Aging Techniques, Juvenile Breeding, and Body Mass in Thomomys bulbivorus

Abstract

The degree of ossification of cranial sutures or the sequence in which they ossify have been used to estimate age of a variety of species of mammals (Sanderson 1961; Hoffmeister and Lee 1963; Hoffmeister and Zimmerman 1967; Mead 1967; Birney and Fleharty 1968; Junge and Hoffmeister 1980; Fiero and Verts 1986) commonly with a high degree of reliability. Thaeler (1968:545) applied the technique to Thomomys bottae and T. townsendii by scoring five cranial sutures (palatine-pterygoid, maxilla-palatine, maxilla-alisphenoid, alisphenoid-squamosal, and parietal-squamosal) as either fused or open; he considered gophers with most or of the five sutures fused to be (born in or before the previous year). Thaeler (1968:545) also indicated that the dorsal surface of the maxilla in the zygomatic arch and the were no longer porous in adults, but were distinctly so in younger animals. Daly and Patton (1986) separated young-of-the-year (juveniles) from older individuals (adults) in T. bottae by totaling scores of the degree of ossification of supraoccipital-exoccipital and basioccipitalbasisphenoid sutures. Scoring was on a scale of one to three with one being open, two being partly closed, and three being fused; midpoints between the three categories were distinguished. Gophers with total scores <4.5 were considered to be young-of-the-year (juveniles), whereas those with total scores -4.5 were considered to be born in or before the previous calendar year (adults). Within a sample of known-aged individuals, 93% and 91% of juvenile females and males, respectively, were classified correctly; among adults, 92% of females and 98% of males were classified correctly (Daly and Patton 1986). In T. bottae, some females, but no males, may breed as juveniles (Daly and Patton 1986). Thomomys bulbivorus is endemic to the Willamette Valley, Oregon (Verts and Carraway 1987), and is related closely to T. bottae (Thaeler 1980). The usual reproductive season (onset of breeding to last parturition) extends from late March (Wight 1918) to late July (Wight 1922), but in an irrigated orchard containing rapidly growing food plants, the reproductive season was extended to early September (Verts and Carraway 1991). Whether female T. bulbivorus in unirrigated habitats ever breed in the year of their birth has not been recorded. However, based on a gestation period of 18-19 days reported for other species of Thomomys (Schramm 1961; Andersen 1978) and on a 42day period from birth to weaning (Wight 1918); the female that produced the young, estimated to be 23 days old when caught 31 August and 1 September (Verts and Carraway 1991), could have been conceived on 1 April of the same year and had 51 days between weaning and conception to become independent, mature, and mate. Also, female pocket gophers (Geomys bursarius, Thomomys talpoides, T. bottae) are known to resorb the pubes before or at first estrus; the pubes are not restored after parturition (Hisaw 1924; Hansen 1960; Daly and Patton 1986). Thus, the three pregnant females with closed pubes captured in the irrigated orchard in mid-August (originally referred to as adults based on attainment of reproductive competence-Verts and Carraway 1991) must have been youngof-the-year. To evaluate the degree of concordance among cranial techniques of determining age, we applied them to a sample of female T. bulbivorus (N = 97) females captured 16 July-21 October, 1987-1990. We also applied criteria of physiological age related to reproduction as a means of evaluating the usefulness of the techniques based on cranial changes. Degree of ossification of cranial sutures for all individuals was recorded by one of us (DLN). Without prior knowledge of the reproductive status of specimens, the five sutures considered by Thaeler (1968) were scored as either open or fused; gophers with two or fewer fused sutures were considered to be young-of-the-year. The exoccipital-supraoccipital and basioccipital-basisphenoid sutures were scored separately by the system described by Daly and Patton (1986). Finally, for a subsample (N = 80), the maxilla and palatine were scored as either porous or nonporous. Body mass and reproductive status were recorded by BJV and LNC at the time of capture. Based on the five-suture technique (Thaeler 1968), 53 (54.7%) of the 97 females were young-ofthe-year and 44 (45.3%) were born in previous years. For the group of 44, application of the twosuture technique (Daly and Patton 1986) produced results in complete agreement; none was classified as young-of-the-year. Of the 53 females classified as young-of-the-year with the five-suture technique, 24 (45.3%) were classified as being born in previous years with the two-suture technique.