Cooperative breeding is a breeding system in which more than a pair of individuals shows parent-like behaviour towards young of a single nest. In its broadest
sense, it includes reproductive systems varying from helping by non-breeding
offspring to various forms of polygamy, in which more than one male or female
share breeding status (Brown 1987; Cockburn 1998; Hatchwell and Komdeur
2000). This is the case of the bearded vulture in the Pyrenees, a species considered
monogamous but with several polyandrous territories found since the beginning of
the 80s (Heredia and Donázar 1990; Bertran et al. 2009). The copulation behaviour
suggests that most of these territories are formed by two males and a female
(Bertran and Margalida 2002a, 2003, 2004) but also polyandrous quartets have
been described (Margalida et al. 1997).
15.4.2 Contrasting Hypotheses to Explain Changes
in Reproductive System
Several hypotheses have been proposed to explain the evolution of this reproductive system. According to the ecological constraints hypothesis, cooperative
breeding appears when opportunities for independent breeding are limited because
of ecological factors, such as low availability of resources or high risk of mortality
during dispersal. The life-history hypothesis, however, states that cooperative
breeding is a consequence of certain life-history traits of a species, such as low adult
mortality, that reduce the opportunities for independent breeding. Beyond their
differences, both hypotheses stress that direct fitness benefits of looking for independent breeding opportunities do not outweigh the indirect fitness benefits of
helping relatives (Pen and Weissing 2000). Although the evolution of cooperative
breeding has often been attributed to kin selection (Emlen 1991; Emlen et al. 1991),
there is increasing evidence that helpers can be unrelated to the young they are
raising (Cockburn 1998; Heg and van Treuren 1998). Other rewards, such as
staying in a restricted area to increase their probabilities to acquire a high-quality
territory (Heg and van Treuren 1998; Kokko and Sutherland 1998) or, as in
polygamous units, to have some chance of breeding (Hartley and Davies 1994)
should tempt birds to join an existing breeding pair rather than search for an
unlikely opportunity of independent reproduction in a saturated environment.
15.4.3 What Happened with the Pyrenean Population?
The Polyandrous Emergence
The analysis of the role of habitat saturation in the mating system of bearded
vultures, a territorial and usually monogamous raptor, was assessed by Carrete et al.
(2006a). Because the bearded vulture population increased progressively since
350
A. Margalida
sense, it includes reproductive systems varying from helping by non-breeding
offspring to various forms of polygamy, in which more than one male or female
share breeding status (Brown 1987; Cockburn 1998; Hatchwell and Komdeur
2000). This is the case of the bearded vulture in the Pyrenees, a species considered
monogamous but with several polyandrous territories found since the beginning of
the 80s (Heredia and Donázar 1990; Bertran et al. 2009). The copulation behaviour
suggests that most of these territories are formed by two males and a female
(Bertran and Margalida 2002a, 2003, 2004) but also polyandrous quartets have
been described (Margalida et al. 1997).
15.4.2 Contrasting Hypotheses to Explain Changes
in Reproductive System
Several hypotheses have been proposed to explain the evolution of this reproductive system. According to the ecological constraints hypothesis, cooperative
breeding appears when opportunities for independent breeding are limited because
of ecological factors, such as low availability of resources or high risk of mortality
during dispersal. The life-history hypothesis, however, states that cooperative
breeding is a consequence of certain life-history traits of a species, such as low adult
mortality, that reduce the opportunities for independent breeding. Beyond their
differences, both hypotheses stress that direct fitness benefits of looking for independent breeding opportunities do not outweigh the indirect fitness benefits of
helping relatives (Pen and Weissing 2000). Although the evolution of cooperative
breeding has often been attributed to kin selection (Emlen 1991; Emlen et al. 1991),
there is increasing evidence that helpers can be unrelated to the young they are
raising (Cockburn 1998; Heg and van Treuren 1998). Other rewards, such as
staying in a restricted area to increase their probabilities to acquire a high-quality
territory (Heg and van Treuren 1998; Kokko and Sutherland 1998) or, as in
polygamous units, to have some chance of breeding (Hartley and Davies 1994)
should tempt birds to join an existing breeding pair rather than search for an
unlikely opportunity of independent reproduction in a saturated environment.
15.4.3 What Happened with the Pyrenean Population?
The Polyandrous Emergence
The analysis of the role of habitat saturation in the mating system of bearded
vultures, a territorial and usually monogamous raptor, was assessed by Carrete et al.
(2006a). Because the bearded vulture population increased progressively since
350
A. Margalida