Canid Reproductive Biology: an Integration of Proximate Mechanisms and Ultimate Causes

SYNOPSIS. The canid reproductive system includes many features that are unusual or even unique among mammals. Focusing on gray wolves, for example, these include monogamy, monestrum with exceptionally long proestrous and diestrous phases, a copulatory lock or tie, incorporation of adult young into the social group, behavioral suppression of mating in these subordinate young, obligate pseudopregnancy in subordinate females, and alloparental care. These features can be analyzed on the levels of both proximate and ultimate causation by considering them in the context of the reproductive system as a whole. First, when assessing possible proximate mechanisms, monestrum appears to be pivotal. It is probably accomplished by the extremely long luteal or diestrous phase, which is followed by a seasonal peak in prolactin. Two sequellae of the extended diestrus (or pseudopregnancy) in non-pregnant subordinate females are to 1) suppress any subsequent cycles, and 2) hormonally prime them to behave maternally. The prolactin peak in all adult pack members, coincident with the birth of pups, also may stimulate parental behavior. The risk of monestrum (limited chance for conception) appears to be reduced by the relatively long proestrous and estrous periods, as well as by monogamy. The adaptive value, or ultimate cause, of this reproductive system is most obvious for the more social canid species, such as the gray wolf. That is, advantages to sociality, such as cooperative hunting, may have driven development of the anomalies of the reproductive system.