Adult age confounds estimates of static allometric slopes in a vertebrate

In many animal groups, the size of male genitalia scales shallowly with individual body size. This widespread pattern appears to admit some exceptions. For instance, steep allometries have been reported for vertebrate genitalia. This exception, however, may be due to a confounding effect arising from the continued growth of some structures during adulthood in vertebrates. Consider the possibility that genitalia continue to grow in adults while body size does not. If so, taking measurements from adults of different ages could yield steeper allometries than would be obtained from measurements of adults of the same age. We used vervet monkeys to test this hypothesis. We found that all body parts continued to grow in adult vervet monkeys, with sexual traits (including genitalia) showing faster growth rates. Traits with faster growth rates over adult ages had steeper allometries. And accounting for variation in adult age yielded shallower allometries, bringing vervet monkey genitalia in line with the predominant pattern observed in other animal groups. These results suggest that steep allometric slope estimates reported for other vertebrates may be due in part to mixing of adult ages, and reinforces one of the most consistent patterns yet detected in the study of static allometry.