Lower investment income and higher catastrophe costs helped drive RenaissanceRe Holdings Ltd.’s second-quarter 2015 net income down 39.4% from the same period a year earlier to $73.2 million, the Pembroke, Bermuda-based reinsurer said.

Second-quarter gross written premiums rose 29.4% year over year to $662.0 million, RenRe said in its Tuesday earnings release. The company generated underwriting income of $94.1 million and a combined ratio of 75.2% in the second quarter of 2015, compared with $99.7 million and 61.7%, respectively, in the second quarter of 2014.

Gross premiums written in the company’s specialty reinsurance segment increased 210.4% from the second quarter of 2014 to $160.0 million, “driven by increases across substantially all lines of business, most notably certain casualty and property other lines of business, principally due to the acquisition of Platinum Underwriters Holdings Ltd. on March 2, 2015.”

Second-quarter underwriting income was driven by increased net premiums earned, mostly due to increased gross premiums written in RenRe’s specialty reinsurance segment, “which was offset by higher current accident year net claims and claim expenses,” the company said.

But RenRe’s total investment result, which principally includes the sum of net investment income, net realized and unrealized losses or gains on investments, was $11.3 million in the second quarter of 2015, compared with $61.6 million in the second quarter of 2014, primarily driven by rising interest rates that affected fixed-maturity investments, resulting in a net loss, the company said.

For the first six months of the year, RenRe’s net income dropped 11.2% year over year to $241.1 million. Gross written premiums increased 7.3% to $1.31 billion, while the combined ratio deteriorated to 66.7% from 54.1% during the same period a year earlier.

“In general, we executed well in the second quarter,” said RenRe CEO Kevin J. O’Donnell during a Wednesday conference call. “I continue to be pleased with the Platinum integration,” he said. Looking ahead, he added that “as we head into hurricane season, we are prepared for any eventuality.”

“While the integration of Platinum appears to be on track, what appeared as big news in late 2014 is now being questioned as to whether it’s enough,” Cliff Gallant, an analyst at Nomura Securities International Inc. in San Francisco, said in a research note.

“The reinsurance business remains under tremendous pressure, particularly in catastrophe, and significantly greater size and diversity may be necessary,” he wrote. “However, due to cultural fit and valuation expectations, merger partners may not be easy to find.”