Integrating molecular techniques with field methods in studies of social behavior: a revolution results

Ecology, March, 1998 by Colin Hughes

Groups of cooperatively breeding birds are often described as having a monogamous pair assisted by helpers, which are offspring from previous years. Red-cockaded Woodpeckers, Picoides borealis, are such a species (Haig et al. 1994). DNA fingerprinting identified only one of 32 progeny as a product of an EPFs, and it was apparently sired by a nongroup mate. Similarly, in Scrub Jays, Aphelocoma coerulescens, the dominants thought to be the breeding pair were confirmed by DNA fingerprinting to be parents of all offspring raised by the group (G. Woolfenden, cited in Birkhead and Moller 1992). Thus, in these species, helpers apparently do not reproduce directly.

In other species, molecular methods have revealed that helpers do sometimes reproduce directly. In Stripebacked Wrens, Campylorhynchus nuchalis, a breeding pair may be aided by male and female offspring from previous years. Delayed dispersal was previously thought to be maintained by indirect reproductive success and delayed direct benefits in the form of production of future helpers and inheriting the position of breeding male in the group (Rabenold 1985). However, DNA fingerprinting showed that when the breeding female (their mother) dies and is replaced by an immigrant (unrelated female), auxiliary males may sire some of the new female's offspring (9% of all offspring produced fell in this category; Rabenold et al. 1990). In the sympatric population of Bicolored Wrens, C. griseus, auxiliary males sire just 2.3% of offspring; again, they only reproduce when they are unrelated to the breeding female (Haydock et al. 1996). In these two species, breeding females only occasionally mate with a male outside their group (1% in C. nuchalis and 2.3% in C. griseus). Thus, the Bicolored Wren genetic mating system approaches monogamy. Allozymes show that auxiliary males reproduce within groups of Acorn Woodpeckers, Melanerpes formicivorous, too (Joste et al. 1985), though the rate may be quite low (Mumme et al. 1985). Indeed, socially monogamous Acorn Woodpeckers are nearly (98%) genetically monogamous, too (Dickinson et al. 1995). However, when Acorn Woodpecker groups contain a behaviorally identifiable breeding pair, it is clear that the benefits of natal philopatry and helping behavior are a more complicated blend of direct and indirect effects than we had previously imagined. More complete data sets must be examined, from a larger number of cooperatively breeding species before we understand the diversity of routes through which this social system has evolved and been maintained.

In long-term studies of individually marked vertebrates, kinship can be estimated from inferred pedigree connections, and molecular methods can be integrated with long-term observations to confirm relationships and resolve unknown pedigree connections. This approach has allowed a thorough examination of the choice that individual dwarf mongooses, Helogale parvula, make as to whether to disperse or remain in their natal pack (Creel and Waser 1994, Keane et al. 1994). The analysis correctly predicted that males should be more likely to disperse than females. Genotyping of chimpanzees, Pan troglodytes, from Gombe allowed the assignment of paternity for offspring of females that mated with multiple males during their fertile period (Morin et al. 1994b). The data were used to confirm relatedness among cooperative males, and therefore a potential role for kin selection in the evolution of their behavior (Morin et al. 1994a). Morin's work shows how microsatellite markers are peculiarly appropriate for such studies; they can be obtained from tiny DNA samples, collected with minimal intrusion (shed hairs from chimp nests in this case; Morin and Woodruff 1992), and yet they yield a large quantity of high-quality information.