Milk composition has been well described for baleen whales, and variations of fat and protein concentration along the lactation period are reported (Oftedal, Reference Oftedal1997). The amount of energy transferred to the offspring during lactation depends directly on the milk composition and on the total amount of milk ingested, and during this period they grow quickly and double in size (Oftedal, Reference Oftedal1997). Studies about milk of odontoceti have been merely descriptive and there is no information available concerning variation in milk composition during lactation or lactating strategies (Oftedal, Reference Oftedal1997). The difficult access to milk samples may explain this lack of information, since collection of these samples is only possible through accidental catches or strandings of lactating females (e.g. Peddemors et al., Reference Peddemors, Muelenaere and Devchand1989; Rosas & Lehti, Reference Rosas and Lehti1996).

The franciscana, Pontoporia blainvillei, is a small cetacean species endemic to the south-western Atlantic Ocean. Its distribution, characteristically coastal, extends from Itaúnas (18°25′S 030°42′W), Espírito Santo, Brazil, to Golfo Nuevo (42°35′S 064°48′W), Patagonia, Argentina (Secchi et al., Reference Secchi, Danilewicz and Ott2003). Very little is known about the physiology of this species, despite the high number of accidental captures reported in all major fishing communities along its area of distribution, as this is probably the most threatened cetacean species in the south-western Atlantic (Secchi et al., Reference Secchi, Ott and Danilewicz2002). In this work the milk composition of P. blainvillei was analysed through the identification of its nutritional content, seasonal variation, and mineral and metal content.

Milk samples were collected from five lactant P. blainvillei females accidentally caught in fishing nets from Rio Grande (32°05′S 52°08′W) (samples CA05, CA010 and CA338) and Imbé fishing communities (29°58′S 50°07′W) (samples GEMARS0547 and GEMARS0828), southern Brazil. Milk was manually collected about 24 h after death and stored at −20°C. Samples CA05 and CA010 were collected in the summer (January 1993 and February 1993) while samples GEMARS0547, GEMARS0828 and CA338 were collected in the winter (August 1998, August 2000 and June 2004, respectively).

Protein nitrogen was determined by the micro-Kjeldahl method (AOAC, 1990). Fat was determined by the Mojonnier ether extraction procedure (AOAC, 1990). Carbohydrate concentration was determined using the phenol sulphuric acid method (Chaplin, Reference Chaplin, Chaplid and Kennedy1986). The caloric value of milk was estimated using fat and protein concentrations (AOAC, 1995). The cryoscopic index was obtained using a digital electronic cryoscope (MV 540, VAPRO® 5520; Wescor) (AOAC, 1975).

It was possible to determine the composition of heavy metals and minerals in the sample CA338 only. A milk fraction was submitted to digestion with nitric acid (Standard Methods 20^TH). The minerals potassium (K), phosphorus (P), sodium (Na), calcium (Ca), magnesium (Mg) and iron (Fe) were quantified by absorption spectrophotometry. In samples CA05 and CA010, only the Ca concentration was determined by oxalate titration. All values were expressed as mg/l. Mercury (Hg), lead (Pb), cadmium (Cd), zinc (Zn) and copper (Cu) were quantified by atomic absorption spectrophotometry and the values were expressed as mg/l, except for Hg (μg/l).

To detect seasonal differences in milk composition, samples collected in summer and winter were compared. Fat and protein concentrations, as well as total length (cm) and total weight (kg) were compared using Student's t-test with Tukey procedures. Data were considered to be significant at P ≤ 0.05.

Milk composition of P. blainvillei is shown in Table 1. Fat concentrations found in summer (15.60%) were statistically different to those observed in winter (8.21%), with P = 0.044. Protein concentration was higher in summer (14.07%) than in winter (10.27%), with P = 0.021. The lactant females had similar total lengths in summer and in winter (summer = 154.80 ±5.94 cm; winter = 155.0 ±2.18 cm; P = 0.958) and similar total weights (summer = 40.30 ±3.96 cm; winter = 38.13±3.20 cm; P = 0.543).

Table 1. Milk composition of the franciscana, Pontoporia blainvillei, in Rio Grande do Sul, Brazil.

n.a., not analysed.

