“Morph-specific patterns of reproductive senescence: connections to discrete reproductive strategies”
Andrea S. Grunst, Melissa L. Grunst, Vince A. Formica, Marisa L. Korody, Adam M. Betuel, Margarida Barcelo-Serra, Sarah Ford, Rusty A. Gonser, and Elaina M. Tuttle (June 2018)
Our results underscore the importance of social dynamics such as shared parental care in reproductive longevity
Morph-specific patterns of reproductive senescence: connections to discrete reproductive strategies
Senescence, the gradual decline in performance with age, was once thought to be confined to humans inhabiting tame environments. In truth, senescence is a nearly universal property of life, manifest in an array of wild organisms. Substantial variation exists in senescence rates, with sex differences in senescence long appreciated. One hypothesis proposes that intense competition for mates, most commonly by males, induces a live-fast-die-young strategy, and rapid senescence. Alternatively, parental care may be associated with rapid senescence. Grunst et al. 2018 offer new insight into the influence of competition and parental care on senescence by studying the genetically-determined morphs of the white-throated sparrow (Zonotrichia albicollis). These morphs show discrete differences in behavioral traits that may affect rates of senescence. White morph birds are more aggressive and less parental than tan counterparts, and white males are promiscuous. In addition, social pairing is disassortative by morph, with white males pairing with tan females (W × T pairs), and tan males with white females (T × W pairs), almost exclusively. Social dynamics within pair types differ. T × W pairs practice biparental care, but in W × T pairs, care is female-biased. The researchers used data on sparrows breeding near Cranberry Lake Biological Station in New York’s Adirondack Mountains. Drs. Elaina Tuttle and Rusty Gonser have studied this population since 1991. Aging white males displayed faster reproductive senescence than tan counterparts, but lived longer. Thus, faster reproductive senescence in white males likely reflects difficulty sustaining a competitive reproductive strategy with age, rather than physiological costs of competitiveness. Tan females exhibited earlier reproductive senescence than white females, and were short-lived, perhaps reflecting challenges of unsupported motherhood. Biparental care may also help older tan males and white females sustain reproductive performance. Thus, social dynamics may play an underappreciated role in determining patterns of senescence.
How reproductive strategies contribute to patterns of senescence in natural populations remains contentious. We studied reproductive senescence in the dimorphic white-throated sparrow, an excellent species for exploring this issue. Within both sexes the morphs use distinct reproductive strategies, and disassortative pairing by morph results in pair types with distinct parental systems. White morph birds are more colorful and aggressive than tan counterparts, and white males compete for extra-pair matings whereas tan males are more parental. Tan males and white females share parental care equally, whereas white males provide little parental support to tan females. We found morph-specific patterns of reproductive senescence in both sexes. White males exhibited greater reproductive senescence than tan males. This result likely reflects the difficulty of sustaining a highly competitive reproductive strategy as aging progresses, rather than high physiological costs of competitiveness, since white males were also long-lived. Moreover, morph was not consistently related to reproductive senescence across the sexes, arguing against especially high costs of the traits associated with white morph identity. Rather, tan females exhibited earlier reproductive senescence than white females, and were short-lived, perhaps reflecting the challenges of unsupported motherhood. Results underscore the importance of social dynamics in determining patterns of reproductive senescence.