, Pseudocaranx dentex (Afonso et al. 2008a ) , as well as other
schooling species.
Although higher mate encounter rates could be demonstrated easily by comparing
the number of intersexual interactions during and away from spawning aggregations, this would not tell us much about the evolution of spawning aggregations. By
defi nition, spawning aggregations involve high population densities (and thus,
presumably, high encounter rates with mates), but it would be diffi cult to determine
whether higher encounter rates at aggregations are actually the evolutionary driver
of formation of spawning aggregations. It is possible that, because individuals that
reproduced in aggregations are guaranteed to encounter a mate at that time, they are
‘free’ to exploit sex-specifi c resources during the non-breeding season. This may be
the case for several shark species, such as nurse sharks, Ginglymostoma cirratum ,
scalloped hammerheads Sphyrna lewini and sand tiger sharks Carcharias taurus ,
which exhibit both mating in aggregations and sexual segregation out of the mating
season (reviewed by Sims 2005 ) . The evolution of spawning aggregations could
therefore be a cause, rather than a consequence, of the distribution of non-spawning
individuals.
3.3.2 Mate Choice Facilitation or Cost Reduction
If spawning aggregations have evolved because females prefer situations that facilitate mate choice, then we should see evidence of greater ease of choice. Females
often select mates by assessing the quality of courtship displays, and courtship commonly occurs at spawning aggregations. For example, males of several epinepheline
groupers exhibit intricate displays involving colour, swimming patterns, sound and
physical contact (e.g. Colin 1994 ; Pelaprat 1999 ; Erisman et al. 2007 ) . Female
dusky groupers, Epinephelus marginatus , spawning in Corsica, France, visit or are
visited by several courting males but spawning occurs with only a small number of
these visitors (Pelaprat 1999 ) , implying that mate selection occurs. Similar displays
and apparent mate selection have been observed in many other aggregating
taxa, including parrotfi shes (Scaridae) (Clavijo 1983 ; Gladstone 1986 ) , angelfi shes
(Pomacanthidae) (Moyer et al. 1983 ) and jacks (Carangidae) (Graham and Castellanos
2005 ) (Fig. 3.3 ) (also see Chaps. 12.2 and 12.18 ).
Courtship is also observed in non-aggregating species, so the presence of displays
at aggregation alone is not strong support for mate choice facilitation. The key question
is whether aggregating females are able to fi nd, inspect and compare potential mates
more easily than their non-spawning relatives. A number of observations made in
relation to aggregation size or density would be consistent with this idea. With
increasing aggregation size/density, the time taken by females to select a mate
should decrease, but mate inspection rate should increase, as should the probability