In summer, the mean protein and fat concentrations in franciscana milk corresponded to 56.26 kcal/100 g and 73.85 kcal/100 g, respectively, totalling 130.11 kcal/100 g. The samples collected in winter corresponded to 181.47 kcal/100 g, a caloric value 1.9 folder higher than in summer, due to higher concentration of fat (140.40 kcal/100 g), while protein concentration corresponded to 41.07 kcal/100 g. Carbohydrates, possibly lactose, showed a mean value of 2.5% in franciscana milk during winter. Values for summer were not available for comparison.

The concentrations of minerals and heavy metals analysed in sample CA338 were K = 1521.0 mg/l, P = 1454.0 mg/l, Na = 1054.0 mg/l, Mg = 89.20 mg/l and Fe = 10.20 mg/l. Calcium presented a concentration of 947.0 mg/l, while samples CA05 and CA10 resulted in concentrations of 1460.0 mg/l and 1440.0 mg/l, respectively. The metals found in sample CA338 were Zn = 4.46 mg/l, Cu = 0.63 mg/l and Hg = 0.22 µg/l. Lead (Pb) and cadmium (Cd) concentrations were below the detection limits of analytical method (<0.05 and <0.002 mg/l, respectively).

The milk of franciscana presented a low concentration of fat, similar to the Amazon River dolphin (Inia geoffrensis) (Rosas & Lehti, Reference Rosas and Lehti1996), bottlenose dolphin (Tursiops truncatus) and Indo-Pacific humpbacked dolphin (Souza plumbea) (Peddemors et al., Reference Peddemors, Muelenaere and Devchand1989). Coastal species or populations of the same species generally have a lower fat concentration in milk compared to those of oceanic habits (Peddemors et al., Reference Peddemors, Muelenaere and Devchand1989). We observed a variation of fat concentration in milk, being lower in summer (8.21%) and higher in winter (15.60%). The presence of proteins in franciscana milk presented a significant variation throughout lactation, higher in winter and lower in summer. However, the mean concentrations were similar to those of other cetacean species (Oftedal, Reference Oftedal1997). There are no references concerning the variation of composition in other odontoceti. In other species, carbohydrates are present at values close to those found for franciscana (0.0–2.5%) (Oftedal, Reference Oftedal1997).

In spite of the low sample size, the data presesented in this study suggested that the milk of P. blainvillei presents a variation according to the lactation period. The differences between the beginning and the end of lactation in franciscanas may be under the influence of the sea temperature changes that are regulated by current regimens, with more energy provided to the calves through fat in colder months. Lactation in franciscanas occurs during nine months, and its ending coincides with the presence of the Falkland's current (15–16°C) in Rio Grande do Sul (Danilewicz, Reference Danilewicz2003). This variation in composition may reflect the pattern of the milk in animals exclusive to the southern range of the species where its reproduction has a well-defined birth period from October to February in southern Brazil. This suggests at least the partial influence of temperature on the water during the time of birth and lactation for the species (Danilewicz, Reference Danilewicz2003). Variation in milk composition of mysticeti during the lactation period was observed, with fat increasing from 20% to 40% throughout the first six estimated months of lactation (mid-lactation) (Oftedal, Reference Oftedal1997).

The mean concentration of Cu, Fe, Zn, Mg and Ca concentrations were similar to other odontoceti (Oftedal, Reference Oftedal1997). However, the concentrations of Na (1788.0 mg/l) and K (7625.0 mg/l) found by Rosas & Lehti (Reference Rosas and Lehti1996) for the Amazon River dolphin (I. geoffrensis) were higher than those found in this study. Zinc concentrations were higher in franciscana than in T. truncatus from the Mediterranean Sea (Frodello et al., Reference Frodello, Viale and Marchand2002), and Pb, Cd and Cu concentrations were lower. Franciscana showed Hg concentrations similar to T. truncatus (0.22 µg/l vs 0.2 µg/kg, respectively), but lower than that found in the Amazon River dolphin (176.0 µg/l) (Rosas & Lehti, Reference Rosas and Lehti1996). This high Hg concentration found in the Amazon River dolphin might be related to gold mining in the Amazon River (Rosas & Lehti, Reference Rosas and Lehti1996).

The variation in the composition of milk throughout lactation, as well as the evaluation of its nutritional components, are important information for the understanding of the energy input in the development and growth of P. blainvillei calves. Therefore, further studies are recommended with larger sample sizes in other regions.