Mating Systems - An Introduction to Behavioural Ecology - page 265

earlier and are better at caring for their eggs and chicks, perhaps through improved
behavioural coordination. The decline in success in later years most likely reflects
reproductive senescence. Experiments confirmed that there
was a causal link between pair-bond duration and
reproductive performance; when either male or female
partner was removed, reproductive success was lower with
the new partner but it increased over the subsequent four
years (van de Pol et al ., 2006).
These results reveal an initial cost of breeding with a new
partner. Nevertheless, in this oystercatcher population there
was an annual divorce rate of 8%. Why? It is important to
distinguish two causes of divorce and to consider the
consequences for each individual concerned (Fig. 9.7,
Table 9.2). Oystercatchers that deserted their mates gained
increased survival and reproductive success because they
often left for better breeding territories (ones nearer good
feeding grounds). By contrast, those that were forced to
change mate because they were usurped by competitors
tended to suffer reduced fitness because they ended up in
poorer territories. Therefore, initiators of divorce gained while
victims lost (Heg et al ., 2003).

(a)

(b)

Initiators and
victims of divorce
in oystercatchers

Fig. 9.7 Two
causes of divorce:
(a) desertion; (b)
usurpation. This
cartoon refers to
a territorial
species, such
as the
oystercatcher,
where the male
remains on his
territory and the
female moves to
another territory.
The territory
where the
divorce occurs is
shaded. From
Ens et al . (1996).

Constrained to be monogamous
However, Lack's hypothesis does not explain the monogamous
mating systems of many songbirds, where both sexes also
commonly rear the young together. If males are removed
during the nestling period then females are usually able to
raise at least some of the young to independence. For example,
in song sparrows ( Melospiza melodia ) male removal caused
success to decrease to 51% of that of pair-fed broods, and in
seaside sparrows ( Ammodramus maritimus ) and dark-eyed

Participant Role Change in territory

Table 9.2
The fitness
consequences of
divorce for each
participant in
desertion and
usurpation
in Fig. 9.7 (Heg
et al ., 2003)

(a) Desertion

Victim of desertion Abandoned by mate Remains

Deserter Abandons mate Moves to another

(b) Usurpation

Victim of usurpation Forced to abandon mate Forced to leave

Bystander Loses old mate, gains new

mate

Remains